Summary
Most LED specifications stop at wattage, and that is where electrical problems begin. A 40 W fixture does not draw 0.17 A just because the arithmetic says so — the driver's power factor decides the real current, and its inrush current decides whether the breaker survives the first switch-on. This guide covers both numbers in the order a contractor needs them: watts to amps, why harmonics overload a neutral conductor, how to read MCB trip curves, how to run the two standard calculations, and what to demand before signing the RFQ.
1. Two Numbers Decide Every Lighting Circuit
A lighting layout tells you how many fixtures a room needs to reach 500 lux. It does not tell you how many you can physically connect to one breaker. That answer comes from two quantities that behave very differently.
The first is steady-state input current — what the circuit carries hour after hour. It sets cable size, voltage drop and the thermal load on the breaker.
The second is inrush current — the spike in the first few hundred microseconds when the driver's input capacitors charge. It is over before anyone notices, but it is the most common reason a new LED installation trips its breaker on commissioning day.
2. Power Factor: Why a 40 W Fixture Is Not 0.17 A
Power factor (PF) is the ratio between the power a fixture actually uses and the power the grid must deliver to it. A poor-PF driver draws more current than the wattage suggests, and you pay for that current in cable size and breaker capacity, not in kilowatt-hours.
For a single-phase circuit:
I (A) = P (W) ÷ [V (V) × PF]
Take a 40 W LED panel on a 230 V circuit:
- With PF = 0.95: I = 40 ÷ (230 × 0.95) = 0.18 A
- With PF = 0.50: I = 40 ÷ (230 × 0.50) = 0.35 A
Same light, same wattage, nearly double the current — on a circuit designed for 30 fixtures, the low-PF version draws what 58 would. On a three-phase board, use I = P ÷ (√3 × V_line × PF).
What to specify
Require PF ≥ 0.9 at full load as a minimum; DLC applies the same threshold to listed luminaires in North America, alongside THD ≤ 20%. Two details buyers routinely miss:
- PF at dimmed level. PF falls as output drops. A driver rated 0.95 at full load may sit near 0.6–0.7 at 10%. If the building runs dimmed most of the evening, size for the dimmed current, not the nameplate one.
- Test condition. PF should state input voltage and load. "0.95" measured at 230 V is not the same as at 277 V or 120 V.
3. THD and the Neutral Conductor: The Overload Nobody Sized
Total harmonic distortion (THD) describes how far the current waveform departs from a clean sine wave. LED drivers are non-linear loads: they draw current in pulses near the voltage peak, and those pulses contain harmonics.
The limits that apply
For lighting equipment, IEC/EN 61000-3-2 Class C sets the harmonic current limits. Above 25 W input power, the limits are expressed as a percentage of the fundamental current:
| Harmonic order | Limit (% of fundamental) |
|---|---|
| 2nd | 2% |
| 3rd | 30% × λ (circuit power factor) |
| 5th | 10% |
| 7th | 7% |
| 9th | 5% |
| 11th–39th (odd) | 3% |
For equipment at or below 25 W there is an alternative route: the 3rd harmonic stays under 86% of the fundamental and the 5th under 61%, with a defined current waveform shape.
Why this matters on site
The 3rd harmonic and its odd multiples (9th, 15th, 21st) are triplen harmonics. On a balanced three-phase system fundamental currents cancel in the neutral; triplens do not cancel, they add. On a large installation built with uncorrected drivers, neutral current can reach 130–150% of the phase current — and the neutral is usually the same size as the phases.
This is a design-level issue, not a product defect: a compliant fixture can still contribute to a facility problem once hundreds of drivers share a distribution path. Require THD per driver model, and on large projects have the designer check the aggregate at the point of common coupling.
4. Inrush Current: Why a Correctly Sized Breaker Still Trips
When AC is applied, the driver's bulk electrolytic capacitor looks momentarily like a short circuit. Peak inrush can be many times normal operating current — a 150 W driver may show 50–70 A for 300–500 µs. That pulse does not damage the driver. It does trip breakers when dozens of drivers switch on together.
MCB trip curves
A breaker's instantaneous magnetic trip threshold depends on its curve:
- Type B: 3–5 × rated current — intended for resistive loads; trips easily on LED inrush.
