For wiring a 60 amp subpanel, use a 60-amp double-pole breaker. The minimum wire size is 6 AWG copper THHN in conduit (rated 65A at 75°C) or 4 AWG copper NM-B cable. For runs over 50 feet, default to 4 AWG copper THHN to mitigate voltage drop. Always include an 8 AWG copper equipment ground.
The Baseline Spec Sheet: Wire, Breaker, and Grounding
- Material: Copper conductors (Aluminum alternatives addressed below)
- Temperature Column: 75°C (Standard for residential panel lugs and breakers per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) or lower
- Raceway: EMT, PVC, or Rigid conduit with a maximum of 3 current-carrying conductors (CCCs)
- System: 120/240V single-phase, 3-wire plus ground
When pulling permits and buying materials, you need exact part specs. The table below outlines the minimum requirements based on NFPA 70 (NEC) Table 310.16 for a standard 60A feeder.
| Conductor Role | Material | Min. AWG (THHN in Conduit) | Min. AWG (NM-B Cable) | 75°C Ampacity |
|---|---|---|---|---|
| Hot (Line 1) | Copper | 6 AWG | 4 AWG | 65A (6 AWG) / 70A (4 AWG) |
| Hot (Line 2) | Copper | 6 AWG | 4 AWG | 65A (6 AWG) / 70A (4 AWG) |
| Neutral (Grounded) | Copper | 6 AWG | 4 AWG | 65A (6 AWG) / 70A (4 AWG) |
| Equipment Ground | Copper | 8 AWG | 8 AWG (or 6 AWG if NM-B) | N/A (Sized per NEC 250.122) |
Note on Neutral Sizing: While NEC 220.61 allows sizing the neutral for the maximum unbalanced load, standard practice for a 60A subpanel feeder is to run a full-size neutral (matching the hots) unless a specific engineered load calculation proves otherwise. Do not downsize the neutral for general-purpose subpanels.
Why 6 AWG THHN or 4 AWG NM-B (And Why Not Smaller?)
A common point of confusion on the jobsite is why 6 AWG copper is legal in conduit, but illegal if you are using Romex (NM-B cable). The answer lies in termination temperature limits and insulation ratings.
Under NEC 110.14(C), you must size your wire based on the lowest temperature rating of any connected device, termination, or conductor. Almost all residential breakers and panel lugs are rated for 75°C.
- In Conduit (THHN/THWN-2): If we look at the 75°C column of Table 310.16, 6 AWG copper is rated for 65 amps. Because 65A is greater than your 60A breaker, 6 AWG THHN is perfectly legal and safe. (Even though THHN insulation is rated 90°C, we cannot use the 90°C column for ampacity unless the breaker lugs are explicitly marked 90°C, which they almost never are).
- In Cable (NM-B): NEC 334.80 mandates that NM-B cable ampacity must be determined using the 60°C column, regardless of the fact that the individual wires inside might have 90°C insulation. In the 60°C column, 6 AWG copper is only rated for 55 amps. Since 55A is less than your 60A breaker, 6 AWG NM-B is a code violation. You must step up to 4 AWG NM-B, which is rated 70A at 60°C.
Voltage Drop: When Distance Changes the Math
Ampacity tells you what size wire will safely carry 60 amps without melting the insulation. It does not tell you if your tools and appliances at the subpanel will actually run correctly. NEC 215.2 (Informational Note) recommends a maximum voltage drop of 3% for feeders.
Let's run the voltage drop math for a 100-foot run from the main panel to the subpanel, assuming a full 60A load.
Scenario A: Using 6 AWG Copper
Using the formula VD = (2 × K × I × D) / CM (where K=12.9 for copper, I=60A, D=100ft, and CM=26,240 for 6 AWG):
VD = (2 × 12.9 × 60 × 100) / 26,240 = 5.89 Volts.
At 240V (for a welder or HVAC), 5.89V is a 2.45% drop. This passes the 3% recommendation.
However, at 120V (for standard receptacles and lighting), 5.89V is a 4.9% drop. This fails the 3% recommendation and will cause noticeable dimming and poor motor performance.
Scenario B: Using 4 AWG Copper
Using the same formula but with the circular mils (CM) for 4 AWG (41,740):
VD = (2 × 12.9 × 60 × 100) / 41,740 = 3.70 Volts.
At 240V, this is a 1.54% drop. At 120V, it is a 3.08% drop. This is vastly superior and keeps 120V loads within acceptable operational tolerances.
