What Size Wire for a 30 Amp Breaker?

Short answer: a 30 amp breaker needs 10 AWG copper wire, whether you use NM-B cable (10/2 or 10/3 with ground) or THHN in conduit. With aluminum, use 8 AWG aluminum. Long runs may need 8 AWG copper to limit voltage drop.

30 amp wire size by wiring method

Ground wire: 10 AWG copper or 8 AWG aluminum.

Why 10 AWG and not smaller? NEC 240.4(D) limits 10 AWG copper to a 30 A breaker, and 12 AWG copper to 20 A, regardless of its table ampacity. Aluminum 10 AWG is limited to 25 A, so aluminum needs 8 AWG. Because 10 AWG copper is rated exactly 30 A in the 60°C column, the answer doesn't change between NM-B cable and THHN with 75°C terminals.

30 amp wire size by distance (240 V)

Smallest conductor that meets both the 75°C ampacity for a 30 A circuit and a 3% voltage drop at 24 A (80% of the rating), 240 V single-phase, one-way distance. Percent drop in brackets. If the circuit uses NM-B cable or 60°C terminations, start from the 60°C column instead.

30 amp wire size by distance (120 V, RV and shop circuits)

Smallest conductor that meets both the 75°C ampacity for a 30 A circuit and a 3% voltage drop at 24 A (80% of the rating), 120 V single-phase, one-way distance. Percent drop in brackets. If the circuit uses NM-B cable or 60°C terminations, start from the 60°C column instead.

At 120 V the same current produces twice the percentage drop. At 100 feet and a 24 A load, 10 AWG copper drops 5.97 V, about 5.0%. 8 AWG copper is just over the guideline at 3.1%, so 6 AWG (2.0%) is the safer choice for a TT-30 RV pedestal at that distance.

Common 30 amp circuits

• Electric dryer: 30 A, 120/240 V, 10/3 with ground (four-wire) to a NEMA 14-30 receptacle. New installations need the four-wire configuration (250.140). Under the 2023 NEC, a dryer receptacle in a dwelling laundry area also needs GFCI protection, because 210.8(A) covers 125–250 V receptacles in laundry areas; many jurisdictions amend this rule, so check the local adoption.
• Electric water heater: a 4,500 W, 240 V heater draws 4,500 ÷ 240 = 18.75 A. Water heaters of 120 gallons or less are treated as continuous loads (NEC 422.13), so the circuit is sized at 125%: 23.4 A. That rules out 12 AWG (limited to 20 A), so the conductor is 10/2 copper, and a 30 A breaker is the usual choice (check the heater's nameplate for its maximum overcurrent device).
• 30 A RV receptacle (TT-30): 120 V only, 10/2 with ground. Don't confuse it with a 14-30 dryer receptacle.
• Level 2 EV charger at 24 A: 30 × 0.8 = 24 A is the most a 30 A circuit may carry continuously, which suits a charger set to 24 A.
• Small subpanel in a shed: 10/3 with ground for a 30 A, 120/240 V feeder, with neutral and ground separated at the subpanel.

Air conditioners: when the nameplate overrides 240.4(D)

Motor-compressor equipment is sized from its nameplate under Article 440, and 240.4(G) lets those circuits skip the small-conductor limits. A condenser marked MCA 18 A, MOCP 30 A can be wired with 12 AWG copper (20 A at 60°C, above the 18 A MCA) on a 30 A breaker. The breaker is sized for motor starting current; the conductor is sized for the running load. This applies only when the nameplate says so. A plain 30 A receptacle circuit still needs 10 AWG.

10 AWG in conduit

Four 10 AWG THHN (two hots, neutral, ground) fit in ½" EMT, which allows up to 5. A dryer circuit and a water heater circuit in the same pipe add up to six 10 AWG conductors (with one shared ground), which needs ¾" EMT (up to 10). They also make four current-carrying conductors, so apply the adjustment from Table 310.15(C)(1). With THHN you may start from the 90°C value, and 10 AWG usually still clears 30 A. See the ampacity chart for the base values.

Common mistakes inspectors flag

• 10/2 to a new dryer receptacle. The dryer needs a neutral and a separate ground: 10/3 with ground and a 14-30R.
• Neutral and ground bonded at the dryer on a four-wire installation. Remove the bonding strap in the dryer terminal block.
• A TT-30 receptacle wired to 240 V. RVs are 120 V on a 30 A service; 240 V at the pedestal can damage the coach's electronics.
• A 40 A breaker on 10 AWG because the dryer "kept tripping." Find the fault instead.
• Aluminum 10 AWG on a 30 A breaker. It's limited to 25 A.

Frequently asked questions

Can I use 10/2 wire for a 30 amp dryer?

Not for a new installation. Dryers need 120 V for timers and controls, so the circuit needs two hots, a neutral and a ground: 10/3 with ground. Existing three-wire dryer circuits may stay in service only under the conditions of the 250.140 Exception.

Can I use 12 gauge wire on a 30 amp breaker?

No, except on equipment such as AC condensers whose nameplate MCA and MOCP permit it. For ordinary circuits, 12 AWG copper is limited to a 20 A breaker by 240.4(D); see 20 amp wire size.

Can I use 8 gauge wire on a 30 amp breaker?

Yes. Larger wire is always allowed on a smaller breaker, as long as it fits the terminals. It's a good choice for long runs.

How many amps can 10 gauge wire handle?

30 A at 60°C, 35 A at 75°C and 40 A at 90°C in the ampacity table, but its breaker is limited to 30 A. For more current, see 40 amp wire size.

What size wire for a 30 amp 240 volt circuit 150 feet away?

At a 24 A load, 8 AWG copper keeps the drop around 2.3%. At a full 30 A load, 8 AWG is right at 2.9%, so many electricians go to 6 AWG copper (1.8%) for margin. The wire size calculator runs other distances.

Related

• Voltage Drop Calculator

---

Last updated: October 6, 2026. Based on NEC 2023. Check results against the code edition and local amendments your jurisdiction enforces and with the AHJ; electrical work should be done by qualified people.