For a standard 100-amp sub-panel, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100-amp double-pole breaker. This assumes 75°C rated terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway. Always run a separate equipment grounding conductor (typically 8 AWG copper).
- Material: Copper (primary) and Aluminum (alternative)
- Temperature Column: 75°C (Standard for residential breakers and panel lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Type: PVC Schedule 80 or EMT, with no more than 3 current-carrying conductors
- Voltage: 240V split-phase (residential standard)
Note: NEC-style guidance is provided here; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
The Core Sizing Rules: Why 3 AWG Copper and 1 AWG Aluminum?
When determining what wire for 100 amp sub panel setups, you cannot simply guess or rely on the 'next size up' rule. The National Electrical Code (NEC) Table 310.16 dictates exact ampacities based on the insulation type and the temperature rating of your equipment terminations.
Most residential breakers (like a standard Square D QO2100 or Homeline 100A) and panel lugs are rated for 75°C. Even if you purchase 90°C THHN wire, NEC 110.14(C) requires you to use the 75°C column for ampacity unless the equipment is explicitly listed for 90°C.
- Copper: In the 75°C column, 4 AWG copper is only rated for 85A. Because 100A is a standard breaker size, you cannot use the 'next size up' rule (NEC 240.4(B)) to protect a 85A wire with a 100A breaker. You must step up to 3 AWG copper, which is rated exactly 100A at 75°C.
- Aluminum: Aluminum has lower conductivity. In the 75°C column, 2 AWG aluminum is rated for 90A. Therefore, you must use 1 AWG aluminum, which hits the 100A mark. If you choose aluminum, you must apply an anti-oxidant compound (like Noalox) to the terminations to prevent galvanic corrosion and high-resistance heating over time.
Ampacity Table & Derating Factors
Wire sizing is not a static number; it changes based on your installation environment. Below is the spec-sheet data for the conductors feeding a 100A sub-panel.
| Material | AWG Size | Insulation Type | 75°C Column Ampacity | 90°C Column Ampacity |
|---|---|---|---|---|
| Copper | 3 AWG | THHN / THWN-2 | 100A | 115A |
| Aluminum | 1 AWG | XHHW-2 / THWN-2 | 100A | 115A |
| Copper (Ground) | 8 AWG | THHN / Bare | N/A (Sized per 250.122) | N/A |
What changes the answer? If you alter the baseline assumptions, you must derate the wire. If you pull multiple circuits through the same conduit, the heat builds up, and the wire's ability to dissipate that heat drops. Use this decision tree to adjust your wire size:
| Installation Condition | NEC Derating Action | Result for 3 AWG Copper (115A @ 90°C base) |
|---|---|---|
| 4 to 6 current-carrying conductors in one raceway | Multiply 90°C ampacity by 0.80 | 115A × 0.80 = 92A. Fails. Must upsize to 2 AWG Copper. |
| Ambient temperature 41°C - 45°C (105°F - 113°F) | Multiply 90°C ampacity by 0.82 | 115A × 0.82 = 94.3A. Fails. Must upsize to 2 AWG Copper. |
| 3 or fewer conductors, 30°C ambient | No derating required | 100A @ 75°C termination limit. Passes. 3 AWG is sufficient. |
Voltage Drop: When Distance Changes the Answer
Ampacity tells you what size wire will prevent a fire; voltage drop tells you what size wire will actually run your tools and appliances properly. The NEC recommends a maximum 3% voltage drop on feeders (National Fire Protection Association (NFPA) NEC guidelines, Informational Note to 210.19(A)). For a 240V feeder, 3% is 7.2V.
Let's run the math for 3 AWG Copper using the standard formula: VD = (2 × K × I × D) / CM.
- K (Copper constant) = 12.9
- I (Current) = 80A (Assuming an 80A continuous load, which is best practice for feeder sizing)
- D (Distance) = 100 feet (one-way)
- CM (Circular Mils for 3 AWG) = 52,620
At 100 feet: VD = (2 × 12.9 × 80 × 100) / 52,620 = 3.92V (1.63% drop). This is well within the 3% limit.
At 150 feet: VD = (2 × 12.9 × 80 × 150) / 52,620 = 5.88V (2.45% drop). Still acceptable, but getting close to the threshold.
At 200 feet: VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84V (3.26% drop). This exceeds the 3% recommendation. If your sub-panel is 200 feet from the main panel, you must upsize to 2 AWG Copper (CM = 66,360), which drops the voltage loss to 6.21V (2.58%). Always verify long runs with a dedicated Southwire Voltage Drop Calculator or equivalent software before pulling wire.
Grounding and Bonding the 100A Sub-Panel
A critical mistake DIYers make is treating a sub-panel like a main panel. In a main panel, the neutral and ground bars are bonded together. In a sub-panel, they must remain strictly isolated.
For a 100-amp feeder, NEC Table 250.122 requires a minimum 8 AWG copper or 6 AWG aluminum equipment grounding conductor (EGC). You must pull four wires total: two ungrounded 'hot' conductors, one grounded 'neutral' conductor, and the EGC.
Frequently Asked Questions
Can I use 4 AWG wire for a 100 amp sub panel?
No. Under the 75°C column of NEC Table 310.16, 4 AWG copper is only rated for 85 amps. Because 100A is a standard breaker size, the 'next size up' overcurrent protection rule (NEC 240.4(B)) does not apply. You must use 3 AWG copper to safely carry 100 amps without violating the breaker-to-wire sizing rules. Using 4 AWG on a 100A breaker is a code violation and a fire hazard.
Does a 100 amp sub panel need a ground rod?
It depends on the location. If the sub-panel is installed in a detached structure (like a garage or shed), NEC 250.32(B) requires you to install a grounding electrode system (typically two ground rods spaced at least 6 feet apart) at the detached building. You still must run the 4-wire feeder (including the EGC) from the main panel. If the sub-panel is in the same building as the main panel, it does not need its own ground rods; it relies on the main panel's grounding electrode system via the equipment grounding conductor.
What size wire for a 100 amp sub panel 200 feet away?
For a 200-foot run, 3 AWG copper will result in a voltage drop of roughly 3.26% at an 80A continuous load, which exceeds the NEC's 3% recommendation for feeders. You should upsize to 2 AWG copper or 1/0 AWG aluminum to keep the voltage drop under 3%. If you are running aluminum, the voltage drop is inherently higher due to increased resistance, making 1/0 AWG the necessary choice for a 200-foot distance.
When must an engineer or AHJ confirm my sub-panel wire size?
You must consult a licensed professional engineer or your local Authority Having Jurisdiction (AHJ) if your installation deviates from standard residential baselines. This includes runs exceeding 300 feet, installations in high-ambient-temperature environments (like attics exceeding 120°F), conduit bundling involving more than six current-carrying conductors, or if you are feeding a sub-panel that services continuous industrial loads or specialized medical equipment. Always pull a permit and schedule a rough-in inspection before closing up walls.
