Inrush Current and Circuit Protection in LED Lighting Installations

When a group of LED drivers is switched on, the miniature circuit breaker protecting the circuit can trip even though the steady-state load is well within its rating. The culprit is inrush current, the brief surge of current that flows as the input capacitors of every driver charge up. Understanding inrush is essential for anyone designing lighting sub-circuits or retrofitting existing installations.

Inside a switch-mode LED driver, the AC input is rectified and stored on a bulk electrolytic capacitor. At the moment of switch-on, that capacitor is discharged, so it appears as a short circuit. The charging current can reach tens of times the driver's rated input current, with peaks in the hundreds of amperes for a large driver, decaying over a few hundred microseconds to several milliseconds. When many drivers are connected to one circuit, their inrush pulses overlap and sum, and the combined pulse is what trips the breaker.

Circuit breakers are classified by their instantaneous trip characteristic, defined in IEC 60898. A Type B breaker trips magnetically between 3 and 5 times its rated current, Type C between 5 and 10 times, and Type D between 10 and 20 times. A Type B breaker on a lighting circuit with modern drivers is prone to nuisance tripping because the inrush pulse exceeds its magnetic threshold even though the thermal element would never operate at the steady-state load. The usual remedy is to move to a Type C breaker, or Type D when inrush is severe.

The British BEAMA guidance on circuit-breaker selection for LED lighting formalizes this process. It bases the selection on the magnitude and duration of the inrush current, typically using a parameter such as the half-value time of the current pulse, and provides factors for comparing the pulse against the breaker's non-tripping characteristics. The takeaway for installers is that the choice between B, C, and D curves should follow the measured or declared inrush data of the drivers, not habit.

Reducing inrush at the source is often better than choosing a heavier breaker. Many drivers include an NTC thermistor or an active inrush-limiting circuit that softens the charging current. Some products offer controlled switching or a relay with delayed connection of the charging resistors. When large numbers of drivers share a circuit, staggering the switching, such as through time-delayed contactors or sequential energization of sub-groups, keeps the combined inrush manageable.

Breaker sizing also has to respect the continuous-load rules used in electrical codes. In North America, continuous loads such as lighting are generally limited to 80 percent of the breaker rating, so a 16 A breaker supports at most about 12.8 A of continuous LED load. The steady-state load calculation and the inrush coordination are two separate checks, and both must pass.

The practical workflow for a lighting circuit is straightforward. First, sum the steady-state input currents of all drivers and apply the continuous-load derating to size the breaker. Second, collect the inrush current specification, peak value and duration, from each driver datasheet. Third, compare the worst-case combined pulse against the trip curve of the candidate breaker, using the manufacturer's guidance or the BEAMA methodology. Finally, verify by test on site with the real load.

For a constant-voltage DALI-2 driver feeding LED strips, the inrush specification should be available in the datasheet, often as a peak current and a duration or half-value time. Specifying drivers with documented inrush data and matching the breaker curve to it avoids the most common nuisance trip in lighting installations and prevents the habit of simply replacing the breaker with a larger one without checking cable sizing.