For a standard 100A sub panel main breaker, use 3 AWG copper THHN wire for the hot and neutral conductors, paired with an 8 AWG copper ground. This direct answer assumes 75°C terminal ratings, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
The Baseline Sizing Chart for Sub Panel Feeders
Before pulling any wire, map your breaker size to the correct conductor gauge and conduit diameter. The table below provides the minimum requirements for standard residential and light commercial sub panel main breakers. Keep in mind that at current 2026 metals pricing, 100 feet of 3 AWG copper THHN runs about $180, while 1 AWG aluminum XHHW is roughly $65. Aluminum is highly cost-effective for long runs, provided your lugs are rated for it and you use anti-oxidant paste.
| Main Breaker | Copper AWG (75°C) | Aluminum AWG (75°C) | Copper Ground (250.122) | Min Sch 40 PVC |
|---|---|---|---|---|
| 60A | 6 AWG (65A) | 4 AWG (65A) | 10 AWG | 1" |
| 100A | 3 AWG (100A) | 1 AWG (100A) | 8 AWG | 1-1/4" |
| 125A | 1 AWG (130A) | 1/0 AWG (120A)* | 6 AWG | 1-1/2" |
| 150A | 1/0 AWG (150A) | 2/0 AWG (150A) | 6 AWG | 1-1/2" |
| 200A | 2/0 AWG (175A)** | 4/0 AWG (205A) | 6 AWG | 2" |
*1/0 Aluminum is rated 120A. NEC 240.4(B) allows the next standard breaker size up (125A) if the calculated load does not exceed 120A.
**2/0 Copper is rated 175A. If your calculated continuous and non-continuous load is under 175A, 240.4(B) allows a 200A breaker. For a true, uncalculated 200A maximum feed, you must upsize to 3/0 Copper.
Why We Pick 3 AWG Over 4 AWG for a 100A Feeder
A frequent point of confusion on the workbench is why we mandate 3 AWG copper for a 100A sub panel main breaker when 4 AWG is often seen on 100A (and even 200A) main service entrances. The answer lies in the distinction between service entrance conductors and feeder conductors.
NEC Article 310.12 provides a special allowance for single-family dwelling service entrance conductors, permitting 4 AWG copper for up to 100A. However, a sub panel feeder does not qualify for this residential service allowance. Sub panel feeders are strictly governed by the standard ampacity tables in NEC 310.16.
Looking at Table 310.16, 4 AWG copper in the 75°C column is rated for only 85A. You cannot terminate an 85A conductor on a 100A breaker for a feeder circuit. You must step up to 3 AWG, which is rated exactly 100A at 75°C. Furthermore, even if you buy THHN wire (which is rated 90°C in the insulation column), NEC 110.14(C) dictates that your termination sizing is limited by the lowest temperature rating of any connected component. Since almost all standard 100A breakers and sub panel lugs are rated 75°C, you must use the 75°C column for your final ampacity check.
Variables That Force an Upsize: Length, Bundling, and Material
The baseline chart assumes ideal conditions. In the real world, physical constraints and physics will frequently force you to upsize your conductors. Here is how to calculate when the baseline fails.
Voltage Drop Over Distance
While the NEC does not strictly mandate voltage drop limits for feeders (it is an Informational Note in 210.19(A)), keeping drop under 3% is the universal standard for preventing motor burnout and dimming lights. Let us run the math for a 100A, 240V feeder using 3 AWG copper.
Using the standard formula VD = (2 × K × I × L) / CM (where K=12.9 for copper, I=100A, and CM=52,620 for 3 AWG):
- At 100 feet: VD = 4.9V. This is a 2.04% drop at 240V. You are clear to use 3 AWG.
- At 150 feet: VD = 7.35V. This is a 3.06% drop. You have crossed the 3% threshold.
If your run is 150 feet, you must upsize to 2 AWG copper (CM = 66,360), which drops the voltage loss to 2.4%. Use a reliable tool like the Southwire Voltage Drop Calculator to verify your exact footage before buying wire.
Conduit Bundling and Derating
Derating is where many DIY builds fail inspection. If you decide to pull two separate 100A sub panel feeders through the same 1.25-inch PVC conduit, you now have four current-carrying conductors (CCCs). According to NEC Table 310.15(C)(1), four CCCs require an 80% derating factor.
When derating, you are allowed to use the 90°C column of Table 310.16 as your starting point. 3 AWG THHN at 90°C is rated 115A. Applying the 80% factor: 115A × 0.80 = 92A. Because 92A is less than your 100A breaker, 3 AWG is now illegal for this run. You must upsize to 2 AWG (130A × 0.80 = 104A) to safely bundle the feeders.
Aluminum vs. Copper Termination Rules
Never treat aluminum and copper as interchangeable at the lug. If you choose 1 AWG aluminum XHHW to save money on a 100A feed, ensure both the main panel breaker and the sub panel main breaker lugs are explicitly marked "AL" or "AL/CU". You must apply an anti-oxidant compound (like Noalox) to the stripped aluminum conductor before torquing the lug to the manufacturer's exact inch-pound specification. Aluminum creeps under pressure over time; a lug torqued to copper specs will eventually loosen and arc.
When to Defer to the AHJ or a Professional Engineer
The sizing matrix and formulas above cover 95% of residential and light commercial sub panel main breaker installations. However, you must pull a permit and defer to your local Authority Having Jurisdiction (AHJ) or a licensed professional engineer under the following conditions:
- Continuous Loads: If the sub panel feeds a workshop where the maximum current draw will run for three hours or more continuously (e.g., a large compressor and welder running simultaneously), NEC Article 100 classifies this as a continuous load. The conductors and breaker must be sized at 125% of the continuous load. A 100A continuous load requires a 125A breaker and 1 AWG copper wire.
- High Ambient Temperatures: If your conduit runs through an unventilated attic in a hot climate, or near a boiler room, the 30°C ambient assumption is void. You must apply the temperature correction factors in Table 310.15(B)(1). At 120°F (49°C), the correction factor for 90°C wire is 0.82, which severely reduces your ampacity and forces an upsize.
- Complex Underground Runs: Direct burial or underground conduit runs subject to high thermal resistivity soil (like dry sand) require specialized engineering to prevent the earth from acting as an insulator and overheating the conductors.
Always verify your local code amendments. While the NEC provides the baseline, your local AHJ has the final authority on whether a specific sub panel main breaker installation meets the safety requirements of your municipality.
