Why do LED drivers cause MCBs to trip on switch-on?

Every 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 is 10–250 A per driver, even though the steady-state current is well under 1 A. When several drivers switch synchronously the combined inrush peak can exceed an MCB magnetic-trip threshold and cause a nuisance trip.

What is the proof factor k in LED inrush sizing?

The proof factor k(T50) is a pulse-duration multiplier read from the MCB manufacturer chart — Siemens 5SY, ABB S200, Schneider C60 and Eaton FAZ all publish similar curves. It expresses how much higher than the steady-state magnetic trip threshold a short pulse can be before the MCB trips. At T50 = 100 µs, k ≈ 20; at 300 µs, k ≈ 9; at 520 µs, k = 5; at 10 ms, k = 1. 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 can I connect to one MCB?

Take the smaller of two limits. Inrush limit: N_inrush = ⌊k × n × In ÷ Ipk⌋ — k is read off the proof-factor chart at the driver T50, n is the MCB magnetic trip multiplier (4 / 8 / 15 for B / C / D), In is the MCB rating, and Ipk is the per-driver peak inrush. Wattage limit: N_wattage = ⌊In ÷ I_fit⌋ where I_fit = P ÷ (V × η). The final answer is min(N_inrush, N_wattage).

What is the difference between Type B, C and D MCB curves for LED lighting?

AS/NZS 60898.1 (= IEC 60898-1) defines three magnetic-trip bands: Type B trips between 3 and 5 × rated current, Type C between 5 and 10 × In, and Type D between 10 and 20 × In. Type B is for residential and resistive loads, Type C is the default for commercial LED lighting because it tolerates the inrush of typical drivers, and Type D is reserved for very high-inrush loads such as transformers. The mid-band magnetic trip multipliers (n = 4, 8 and 15 respectively) are the default proof-factor inputs in this calculator.

Where do I find Ipk and T50 on an LED driver datasheet?

Both values are listed in the "Input" or "Electrical characteristics" section of the driver datasheet. Typical labels are "Inrush current Ipk = 30 A at 230 V AC" and "Pulse width at 50 % Ipk (T50) = 250 µs". Tridonic, OSRAM, Philips Xitanium and Mean Well all publish them. Use the value quoted at 230 V AC — Ipk scales with mains voltage. If the datasheet only quotes Ipk, default T50 to 250–300 µs as a conservative starting point.

Is the proof-factor method only valid for Siemens MCBs?

No. ABB S200, Schneider C60 / iC60, Eaton FAZ and NHP DOM all publish proof-factor curves with the same shape; the Siemens 5SY chart is widely treated as the industry reference. Curves vary slightly between manufacturers — for design-critical sign-off cross-check against the specific MCB datasheet. The k(T50) lookup in this calculator is digitised from the Siemens 5SY curve with the example anchor T50 = 520 µs → k = 5.

Is "compare peak inrush to magnetic trip threshold" the same as the proof-factor method?

No, and the older comparison is far too conservative. Comparing peak inrush to the steady-state magnetic trip threshold (3–5, 5–10 or 10–20 × In) ignores pulse duration entirely — a 600 A pulse for 300 µs has very different let-through energy from 600 A sustained, and the MCB magnetic mechanism cannot react quickly enough to trip on the short pulse. The proof-factor k captures this and produces realistic counts that match what installers see in service.

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.

ElecAS

LED Inrush Current Calculator

Why this page matters

Find the maximum number of LED fittings you can safely connect to a single MCB using the industry-standard 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) and shows every step. This static content is published so the canonical route has meaningful crawlable HTML even before the interactive application hydrates.

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 helps with

Proof-factor (k) method — the industry-standard pulse-response approach used by Siemens, ABB, Schneider, Eaton and the Stantec spreadsheet template.
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⌋) — answer is the smaller of the two.
Driver-efficiency-based per-fitting current: I_fit = P ÷ (V × η), matching how Australian lighting designers size circuits in practice.
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

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 (typically 18–75 A per driver).
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 100–600 µs. If not published, use 250–300 µs as a conservative default.
Enter the fitting wattage and driver efficiency — Fitting wattage P is the lamp output power (W). Driver efficiency η is typically 0.80–0.95; use 0.85 if unknown.
Select the MCB rating and curve — Pick the breaker rated current In from the standard ladder (2–125 A) and click Type B, C or D to auto-fill the magnetic trip multiplier n (4, 8 or 15). Override n if the MCB datasheet publishes a specific value.
Read the maximum number of fittings — The headline result is the smaller of the inrush limit and the wattage limit. 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 10–250 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 (3–5 × In for B, 5–10 × In for C, 10–20 × In for D) — is far too conservative for pulses that only last 100–600 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 captures this and produces realistic maximum-fitting counts that match what installers actually see in service.

