For a standard 60-amp subpanel, the correct wire size for sub panel feeds is 6 AWG copper THHN/THWN-2 paired with a 60A double-pole breaker. If upgrading to a 100-amp subpanel, step up to 3 AWG copper or 1/0 AWG aluminum. These baselines assume standard residential conditions.

Baseline Assumptions for this Guide:
  • Conductor Material: Copper (unless Aluminum is explicitly stated)
  • Temperature Column: 75°C (Standard for residential terminations)
  • Ambient Temperature: 30°C (86°F)
  • Conduit Type: PVC Schedule 80 or EMT, maximum 3 current-carrying conductors

The 75°C Rule and Ampacity Table Realities

A common mistake on the bench and in the field is looking at the 90°C column on NEC Table 310.16 because THHN wire is stamped with a 90°C rating. You must ignore the 90°C column for final sizing. NEC 110.14(C) dictates that unless your equipment is explicitly listed and marked for 90°C terminations, you must use the 75°C column. Almost all residential breakers and panel lugs are rated for 75°C.

Why 6 AWG and Not One Size Smaller?

If you look at the 75°C column for copper, 8 AWG is rated for 50 amps. It physically cannot handle a 60-amp continuous or non-continuous load without risking thermal degradation of the breaker terminals. Therefore, 8 AWG is strictly forbidden for a 60A feeder.

Moving up, 6 AWG copper in the 75°C column is rated for 65 amps. Because 65A is not a standard breaker size (standard sizes are 15, 20, 30, 40, 50, 60, 70, etc.), NEC 240.4(B) allows you to round up to the next standard size, which is 70A. However, if your subpanel's main lugs or calculated load are strictly rated for 60A, you pair the 65A-capable 6 AWG wire with a 60A breaker. This provides a perfect, code-compliant match with a built-in 5-amp thermal buffer.

Bench Tip: Never use 4 AWG copper for a 60A panel just because "bigger is better." 4 AWG is significantly stiffer, making it a nightmare to route through tight 1-inch conduit sweeps and crowded panel gutters. Stick to 6 AWG unless voltage drop forces your hand.

Voltage Drop: When Distance Forces a Larger Wire

Ampacity tells you what the wire can handle thermally; voltage drop tells you what the wire can deliver electrically. The NEC recommends (via Informational Notes in 310.15) that feeder voltage drop not exceed 3% for optimal efficiency.

Let us run the math for a 60A subpanel located 100 feet from the main panel, using 240V and 6 AWG copper. Using the standard single-phase voltage drop formula and referencing the Southwire Voltage Drop Calculator parameters:

  • Formula: VD = (2 × K × I × L) / CM
  • K (Copper): 12.9 ohms
  • I (Current): 60A
  • L (Length): 100 feet
  • CM (Circular Mils for 6 AWG): 26,240

Calculation: (2 × 12.9 × 60 × 100) / 26,240 = 5.89 Volts dropped.

On a 240V circuit, 5.89V represents a 2.45% drop. This is well under the 3% threshold. However, if your subpanel is 150 feet away, the drop jumps to 3.68%. At that distance, you must bump your wire size for sub panel feeds up to 4 AWG copper to maintain efficiency and prevent motor-starting voltage sags.

Decision Matrix: Sizing for 60A, 100A, and 200A Feeds

Use this decision-tree-table to select your exact materials based on your subpanel's main breaker rating. This table assumes a run under 100 feet and standard 75°C terminations.

Subpanel Rating Copper Wire Size (THHN) Aluminum Wire Size (XHHW) Feeder Breaker Size Minimum Conduit Size
60 Amp 6 AWG 4 AWG 60A (2-pole) 3/4 inch
100 Amp 3 AWG 1/0 AWG 100A (2-pole) 1 inch
125 Amp 1 AWG 2/0 AWG 125A (2-pole) 1-1/4 inch
200 Amp 2/0 AWG 4/0 AWG 200A (2-pole) 1-1/2 inch

What Changes the Answer: Aluminum vs. Copper

Aluminum wire is roughly 40% cheaper than copper by the foot, making it highly attractive for 100A and 200A feeds where copper prices become punishing. However, aluminum has a lower ampacity per cross-sectional area. Notice in the table above that a 100A feed requires 3 AWG copper, but demands 1/0 AWG aluminum.

