Motor FLA & Conductor Sizing
Three-phase full-load current from the NEC 430.250 table, the 125% conductor rule, wire size, maximum breaker and fuse, and the overload heater setting.
Table current, not nameplate current, sizes the conductors and the short-circuit device (430.6(A)(1)). The overload heater is the exception: it is sized on the nameplate FLA.
How the math works
- Table 430.250 three-phase FLA: at 208 V: 5 HP 16.7, 10 HP 30.8, 20 HP 59.4, 40 HP 114, 100 HP 236. At 230 V: 5 HP 15.2, 10 HP 28, 20 HP 54, 40 HP 104, 100 HP 212. At 460 V, halve the 230 V values: 5 HP 7.6, 40 HP 52.
- 125% rule (430.22(A)): conductors supplying one continuous-duty motor are sized at not less than 125% of the table full-load current. A 5 HP at 208 V is 16.7 A, so 20.9 A of ampacity: 8 AWG copper (55 A at 75°C) is generous, 10 AWG (40 A) is the practical minimum for a 3-phase run.
- Short-circuit protection (430.52): inverse-time circuit breakers 250% of FLA, dual-element time-delay fuses 175%, instantaneous-trip breakers 300% (with motor ratings), and the next standard size up is allowed when the computed value does not correspond to a standard rating (430.52(C)(1) exception).
- Overload protection (430.32): heaters on a continuous-duty motor are set at 115% of nameplate FLA where the nameplate FLA is 100 A or less, and 125% where it is over 100 A. This is the one place the nameplate number governs, not the table.
- Locked rotor (430.7(B)): the code letter gives kVA per HP of inrush: B 1,600 max, F 3,150, G 5,000 to 5,600, E 6,300 to 7,100. A 50 HP F-code motor starts at 157 kVA, which is a voltage-sag and conductor-heating problem on a long run.
What size breaker for a 5 HP 230 volt motor?
Table FLA is 15.2 A, so an inverse-time breaker goes to 250% = 38 A, and the next standard size is 40 A. A dual-element time-delay fuse computes to 26.6 A and takes the next standard 30 A. Conductors at 125% = 19 A, so 10 AWG copper.
Do I use nameplate amps or table amps?
Table current for the conductors, disconnect, and short-circuit protection (430.6(A)(1)); nameplate current for the overload heaters (430.6(A)(2)). Nameplate FLA is usually lower than the table, which is why heaters on the table value nuisance-trip.
How many amps does a 10 HP 480 volt motor draw?
At 460 V the table says 14 A, so 17.5 A of conductor ampacity (125%) and 8 AWG copper at 55 A. At 575 V it is 11 A, and at 208 V the same motor is 30.8 A, which is why 208 V motors need bigger wire.