Inrush Current and Branch Circuit Protection

When power is first applied to an LED driver, there is a moment when it behaves nothing like the steady-state device described on its label. The input filter capacitors and the PFC bus capacitor are completely discharged, so the initial current is limited only by the resistance of the wiring and the inrush-limiting components. This surge lasts only a few hundred microseconds, but its peak can reach tens of times the driver's steady-state input current, and it can disrupt the branch circuit around it.

The mechanism is simple physics. The input bridge rectifier charges the bus capacitance through the rectifier and the EMI filter at the moment of switch-on. The peak inrush current is roughly the line voltage divided by the total series impedance of the wiring, filter, and any inrush limiter. On a 230V supply, a 100W driver with a poorly limited inrush can draw a spike of many tens of amperes, compared with its steady-state input current of about 0.5A. The pulse is brief, but energy is energy, and contact wear and magnetic trips respond to peaks, not averages.

The lighting industry standardized this problem in NEMA 410, a voluntary test standard that defines limits for inrush current from lighting control gear so that switches, relays, and dimmers can be designed to survive it. The standard, updated and reissued as NEMA JSC 10410-2023, limits both the peak current and the pulse width of the inrush event. A driver marked as meeting NEMA 410 limits has been verified to start within defined current and time boundaries, which is exactly what a specifier needs when dozens of drivers share one contactor.

The consequences of uncontrolled inrush are specific and reproducible. A miniature circuit breaker with a magnetic trip can see a 60-amp peak from a row of drivers and trip even though the steady-state load is a fraction of its rating; this is the classic mystery of a breaker that trips only when lights first switch on. Relay and contactor contacts can weld shut after years of making and breaking the surge, leaving the circuit permanently energized. Dimmer switches fail the same way. None of these components is defective; they are simply being asked to switch a current waveform they were not designed for.

Mitigation happens at two levels. Inside the driver, an NTC thermistor, a series resistor bypassed after startup, or an active soft-start circuit limits the inrush peak and extends the pulse. At the system level, the specifier counts the total inrush of all drivers on a branch, adds the steady-state load, and selects a breaker, relay, or contactor rated for both. Some manufacturers publish inrush data per driver precisely so that this calculation can be done. Where a branch carries many drivers, staggered or delayed switching, sometimes called sequential power-up, spreads the surges so the branch never sees them simultaneously.

The practical rule is to treat inrush as a design input, not an afterthought. Verify that the driver has inrush limiting and, if the project uses relays or contactors, that the total branch inrush is within the switching device's rating. Add a safety margin for cold temperature, where inrush peaks are highest. A few minutes of calculation at the design stage prevents the intermittent breaker trips and welded contacts that otherwise surface years later, at the worst possible moment.