Input watts of a driver
Input W = DC load W / driver efficiency
Efficiency is the ratio on the driver datasheet, typically 0.85 to 0.92 for constant voltage supplies.
Guide
LED Drivers and Loads: Sizing, Power Factor, and Inrush
What a driver label really tells you, how many of them a circuit can carry, and why the inrush figure trips breakers that the running current never would.
Size on the output rating, with a margin
A constant voltage driver is rated in output watts at a stated output voltage, and a constant current driver in output current over a voltage range. Add up the DC load first: metres of tape times watts per metre, or the module count times module watts. Then hold the load to about eighty percent of the driver output, because the margin buys derating at high ambient temperature and the tolerance in the tape length.
The enclosure decides the real rating
Driver datasheets carry a derating curve: the rated output holds to a stated case temperature, then falls. Inside a closed cabinet, in a plenum, or on a roof in summer, the achieved rating can be a fraction of the label figure. Derating is also why two drivers in the same small enclosure run hotter than one, and why the second driver in a cabinet may not reach its rating at all.
Circuits care about VA, not watts
Input watts equal DC watts divided by efficiency, and the current a circuit carries is VA divided by voltage. A driver with a power factor of 0.9 draws about eleven percent more current than its wattage suggests. Since branch circuits are limited to about eighty percent of the breaker rating for continuous loads, the fixtures per circuit figure comes from VA, not from nameplate watts.
Inrush is the one that trips the breaker
Every driver charges its input capacitors when the circuit is energised, so switch-on current can peak at tens of amps per driver for a millisecond or two. Twenty drivers switched together can produce an instantaneous current in the hundreds of amps, which is inside the magnetic trip band of an ordinary breaker even though the running load is a few amps. The remedies are ordinary: fewer drivers per switched group, staggered switching, a breaker with a higher magnetic threshold, or an inrush current limiter.
Two failure modes worth designing out
An open output on a constant current driver pushes full voltage into the load and ends in a dead driver or dead string; a loose connection on a constant voltage tape run causes voltage drop and warm joints. Keep drivers accessible, keep secondary drops short and rated, and label the circuits so the next person can see which driver feeds which run.
Formulas used
Current on the circuit
VA = Input W / power factor; I = VA / V
Power factor converts real power to apparent power, which is what the wire and breaker actually carry.
Drivers per circuit
Drivers per circuit = floor(breaker VA x 0.8 / VA per driver)
Continuous loads are held to eighty percent of the branch circuit rating, then the result is checked against the inrush figure.
Worked examples
A 480 watt tape run on 96 watt drivers
Inputs: 480 W of tape, 96 W drivers, 80 percent load target, 88 percent efficiency, power factor 0.9, 120 V, 20 A breaker.
A driver carries 76.8 watts at the load target, so the run needs 7 drivers and each sits at 71 percent of its rating. Input watts are 480 / 0.88 = 545 W, or 606 VA, which is about 5 amps: the running current comfortably fits a 20 amp circuit with room for 22 drivers.
The same run, and the same circuit, at switch on
Inputs: 22 drivers per circuit, 30 A inrush each.
The running load is fine, but switched together the drivers present about 660 amps at the instant of energising. That is a nuisance trip waiting to happen, so the drivers go on two or three switched groups rather than one.
Questions readers ask
Can I run a driver at one hundred percent of its rating?
Only if the ambient temperature in the enclosure stays below the point where the derating curve begins. In a closed cabinet or above a ceiling, holding the load near eighty percent is what keeps the driver inside its temperature rating and inside its warranty terms.
Why does my driver draw more current than its wattage suggests?
Because the datasheet wattage is real power and the wiring carries apparent power. Dividing by the power factor of 0.9 adds about eleven percent, which matters when a circuit is nearly full.
A single driver on its own breaker trips the breaker on switch on. Why?
Inrush. A large driver can present hundreds of amps for a millisecond, which is inside the magnetic trip band of a small breaker. The fix is a breaker with a suitable curve or an inrush limiter, not a bigger breaker.
Size the drivers and the circuit
Sources
- Lighting, fans, and controls product resources U.S. Environmental Protection Agency, ENERGY STAR · 2026-10-15
- Lighting systems and drivers National Electrical Manufacturers Association · 2026-10-15
- Lighting and energy saving resources U.S. Department of Energy, Energy Saver · 2026-10-15