The Verdict for Distance: If your subpanel is more than 50 feet away, or if you plan to run heavy 120V loads (like a large dust collector, multiple space heaters, or high-draw power tools), skip 6 AWG entirely and pull 4 AWG copper.
Decision Tree: Choosing Your Exact Conductor
Use this decision matrix to finalize your materials list. Find your installation scenario in the left column and buy the wire specified in the right column.
| If Your Installation Is... | Then Buy This Wire (Hots & Neutral) | Why? |
|---|---|---|
| Under 50 ft, THHN in EMT/PVC conduit | 6 AWG Copper THHN | Meets 75°C ampacity (65A); voltage drop is negligible at short distances. |
| Any distance, using NM-B (Romex) cable | 4 AWG Copper NM-B | NEC 334.80 forces 60°C column; 6 AWG is only 55A, so 4 AWG (70A) is mandatory. |
| 50 to 120 ft, THHN in conduit | 4 AWG Copper THHN | Mitigates 120V voltage drop over longer distances; provides thermal headroom. |
| Over 120 ft, or heavy 120V continuous loads | 3 AWG or 2 AWG Copper THHN | Required to keep 120V drop strictly under 3% at maximum draw. |
| Budget-constrained, conduit, >50 ft | 2 AWG Aluminum XHHW-2 | Aluminum is cheaper; 2 AWG Al (75°C col) is rated 90A, easily covering 60A + drop. |
| Bundling 4 to 6 CCCs in one conduit | Step up one AWG size from baseline | NEC 310.15(C)(1) requires an 80% derating factor for 4-6 CCCs, reducing ampacity. |
Aluminum vs. Copper: Cost and Termination Realities
Copper prices fluctuate wildly, making aluminum an attractive option for subpanel feeders. If you choose aluminum, you must follow different rules. Never treat aluminum and copper interchangeably.
For a 60A subpanel, the minimum aluminum size in conduit is 2 AWG XHHW-2 or THWN-2. In the 75°C column, 2 AWG aluminum is rated for 90 amps, which easily covers the 60A breaker requirement and provides excellent voltage drop mitigation.
1. You must use an anti-oxidant compound (like Noalox) on the stripped aluminum wire before terminating it in the lugs to prevent galvanic corrosion and high-resistance heating.
2. You must use a calibrated torque screwdriver or wrench to tighten the lugs to the exact inch-pound specification printed on the panel label. Aluminum creeps (expands and contracts) more than copper; under-torqued aluminum lugs will loosen over time and cause a fire.
Furthermore, ensure your main panel's 60A breaker and the subpanel's main lugs are explicitly rated for aluminum (marked 'AL' or 'AL/CU'). Most modern breakers are, but older panels from the 1970s or 1980s may only be rated for copper.
When the AHJ or an Engineer Must Confirm
While the math and code references above cover 95% of residential and light-commercial garage/shop subpanels, there are specific scenarios where you must pause and consult your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer.
- Continuous Loads (The 125% Rule): If the primary purpose of this subpanel is to feed a continuous load (defined by the NEC as a load expected to run for 3 hours or more, such as an EV charger, a large kiln, or server rack cooling), NEC 215.2(A)(1) requires the conductors to be sized at 125% of the continuous load. If your continuous load is 48A, 48 × 1.25 = 60A. You can no longer use 6 AWG THHN (65A); you must step up to 4 AWG THHN (85A) to provide the required 125% buffer (75A) before the breaker trips.
- Local Amendments: Some municipalities have blanket amendments that override baseline NEC minimums. For example, certain jurisdictions mandate a minimum of 4 AWG copper for any subpanel feeder, regardless of the calculated load or distance, to ensure future-proofing and mechanical durability. Always check your local AHJ's specific amendment sheet before pulling wire.
- High Ambient Temperatures: If your conduit runs through an attic that reaches 120°F (49°C) in the summer, or runs directly above a hot roof, the 30°C baseline assumption is void. You must apply the temperature correction factors from the bottom of Table 310.16, which will severely derate your wire's ampacity and likely force you to step up to 3 AWG or 2 AWG copper.
For further reading on conductor ampacity and derating factors, the Electrical Contractor Magazine (EC&M) codes and standards section provides excellent field interpretations of NEC Table 310.16 and bundling derations.
By sticking to 4 AWG copper THHN as your default for conduit runs, or 4 AWG NM-B for cable runs, you guarantee code compliance, minimize voltage drop, and ensure your subpanel can handle the reality of modern workshop and garage loads without nuisance tripping or overheated lugs.
