For a standard 30-amp circuit, you need 10 AWG copper wire or 8 AWG aluminum wire. This baseline is established by the 60°C and 75°C temperature columns in NEC Table 310.16. While 10 AWG copper is technically rated for 35 amps in the 75°C column, NEC Article 240.4(D) strictly limits the overcurrent protection for 10 AWG copper to 30 amps for most general branch circuits. If you are pulling individual conductors in conduit (like THHN), 10 AWG copper is your go-to. If you are running aluminum feeder (like SER cable), you must step up to 8 AWG to safely carry 30 amps.

NEC Table 310.16: Ampacity Chart for 30A Circuits

Before picking your wire off the spool, you need to know how to read the ampacity table. The National Electrical Code (NEC) Table 310.16 organizes wire ampacity by material (Copper vs. Aluminum) and insulation temperature rating (60°C, 75°C, and 90°C).

Which column applies to your installation? According to NEC 110.14(C), for circuits rated 100A or less, or conductors sized 14 AWG through 1 AWG, you must use the 60°C column to determine your baseline ampacity. The only exception is if your equipment terminals are specifically tested, listed, and identified for 75°C. Most modern 30-amp breakers and NEMA 14-30 receptacles are rated for 75°C, but if you are using NM-B (Romex) cable, NEC 334.80 forces you to use the 60°C column regardless of the terminal rating because the cable jacket cannot dissipate heat as efficiently as individual wires in free air.

Bookmark Quick-Jump: Looking for the standard residential answer? Jump straight to the 10 AWG Copper / 8 AWG Aluminum row in the table below.
Source Standard: NFPA 70 (NEC) Table 310.16 - Allowable Ampacities of Insulated Conductors (Not more than three current-carrying conductors in raceway, cable, or earth, based on ambient temperature of 30°C / 86°F).
AWG Size Copper (60°C) Copper (75°C) Copper (90°C)* Aluminum (75°C)
14 AWG 15A — 25A —
12 AWG 20A 25A 30A —
10 AWG 30A 35A 40A 30A
8 AWG 40A 50A 55A 40A
6 AWG 55A 65A 75A 50A

*Note: The 90°C column is primarily used as the starting point for calculating derating factors, not for final overcurrent protection sizing.

Derating and Installation Conditions

The ampacity values in the table above assume you have no more than three current-carrying conductors in a raceway and an ambient temperature of 30°C (86°F). When your installation deviates from these baseline conditions, derating rows modify the base value, often forcing you to upsize your wire.

Conduit Fill (More than 3 conductors): When you pull multiple circuits through the same conduit, the wires heat each other up. If you have 4 to 6 current-carrying conductors, you must multiply the 90°C ampacity by 80%. If you have 7 to 9 conductors, the multiplier drops to 70%.

Workbench Example: You are pulling four 10 AWG THHN wires (two hots, one neutral, one ground) through a single EMT conduit for a multi-wire branch circuit. The ground does not count as a current-carrying conductor, leaving you with 3. No derating is required. However, if you add a second circuit (adding two more hots and a neutral), you now have 6 current-carrying conductors. You must use the 90°C column (40A) and apply the 80% derating factor: 40A × 0.80 = 32A. Your 10 AWG wire is now only good for 32 amps, which still safely covers a 30-amp breaker. But if you added a third circuit (9 current-carrying conductors), the 70% factor applies: 40A × 0.70 = 28A. Your 10 AWG wire is now undersized, and you must upsize to 8 AWG.

Ambient Temperature: If you are routing wire through a hot attic in the summer where temperatures regularly hit 110°F (43°C), you must apply a temperature correction factor. For 90°C THHN wire at 41-45°C ambient, the correction factor is 0.87. Always check the NFPA National Electrical Code Table 310.15(B)(1) for exact temperature correction multipliers before finalizing your wire pull.

What the Ampacity Table Cannot Tell You

Ampacity tables only tell you the maximum current a wire can carry before its insulation begins to degrade from heat. They do not account for voltage drop.

The NEC recommends (in Informational Note to 210.19(A)) that branch circuit voltage drop be limited to 3% for maximum efficiency. On a 120V circuit, a 3% drop is 3.6 volts. On a 240V circuit, it is 7.2 volts. If you are installing a 30-amp RV pedestal (NEMA 14-30) or a 240V window air conditioner at the far end of a 150-foot trench, 10 AWG copper will result in a voltage drop exceeding 5%. The equipment will run hot, motors will struggle to start, and electronics may brown out.

To calculate voltage drop for single-phase circuits, use the formula: VD = (2 × K × I × D) / CM.

  • K = 12.9 for Copper, 21.2 for Aluminum
  • I = Current (30 Amps)
  • D = One-way distance in feet
  • CM = Circular Mils of the wire (10 AWG = 10,380 CM)

For a 150-foot run at 30 amps using 10 AWG copper: (2 × 12.9 × 30 × 150) / 10,380 = 11.19 volts. On a 240V circuit, that is a 4.6% drop. To fix this, you must ignore the ampacity table's minimums and upsize to 8 AWG or even 6 AWG copper to keep the drop under 3%. For deeper calculations, the Copper Development Association provides excellent resources on conductor sizing for long runs.

Frequently Asked Questions About 30 Amp AWG Wire

What size 30 amp AWG wire do I need for a 240V dryer or RV outlet?

For a standard 30-amp, 240V receptacle (like a NEMA 14-30R used for dryers or RV hookups), you need 10 AWG copper wire. Specifically, you need a 4-wire setup: two 10 AWG hot wires (usually Black and Red), one 10 AWG neutral (White), and one 10 AWG equipment grounding conductor (Bare or Green). Do not use the older 3-prong NEMA 10-30 configuration for new installations; the NEC has required a separate equipment ground for these circuits since 1996 to prevent the dryer chassis from becoming energized if the neutral fails.

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

Absolutely not. 12 AWG copper wire is rated for a maximum of 20 amps (in the 60°C column). Placing it on a 30-amp breaker is a severe fire hazard. If a fault or overload draws 25 amps, the 12 AWG wire will overheat, melt its insulation, and potentially ignite surrounding framing long before the 30-amp breaker trips. NEC Article 240.4 strictly requires the overcurrent device to match the wire ampacity. The only rare exception is for specific motor circuits where startup inrush currents require a larger breaker, but the wire must still be sized for the motor's full load amps.

Do I need to upsize my 30 amp AWG wire ground if I increase the hot wire size?

Yes. NEC Table 250.122 dictates that a 30-amp circuit requires a minimum 10 AWG copper equipment grounding conductor. However, NEC 250.122(B) states that if you increase the size of your ungrounded (hot) conductors to compensate for voltage drop, you must increase the equipment grounding conductor proportionally. If you upsized your hot wires from 10 AWG to 6 AWG for a long RV run, you must also upsize your ground wire from 10 AWG to 6 AWG to ensure it can safely clear a fault at the far end of the circuit.

Is 10 AWG wire always rated for 30 amps in every installation?

No. The 30-amp rating assumes standard conditions: an ambient temperature of 86°F (30°C), no more than three current-carrying conductors bundled together, and proper termination. If you route 10 AWG NM-B cable through insulation in a hot attic, or bundle multiple 10 AWG cables tightly together in a bored hole through a framing stud without maintaining spacing, the wire cannot dissipate heat. In those high-temperature or high-congestion scenarios, the effective ampacity of 10 AWG wire drops below 30 amps, and you must upsize to 8 AWG to maintain safety margins.