Aluminum Installation Rules: If you choose aluminum, you must apply an anti-oxidant compound (like Noalox) to the stripped wire ends before torquing them into the lugs. Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that causes terminal fires. Furthermore, aluminum expands and contracts more than copper under thermal cycling; you must re-torque the lugs after 24 hours of use.

Derating and Bundling: What Shrinks Your Ampacity

The sizes listed above assume you are pulling a single feeder circuit (two hots, one neutral, one ground) through a conduit. That equals three current-carrying conductors (the ground does not count). Three conductors require no derating.

But what if you decide to pull a second circuit—say, a 20A buried landscape lighting feed—through the same conduit to save on trenching? You now have four hots and two neutrals, totaling six current-carrying conductors. According to NEC Table 310.15(C)(1), 4 to 6 conductors require an 80% derating factor.

Here is where the 90°C column finally gets used. You calculate derating from the 90°C column, then verify the final number against the 75°C termination limit.

  • 6 AWG THHN at 90°C: 75 Amps
  • Derated (75A × 0.80): 60 Amps

It barely survives. Your 6 AWG wire is now legally capped at exactly 60A. If you pull a third circuit (7-9 conductors, 70% derating), your 6 AWG wire drops to 52.5A, and you will trip a 60A breaker. Decision path: If bundling more than one feeder circuit in a single conduit, automatically bump your wire size up one full tier (e.g., use 4 AWG for a 60A feed) to preserve your safety margin.

Continuous Loads and Torque Specifications

Sizing the wire is only half the battle; terminating it correctly prevents the panel from burning down. NEC 110.14(D) mandates that all terminations be torqued to the manufacturer's specified values using a calibrated torque screwdriver or wrench. Guessing "finger tight plus a quarter turn" is a code violation and a leading cause of high-resistance thermal failures.

The 125% Continuous Load Rule

If your subpanel is being installed specifically to feed a continuous load—defined by the NEC as any load expected to run for 3 hours or more, such as an EV Level 2 charger, a basement server rack, or a large HVAC system—you must multiply the continuous load by 1.25 before sizing the wire.

For example, if your subpanel calculates out to a 48A continuous load from a heavy-duty workshop dust collection system and EV charger running simultaneously, 48A × 1.25 = 60A. You must size the wire and breaker for 60A minimum, meaning 6 AWG copper is your absolute floor. If the calculated continuous load is 50A, you must size for 62.5A, forcing you to step up to 4 AWG copper and a 70A breaker.

When to Pull the Permit and Call the AHJ

While this guide provides NEC-style guidance for standard residential applications, local Authority Having Jurisdiction (AHJ) rules always supersede general advice. You must consult a licensed electrical engineer or your local inspector under the following conditions:

  • Runs exceeding 200 feet: Voltage drop calculations become highly sensitive to ambient soil temperatures (if buried direct) and specific load power factors.
  • Subpanels over 200A: Feeders of 4/0 AWG and larger often require parallel runs or specialized lug kits that must be engineered and stamped.
  • Ambient temperatures above 30°C (86°F): If your conduit runs across a sun-baked black roof or through an unventilated attic in a southern climate, you must apply the temperature correction factors in Table 310.15(B)(1), which will drastically reduce your wire's ampacity and force a size increase.

For the vast majority of garage, shed, or basement additions, sticking to 6 AWG copper for 60A and 3 AWG copper for 100A—pulled in properly sized conduit and torqued to spec—will yield a safe, code-compliant installation that passes inspection on the first visit.