All wire sizes and breaker pairings below assume:
• Conductor Material: Copper (unless explicitly stated as aluminum).
• Insulation Type: THHN/THWN-2.
• Temperature Column: 75°C (per NEC 110.14(C) termination limits).
• Ambient Temperature: 30°C (86°F).
• Conduit Fill: No more than three current-carrying conductors (CCCs) in the raceway.
Note: Local AHJ (Authority Having Jurisdiction) and NEC-style guidance apply; your local inspector has final authority.
Core Sizing Rules and Baseline Assumptions
Sizing a breaker for sub panel feeders is not just about matching the breaker rating to the wire's absolute maximum melting point. It is about matching the breaker to the weakest link in the circuit, which is almost always the termination lug on the breaker or the panel busbar.
Most modern breakers and panel lugs are rated for 75°C. Even though THHN wire is printed with a 90°C rating on the jacket, the National Electrical Code (NEC) requires you to size the overcurrent protection based on the 75°C column of Table 310.16. You are only permitted to use the 90°C column for derating adjustments (like ambient temperature or conduit bundling), but the final derated ampacity cannot exceed the 75°C baseline limit.
| Subpanel Rating | Breaker Size (2-Pole) | Copper Wire Size (THHN) | Minimum Ground (Copper) |
|---|---|---|---|
| 60 Amp | 60A | 6 AWG | 10 AWG |
| 100 Amp | 100A | 3 AWG | 8 AWG |
| 125 Amp | 125A | 1 AWG | 6 AWG |
| 200 Amp | 200A | 2/0 AWG | 4 AWG |
Ampacity, Voltage Drop, and Real-World Derating
Why use 3 AWG for a 100A breaker and not one size smaller (4 AWG)? According to the 75°C column, 4 AWG copper is rated for 85 amps. Because 85A is not a standard breaker size, you might be tempted to use the 'next size up' rule (NEC 240.4(B)) and put it on a 90A or 100A breaker. However, if your calculated continuous load exceeds 80A, the wire will overheat at the lugs before the breaker trips. You must use 3 AWG (rated 100A at 75°C) to safely protect a 100A circuit.
The Voltage Drop Check
Ampacity tables assume a short run. When you push 100 amps over a long distance, resistance causes voltage drop, which can damage motors and trip sensitive electronics. The NEC recommends a maximum 3% voltage drop for feeders.
Let's calculate the voltage drop for a 100A subpanel located 120 feet away from the main panel, using 3 AWG copper on a 240V circuit:
- Formula: VD = (2 × K × I × D) / CM
- K (Copper resistivity) = 12.9
- I (Current) = 100A
- D (Distance) = 120 ft
- CM (Circular Mils for 3 AWG) = 52,620
- Calculation: (2 × 12.9 × 100 × 120) / 52,620 = 5.88 Volts
- Percentage: 5.88V / 240V = 2.45%
At 2.45%, 3 AWG copper passes the 3% feeder recommendation at 120 feet. If your run was 160 feet, the drop would hit 3.27%, and you would need to step up to 2 AWG copper to maintain power quality. You can verify your specific runs using the Southwire Voltage Drop Calculator.
What Changes the Answer? (Derating Decision Tree)
| Condition | Impact on Sizing | Required Action |
|---|---|---|
| Run exceeds 100 feet | Voltage drop exceeds 3% threshold | Increase wire gauge by one size; breaker size remains the same. |
| More than 3 CCCs in conduit | Ampacity must be derated (e.g., 80% for 4-6 conductors) | Calculate derated ampacity using the 90°C column, then verify it still meets the 75°C termination requirement. |
| Using Aluminum Wire | Aluminum has higher resistance and lower ampacity per AWG | Must use larger gauge (e.g., 1 AWG AL for 100A) and apply anti-oxidant paste to lugs. |
| Ambient temp > 30°C (86°F) | Wire insulation degrades faster; ampacity drops | Apply ambient temperature correction factors from NEC Table 310.15(B)(1) before finalizing wire size. |
When to Escalate to an Engineer or AHJ
While the tables above cover 90% of residential and light commercial subpanel installations, certain conditions require a formal load calculation and sign-off from a licensed professional engineer (PE) or your local electrical inspector.
- High Ambient Environments: If your conduit runs through an unventilated attic in a southern climate where ambient temperatures regularly exceed 110°F (43°C), standard derating tables may force you to jump up two or three wire sizes. An engineer can perform a thermal analysis to optimize this.
- Complex Continuous Loads: If the subpanel will serve heavy continuous loads (like a commercial woodshop or EV charging station running 3+ hours continuously), NEC Article 210.20 requires the breaker to be sized at 125% of the continuous load. A 100A continuous load requires a 125A breaker and 1 AWG copper.
- Utility Service Limits: If adding a 200A subpanel pushes your total home load calculation past your main service entrance rating (e.g., a 200A main service with a 200A subpanel plus existing loads), the utility company and AHJ must approve a service upgrade.
For comprehensive code compliance and safety standards, always refer to the latest NFPA National Electrical Code guidelines and consult your local building department.
Frequently Asked Questions About Subpanel Breakers
What size breaker for sub panel in a detached garage?
For a typical detached garage subpanel powering lights, receptacles, and a small compressor, a 60A breaker with 6 AWG copper wire is usually sufficient. However, the physical size of the breaker is secondary to the wiring method and grounding rules. Because it is a detached structure, NEC 250.32 requires a 4-wire feeder (two hots, one neutral, one equipment grounding conductor). You must also install a separate ground bar in the subpanel, keep the neutral and ground isolated, and install a local disconnecting means if the garage has more than one branch circuit.
Can I use a 100A breaker for sub panel with 4 AWG wire?
No, this is a code violation and a fire hazard. 4 AWG copper THHN is rated for 85 amps in the 75°C column. If you protect 85A-rated wire with a 100A breaker, a sustained load of 95 amps will overheat the wire and melt the insulation long before the breaker trips. If your calculated load is strictly under 80 amps, you can use 4 AWG wire, but you must protect it with an 80A breaker. If your load requires a 100A breaker, you must step up to 3 AWG copper or 1 AWG aluminum.
How do I calculate the breaker for sub panel if I use aluminum wire?
Aluminum wire is lighter and significantly cheaper than copper, but it has lower conductivity and expands/contracts more under thermal cycling. To calculate the breaker and wire size for aluminum, you must use the aluminum-specific columns in NEC Table 310.16. For a 100A subpanel, you cannot use 3 AWG aluminum (rated 75A); you must step up to 1 AWG aluminum (rated 100A at 75°C). When terminating aluminum, you must brush the wire with a wire brush, apply an anti-oxidant compound like Noalox to prevent galvanic corrosion, and torque the lugs to the exact inch-pound specification printed on the panel label using a calibrated torque screwdriver.