- Type C: 5–10 × rated current — the practical default for lighting circuits.
- Type D: 10–20 × rated current — for high inrush or motor loads; use with care because short-circuit sensitivity is lower.
Two calculations, take the lower number
Driver manufacturers and MCB makers publish two checks:
(a) Steady-state current check:
Drivers = derating % × I_mcb ÷ I_input
Typical derating: 60% for Type B, 70% for Type C, 80% for Type D.
(b) Inrush check:
Drivers = (In × trip multiplier × K) ÷ I_peak
where K is a proof factor based on pulse duration — roughly 100 at 10 µs, 30 at 100 µs, 5 at 1,000 µs and 1 at 10,000 µs.
Worked example
A C16 breaker feeding 40 W fixtures (230 V, PF 0.95, input 0.18 A, peak inrush 30 A at 500 µs):
- Steady-state: 0.70 × 16 ÷ 0.18 = 62 fixtures
- Inrush: (16 × 5 × 5) ÷ 30 = 13 fixtures
The circuit is limited to 13 fixtures — by the switch-on spike, not by the running load. That is the most useful thing to understand about LED circuit design: the wiring usually has capacity to spare; the inrush does not.
To get more per circuit, in order of practicality: stagger the switch-on with time-delay contactors or a sequencing controller; move to Type D where local wiring rules allow; add an inrush current limiter; or split the zone across more breakers. Relay and contactor ratings are often the real bottleneck — check them before blaming the breaker.
5. North America: 120/277 V and the Continuous Load Rule
The arithmetic is the same, but the code framing differs. Branch circuits are commonly 120 V or 277 V at 20 A, and lighting is treated as a continuous load: under the NEC, overcurrent devices and conductors for continuous loads are sized at not less than 125% of the load. Confirm details with the local authority having jurisdiction — but do not size a lighting breaker as if it were a general-purpose outlet.
6. What to Demand in the RFQ
| Parameter | Why you need it |
|---|---|
| Input current (A) at 120 / 230 / 277 V | Actual circuit loading |
| PF at 100% load and at 10% dim | Dimmed operation is often worse |
| THD (input current) at full load | Neutral sizing and code compliance |
| Peak inrush current (A) with pulse width (µs) and test condition | Breaker sizing — a peak without a duration is useless |
| Max drivers per MCB, by curve type | Manufacturer's own verified table |
| Driver efficiency | System watts ≠ LED watts |
| Ambient rating (Ta / Tc) | Derating in hot ceilings and enclosed fixtures |
| Surge protection (kV, per ANSI/IEEE C62.41) | Grid transients, especially outdoor and transit |
Ask for the inrush figure with its duration and test conditions. A datasheet quoting "Ipeak 40 A" with no µs value and no stated source impedance cannot be used for a breaker calculation.
7. Six Failures That Repeat on Site
- Type B breakers on LED circuits. Cheapest option, first to trip.
- Neutral sized as if triplens cancelled. The hottest conductor in the board.
- Relay and contactor ratings ignored. The control device often trips before the MCB.
- PF checked at full load only. Dimmed buildings behave differently.
- Inrush assumed additive without staggering. Splitting the zone is cheaper than re-cabling.
- Mixed driver brands on one circuit. Different inrush profiles make faults intermittent.
Conclusion
Watts tell you what a fixture consumes. Power factor tells you what the cable carries. Inrush tells you whether the installation survives its own switch-on. Running all three before the panel is built costs an hour; discovering them on commissioning day costs a re-test, a change order and the client's confidence.
Solarix Lighting supplies commercial, industrial and rail-transit LED luminaires and drivers with published power factor, THD and inrush data, plus per-MCB loading tables for B, C and D curves. Our engineering team supports contractors and consultants with circuit-level calculations, photometric layouts (IES/LDT) and project-specific driver selection across metro stations, airports, tunnels, warehouses and office fit-outs. Send us your single-line diagram and fixture schedule — we will return a verified loading calculation before you order the panel.
Contact Solarix Lighting for datasheets, IES files and a project quotation: https://www.gzsolarix.com