---
title: "Switchboard Sizing Calculator: LV Switchboard Dimensions, Bays & Form of Separation to AS/NZS 61439"
url: "https://scaleset.com.au/calculator/switchboard-sizing"
description: "Free switchboard sizing calculator: width, height, depth and bays of an LV switchboard to AS/NZS 61439, with cable zones, a to-scale drawing and a PDF report."
updated: "2026-10-13"
standards:
  - "AS/NZS 61439.1:2016 (Low-voltage switchgear and controlgear assemblies: General rules)"
  - "AS/NZS 61439.2:2016 (Low-voltage switchgear and controlgear assemblies: Power switchgear and controlgear assemblies, including Forms of internal separation 1 to 4)"
  - "IEC 61439-2 (Low-voltage switchgear and controlgear assemblies: Power switchgear and controlgear assemblies)"
  - "AS/NZS 3000:2018 (Wiring Rules: switchboard construction, clearances and access requirements)"
author: "Wisam Tozah (B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus))"
site: "ScaleSet"
license: "https://scaleset.com.au/terms"
usage: "Cite and link. Do not reproduce or train on this content."
---

# Switchboard Sizing Calculator: LV Switchboard Dimensions, Bays & Form of Separation to AS/NZS 61439

Source: https://scaleset.com.au/calculator/switchboard-sizing

Free, browser-based switchboard sizing calculator for Australian and New Zealand electrical engineers, switchboard designers and builders. It estimates the physical footprint of a free-standing low-voltage switchboard assembly, the overall width, height and depth in millimetres and the number of bays, from its functional sections. Enter the main rating, maximum demand and fault level, the incoming and metering arrangement (main switch, supply authority meter or CT chamber, MEN link, surge protection), each distribution section with its final subcircuits and MCCB or ACB feeders, a spare-capacity allowance, any generator, changeover, power factor correction or spare sections, the Form of separation (Form 1 to Form 4b to AS/NZS 61439.2), the IP rating, the cable entry and any side cable ways. The calculator stacks the functional sections into standard bays 400 to 1200 mm wide, up to the usable height: the overall height (2200 mm unless you set 1800 to 2400 mm) less a 100 mm plinth, a 100 mm busbar zone and a 200 mm cable zone at each end the cables come in at. A section that does not fit the stack takes another bay, and a side cable way adds its own width. Depth is the greatest of the rating-band envelope, the deepest switchgear compartment, the IP-rating depth floor and any manufacturer minimum, plus 300 mm for a rear-connected cable alley. From Form 2 the busbars are partitioned off from the functional units and the partition takes a 150 mm busbar zone over the stack rather than 100 mm; Form 4b also puts each unit's terminals in a compartment of their own, so a 300 mm cable compartment sits beside every outgoing column. Forms 3b and 4a separate each outgoing unit's terminals from the unit, so every feeder takes a 150 mm terminal zone beside its breaker, which widens a section of larger feeders. Form 2a, 2b and 3a barriers are built inside the same sections and add nothing more. Beside the size it reads the ratings the assembly declares under AS/NZS 61439 (the product part, Icw and the AS/NZS 61439.1 Table 7 peak withstand, the AS/NZS 61439.1 Annex N busbar guide, the AS/NZS 61439.2 or 61439.3 Table 101 loading factor and the insulation figures) and flags a main switch whose handle would sit more than 2 m above the floor (AS/NZS 3000 Clause 2.3.3.4). It draws a to-scale general arrangement, front and side elevation with every compartment numbered, exports a PDF report, and on Pro downloads the drawing as a DXF at true size. Outputs are preliminary functional-section estimates for space planning, switchroom layout and tender allowances: AS/NZS 61439 has no universal enclosure-size formula, so the final busbar, terminal, thermal, clearance and enclosure dimensions come from the manufacturer's verified assembly before construction.

The ScaleSet Switchboard Sizing Calculator estimates the preliminary physical dimensions (width, height and depth in millimetres) and bay count of a free-standing low-voltage switchboard assembly from its functional sections (incoming switchgear, metering, distribution chassis, outgoing feeders and spare capacity) to support early-stage space planning under AS/NZS 61439. The selected Form of internal separation sets the busbar zone, a terminal zone beside every feeder at Forms 3b and 4a, and a cable compartment beside every outgoing column at Form 4b. It is a free browser-based early-design estimator and does not replace a manufacturer's verified assembly (design verification) to AS/NZS 61439. It is not the Switchroom Sizing calculator, which lays the boards out in the room around them and checks the AS/NZS 3000 clearances and exits.

