To wire a sub panel for a standard 60-amp detached garage or workshop, use 6 AWG copper THHN/THWN-2 wire protected by a 60-amp double-pole breaker at the main panel. For a 100-amp sub panel, step up to 3 AWG copper or 1 AWG aluminum. These sizes assume a standard 240V residential split-phase system.
All sizing below assumes copper conductors (unless aluminum is explicitly specified), THHN/THWN-2 insulation, the 75°C ampacity column of NEC Table 310.16, an ambient temperature of 30°C (86°F), and installation in standard EMT or PVC conduit with no more than three current-carrying conductors. Local codes and the Authority Having Jurisdiction (AHJ) always have final say.
The Core Sizing Matrix for Sub Panel Feeders
Before you pull any wire, you need to match the sub panel's main lug rating to the feeder breaker at your main service. The table below provides the baseline minimums based on the National Electrical Code (NEC) Table 310.16. Note that the maximum distance column calculates the absolute limit before voltage drop exceeds the recommended 3% threshold for feeders at full breaker capacity.
| Sub Panel Rating | Main Breaker Size | Copper AWG (75°C) | Aluminum AWG (75°C) | Max Distance for <3% VD (240V) |
|---|---|---|---|---|
| 60 Amp | 60A 2-Pole | 6 AWG | 4 AWG | 122 feet |
| 100 Amp | 100A 2-Pole | 3 AWG | 1 AWG | 146 feet |
| 125 Amp | 125A 2-Pole | 1 AWG | 2/0 AWG | 186 feet |
| 200 Amp | 200A 2-Pole | 2/0 AWG | 4/0 AWG | 185 feet |
Note: A sub panel does not require a main breaker if it is in the same building as the main service, but detached structures require a local disconnecting means (NEC 225.32). A main breaker in the sub panel satisfies this requirement.
Why We Choose 6 AWG Copper for a 60-Amp Feed
A common mistake on the jobsite is trying to save money by using 8 AWG copper wire on a 60-amp breaker. This is a direct code violation and a fire hazard. Here is the exact breakdown of why 6 AWG is the minimum.
While 8 AWG THHN wire has an ampacity of 55 amps in the 90°C column, NEC 110.14(C) requires us to size conductors based on the temperature rating of the equipment terminations. Most standard residential breakers and panel lugs are rated for 75°C. In the 75°C column of NEC Table 310.16, 8 AWG copper is only rated for 50 amps. You cannot push 60 amps through a 50-amp conductor.
Stepping up to 6 AWG copper gives us an ampacity of 65 amps at 75°C. Because 60 amps is a standard breaker size listed in NEC 240.6, we pair the 65-amp wire with a 60-amp breaker. The breaker protects the wire, and the wire safely carries the maximum intended load.
Bench Tip: When terminating 6 AWG wire into a 60-amp breaker, do not just tighten the lug until it 'feels tight.' Use an insulated torque screwdriver. Most modern 60A breakers require between 35 and 40 in-lbs of torque. Under-torqued lugs cause high resistance, leading to thermal expansion, arcing, and eventually a melted breaker bus stab.
Variables That Force You to Upsize Your Wire
The matrix above is your starting point, not your final answer. Real-world conditions rarely match the pristine 30°C ambient assumptions of the NEC tables. Here is a decision framework for when you must abandon the baseline and upsize your feeders.
| Condition | NEC Reference | Action Required |
|---|---|---|
| Long Distance Runs | NEC 310.15(B) (Informational Note) | If your 60A run exceeds 122 feet, upsize to 4 AWG copper to keep voltage drop under 3% (7.2V). At 150 feet, 6 AWG yields a 3.6% drop, which can cause dimming lights and motor overheating. |
| Conduit Bundling | NEC 310.15(C)(1) | If you pull two separate sub panel feeders in the same conduit (4 current-carrying conductors), you must apply an 80% derating factor. 6 AWG at 90°C is 75A. 75A x 0.80 = 60A. It barely passes, but upsizing to 4 AWG provides a safer thermal margin. |
| High Ambient Heat | NEC Table 310.15(B)(1) | If the conduit runs through an unconditioned attic in a hot climate where ambient temps reach 110°F (43°C), you must apply a 0.82 correction factor to the 90°C column. Upsizing one full wire size is mandatory. |
| Switching to Aluminum | NEC 310.16 | Aluminum is lighter and cheaper (at current 2026 commodity rates, 100ft of 3 AWG Cu is ~$280, while 1 AWG Al is ~$85), but it requires a larger physical size. You must also apply an anti-oxidant compound like Noalox to all aluminum terminations to prevent galvanic corrosion and high-resistance joints. |
The Voltage Drop Math in Practice
Let's look at a concrete scenario. You are wiring a sub panel to a detached shop that is 150 feet away from the main house panel. You want a 60-amp feed. Using the standard voltage drop formula VD = (2 × K × I × D) / CM (where K=12.9 for copper, I=60A, D=150ft, and CM=26,240 for 6 AWG), the voltage drop calculates to 8.8 volts.
Divide 8.8V by 240V, and you get a 3.68% drop. This exceeds the 3% recommendation for feeders. To fix this, you must upsize to 4 AWG copper (CM = 41,740). Running the math again with 4 AWG yields a 5.5V drop, or 2.3%, which is well within acceptable limits. For deeper calculations on complex runs, referencing a dedicated wire size calculator can save you from costly teardowns.
When to Call an Engineer or the AHJ
While sizing a standard 60A to 200A residential sub panel is straightforward math, certain scenarios push the project out of standard DIY or journeyman territory and require professional engineering or explicit AHJ approval.
- Parallel Conductors: If your sub panel requires a 400-amp feed, you will likely need to run parallel sets of wire (e.g., two sets of 3/0 AWG copper per phase). NEC 310.10(H) has strict rules about parallel conductor length, routing, and termination. The AHJ will scrutinize this heavily.
- Continuous Loads Exceeding 80%: If your sub panel will power a commercial-grade woodshop with dust collectors and welders that run for 3 hours or more continuously, the feeders and breaker must be sized at 125% of the continuous load. A 100A continuous load requires 125A of wire ampacity and a 125A breaker.
- Service Entrance Confusion: Never confuse a sub panel feeder with a service entrance. If you are upgrading the actual utility service drop from the meter to the main panel, this is utility territory. OSHA and local utility regulations dictate that only licensed professionals handle the line side of the service disconnect.
Ultimately, when you wire a sub panel, the goal is to ensure the breaker trips before the wire melts, and the voltage at the far end remains stable enough to run your tools. Stick to the 75°C column, respect the voltage drop limits, and torque your lugs to spec.
