--- title: "LED Inrush Current Calculator: Fittings per MCB to AS/NZS 60898.1" url: "https://scaleset.com.au/calculator/led-inrush" description: "How many LED fittings can one MCB take before it nuisance-trips? Free LED inrush current calculator: enter driver Ipk, T50 and the MCB curve, get the answer." updated: "2026-10-13" standards: - "AS/NZS 60898.1" - "IEC 60898-1" - "AS/NZS 3000:2018" 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." --- # LED Inrush Current Calculator: Fittings per MCB to AS/NZS 60898.1 Source: https://scaleset.com.au/calculator/led-inrush Find the maximum number of LED fittings you can safely connect to a single MCB using the proof-factor method. Inputs are the driver peak inrush Ipk, pulse width T50, fitting wattage and MCB rating; the calculator returns max fittings (inrush-limited and wattage-limited, both counted per phase on a three-phase circuit and multiplied out to the circuit) and shows every step. ## Key facts - LED driver inrush trips MCB magnetic elements long before the thermal wattage limit is reached. - The proof-factor method computes an inrush limit and a wattage limit; the smaller of the two governs the fittings count. - The magnetic-trip multiplier n defaults to mid-band: 4, 8 and 15 for Type B, C and D. AS/NZS 60898.1 guarantees no instantaneous trip only up to the lower limit (3, 5 and 10 × In), so for a guaranteed no-trip count enter the lower limit or the MCB datasheet minimum. - The wattage limit loads the MCB to its full rating at a power factor of about 1. A small driver often runs at 0.5 to 0.9, draws more current than its wattage suggests, and fits fewer fittings than the wattage limit shows. - The peak inrush Ipk and pulse width T50 come from the LED driver datasheet. ## Who this page is for Electrical engineers, lighting designers, contractors and electricians specifying MCBs for LED lighting circuits in Australian and New Zealand commercial, industrial and residential installations. ## Relevant standards - AS/NZS 60898.1 - IEC 60898-1 - AS/NZS 3000:2018 ## What this tool does - Proof-factor (k) method: the MCB magnetic trip threshold is multiplied by a pulse-duration factor k, read off a proof-factor chart at the driver pulse width T50. - Built-in k(T50) chart digitised from the Siemens 5SY proof-factor curve, with log-interpolation between anchor points. - Type B / C / D one-click presets auto-populate the MCB magnetic-trip multiplier n (4 / 8 / 15): editable for the specific MCB datasheet. - Two binding limits computed in parallel: inrush limit (N_inrush = ⌊k·n·In ÷ Ipk⌋) and wattage limit (N_wattage = ⌊In ÷ I_fit⌋), and the answer is the smaller of the two. - Driver-efficiency-based per-fitting current: I_fit = P ÷ (V × η), with V the voltage the fitting sits across. On a three-phase circuit each fitting is line to neutral, so both limits are counted per phase at 400 ÷ √3 V, and the circuit takes three times that, balanced. - Per-field guidance with info modals explaining exactly where to find Ipk and T50 on the LED driver datasheet. - Full step-by-step working shown in the result so the calculation is auditable and reviewable. - Export a branded PDF report with project header, inputs, calculation steps, max fittings and method / standards references. ## How to calculate the maximum number of LED fittings on an MCB 1. **Find the driver peak inrush (Ipk)** — Open the LED driver datasheet and locate the "Inrush current" or "Ipk" entry: use the value quoted at 230 V AC. 2. **Find the pulse width (T50)** — On the same datasheet locate the pulse width at 50 % of Ipk: usually labelled T50 or "Pulse width at 50 % Ipk", typically 100 to 600 µs. A longer T50 gives fewer fittings, so a short assumed value overstates the count: if it is not published, get it from the driver maker, or assume the long end of the range (600 µs). 3. **Enter the fitting wattage and driver efficiency** — Fitting wattage P is the lamp output power (W). Driver efficiency η is typically 0.80 to 0.95; use 0.85 if unknown. 