## Key facts

- AS/NZS 61439 has no universal enclosure-size formula; final switchboard dimensions must come from the manufacturer's verified assembly.
- The calculator builds a preliminary width from functional sections: incomer, metering, distribution chassis, feeders and spare capacity.
- Higher Forms of separation (Form 3b, Form 4b) need more internal space in practice: from Form 2 the busbar partition takes a 150 mm busbar zone, Forms 3b and 4a give every outgoing feeder a 150 mm terminal zone beside its breaker, and Form 4b adds a 300 mm cable compartment beside every outgoing column for the units' separate terminal compartments.
- Board depth follows the greatest of the rating envelope, the switchgear compartment depth and the IP-rating depth floor, plus a rear alley for rear-connected boards.
- A 20-25% spare-capacity allowance is the common convention for new switchboards.
- Beside the footprint the calculator reads the ratings an assembly declares under AS/NZS 61439: the product part (61439.2 or 61439.3), the AS/NZS 61439.1 Table 7 peak withstand for the entered fault level, the AS/NZS 61439.2 or 61439.3 Table 101 assumed loading factor, the least AS/NZS 61439.1 Annex N copper bar for the main rating, and the impulse withstand, clearance, creepage and dielectric test voltage of AS/NZS 61439.1 Tables G.1, 1, 2 and 8.
- Every bay keeps a 200 mm cable zone clear at each end the cables come in at, so on a 2200 mm board the switchgear stacks into 1800 mm, or 1600 mm with top and bottom entry.
- Supply authority metering is direct connected up to 100 A; above that it is CT connected, and the board needs a sealed CT chamber 600 to 1000 mm wide by rating.
- A main switch must be operated from no more than 2 m above the floor (AS/NZS 3000 Clause 2.3.3.4), so the calculator flags a main switch placed higher than that.
- An assembly with a short-circuit rating not exceeding 10 kA, or protected by a current-limiting device cutting off at 17 kA or less, is exempt from short-circuit withstand verification (AS/NZS 61439.1 Clause 10.11.2).

## Who this page is for

Electrical engineers, switchboard designers, switchboard builders and manufacturers, electrical contractors and estimators who need an early-stage switchboard footprint (width, height, depth and bays) for space allocation, switchroom and riser planning, tender allowances and coordination on Australian and New Zealand low-voltage installations.

## Relevant standards

- AS/NZS 61439.1:2016 (Low-voltage switchgear and controlgear assemblies: General rules)
- AS/NZS 61439.2:2016 (Low-voltage switchgear and controlgear assemblies: Power switchgear and controlgear assemblies, including Forms of internal separation 1 to 4)
- IEC 61439-2 (Low-voltage switchgear and controlgear assemblies: Power switchgear and controlgear assemblies)
- AS/NZS 3000:2018 (Wiring Rules: switchboard construction, clearances and access requirements)

## What this tool does

- Estimate the overall width, height and depth of a free-standing LV switchboard in millimetres, and its bays, from your functional sections, for space planning and tender allowances.
- Build the board from the incoming switchgear and metering (a sealed CT chamber above the 100 A direct-connected limit, the MEN link, surge protection), each distribution section's chassis and feeders, dedicated generator, changeover, PFC and spare sections, and a spare-capacity allowance.
- Keep a 200 mm cable zone clear at each cable-entry end of every bay for the gland plate and the cables' bending room, and add side cable ways at either end of the board or beside every outgoing column.
- Record the AS/NZS 61439.2 / IEC 61439-2 Form of separation (Form 1, 2a/2b, 3a/3b, 4a/4b) and size the board for it: a 150 mm partitioned busbar zone from Form 2, a 150 mm terminal zone beside every feeder breaker at Forms 3b and 4a, and a 300 mm cable compartment beside every outgoing column for Form 4b, where each unit's terminals have a compartment of their own.
- Size the depth from the greatest of the rating-band envelope, the actual switchgear compartment depth and the IP-rating depth floor (IP40 through IP66), with a rear alley added automatically for rear-connected boards.
- Read the ratings the assembly declares under AS/NZS 61439 beside the size: the product part, Icw and the AS/NZS 61439.1 Table 7 peak withstand, the AS/NZS 61439.1 Annex N busbar guide, the AS/NZS 61439.2 or 61439.3 Table 101 loading factor and the insulation figures.
- Get a check when the main switch would be operated from more than 2 m above the floor, the limit of AS/NZS 3000 Clause 2.3.3.4.
- See a to-scale general arrangement, front and side elevation with every compartment numbered, export it in a PDF report, and on Pro download it as a DXF at true size for any CAD package.
- Understand up front that the result is a preliminary functional-section estimate for concept design: AS/NZS 61439 has no universal enclosure-size formula, so final dimensions must come from the manufacturer's verified assembly.

