What Size Wire for a 60 Amp Sub Panel? Copper, Aluminum, and Distance Rules

To feed a 60 amp sub panel, use 6 AWG copper THHN/THWN-2 wire or 4 AWG aluminum XHHW wire, protected by a 60A double-pole breaker. This assumes 75°C rated lugs, a 30°C ambient temperature, and a run under 120 feet.

The Core Sizing Data: Copper vs. Aluminum Feeders

Choosing the right wire for a 60 amp sub panel starts with NEC Table 310.16. The most common mistake DIYers make is looking at the 90°C column of the ampacity table. Because the termination lugs inside standard residential load centers are rated for 75°C, NEC 110.14(C) forces you to use the 75°C column for your final ampacity sizing, regardless of how high the wire's insulation is rated.

Feeder Wire Sizing for 60A Subpanel (4-Wire, 120/240V)

Conductor Material	Insulation Type	AWG Size	Ampacity (75°C Col)	Max Run (3% VD)
Copper	THHN / THWN-2	6 AWG	65 Amps	120 feet
Copper	NM-B (Romex)	4 AWG*	70 Amps (60°C col)	95 feet
Aluminum	XHHW / USE-2	4 AWG	65 Amps	150 feet

*See the installation variables section below for why NM-B requires 4 AWG instead of 6 AWG.

Why 6 AWG Copper (and When You Must Upsize)

You might wonder why we don't use 8 AWG copper, which is cheaper and easier to pull. According to Table 310.16, 8 AWG copper at 75°C is rated for 50 amps. Because 50A is a standard breaker size listed in NEC 240.6(A), you are strictly limited to a 50A breaker. You cannot use the 'next size up' rule (NEC 240.4(B)) to protect 8 AWG wire with a 60A breaker. Therefore, 6 AWG (rated 65A at 75°C) is the absolute minimum size for a 60A feeder.

The Voltage Drop Check

Ampacity tells you the wire won't melt; voltage drop tells you your appliances will actually work. NEC 310.15(B) recommends a maximum 3% voltage drop on feeders. For a 240V system, 3% equals 7.2 volts.

Using the standard voltage drop formula ($VD = \frac{2 \times K \times I \times L}{CM}$) and the Southwire Voltage Drop Calculator methodology:

• At 60 amps and 100 feet: 6 AWG copper drops 5.8V (2.4%). Pass.
• At 60 amps and 120 feet: 6 AWG copper drops 7.0V (2.9%). Pass.
• At 60 amps and 150 feet: 6 AWG copper drops 8.7V (3.6%). Fail.

If your trench or conduit run exceeds 120 feet, you must upsize to 4 AWG copper to maintain the 3% feeder threshold.

Decision Tree: Do I Need to Upsize from 6 AWG Copper?

Condition	Required Action
Run is under 120 ft, in conduit, 30°C ambient	Use 6 AWG Copper THHN
Run is 121 ft to 190 ft	Upsize to 4 AWG Copper THHN
Run is over 190 ft	Upsize to 3 AWG or 2 AWG Copper (calculate exact VD)
Wire is pulled through attic in Phoenix (50°C+ ambient)	Apply temperature correction factors; likely upsize to 4 AWG

Installation Variables That Change the Math

The 6 AWG baseline assumes ideal conditions. Real-world jobsite variables frequently force an upsize. Here is what changes the answer.

The NM-B (Romex) Trap

If you are running NM-B cable through wall cavities instead of pulling THHN through conduit, the rules change. NEC 334.80 mandates that NM-B ampacity is limited to the 60°C column of Table 310.16, regardless of the fact that the wire's insulation is technically rated for 90°C. In the 60°C column, 6 AWG copper is only rated for 55 amps. While some inspectors might allow the next-size-up rule to hit 60A on a non-continuous load, many AHJs flatly reject 6 AWG NM-B for a 60A feeder due to heat dissipation issues in insulated walls. To guarantee you pass inspection anywhere in the country, use 4 AWG NM-B (rated 70A at 60°C) for a 60A subpanel.

Bundling and Conduit Fill Derating

If you pull multiple circuits through the same conduit, you must apply derating factors from NEC 310.15(B)(1)(a). However, there is a vital exception for single-phase residential feeders: the neutral wire in a 120/240V system only carries unbalanced current and does not count as a current-carrying conductor (CCC) for derating purposes. Therefore, a standard 4-wire subpanel feeder only has 2 CCCs (the two hot legs). Derating does not apply unless you have more than 3 CCCs in the conduit. If you are pulling feeders for two subpanels in one pipe (4 hot legs), you hit the 80% derating factor, and 6 AWG (65A x 0.8 = 52A) is no longer sufficient. You must upsize to 4 AWG.

When an Engineer or AHJ Must Confirm

Stop and consult your local electrical inspector or a licensed professional engineer if:

• Your run exceeds 250 feet (voltage drop mitigation may require transformer step-up/step-down or parallel conductors).
• The subpanel is located in an environment where ambient temperatures regularly exceed 40°C (104°F), requiring strict application of Table 310.15(B)(1)(1) correction factors.
• You are utilizing aluminum wire and the subpanel manufacturer's documentation explicitly forbids aluminum terminations (rare for modern main lugs, but possible on older or specialized industrial panels).