4. **Select the MCB rating and curve** — Pick the breaker rated current In from the standard ladder (2 to 125 A) and click Type B, C or D to auto-fill the magnetic trip multiplier n (4, 8 or 15, mid-band). Override n with the MCB datasheet figure, or with the lower limit (3, 5 or 10) for a guaranteed no-trip count. 5. **Read the maximum number of fittings** — The headline result is the smaller of the inrush limit and the wattage limit. On a three-phase circuit both limits are counted per phase, each pole carrying the fittings on its own phase, and the headline is three times that, balanced. The calculation steps panel shows k, the trip threshold k·n·In, the per-fitting current, and both limit counts, with the binding constraint highlighted. ## LED Inrush Sizing Guide: Proof-Factor Method (AS/NZS 60898) ### Why LED lighting circuits nuisance-trip MCBs Every modern LED driver contains an input bulk capacitor that has to charge from 0 V to the peak line voltage in a few hundred microseconds at switch-on. The instantaneous current to charge that capacitor (the peak inrush Ipk) is roughly 10 to 250 A per driver, even though the driver's steady-state input current is well under 1 A. When several drivers on one circuit switch on synchronously the combined inrush briefly looks like a short-circuit to the MCB, and a curve B or even curve C breaker can trip on what is otherwise a perfectly healthy load. The traditional shortcut, comparing the inrush peak directly to the steady-state magnetic-trip threshold (3 to 5 × In for B, 5 to 10 × In for C, 10 to 20 × In for D), is far too conservative for pulses that only last 100 to 600 µs. The MCB magnetic mechanism has a finite response time and the let-through I²t of a short pulse is too small to actuate it. The proof-factor method accounts for this. ### The proof-factor (k) method explained A proof-factor chart plots k = I_surge / I_hold as a function of pulse duration T50 on log-log axes, and this calculator uses the Siemens 5SY chart. At T50 ≈ 10 µs the chart shows k ≈ 100 (a 100× peak can pass without tripping). At T50 ≈ 520 µs the chart drops to k = 5, and at T50 ≥ 10 ms it asymptotes to k = 1, where the pulse looks like a sustained fault. Its anchor points are digitised into this calculator with log-interpolation between them. Charts for other MCBs differ, so for design-critical work check against the chart for the MCB being installed. Multiplying k by the MCB's magnetic-trip multiplier n (a datasheet value: typically 4 for B-curve, 8 for C-curve and 15 for D-curve, midway through the AS/NZS 60898 tolerance band) and the rated current In gives the effective pulse trip threshold: I_trip = k × n × In. Divide that by the per-driver Ipk and you get the maximum number of fittings the MCB can withstand on a synchronous switch-on event. ### Why we also check the wattage (thermal) limit A 10 A Type-C MCB can absolutely survive a 600 A inrush pulse for 300 µs, but it cannot carry 600 A continuously: that would melt the cable and trip the breaker on thermal overload. The wattage limit converts the fitting wattage P to a steady-state input current I_fit = P ÷ (V × η) and divides the MCB rating by it. For an 82 W fitting at 230 V with η = 0.85, I_fit ≈ 0.42 A, so a 10 A MCB carries roughly 23 fittings continuously. The final answer is the lower of the two limits: almost always inrush for short low-wattage drivers, but wattage for larger drivers where the steady-state current dominates. ### Worked example: 82 W fitting on a 10 A Type-C MCB A common audit scenario: a UFO highbay fitting with a published Ipk of 50.5 A and T50 of 300 µs, 82 W output, 0.85 driver efficiency on a 10 A Type-C MCB (n = 8). Step 1: read k(300 µs) = 8.7 from the chart. Step 2: I_trip = 8.7 × 8 × 10 = 696 A. Step 3: N_inrush = ⌊696 ÷ 50.5⌋ = 13 fittings. Step 4: I_fit = 82 ÷ (230 × 0.85) = 0.419 A. Step 5: N_wattage = ⌊10 ÷ 0.419⌋ = 23 fittings. Step 6: max fittings = min(13, 23) = 13. Limited by inrush. Comparing the 50.5 