## How to estimate a switchboard size with the ScaleSet Switchboard Sizing Calculator

1. **Set the supply and main switch** — Choose the main rating in amps. The main device follows from it: a switch-disconnector to 250 A, an MCCB to 630 A and an ACB from 800 A. Enter the maximum demand, which is checked against the rating (a main below it is flagged with the next standard frame to use), and the prospective fault level, which sets the Icw band and the AS/NZS 61439.1 Table 7 peak withstand. Say whether it is a main switchboard (MEN link, overvoltage category IV) or a distribution board.
2. **Set the incoming and metering** — Choose the supply authority metering: none, a meter in the board (CT connected above 100 A, with a sealed CT chamber) or CTs only with the meter elsewhere. Count the surge protection, private or check meters, customer CT sets and any spare CT chamber.
3. **Enter the distribution sections** — Add each distribution section (unmetered, house essential, house non-essential and so on) as its own bus. Count its final subcircuits, which become DIN chassis ways, and add its MCCB or ACB feeders by rating, each in its own compartment. Set the spare capacity, applied to both the chassis ways and the feeder space.
4. **Add the dedicated sections** — Count any generator incomer, changeover (ATS), power factor correction, essential services, solar or battery, EMS and controls sections, and spare bays for the future. Each is a full-height section of its own.
5. **Describe the enclosure** — Choose the AS/NZS 61439.2 Form of separation and its construction suffix, the IP rating, the cable entry and whether the cables are front or rear connected. From Form 2 the busbar zone is 150 mm, Forms 3b and 4a give every outgoing feeder a 150 mm terminal zone beside its breaker, and Form 4b adds a 300 mm cable compartment beside every outgoing column; the IP rating sets a planning depth floor; a rear connection adds a 300 mm cable alley. A 200 mm cable zone is kept at each cable-entry end of every bay; add side cable ways at either end or beside every outgoing column where the cables run the height of the board.
6. **Read the board and its drawing** — Read the width, height and depth, the bay count and the checks, then the general arrangement: a to-scale front elevation with every compartment drawn and numbered, and a side elevation with the depth and cable entry. The bay schedule, the declared AS/NZS 61439 ratings and the note for the drawings sit under it, and the whole report exports as a PDF. On Pro the drawing downloads as a DXF at true size.
7. **Confirm with the manufacturer before construction** — Treat the result as a preliminary estimate. Before construction, confirm the final busbar, terminal, thermal, clearance and enclosure dimensions with the switchboard builder's verified assembly system to AS/NZS 61439.

## How switchboard sizing works: estimating LV switchboard dimensions to AS/NZS 61439

### What the switchboard sizing calculator actually estimates

The ScaleSet Switchboard Sizing Calculator produces a preliminary physical footprint for a free-standing low-voltage switchboard assembly: the overall width, height and depth in millimetres, the number of bays, and the layout implied by the chosen Form of separation. It is a concept-stage space-planning tool: you use it to reserve floor area in a switchroom, size a riser, set a tender allowance and coordinate the board with the architecture long before the switchboard builder confirms the final assembly.

What it deliberately does not do is invent a precise enclosure. AS/NZS 61439 is a performance and verification standard, not a dimensional catalogue: it does not publish a universal formula that turns a load schedule into an exact enclosure size. The real dimensions come out of a manufacturer's verified assembly system, where busbar bracing, terminal space, thermal rise, minimum clearances and the chosen switchgear range all interact. This calculator gives you a defensible early number; the verified assembly gives you the built one.

### How the width is built up from functional sections

Switchboard width is the dimension that actually scales with the design, so it is built up section by section. The calculator starts with the incoming switchgear (the main switch or incomer inferred from the switchboard rating), adds the metering arrangement you select (a CT metering chamber, the MEN link section and any surge-protection allowance) then adds the distribution chassis that carries the outgoing final subcircuits, and finally the dedicated feeder sections for larger outgoing ways.

Outgoing final subcircuits are converted into chassis ways, and larger outgoing items are given feeder space. The total width is the sum of these functional sections, packed into standard bays, which is why the result is reported both as an overall millimetre width and as a bay count with an elevation sketch. Because the width tracks the sections you enter rather than circuit names, adding metering, more feeders or a higher spare allowance visibly widens the board.

### Typical switchboard sizes by rating

The width follows what the board carries, so two boards of the same rating can differ by a bay or more. As a starting point, a typical Australian main switchboard, carrying sub-main feeders totalling about 1.6 times its rating and a final-circuit section, comes out of the calculator as follows, bottom entry, front connected, IP54. A 250 A board is about 1250 mm wide, 2200 mm high and 450 mm deep. A 630 A board is about 1950 mm wide, 2200 mm high and 450 mm deep. A 1600 A board is about 3700 mm wide, 2200 mm high and 600 mm deep. A 3200 A board is about 6950 mm wide, 2200 mm high and 800 mm deep.

The depth steps with the switchgear: a switch-disconnector or MCCB board sits on the 450 mm IP54 planning floor, an ACB board from 800 A is 600 mm deep, and from 2000 A it is 800 mm. Rear connection adds a 300 mm cable alley to any of them. Enter your own sections for your own board: the figures above are where a design starts, not where it ends.

