Wire Sizing Guide: Feeding a Sub Panel with Main Breaker

For a standard 100A sub panel with main breaker located 50 feet away, use 3 AWG copper THHN wire and a 100A breaker. For aluminum, step up to 1 AWG. This assumes standard residential conditions, 75°C terminations, and a maximum 3% voltage drop.

The Baseline Sizing Matrix for a Sub Panel with Main Breaker

When installing a sub panel with a main breaker, the main breaker serves a critical code function: it acts as the required service disconnect for a detached structure (per NFPA 70 / NEC Article 250.32). However, the main breaker’s amp rating does not automatically dictate your wire size. The feeder conductors must be sized to carry the calculated load while being protected by the upstream breaker at the main service panel.

The most common mistake DIYers make is using the 90°C column on the wire ampacity chart to pick a smaller wire. While THHN wire is rated for 90°C, the lugs inside almost all residential and light-commercial breakers and panels are only rated for 75°C. Per NEC 110.14(C), you must use the 75°C column for your final termination ampacity. This is exactly why we use the sizes in the table below, and not one size smaller.

Why not use aluminum interchangeably with copper? Aluminum is significantly cheaper and lighter, making it the default choice for 150A and 200A feeders. However, aluminum expands and contracts at a different rate than copper and is prone to oxidation. If your sub panel lugs are not explicitly marked "AL/CU" or "CU-AL", you cannot terminate aluminum wire. Furthermore, aluminum requires the application of an antioxidant compound (like Noalox) and specific torque values to prevent high-resistance connections that lead to melted lugs and fires.

Voltage Drop and Distance: When to Upsize Your Feeders

Ampacity tables only tell half the story. If your sub panel is located far from the main service, the resistance of the wire will cause voltage drop (VD). While the NEC treats voltage drop as an Informational Note (recommendation) rather than a strict mandate for most feeders, best practice—and many local codes—enforce a maximum 3% drop on feeders and a 5% combined drop on feeders plus branch circuits.

Let’s run a voltage drop check for our 100A sub panel using 3 AWG Copper at a 240V nominal supply. The formula is: VD = (2 × K × I × D) / CM, where K is 12.9 for copper, I is current (100A), D is one-way distance, and CM is the circular mil area of 3 AWG (52,620).

When you upsize wire for voltage drop, you do not necessarily need to upsize the breaker. A 100A breaker can safely protect 2 AWG or 1 AWG wire; the breaker protects the wire from overcurrent, and the larger wire simply ensures the voltage reaches the panel intact. However, physically fitting a 1/0 AWG wire into a 100A breaker lug can be difficult. In those cases, you may need to use a Polaris connector or a terminal lug adapter to step down the wire size right at the breaker termination.

Derating, Material Swaps, and AHJ Thresholds

The baseline matrix assumes ideal conditions. In the real world, jobsite variables force you to adjust your wire size. Here is what changes the answer and when you must bring in a professional.

Conduit Fill and Bundling Derating

If you pull more than three current-carrying conductors (CCCs) through a single raceway, the wires heat each other up. NEC 310.15(C)(1) mandates derating. For example, if you run a 4-wire subpanel feeder (2 hots, 1 neutral, 1 ground) plus a separate 3-wire circuit for a detached garage light in the same PVC conduit, you have 5 CCCs (the equipment grounding conductor does not count). Five CCCs require an 80% derating multiplier.

If you are using 3 AWG Copper (100A at 90°C column for derating purposes: 115A × 0.80 = 92A), your derated ampacity drops below your 100A breaker. You would be forced to upsize to 2 AWG Copper to maintain code compliance.

Ambient Temperature Corrections

If your conduit runs across an unventilated attic in the summer or along a sun-baked exterior wall, the ambient temperature easily exceeds 30°C (86°F). At 40°C (104°F), you must apply a 0.88 correction factor to the 90°C column before applying any bundling derating. High-ambient environments compound quickly with bundling derating, often forcing a wire size increase of two full AWG steps.

When an Engineer or AHJ Must Confirm

While the rules above cover 95% of residential and light-commercial subpanel installations, you must defer to a licensed electrical engineer or your local AHJ inspector under the following conditions:

• Feeders over 200A: Sizing wire for 300A or 400A subpanels usually requires parallel conductors (multiple wires per phase). This invokes NEC 310.10(G), which has strict rules regarding equal length, same material, and identical routing for parallel feeds.
• Continuous Industrial Loads: If the sub panel will serve continuous loads (running for 3 hours or more, like a large CNC machine or server rack), the feeder and breaker must be sized at 125% of the continuous load, fundamentally changing the math.
• Complex Grounding Electrode Systems: If the detached structure has its own grounding electrodes (like a concrete-encased electrode or ground ring) in addition to the equipment grounding conductor, the bonding jumper sizing requires specific calculations per NEC Table 250.102(C)(1).

Getting the wire size right for a sub panel with a main breaker is about balancing terminal temperature limits, voltage drop over distance, and environmental derating. Stick to the 75°C column for your terminations, run the voltage drop math for any run over 50 feet, and keep your neutral and ground isolated at the sub panel.