Inrush & Fixtures per Circuit

LED fixtures, moving lights and switch-mode power supplies draw a very large, very short current spike when power is applied. Put enough of them on one MCB and it trips at power-up even though the running load is fine. Work out how many fixtures a circuit can take for steady-state load and for inrush, and how to stagger power-up.

Guidance only. Results are estimates based on the figures you enter and published tables. All electrical work must be designed, checked and tested by a qualified electrician to the regulations that apply where you are (e.g. BS 7671, NEC).

Fixture

Examples:
The “illustrative” buttons are made-up round numbers to show how the tool works, not real products. Always use the inrush figures from your fixture's datasheet (peak current and pulse width at 230 V, ideally to IEC 63129).
A
µs
Width at 50 % of the peak.
qty
W
Maximum input power, not light output.
PF
V

Circuit Breaker

% of In
80 % is common practice for continuous load.

Short-pulse correction (optional)

A
%
Some MCB manufacturers publish the peak current a breaker withstands for a given pulse width (e.g. ABB S200 B16: 672 A at 200 µs and 533 A at 250 µs, as quoted by eldoLED). It only applies to that breaker model. Leave blank if you don't have it.

Fixtures per Circuit

Steady-state limit
—
Inrush limit, naive
—
Inrush limit, corrected
—
Max per circuit
—
Combined peak, all at once
—
Running current
—
Circuits needed
—
Power-up groups per circuit
—

B, C and D Curves Compared

Curve Instant trip band Naive inrush limit Steady-state limit Max Zs (230 V)

Max Zs = Cmin × U0 ÷ Ia with Cmin = 0.95, U0 = 230 V and Ia the top of the trip band (5, 10 or 20 × In), as BS 7671 Table 41.3. Measured values must be lower. Check yours with the Earth Fault Loop Impedance Calculator.

If the Breaker Trips at Power-Up

  • Stagger the power-up. Bring fixtures on in groups no bigger than the inrush limit, a second or so apart, using a sequenced distro, DMX- or network-controlled relay packs, or simply switching breakers one at a time. Inrush pulses last well under a millisecond, so a short delay between groups is enough.
  • Inrush current limiters (NTC or relay-bypassed resistor modules) in series with a group cut the peak. Check their continuous rating and that they have time to cool and reset between power cycles; a hot NTC limits much less.
  • Spread fixtures over more circuits, even if the running load would fit on fewer.
  • A C- or D-curve breaker tolerates more inrush, but needs a much lower earth fault loop impedance to disconnect in time (see the Zs column). On long temporary cable runs or a generator supply a D-curve often can't meet BS 7671 disconnection times, so measure Zs before changing curve. The cable must still be protected for overload and fault current.
  • RCDs and RCBOs. An RCBO's overcurrent part has the same B/C/D behaviour as an MCB, so the same limits apply. Separately, switch-mode supplies have filter capacitors to earth that leak current at switch-on and in normal running; many fixtures on one 30 mA RCD can cause nuisance trips. BS 7671 Regulation 531.3.2 asks that the accumulated protective conductor current is no more than 30 % of the RCD's rated residual current.
  • Keep the fixtures' power switches on and power up from the distro, so that every fixture's inrush isn't also landing on the same internal fuse or relay.

Notes & References

  • Steady state. Fixtures = floor(limit % × In ÷ Ifixture), with Ifixture = W ÷ (V × PF).
  • Naive inrush (conservative). Fixtures = floor(mmin × In ÷ Ipeak), where mmin is the bottom of the instantaneous trip band in BS EN 60898-1: B 3 ×, C 5 ×, D 10 × In. It assumes peaks add directly, and compares the peak with a threshold that the standard defines for sustained 50 Hz current. That is very pessimistic for sub-millisecond pulses (it often allows zero or one fixture), but it is the only figure that needs no manufacturer data.
  • Corrected inrush. MCBs are much less sensitive to pulses lasting a few hundred microseconds than to a sustained fault, because the magnetic release and latch need time and energy to operate. There is no generic correction factor in the product standard, so the corrected figure uses the MCB manufacturer's own peak-withstand for the pulse width: fixtures = floor(safety factor × Iwithstand ÷ Ipeak). This is the method in eldoLED's guide How to choose the right circuit breaker (ABB S200 B16: 672 A at 200 µs, 533 A at 250 µs; 67 A at 240 µs driver → 8 drivers), and it is how driver makers such as Mean Well, Tridonic and Philips derive their “drivers per MCB” tables. If the fixture or driver maker publishes a per-MCB count, use that.
  • Peaks don't add perfectly: cable and supply impedance flatten the combined pulse, but they also stretch it (a longer pulse is harder on the MCB). Adding peaks linearly is the usual conservative assumption. IEC 63129 defines how peak inrush and pulse width (tH50) are measured for lighting products, including for multiple units.
  • Max Zs: BS 7671:2018+A2:2022 Regulation 411.4 and Table 41.3 (MCBs to BS EN 60898 / RCBOs to BS EN 61009).
  • Related tools: Power Calculator, Cable Gauge Calculator, Three-Phase Load Balancer.
  • This is guidance only. It does not replace manufacturers' data, BS 7671, or the judgement of a competent person. Test the actual combination before the show.