### Form of separation and how the calculator treats it

The Form of separation describes how far the internal barriers go in segregating busbars, functional units and terminals from one another, as set out in AS/NZS 61439.2 Table 104: Form 1 has no internal separation; Form 2 separates the busbars from all functional units, with the terminals for external conductors not separated from the busbars (2a) or separated from them (2b); Form 3 also separates the functional units from one another and the terminals from the units, again with the terminals not separated from the busbars (3a) or the terminals and external conductors separated from the busbars (3b); Form 4 separates every unit's terminals from every other unit as well, with the terminals in the same compartment as their unit (4a) or in their own separate compartments (4b). Australia and New Zealand add a construction suffix (AS/NZS 61439.2 Appendix ZZ 8.101.1): h where a device's integral housing provides the separation, i where insulation does, ih for both, so a board can be specified as Form 3bh or 3bi.

Higher Forms do cost space in practice: but how much depends entirely on the manufacturer's tested cubicle system, because the barriers are achieved inside a verified assembly rather than by a universal external increment. The calculator applies the two effects the manufacturers' functional systems show from the outside. From Form 2 the busbars are partitioned off from the functional units and the partition takes a 150 mm busbar zone over the stack rather than 100 mm; Form 4b also puts each unit's terminals in a compartment of their own, so a 300 mm cable compartment sits beside every outgoing column. Forms 3b and 4a separate each outgoing unit's terminals from the unit, so every feeder takes a 150 mm terminal zone beside its breaker, which widens a section of larger feeders. Form 2a, 2b and 3a barriers are built inside the same sections and add nothing more. Choose the Form early (often Form 3b or Form 4b for main switchboards where maintenance on one circuit must be possible without isolating the whole board), and confirm its dimensional effect against the switchboard builder's range, because that is the only place the real number exists.

### How the height and depth are derived

Unlike width, the height and depth of a switchboard do not scale smoothly with the number of circuits: they step with the switchgear range, the busbar system and the cable-entry strategy. Height is therefore an envelope rather than a sum: the calculator takes the overall height as 2200 mm unless you set 1800 to 2400 mm, takes off the 100 mm plinth, the 100 mm busbar zone at the top and a 200 mm cable zone at each end the cables come in at, and packs the functional sections into the usable stack height that leaves: 1800 mm on a 2200 mm bottom-entry board. When a column of sections will not fit the usable height, the board grows sideways into another bay instead of growing taller.

Depth is taken as the greatest of four requirements rather than a single figure: the depth band for the selected main rating, the actual depth of the incoming switchgear compartment (an ACB board is deeper than an MCCB or switch-fuse board of the same rating), the depth floor implied by the selected IP rating, and any minimum depth you specify yourself. A rear-connected board then adds a rear alley on top of that front-connected depth, and the result is rounded up to the nearest 50 mm. Treat the returned height and depth as a safe box to plan around, then refine them against the manufacturer's selected range once the design firms up.

### Cable zones and side cable ways

Every bay keeps a cable zone clear of switchgear at the end the cables come in at: 200 mm for the gland plate and the room the cables need to bend up to their terminals. A bottom-entry board keeps it at the bottom of every bay, a top-entry board at the top, and a board with top and bottom entry keeps one at each end, so the stack on a 2200 mm board is 1800 mm, or 1600 mm with entry at both ends. The zone is reserved before anything is packed, so a full bay never leaves its cables nowhere to go.

Where the outgoing cables run the height of the board to reach their feeders, a side cable way gives them a full-height bay of their own with no switchgear in it: at the left end, the right end, both ends or beside every outgoing column, 200, 300, 400 or 600 mm wide. Each one adds its width to the board, and the checks ask the manufacturer to confirm it against the number and size of the cables, their bending radius and the cable supports.

### Spare capacity: designing room to grow

Most switchboards are specified with spare capacity so future circuits can be added without a new board. The calculator takes a spare-capacity percentage and applies it to both the chassis ways (spare pole positions on the distribution chassis) and the feeder space (room for additional outgoing feeder sections), so the width you see already includes the growth allowance.

A common convention is around 20 to 25% spare, but the right figure depends on the client brief and how likely the installation is to expand. Setting it explicitly means the early footprint you reserve in the switchroom already carries the spare, rather than discovering at construction that there is nowhere to add the next circuit.

### Where the main switch can sit

AS/NZS 3000 Clause 2.3.3.4 requires a main switch to be readily accessible and operated from no more than 2 m above the floor or a platform. The calculator places the main switch in its bay the way the drawing does, with the cable zone at the entry end, takes its handle at the middle of its compartment, and flags a main switchboard where that comes out above 2 m: a separate incoming bay on a 2400 mm board puts the main of a 400 A board about 2150 mm up. A lower board, or the main placed lower in the section, clears it.

### The general arrangement drawing

The result is drawn as a general arrangement: a to-scale front elevation with the doors off, every compartment numbered and drawn as what it is (a breaker body, DIN rails with their modules, three CTs on the bus, the MEN link), and a side elevation with the depth, the cable entry and any rear cable alley, with dimension chains in millimetres. It goes into the PDF report with the bay schedule and the declared ratings.

