If the appliance runs for hours at full power, a water heater for example, the circuit has to be sized for more than its nameplate current, and the calculator handles that when you mark it as continuous.
How to use this calculator
- Enter 30 amps for a circuit sized to its breaker, or the appliance's nameplate current if you are working out what breaker it needs.
- Pick 240 volts for a dryer, a water heater or an air conditioner, and 120 volts for the common TT-30 RV receptacle.
- Enter the one-way distance from the panel to the appliance or the pedestal.
- Mark the load as continuous when it runs at full current for 3 hours or more: the wire must then carry 125 percent of it.
Worked examples
A clothes dryer in the laundry room
Load current: 30 A · Circuit voltage: 240 V · One-way distance: 35 ft
- Wire size
- 10 AWG copper
- Current the wire must carry
- 30.0 A
- Rated for
- 30 A
- Voltage drop
- 1.09%
- Longest run within 3%
- 97 ft
An electric water heater in the basement
Load current: 18.75 A · Circuit voltage: 240 V · One-way distance: 25 ft · Runs for 3 hours or more at a time: Yes, a continuous load
- Wire size
- 10 AWG copper
- Current the wire must carry
- 23.4 A
- Rated for
- 30 A
- Voltage drop
- 0.48%
- Longest run within 3%
- 155 ft
A TT-30 RV receptacle by the driveway
Load current: 30 A · Circuit voltage: 120 V · One-way distance: 90 ft
- Wire size
- 6 AWG copper
- Current the wire must carry
- 30.0 A
- Rated for
- 55 A
- Voltage drop
- 2.21%
- Longest run within 3%
- 122 ft
Wire size for a 30 amp circuit by one-way distance
| One-way distance | 240 V, Copper | 120 V, Copper | 240 V, Aluminum |
|---|---|---|---|
| 25 ft | 10 AWG | 10 AWG | 8 AWG |
| 50 ft | 10 AWG | 8 AWG | 8 AWG |
| 75 ft | 10 AWG | 8 AWG | 8 AWG |
| 100 ft | 8 AWG | 6 AWG | 6 AWG |
| 150 ft | 8 AWG | 4 AWG | 4 AWG |
Each cell: the smallest size that carries the current within the usual branch-circuit drop. A 120 volt circuit loses twice the percentage of a 240 volt one over the same wire, so it needs a thicker size sooner.
Why 10 AWG is the answer for most 30 amp circuits
The code caps the breaker on 10 AWG copper at 30 amps, the same as its ampacity in the cable column, so the size and the circuit match exactly. 12 AWG is limited to 20 amps and cannot be used, even for a short run. Aluminum is capped lower: 10 AWG aluminum may carry only 25 amps, which is why an aluminum 30 amp circuit needs 8 AWG.
Dryers, water heaters and RV hookups
A dryer circuit needs a neutral for the drum motor and controls, so it is run in 10/3 cable with a ground. A water heater uses only the two hots and a ground, and because it heats for long stretches it counts as continuous: its nameplate current times 125 percent must fit the wire. A TT-30 RV receptacle is 120 volts, and a camper at the far end of a driveway often needs 8 AWG or more to keep the air conditioner from tripping on low voltage. The table shows how quickly the 120 volt column grows.
Before you buy the cable
Measure along the real route, add the drops up and down the walls, and use the one-way length in the calculator. The breaker stays at 30 amps when you go up a wire size for distance, and the amps calculator turns an appliance's watts into the current to enter.
Common questions
What size wire for a 30 amp dryer?
10 AWG copper in 10/3 cable with a ground, which is rated 30 amps and is the largest size the code allows on that breaker. A very long run may call for 8 AWG.
Can I use 12 gauge wire on a 30 amp breaker?
No. 12 AWG copper is limited to a breaker of 20 amps. A 30 amp breaker needs 10 AWG copper or 8 AWG aluminum at least.
What size aluminum wire for 30 amps?
8 AWG aluminum, rated 40 amps. 10 AWG aluminum is limited to a breaker of 25 amps, so it is too small for a 30 amp circuit.
Is 10 gauge wire good for 30 amps at 100 feet?
Not at the full 30 amps. Over that distance 10 AWG loses more than 3 percent even at 240 volts, so 8 AWG is the size to use, and at 120 volts the drop doubles and calls for 6 AWG. The table shows each step.