Motor full load amps chart

Three phase motor full load amps, NEC Table 430.250

These are the full load currents the NEC requires you to use for three phase induction motors running at usual speeds with normal torque. Read the column for the motor nameplate voltage. The code lets each column cover a range of system voltages, 110 to 120, 220 to 240, 440 to 480 and 550 to 600 volts, so a motor on a 480 volt system uses the 460 V column and a motor on a 240 volt system uses the 230 V column. The 208 V and 200 V columns are separate because those motors are wound for the lower voltage.

Amps, from NEC Table 430.250 as published by Schneider Electric (based on the 2017 NEC) for 115 to 575 V, and by VoltageLab for the 2300 V column. n/a means the table gives no value. Published copies differ on a few cells, VoltageLab gives 4.8 A for 1 HP at 200 V, 2.1 A for 1-1/2 HP at 575 V, 512 and 579 A at 460 V for 450 and 500 HP, and 230 V values above 200 HP. Use the NEC edition your inspector enforces for those cells.

Single phase motor full load amps, NEC Table 430.248

Single phase motors are listed at 115, 200, 208 and 230 V. A motor on a 120 volt circuit uses the 115 V column and a motor on a 240 volt circuit uses the 230 V column. Table 430.248 stops at 10 HP.

Amps, from NEC Table 430.248 as reproduced by Open Exam Prep (1/2 to 10 HP) and Engineering ToolBox (1/6 to 1/3 HP). See NEC means our sources do not give that cell.

Why the NEC table, not the nameplate, sizes the circuit

NEC 430.6(A)(1) says the table values, not the nameplate amps, are used to size the branch circuit conductors, the feeder, the short circuit and ground fault protection and the disconnect. The reason is interchangeability. When a motor is replaced with another brand of the same horsepower, the wiring and breaker still fit it.

The nameplate current, often written FLA, is used for one thing, the overload protection under 430.32, because the overload has to protect the windings of that particular motor. The table does not apply to motors built for low speed (under 1,200 RPM), high torque motors or multispeed motors. For those the nameplate is used.

Wire and breaker size for three phase motors at 460 V

Branch circuit conductors for a single motor must carry at least 125 percent of the table current (NEC 430.22). An inverse time breaker may be up to 250 percent of the table current under NEC Table 430.52, rounded up to the next standard size when 250 percent falls between sizes. That is a maximum to let the motor start, not a target, and a separate overload relay sized from the nameplate protects the motor itself.

Computed. Conductor is 125 percent of the table amps, matched to the copper 75 C column of NEC Table 310.16 from our wire ampacity chart. Where the equipment terminals are rated 60 C, use the 60 C column instead. Breaker is 250 percent rounded up to the next standard size. Long runs may need larger wire for voltage drop.

Motor kVA and the transformer that serves it

The running kVA of a three phase motor is the square root of 3 times the voltage times the table amps, divided by 1,000. That sets the smallest transformer that can carry the motor while it runs. Starting is the harder case. Locked rotor current is often around six times full load current, and the nameplate code letter gives the locked rotor kVA per horsepower under NEC 430.7(B). Code letter G, for example, is 5.6 to 6.29 kVA per HP, which puts the maximum locked rotor current of a 20 HP, 460 V code G motor at about 158 A.

For a transformer that serves one large motor, check the voltage dip at starting with the motor and transformer data, or use a soft starter or drive to cut the inrush. The column below is only the running floor.

Computed from the 460 V table amps. Transformer sizes are the standard three phase ratings on this site. Several motors on one transformer add up, and the starting of the largest usually decides the size.

Amps per horsepower, the rule of thumb and where it breaks

Engineering ToolBox gives quick rules of about 2.5 A per HP for a 230 V three phase motor, 1.25 A per HP at 460 V, 14 A per HP for a 115 V single phase motor and 7 A per HP at 230 V single phase. The table shows why they are only rough. Small motors draw far more per horsepower because their efficiency and power factor are lower, so the rule undersizes them.

Computed from NEC Table 430.250 values above.

Worked examples at 460 V

A 10 HP pump has a table current of 14 A. The conductors need 17.5 A of ampacity, so 12 AWG copper at 75 C, and the largest inverse time breaker is 35 A. Its running kVA is 11.2.

A 50 HP air compressor has a table current of 65 A. The conductors need 81.25 A, so 4 AWG copper at 75 C, and the breaker can be up to 175 A. Its running kVA is 51.8, so a 75 kVA transformer carries it running and starting decides whether that is enough.

A 200 HP chiller has a table current of 240 A. The conductors need 300 A, so 350 kcmil copper at 75 C or parallel sets, and the breaker can be up to 600 A. Its running kVA is 191, which points to a 225 kVA transformer as the floor, and in practice a larger one once starting and other loads are counted.

A feeder for several motors needs 125 percent of the largest motor plus 100 percent of the rest (NEC 430.24). For a 25 HP, a 15 HP and a 10 HP motor at 460 V that is 34 x 1.25 + 21 + 14, or 77.5 A.

How many amps does a 10 HP 3 phase motor draw at 480 V

Use 14 A, the 460 V value in NEC Table 430.250, for sizing the wire and breaker. The nameplate may say a little more or less, and the nameplate figure is used for the overload setting.

Which column do I use for a 480 volt motor

The 460 V column. Motor nameplates say 460 V, and the NEC lets that column cover systems from 440 to 480 V. Likewise 230 V covers 220 to 240 V and 575 V covers 550 to 600 V.

Why are there separate 200 V and 208 V columns

Motors wound for 200 or 208 V draw more current than 230 V motors of the same horsepower, so the table lists them separately.

How many amps per horsepower

About 1.2 to 1.4 A per HP at 460 V and 2.4 to 2.8 A per HP at 230 V for motors from 10 to 200 HP, more for small ones. A 1 HP motor at 460 V is 2.1 A per HP. Use the table, not the rule, for anything you will wire.

Should I size the wire from the nameplate amps

No. NEC 430.6(A)(1) requires the table current for conductors, short circuit protection and disconnects. The nameplate is used for overload protection.

What size transformer do I need for a 100 HP motor

A 100 HP motor at 460 V is 124 A, about 98.8 kVA running, so 112.5 kVA is the smallest standard three phase size that carries it running. Across the line starting can draw several times that, so check the voltage dip or use a larger transformer, a soft starter or a drive.