On Pro the same drawing downloads as a DXF at true size, so it drops into a drawing set in any CAD package as the board itself, 1:1 in millimetres.

### Why this is a preliminary estimate, not a verified assembly

AS/NZS 61439.1 and 61439.2 require that a low-voltage assembly be demonstrated to comply through design verification (by testing, by calculation/comparison with a tested reference design, or by satisfying design rules) and routine verification of each built assembly. Dimensions are an outcome of that process, not an input you can compute in isolation: the same load schedule can fit different enclosure sizes depending on the manufacturer's tested system, busbar ratings and temperature-rise results.

That is why every result in this tool is labelled a preliminary functional-section estimate for early design, and why the final busbar, terminal, thermal, clearance and enclosure dimensions must come from the manufacturer's verified assembly system before construction. Use the calculator to plan space and price a tender with confidence; confirm the built board with the switchboard builder.

### The ratings an assembly declares, and where the calculator reads them

AS/NZS 61439.1 Clause 5 lists what an assembly manufacturer has to declare, and several of those figures can be read directly from the design basis entered here. The prospective fault level sets the rated short-time withstand current Icw (AS/NZS 61439.1 Clause 5.3.4) and, through AS/NZS 61439.1 Table 7, the rated peak withstand current Ipk (AS/NZS 61439.1 Clause 5.3.3): the r.m.s. fault is multiplied by n, 1.5 up to 5 kA, 1.7 to 10 kA, 2.0 to 20 kA, 2.1 to 50 kA and 2.2 above, so a 25 kA fault means a 52.5 kA peak that the busbar supports must hold. AS/NZS 61439.1 Clause 9.3.2 lets a rating declared for one duration be restated for another with I squared t constant up to 3 s, which is how the 1 s band is shown for 0.2 s and 3 s as well. AS/NZS 61439.1 Clause 10.11.2 exempts an assembly whose rating does not exceed 10 kA, or a circuit behind a current-limiting device with a cut-off of 17 kA or less, from short-circuit withstand verification; everything else must be verified by test or by comparison with a tested reference design.

The main rating is read against AS/NZS 61439.1 Annex N Table N.1, the operating current of bare copper bars at 55 degrees enclosure air and 70 degrees conductor, to suggest the least copper that carries it, one bar per phase before two, with the AS/NZS 61439.1 Annex N loss formula giving the watts per metre the bar dissipates. A bar the manufacturer has verified by test may be smaller, and a hotter enclosure needs more (AS/NZS 61439.1 Table N.2). AS/NZS 61439.1 Table 12 names the bar arrangement a rating band is type-tested with, from two 30 by 5 mm bars above 400 A to four 100 by 10 mm bars at 4000 A. Table 101 of the applicable part (AS/NZS 61439.2 or 61439.3) gives the assumed loading of the outgoing circuits when actual currents are not agreed: 0.9 for two or three circuits down to 0.6 for ten or more in a power switchgear assembly, one step lower in a distribution board for ordinary persons. And for a 230/400 V system the insulation figures follow: AS/NZS 61439.1 Table G.1 puts a main switchboard at the origin of the installation in overvoltage category IV at 6 kV impulse withstand and a distribution board at category III at 4 kV; AS/NZS 61439.1 Table 1 turns those into 5.5 mm and 3 mm minimum clearances, times 1.5 when verified by measurement rather than by impulse test; AS/NZS 61439.1 Table 2 gives 6.3 mm creepage at 400 V for pollution degree 3 and material group IIIa; AS/NZS 61439.1 Table 8 sets the 1890 V dielectric test.

Which part applies is decided by the scope of AS/NZS 61439.3: a board operated by ordinary persons, rated up to 250 A, with outgoing circuits up to 125 A and no more than 300 V to earth, is a distribution board (DBO) to Part 3; anything larger, or anything in a switchroom operated by skilled persons, is a power switchgear and controlgear assembly (PSC) to Part 2. The parts differ in the assumed loading table, in the neutral rule (Part 3 in Australia and New Zealand rates the neutral at 100 % of the board rating with single-pole devices only and 50 % with multi-pole devices) and in the minimum overvoltage category, which is III for a DBO. The calculator sizes switchroom boards operated by skilled persons, so it reports Part 2.

### Standards reference set and how the tool uses them

AS/NZS 61439.1:2016 sets the general rules for low-voltage switchgear and controlgear assemblies: ratings, temperature rise, short-circuit withstand and the verification framework. AS/NZS 61439.2:2016 (aligned with IEC 61439-2) covers power switchgear and controlgear assemblies specifically and defines the Forms of internal separation (Form 1 to Form 4) that drive the internal space allowances in this calculator.

AS/NZS 3000:2018 (the Wiring Rules) governs how the switchboard is installed and accessed (construction, clearances, working space and main-switch/MEN requirements), which is why the calculator handles the incomer, MEN link and metering as distinct functional sections. The switchboard rating and outgoing schedule you enter typically come from an AS/NZS 3000 maximum-demand assessment and AS/NZS 3008.1.1 cable sizing, so the ScaleSet Maximum Demand and Cable Size calculators feed naturally into this one.

