The breaker and building wiring are sized on input current, and input current equals output power divided by input voltage times efficiency times power factor. This guide maps common YFX 60, 100, 150, 200, 300 and 400 W units at 230 V to input current, then to minimum cable cross-section and a C-curve breaker, with the two traps of 80 percent continuous derating and inrush current.
Many people size the cable and breaker from the driver's "output power" and end up over- or under-sized. The correct move is: the AC side and the building wiring are both sized on input current, and input current must be worked back from output power - the LED driver keeps the low-voltage high current on its output side, so the AC draw is actually small, yet it is often sized wrong.
The one-sentence rule: read input current first, then cable and breaker
Input current I_in = P_out / (V_in x eta x PF) - do not size the AC wire from output current
A single standard YFX unit at 230 V, eta 0.9, PF 0.95 draws roughly 0.3 to 2 A; 0.75 mm2 wire is enough
What actually gets tight is many units on one circuit: 20 units of 200 W add to about 20 A and both cable and breaker must step up
I_in = P_out / (V_in x eta x PF)
P_out rated output power (W)
V_in input voltage (V); use your lowest site value for worst-case current
eta efficiency (from spec; 0.9 if unknown)
PF power factor (high-PF units about 0.95; standard units may be 0.6 to 0.7)
Two sizing notes: compute current at the lowest input voltage (lower voltage means higher current); for a multi-unit circuit use the sum of each unit's worst-case current plus inrush headroom.
Input current for common units (230 V, eta 0.9, PF 0.95)
The table tells one story: a single LED driver almost never needs thick wire. Even at 400 W the AC side is only 2 A, and 0.75 mm2 (about 6 A rated in conduit at 60 degrees C) still covers up to about 4.8 A after the 80 percent continuous derate, comfortably past 300 W. So the bottleneck is never one unit - it is "how many units sit under one breaker".
Sizing chain: output power, divide by voltage times eta times PF, get input current, then cable and breaker, with inrush checked on its own
Mapping current to cable (80 percent continuous derate)
Typical flexible conductor ampacity in conduit at about 60 degrees C (conservative):
Cross-section
Rated ampacity
Usable at 80 percent
0.75 mm2
~6 A
~4.8 A
1.0 mm2
~10 A
~8 A
1.5 mm2
~16 A
~12.8 A
2.5 mm2
~25 A
~20 A
4.0 mm2
~32 A
~25.6 A
Use it as: take I_in / 0.8 and pick the smallest cross-section whose rated ampacity meets it. Single units land in the 0.75 mm2 band; only when summed current crosses 4.8 A do you step up to 1.0 or 1.5 mm2. The driver's own input lead is already sized by the maker; this is the building branch wiring.
How to choose the breaker (C curve, sum, inrush)
C curve: LED drivers have small steady inrush, so C is enough; no need for D
Rating at least the circuit steady total current, but below the cable ampacity (the breaker protects the wire, not the reverse)
Sum for many units: N identical units means total about N times I_in; 20 units of 200 W (1.02 A) is about 20.4 A, needing a 25 A breaker and 4 mm2 wire
Inrush must be checked separately: startup transient can be tens of times steady current and stacks when many units power on together; a breaker sized on steady current may trip every morning (criteria in inrush current and breaker sizing)
Split circuits to cut risk: on large counts group 8 to 12 units per circuit rather than cramming one; easier to service too
Site acceptance checklist
Measure the lowest input voltage: clamp the supply at peak demand and recompute I_in (lower voltage means higher current)
Sum total current: add every driver (and any other load) on that breaker and confirm it stays under the cable's 80 percent ampacity
Check curve and rating: C curve, rated between "at least total current" and "below cable ampacity"
All-on test: close the whole circuit at once and watch for an instantaneous trip - if it trips, add circuits or change curve per the inrush article
Feel the temperature: after 2 hours at full load, touch the branch terminals; clearly hot to hold (above 60 degrees C) means the cable is too small
Prefer high PF: on shared circuits and long runs, use high-PF units such as H14, H16 or S05 so summed current stays smaller and the wire stays thinner
The ampacity figures above are common conservative references; use the conductor maker's data and local wiring rules for the actual value, and breaker selection also needs short-circuit breaking capacity, which is outside this article. AC-side drop is usually negligible because the current is small; the drop that matters is on the low-voltage output side (see voltage drop and cable size table).
Several drivers on one circuit add up, and inrush matters more than the running current. Tell us the model and count, run length and conductor size and we will set the breaker and cable and check the inrush limit. Contact →