A peak directly with the magnetic trip threshold, with no proof factor, allows ⌊8 × 10 ÷ 50.5⌋ = 1 fitting at the mid-band n, and none at all against the 5 × 10 = 50 A lower limit AS/NZS 60898.1 guarantees. The proof factor raises the count to 13 because a 300 µs pulse carries far less energy than a sustained current of the same peak. ## Key terms - **Inrush current** — The brief, very high current an LED driver draws at switch-on as its input capacitors charge. It can be tens of amps for a fitting whose steady-state draw is well under one amp, which is why wattage alone does not predict tripping. - **Ipk (peak inrush current)** — The maximum instantaneous current of the inrush pulse, from the driver datasheet. Fittings in parallel on one circuit add their peaks, because they energise together. - **T50 (pulse width)** — The duration of the inrush pulse measured at half its peak value. Together with Ipk it describes the energy the breaker actually sees, which a peak figure alone does not. - **Proof factor method** — The pulse-response method for LED inrush: the MCB's magnetic trip threshold n × In is multiplied by a proof factor k, read off the MCB's chart at the pulse width T50, and the combined peak of the fittings on a pole must stay below k × n × In. - **MCB tripping curve (B, C, D)** — The magnetic trip band of a miniature circuit-breaker, expressed as a multiple of rated current: roughly 3-5× for type B, 5-10× for C and 10-20× for D. A higher curve tolerates more inrush before tripping instantaneously. - **Wattage-limited vs inrush-limited** — The two ceilings on fittings per breaker. Wattage-limited is the steady-state current limit; inrush-limited is the switch-on pulse limit. The smaller of the two governs, and on LED circuits it is usually inrush. ## Frequently asked questions ### Why do LED lights nuisance-trip MCBs on switch-on? Every LED driver has an input capacitor that charges from 0 V to the peak line voltage in a few hundred microseconds at switch-on, drawing a peak inrush of roughly 10 to 250 A per driver against a steady current well under 1 A. When many drivers on one circuit switch together, the combined peak can cross the MCB's magnetic trip threshold and trip it, even though the running load is well inside the rating. ### What is the proof factor k in LED inrush sizing? The proof factor k(T50) is a pulse-duration multiplier read from an MCB manufacturer's chart: how many times higher than the steady-state magnetic trip threshold a short pulse can be before the MCB trips. On the Siemens 5SY chart the calculator uses, k ≈ 26 at T50 = 100 µs, 8.7 at 300 µs, 5 at 520 µs and 1 at 10 ms. Multiply k by the magnetic trip multiplier n and the MCB rating In to get the effective inrush trip threshold, I_trip = k × n × In. ### How many LED fittings or drivers can I put on one MCB? The smaller of two limits. Inrush limit: N_inrush = ⌊k × n × In ÷ Ipk⌋, with k read off the proof-factor chart at the driver's T50, n the magnetic trip multiplier (4, 8 or 15 for Type B, C or D), In the MCB rating and Ipk the peak inrush per driver. Wattage limit: N_wattage = ⌊In ÷ I_fit⌋, with I_fit = P ÷ (V × η) on a single-phase circuit. An 82 W fitting with Ipk = 50.5 A and T50 = 300 µs on a 10 A Type C MCB gives N_inrush = ⌊8.7 × 8 × 10 ÷ 50.5⌋ = 13 and N_wattage = 23, so 13 fittings. On a three-phase circuit each fitting sits line to neutral, so both limits are counted per phase at 400 ÷ √3 V, and the circuit takes three times that count, balanced. ### Should I use a Type B, C or D MCB for LED lighting? Type C is the usual choice for commercial LED lighting, because its higher magnetic trip band tolerates driver inrush. AS/NZS 60898.1 (IEC 60898-1) sets the instantaneous trip bands at 3 to 5 × In for Type B, 5 to 10 × In for Type C and 10 to 20 × In for Type D. Type B suits resistive and domestic circuits, and Type D is kept for very high-inrush loads such as transformers. The calculator's presets use the mid-band n of 4, 8 and 15; the standard