## Key terms

- **Functional unit** — A part of an assembly that performs one function (an incomer, an outgoing feeder or a final-subcircuit way) together with the switchgear and terminals belonging to it. AS/NZS 61439 sizes and separates an assembly by its functional units, which is why this calculator builds the board width from functional sections rather than from a circuit count.
- **Form of internal separation** — The degree to which an assembly's busbars, functional units and terminals are segregated by internal barriers, defined in AS/NZS 61439.2 Table 104: Form 1 (none), Form 2a/2b (busbars separated from the units, terminals not separated / separated from the busbars), Form 3a/3b (units also separated from one another, terminals not separated / separated from the busbars) and Form 4a/4b (each unit's terminals separated from every other unit, in the unit's compartment / in their own compartments). AS/NZS 61439.2 Appendix ZZ adds the suffixes h, i and ih for separation by a device housing, by insulation or both.
- **Rated peak withstand current (Ipk)** — The peak value of short-circuit current an assembly can withstand mechanically, which must be at least the peak of the prospective fault (AS/NZS 61439.1 Clause 5.3.3). It is the r.m.s. fault times the factor n of AS/NZS 61439.1 Table 7: 1.5 up to 5 kA, 1.7 to 10 kA, 2.0 to 20 kA, 2.1 to 50 kA and 2.2 above.
- **Rated short-time withstand current (Icw)** — The r.m.s. short-circuit current an assembly can carry for a stated time, usually 1 s, without damage (AS/NZS 61439.1 Clause 5.3.4). It must be at least the prospective fault at the point of connection, and up to 3 s it converts between durations with I squared t constant (AS/NZS 61439.1 Clause 9.3.2).
- **Assumed loading factor** — The fraction of their rated current the outgoing circuits of an assembly are assumed to draw together when the manufacturer and the user have not agreed actual currents: AS/NZS 61439.2 Table 101 gives 0.9 for two or three circuits, 0.8 for four or five, 0.7 for six to nine and 0.6 for ten or more; AS/NZS 61439.3 Table 101 is one step lower for a distribution board for ordinary persons. The rated diversity factor times the circuit ratings must cover it (AS/NZS 61439.1 Clause 5.4).
- **Rated impulse withstand voltage (Uimp)** — The 1.2/50 microsecond impulse an assembly's insulation withstands, chosen from AS/NZS 61439.1 Table G.1 by the system voltage and the overvoltage category of the installation point: 6 kV at the origin of a 230/400 V installation (category IV), 4 kV at distribution circuit level (III). AS/NZS 61439.1 Table 1 turns it into the minimum clearance in air.
- **Bay (section)** — One vertical cubicle of a floor-standing switchboard, bolted alongside its neighbours to form the assembly. The calculator packs the functional sections into bays of standard width and reports the bay count alongside the overall width, because switchrooms and transport access are planned bay by bay.
- **Distribution chassis** — The DIN-rail assembly inside a section that carries the small outgoing final subcircuits. Circuits are converted into chassis "ways" on an 18 mm planning pitch, banded into a standard chassis width; ways beyond one rail row add height rather than width.
- **Way** — One pole position on a distribution chassis. A 1P device occupies one way, a 2P or 1P+N device two, and a 3P+N or 4P device four, so the way count (not the circuit count) is what drives chassis width.
- **Incomer** — The incoming main switch or main protective device at the origin of the switchboard, sized from the switchboard rating. Whether it is a switch-fuse unit, a moulded-case circuit breaker (MCCB) or an air circuit breaker (ACB) sets its compartment depth, which in turn can govern the depth of the whole board.
- **MEN link** — The multiple earthed neutral link that bonds the neutral bar to the earth bar at the main switchboard, required by AS/NZS 3000:2018 for a MEN system. It needs an accessible zone in the board, so the calculator allows for it as its own functional section with a height that scales with the main rating.
- **CT metering chamber** — A dedicated compartment housing current transformers and metering for supplies too large to meter directly. Its width allowance depends on the metering purpose (EMS/BMS monitoring, private metering, tenant sub-metering or revenue-grade metering), and above a threshold number of metered feeders it becomes a shared board-level section rather than CTs inside each feeder.
- **Design verification** — The AS/NZS 61439.1 process by which a manufacturer demonstrates that an assembly design complies: by testing, by calculation or comparison with a tested reference design, or by satisfying design rules. Enclosure dimensions are an outcome of design verification, not an input that can be computed independently of the tested system.
- **Verified assembly** — A switchboard built within a manufacturer's design-verified system, so its busbar bracing, temperature rise, short-circuit withstand, clearances and enclosure dimensions are all covered by that verification. The final dimensions of a real board come from the verified assembly; a functional-section estimate like this one is a planning figure that precedes it.
- **Spare capacity** — The allowance, usually expressed as a percentage and commonly 20-25%, for circuits added after handover. In this calculator it expands the chassis design ways and reserves additional feeder space, and discrete spare bays can be added on top for whole future sections.
- **IP rating** — The AS 60529 ingress protection classification of the enclosure (for example IP40 indoors, IP55 or IP66 outdoors). A higher IP rating requires deeper gland and sealing zones, so it sets a depth floor that the calculator applies when it exceeds the depth implied by the rating band and the switchgear compartment.
- **Cable zone** — The height kept clear of switchgear at the end of a bay where the cables come in: the gland plate and the room the cables need to bend to their terminals. The calculator keeps 200 mm at each cable-entry end of every bay.
- **Side cable way** — A full-height bay with no switchgear in it, beside the sections, for outgoing cables to run up or down to their terminals. It sits at either end of the board or beside every outgoing column, and adds its own width.
- **General arrangement (GA)** — The drawing of a switchboard as it will be built: its front elevation with the doors off, its side elevation and its dimensions. The calculator draws a preliminary GA to scale from its own result.

