Designing Multi-Driver Circuits: Load Balancing, Inrush, and Circuit Protection

A single LED driver is a simple device. Twenty LED drivers on one circuit are a system — and the system has properties that none of the individual drivers exhibit. Total load, cumulative inrush current, harmonic summation, and the interaction between the drivers and the circuit protection all must be considered when designing a multi-driver installation. These are the calculations that determine whether a lighting circuit trips on startup, whether the transformer runs hot, and whether the circuit breakers protect the installation or cause nuisance outages.

The starting point is the steady-state load. Each driver's input current is stated on its datasheet — the PV100 lists 0.56A maximum at 230V. Ten drivers on one circuit draw 5.6A steady-state; twenty draw 11.2A. The circuit must be protected by a breaker rated for the total load with margin, and the wiring must be sized for the connected load. A 16A B-curve breaker feeding a 20-driver circuit at 11.2A steady-state operates at 70 percent of rating — acceptable for continuous loads (continuous loads are typically limited to 80 percent of breaker rating in most codes). If the installation may grow, the circuit should be sized for the future load from the start.

Inrush current is where multi-driver circuits fail in practice. Each constant voltage driver has input capacitors that charge rapidly at power-on, drawing a transient current 20 to 50 times the steady-state value for a few milliseconds. A 100W driver with 0.56A steady-state may draw 15 to 25A inrush. When 20 drivers switch on simultaneously — via a contactor, a time clock, or the main switch — the combined inrush can reach 300 to 500A. A standard B-curve breaker allows 3 to 5 times its rating for transient periods, which means a 16A breaker can pass 48 to 80A briefly — far below 500A. The breaker will trip even though the steady-state load is well within rating.

The solutions are well established. First, choose a C-curve or D-curve breaker, which allow 5 to 10 times (C) or 10 to 20 times (D) the rated current for transients. A C16 breaker can pass 80 to 160A transiently, which covers 20 drivers' combined inrush in most cases. Second, stagger the startup: rather than switching all drivers simultaneously, use a sequential switch or a contactor with a time-delay relay that closes each group a few hundred milliseconds apart, reducing the peak inrush to one group at a time. Third, use drivers with active inrush limiting — an NTC thermistor or an active soft-start circuit in the driver reduces the inrush to 2 to 3 times steady-state, which is far easier for the circuit protection to handle. The PV100's inrush behavior should be checked in its datasheet; drivers with NTC-based inrush limiting are common at the 100W level.

Harmonic summation is the third design factor. Each driver with active PFC injects harmonics into the circuit, and while the individual contribution is small, the cumulative third harmonic in a three-phase system appears on the neutral conductor, where it does not cancel as fundamental currents do. For a heavily loaded circuit with many single-phase drivers on different phases, the neutral current can approach or exceed the phase current. The neutral conductor should be sized at least equal to the phase conductors, and many engineers specify 1.5 to 2 times neutral for high-density non-linear loads. The total harmonic load on the supply transformer should also be checked — harmonic content may require derating if the transformer is already loaded.

Circuit protection selection goes beyond the breaker curve. Each branch feeding a group of drivers should have its own protection, so a fault in one branch does not take down the whole installation. The branch breaker size should be based on the steady-state load of that branch with the 80 percent continuous-load rule applied. Residual current devices (RCDs) should be installed where required by code — note that some RCD types can misoperate with harmonic-rich currents, so a type that tolerates the expected harmonic content should be selected. And the final check is empirical: after installation, switch the circuit on and verify the breakers hold; measure the steady-state current on each phase and the neutral; and check the voltage at the farthest driver under full load. The calculations predict, but measurement confirms — and a few minutes of measurement at commissioning prevents years of nuisance tripping.