Sizing
240V circuit for a 3 HP motor
A 3 HP single-phase motor draws 17 A at 240V (NEC Table 430.248), so the conductor must carry 21.3 A: a 25 A breaker and 10 AWG copper on a short run, NEMA 6-30R. On 120V the same motor draws 34 A, more than a 20 A household circuit can feed.
Breaker and wire by run length
| One-way run | Breaker | Wire | Voltage drop | Receptacle |
|---|---|---|---|---|
| 25 ft | 25 A | 10 AWG | 0.4 percent | NEMA 6-30R |
| 50 ft | 25 A | 10 AWG | 0.9 percent | NEMA 6-30R |
| 100 ft | 25 A | 10 AWG | 1.8 percent | NEMA 6-30R |
| 150 ft | 25 A | 10 AWG | 2.6 percent | NEMA 6-30R |
Wire is upsized from the breaker minimum whenever the voltage drop would pass 3 percent, which is why the gauge steps up with distance. Gauges are copper on the 60 C column of NEC Table 310.16, which is how NM-B cable is rated.
What 3 HP looks like on 120V
At 120V the motor pulls 34 A. The conductor must carry 42.5 A, which means a 45 A breaker, beyond the 20 A that a standard 120V receptacle circuit allows. That is the whole case for 240V: half the current in the same wire. Many 1-1/2 and 2 HP machines ship prewired for 120V and can be rewired for 240V, so check the nameplate.
Use the 240V circuit calculator for your own run length, and the voltage drop calculator for a cord.
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- 240V circuit for a 1 HP motor
- 240V circuit for a 1-1/2 HP motor
- 240V circuit for a 2 HP motor
- 240V circuit for a 5 HP motor
- 240V circuit for a 7-1/2 HP motor
- 240V circuit for a 10 HP motor
Frequently asked questions
What size breaker for a 3 HP 240V motor?
The full-load current is 17 A, times 1.25 is 21.3 A, so the next standard breaker is 25 A. The code allows a larger breaker to ride through motor starting, but this is the conservative size.
This page is researched guidance, not professional electrical, engineering or safety advice. Circuits, loads and clearances depend on your house, your service and your town: have a licensed electrician confirm any wiring, and check local code and each manufacturer's manual before you act.