## Frequently asked questions

### Does AS/NZS 61439 tell me how big a switchboard should be?

No. AS/NZS 61439 is a performance and verification standard, not a dimensional catalogue: it has no universal formula that turns a load schedule into an enclosure size. The ScaleSet calculator builds a preliminary footprint from your functional sections (incomer, metering, chassis, feeders, spare) for early space planning; the final dimensions must come from the switchboard manufacturer's verified assembly system.

### What Form of separation should I specify: Form 1, 2, 3b or 4b?

Form 3b or Form 4b is common for a main switchboard, where one circuit has to be worked on without isolating the whole board; Form 1 or Form 2a/2b suits a smaller distribution board. Higher Forms add internal barriers and terminal segregation, so the same electrical content takes more space, and how much depends on the manufacturer's tested cubicle system. The calculator applies what each Form does from the outside: a 150 mm partitioned busbar zone from Form 2 instead of 100 mm, a 150 mm terminal zone beside every feeder breaker at Forms 3b and 4a, and at Form 4b a 300 mm cable compartment beside every outgoing column, because each unit's terminals need a compartment of their own. Form 2a, 2b and 3a barriers are built inside the same sections and add nothing more. Confirm the arrangement with the switchboard builder for the range you intend to use.

### How much spare capacity should a switchboard have?

A common convention is 20-25% spare, applied to both the chassis ways and the feeder space. The right figure depends on the client brief and how likely the installation is to grow. The calculator takes the percentage as an input and includes it in the reported width, so the footprint you reserve already carries the growth allowance.

### What peak current must the switchboard busbars withstand?

The rated peak withstand current Ipk has to be at least the peak of the prospective fault, and AS/NZS 61439.1 Table 7 gives the factor between the r.m.s. fault and its peak: n is 1.5 up to 5 kA, 1.7 up to 10 kA, 2.0 up to 20 kA, 2.1 up to 50 kA and 2.2 above. A 25 kA fault therefore needs a 52.5 kA peak rating. The calculator shows both figures beside the Icw band, and restates the band for 0.2 s and 3 s using the I squared t constant rule of AS/NZS 61439.1 Clause 9.3.2.

### Is my switchboard a distribution board to AS/NZS 61439.3 or an assembly to 61439.2?

It is a 61439.3 distribution board (DBO) only if ordinary persons operate it and it sits inside the Part 3 scope: rated up to 250 A, outgoing circuits up to 125 A, no more than 300 V to earth. A main switchboard in a switchroom, or any board above those limits, is a power switchgear and controlgear assembly to 61439.2. The parts differ in their assumed loading table, their neutral rule and the minimum overvoltage category. The calculator sizes switchroom boards operated by skilled persons, so it reports Part 2.

### How big should the main busbar be?

The calculator suggests the least bare copper in AS/NZS 61439.1 Annex N Table N.1 whose operating current covers the main rating at 55 degrees enclosure air and 70 degrees conductor, one bar per phase before two, and reports the watts per metre it dissipates. It is a guide: a bar verified by test in the manufacturer's system may be smaller, and a hotter enclosure needs more copper by the AS/NZS 61439.1 Table N.2 factor.

### Can I use the result for tender pricing and switchroom planning?

Yes, that is exactly what it is for: reserving floor area in a switchroom, sizing a riser and setting a tender allowance at concept stage. Treat the output as a preliminary functional-section estimate, then confirm the final busbar, terminal, thermal, clearance and enclosure dimensions with the switchboard builder's verified assembly before construction.

### How big is a main switchboard?

It depends on what the board carries, so enter your own sections for your own board. As a starting point, a typical Australian main switchboard, carrying sub-main feeders totalling about 1.6 times its rating and a final-circuit section, bottom entry, front connected and IP54, comes out of the calculator as follows. A 250 A board is about 1250 mm wide, 2200 mm high and 450 mm deep. A 400 A board is about 1950 mm wide, 2200 mm high and 450 mm deep. A 630 A board is about 1950 mm wide, 2200 mm high and 450 mm deep. A 1600 A board is about 3700 mm wide, 2200 mm high and 600 mm deep. A 3200 A board is about 6950 mm wide, 2200 mm high and 800 mm deep. The final dimensions come from the manufacturer's verified assembly.

