What Gauge Wire for 30 Amps?
Dryers, central air conditioners, water heaters, and 30 amp RV plugs all live on 30 amp circuits. The answer is 10 AWG copper in almost every case, but the breaker rules, the 80 percent continuous-load limit, and long-run voltage drop all shape the final call.
A 30 amp circuit needs 10 AWG copper (30 amps at 60C, breaker capped at 30 amps by NEC 240.4(D)) or 8 AWG aluminum. Continuous loads are limited to 24 amps. Voltage drop math: a 150 ft run at 24 amps on 10 AWG copper drops 8.95 volts (3.73 percent), so 8 AWG brings it to 5.63 volts (2.34 percent).
The short answer: 10 AWG copper
10 AWG copper is the standard wire for a 30 amp circuit. It is rated 30 amps at the 60 degree column of NEC 310.16, and NEC 240.4(D) caps the overcurrent protection at 30 amps for 10 AWG small conductors anyway, so the wire and the breaker line up exactly: 10 AWG wire, 30 amp breaker. The aluminum equivalent is 8 AWG (40 amps at 75 degrees, which covers 30 amps with margin).
Where 30 amp circuits show up
The usual suspects: electric clothes dryers (240 volt), central air conditioners and heat pumps (240 volt), electric water heaters (240 volt), and 30 amp RV pedestal outlets (120 volt, NEMA TT-30). Dryers and ranges need a neutral in addition to the two hots, which is why dryer circuits run 10/3 cable (two hots, neutral, ground) while a water heater can use 10/2 (one hot pair plus ground, no neutral).
The 80 percent continuous rule
NEC 210.19 limits continuous loads, anything running 3 hours or more, to 80 percent of the circuit rating. On a 30 amp circuit that means 24 amps maximum continuous. A water heater with a 4,500 watt element draws 18.75 amps at 240 volts (4,500 / 240), comfortably under the 24 amp line. If your load is genuinely continuous and above 24 amps, the circuit needs to be upsized to 40 amps on 8 AWG.
Distance math: when 10 AWG is not enough
Take a 240 volt, 24 amp continuous load on 10 AWG copper and run the drop formula (VD = 2 x 12.9 x I x L / CM):
75 foot one-way run: 2 x 12.9 x 24 x 75 / 10380 = 46,440 / 10,380 = 4.47 volts, or 1.86 percent. Fine.
150 foot one-way run: 2 x 12.9 x 24 x 150 / 10380 = 92,880 / 10,380 = 8.95 volts, or 3.73 percent. Over the 3 percent guidance. Upsize to 8 AWG: 92,880 / 16,510 = 5.63 volts, or 2.34 percent. That is the right wire for the long run.
Outbuildings and well pumps are where this bites: the amps say 10 AWG, the distance says 8 AWG, and the distance wins.
10/2 vs 10/3: which cable?
The second number counts insulated current-carrying conductors, and the bare ground comes free. 10/2 (black, white, bare ground) suits 240 volt loads with no neutral: water heaters, well pumps, some AC units. 10/3 (black, red, white, bare ground) suits loads that need both 240 and 120 volts: dryers, RV pedestals wired for a subpanel. When in doubt, pull 10/3; an unused neutral is cheap insurance, but a missing neutral means re-pulling the cable.
A note on safety: wire sizing involves the National Electrical Code, breaker rules, and local amendments. The examples below are planning estimates. Have a licensed electrician review any real installation before you buy wire or pull a permit.
Skip the arithmetic
Running a 30 amp circuit to an outbuilding or a new dryer outlet? Check the exact voltage drop on 10 AWG or 8 AWG for your run length before you buy.
30 amp wire size questions
Can you run 30 amps on 12 gauge wire?
No. 12 AWG copper is rated 20 amps at the 60 degree column, and NEC 240.4(D) caps its breaker at 20 amps. A 30 amp load on 12 AWG would overheat the wire before the breaker ever tripped. Use 10 AWG copper minimum.
What size breaker for 10 gauge wire?
30 amps maximum, per NEC 240.4(D). The breaker protects the wire, so the breaker rating must not exceed the wire's ampacity, and 30 amps is the small-conductor ceiling for 10 AWG in residential work.
How many watts can 10 AWG wire handle?
On a 30 amp circuit: 3,600 watts at 120 volts (120 x 30) or 7,200 watts at 240 volts (240 x 30). For continuous loads, apply the 80 percent rule: 2,880 watts at 120 volts or 5,760 watts at 240 volts.
What wire do I need for a 30 amp RV plug?
A NEMA TT-30 RV outlet is 120 volt, 30 amp: run 10 AWG (10/2 with ground is enough, since there is no 240 volt leg). Keep the run short if you can; RV pedestals at the far end of a property are prime candidates for an 8 AWG upsize on voltage drop.
Does a 30 amp dryer need 10/3 wire?
Yes, in almost every case. A modern electric dryer needs two hots for the 240 volt heating element plus a neutral for the 120 volt drum motor and controls, so 10/3 with ground is the standard cable. Older 3-wire dryer installations are grandfathered in some jurisdictions but new work needs the 4-wire 10/3.