guarantees no instantaneous trip only up to the lower limit (3, 5 and 10 × In), so for a guaranteed no-trip count enter that limit or the MCB datasheet minimum. ### What size MCB do I need for an LED lighting circuit? The smallest rating and curve whose maximum fitting count, on both the inrush and the wattage limit, covers every fitting on the circuit, and which the cable can still protect: AS/NZS 3000:2018 Clause 2.5.3.1 requires Ib ≤ In ≤ Iz. Enter the driver's Ipk, T50, wattage and efficiency, then step the MCB rating and the B, C or D preset until the count is enough. The result also names the smallest MCB of the selected curve that holds a single fitting at that peak inrush. ### Why does the calculator ask for fitting wattage and driver efficiency? They set the wattage limit, the steady-state number of fittings the MCB can carry. Each fitting draws I_fit = P ÷ (V × η), with P the fitting wattage, V 230 V and η the driver efficiency (typically 0.80 to 0.95, 0.85 by default), at a power factor of about 1. N_wattage = ⌊In ÷ I_fit⌋: an 82 W fitting at η = 0.85 draws 0.42 A, so a 10 A MCB carries 23. Low-wattage drivers are almost always limited by inrush; large high-bay drivers are often limited by wattage. ### Where do I find Ipk and T50 on an LED driver datasheet? Both are in the input or electrical characteristics section of the driver datasheet, typically as "Inrush current Ipk = 30 A at 230 V AC" and "Pulse width at 50 % Ipk (T50) = 250 µs". Use the figure quoted at 230 V AC, since Ipk scales with mains voltage. If the datasheet gives only Ipk, get T50 from the driver maker. A longer T50 allows fewer fittings, so an assumed short value overstates the count; failing a published figure, assume the long end of the typical range, about 600 µs. ### Does the proof-factor method only apply to Siemens MCBs? No. The method, a pulse trip threshold of k × n × In with k read off a proof-factor chart at the pulse width, applies to any MCB with such a chart. The calculator's k(T50) lookup is digitised from the Siemens 5SY chart, anchored at T50 = 520 µs and k = 5, with log interpolation between points. Charts differ between MCBs, so for design-critical sign-off cross-check against the chart for the MCB being installed. ### Why not just compare peak inrush to the MCB magnetic trip current? Because it ignores pulse duration and is far too conservative. The magnetic trip bands (3 to 5, 5 to 10 or 10 to 20 × In) describe a sustained current; a 600 A pulse lasting 300 µs carries far less let-through energy (I²t) than 600 A sustained, and the MCB's magnetic mechanism cannot respond to it in time. The proof factor k accounts for that. ### Which Australian Standards apply to LED lighting circuit protection? AS/NZS 3000:2018 (Wiring Rules) sets the overall installation and protection requirements including overcurrent protection, voltage drop and cable sizing. AS/NZS 60898.1 (equivalent to IEC 60898-1) defines the MCB tripping characteristics (curves B, C, D) used by this calculator. Manufacturer datasheets remain the source of truth for individual driver inrush figures, and the engineer of record is responsible for matching breaker curve and rating to the installed driver population. ## Related ScaleSet pages - [Cable Selection](https://scaleset.com.au/calculator/cable-selection) - [Voltage Drop](https://scaleset.com.au/calculator/voltage-drop) - [Maximum Demand](https://scaleset.com.au/calculator/max-demand) - [Power Factor Correction](https://scaleset.com.au/calculator/pfc) - [Generator Sizing](https://scaleset.com.au/calculator/generator-sizing) - [Selectivity Calculator (Breaker Discrimination)](https://scaleset.com.au/calculator/selectivity) - [Emergency Lighting Design Guide](https://scaleset.com.au/design-guide/emergency-lighting) - [Design Guides](https://scaleset.com.au/design-guide) ## About this page Methods built and reviewed by Wisam Tozah (CPEng, NER, MIEAust), Sydney, Australia. 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