### How is the size of a switchboard calculated?

From its functional sections, stacked into bays. Each section (the incoming switchgear and metering, every distribution chassis with its final subcircuits, the MCCB or ACB feeders, any generator, changeover, PFC or spare section, and the spare-capacity allowance) takes a standard bay width of 400 to 1200 mm and a height in the stack. The usable stack is the overall height, 2200 mm unless you set 1800 to 2400 mm, less a 100 mm plinth, the busbar zone and a 200 mm cable zone at each cable-entry end. A section that does not fit the stack takes another bay, so the width is the sum of the bays plus any side cable ways. The result is a preliminary footprint for space planning; the final dimensions come from the manufacturer's verified assembly.

### How deep is a switchboard?

From 300 mm for a small IP40 board to 800 mm for a large main switchboard, before any rear cable alley. The calculator takes the greatest of the depth band for the main rating (300 mm to 250 A, 350 mm to 400 A, 400 mm to 630 A, 600 mm to 1600 A, 800 mm above), the deepest switchgear compartment on the board, the IP-rating depth floor (300 mm at IP40, 350 mm at IP42, 450 mm from IP44 to IP55, 500 mm at IP65, 600 mm at IP66) and any manufacturer minimum you set. A rear-connected board adds 300 mm for its cable alley, and the total rounds up to 50 mm. A 630 A IP54 front-connected board comes out 450 mm deep. An IP rating does not itself set dimensions, so confirm the depth with the manufacturer.

### How much cable zone and gland plate space does a switchboard need?

The calculator keeps a 200 mm cable zone clear of switchgear at each end of every bay where the cables come in, for the gland plate and the cables' bending room: one at the bottom for bottom entry, one at the top for top entry, one at each end for both. On a 2200 mm bottom-entry board, after the 100 mm plinth and the 100 mm busbar zone at the top, that leaves an 1800 mm stack for the switchgear; from Form 2 the busbar zone is 150 mm. Where the outgoing cables run the height of the board, add a side cable way, a full-height bay of its own, 200 to 600 mm wide.

### What is a side cable way on a switchboard?

A full-height bay with no switchgear in it, set beside the sections so the outgoing cables can run up or down to their terminals without crossing other functional units. The calculator places one at the left end, the right end, both ends or beside every outgoing column, at 200, 300, 400 or 600 mm, and adds its width to the board.

### How high can the main switch be on a switchboard?

No more than 2 m above the floor or a platform: AS/NZS 3000 Clause 2.3.3.4 requires a main switch to be readily accessible and its means of operation no more than 2 m up. The calculator takes the main switch's handle at the middle of its compartment, as placed in the bay, and flags a main switchboard where it comes out higher, which happens on a 2400 mm board with the main at the top of a separate incoming bay.

### When does a switchboard need CT metering?

When the supply is above the 100 A limit for a direct-connected meter. Up to 100 A the supply authority meter is wired straight in, a 600 by 400 mm meter panel; above it the meter reads current transformers on the incoming conductors, and the board needs a sealed CT chamber, about 600 mm wide to 250 A, 650 mm to 1000 A, 800 mm to 2000 A and 1000 mm above. The metering provider confirms the CT type and the chamber.

### Can I get the switchboard general arrangement drawing as a DXF?

Yes, on Pro. The general arrangement, front and side elevation with every bay, compartment and dimension, downloads as a DXF at true size in millimetres, ready for any CAD package that opens DXF. On any plan, a free account downloads the same drawing in the PDF report.

## Related ScaleSet pages

- [Switchroom Sizing Calculator (the room around the board)](https://scaleset.com.au/calculator/switchroom-sizing)
- [Maximum Demand Calculator](https://scaleset.com.au/calculator/max-demand)
- [Cable Size Calculator](https://scaleset.com.au/calculator/cable-selection)
- [Arc Flash Calculator](https://scaleset.com.au/calculator/arc-flash)
- [Selectivity Calculator (Breaker Discrimination)](https://scaleset.com.au/calculator/selectivity)
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- [Voltage Drop Calculator](https://scaleset.com.au/calculator/voltage-drop)
- [Design Guides](https://scaleset.com.au/design-guide)
- [AS/NZS 3000 Table C2 Non-Domestic Maximum Demand](https://scaleset.com.au/calculator/c2)
- [AS/NZS 3000 Table C3 Energy Demand (VA/m²)](https://scaleset.com.au/calculator/c3)
- [Emergency Lighting Design Guide](https://scaleset.com.au/design-guide/emergency-lighting)
- [Calculation Verification](https://scaleset.com.au/verification)

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