The proof-factor (k) method explained

Every major MCB manufacturer (Siemens 5SY, ABB S200, Schneider C60 / iC60, Eaton FAZ, NHP DOM) publishes a proof-factor chart that plots k = I_surge / I_hold as a function of pulse duration T50 on log-log axes. 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. The Siemens 5SY chart is the de-facto industry reference and is digitised into this calculator with log-interpolation between anchor points.

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 (HLG-240 and similar) 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.

Note that the older "compare 600 A peak to 5×10 = 50 A trip threshold" method would have allowed only 1 fitting on the same breaker. The proof-factor method correctly delivers 13. This is why the same circuit that fails the conservative check works flawlessly in practice.

Common LED driver inrush figures (verify against your specific datasheet)

Tridonic LCI 30 W compact: Ipk ≈ 18 A, T50 ≈ 230 µs. Tridonic LCA 75 W linear: Ipk ≈ 38 A, T50 ≈ 250 µs. OSRAM Optotronic OT 30 W: Ipk ≈ 22 A, T50 ≈ 200 µs. OSRAM OT 75 W: Ipk ≈ 42 A, T50 ≈ 250 µs. Philips Xitanium 36 W: Ipk ≈ 25 A, T50 ≈ 250 µs. Philips Xitanium 75 W: Ipk ≈ 45 A, T50 ≈ 300 µs. Mean Well ELG-100: Ipk ≈ 65 A, T50 ≈ 300 µs. Mean Well HLG-240H: Ipk = 75 A, T50 = 570 µs (per official datasheet).

These are indicative midpoints — actual values vary by part number, batch and mains voltage. Always use the value quoted at 230 V AC on the specific driver datasheet for design submissions.

Reviewed by

Wisam Tozah — Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). LinkedIn.

Frequently asked questions

Why do LED drivers cause MCBs to trip on switch-on?: Every 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 is 10–250 A per driver, even though the steady-state current is well under 1 A. When several drivers switch synchronously the combined inrush peak can exceed an MCB magnetic-trip threshold and cause a nuisance trip.
What is the proof factor k in LED inrush sizing?: The proof factor k(T50) is a pulse-duration multiplier read from the MCB manufacturer chart — Siemens 5SY, ABB S200, Schneider C60 and Eaton FAZ all publish similar curves. It expresses how much higher than the steady-state magnetic trip threshold a short pulse can be before the MCB trips. At T50 = 100 µs, k ≈ 20; at 300 µs, k ≈ 9; at 520 µs, k = 5; at 10 ms, k = 1. 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 can I connect to one MCB?: Take the smaller of two limits. Inrush limit: N_inrush = ⌊k × n × In ÷ Ipk⌋ — k is read off the proof-factor chart at the driver T50, n is the MCB magnetic trip multiplier (4 / 8 / 15 for B / C / D), In is the MCB rating, and Ipk is the per-driver peak inrush. Wattage limit: N_wattage = ⌊In ÷ I_fit⌋ where I_fit = P ÷ (V × η). The final answer is min(N_inrush, N_wattage).
What is the difference between Type B, C and D MCB curves for LED lighting?: AS/NZS 60898.1 (= IEC 60898-1) defines three magnetic-trip bands: Type B trips between 3 and 5 × rated current, Type C between 5 and 10 × In, and Type D between 10 and 20 × In. Type B is for residential and resistive loads, Type C is the default for commercial LED lighting because it tolerates the inrush of typical drivers, and Type D is reserved for very high-inrush loads such as transformers. The mid-band magnetic trip multipliers (n = 4, 8 and 15 respectively) are the default proof-factor inputs in this calculator.
Where do I find Ipk and T50 on an LED driver datasheet?: Both values are listed in the "Input" or "Electrical characteristics" section of the driver datasheet. Typical labels are "Inrush current Ipk = 30 A at 230 V AC" and "Pulse width at 50 % Ipk (T50) = 250 µs". Tridonic, OSRAM, Philips Xitanium and Mean Well all publish them. Use the value quoted at 230 V AC — Ipk scales with mains voltage. If the datasheet only quotes Ipk, default T50 to 250–300 µs as a conservative starting point.
Is the proof-factor method only valid for Siemens MCBs?: No. ABB S200, Schneider C60 / iC60, Eaton FAZ and NHP DOM all publish proof-factor curves with the same shape; the Siemens 5SY chart is widely treated as the industry reference. Curves vary slightly between manufacturers — for design-critical sign-off cross-check against the specific MCB datasheet. The k(T50) lookup in this calculator is digitised from the Siemens 5SY curve with the example anchor T50 = 520 µs → k = 5.
Is "compare peak inrush to magnetic trip threshold" the same as the proof-factor method?: No, and the older comparison is far too conservative. Comparing peak inrush to the steady-state magnetic trip threshold (3–5, 5–10 or 10–20 × In) ignores pulse duration entirely — a 600 A pulse for 300 µs has very different let-through energy from 600 A sustained, and the MCB magnetic mechanism cannot react quickly enough to trip on the short pulse. The proof-factor k captures this and produces realistic counts that match what installers see in service.
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.