For a standard 100-amp circuit, use 3 AWG copper or 1 AWG aluminum wire paired with a 100A double-pole breaker. This baseline assumes standard 75°C terminations, a 30°C (86°F) ambient temperature, and no more than three current-carrying conductors in a single raceway.
- Material: Copper (default) or Aluminum (alternative)
- Insulation: THHN/THWN-2 (rated 90°C, but terminated at 75°C)
- Temperature Column: 75°C (per NEC 110.14(C) for equipment rated 100A)
- Ambient Temp: 30°C (86°F)
- Conduit Fill: Max 3 current-carrying conductors (no bundling derating)
The Core Ampacity Data: Why 3 AWG Copper?
The most common mistake DIYers and junior apprentices make is looking at the 90°C column in the ampacity tables because THHN wire is stamped with a 90°C rating. However, the National Electrical Code (NEC) dictates that unless your breaker and lugs are explicitly rated for 90°C, you must size the wire based on the 75°C column. Almost all standard 100A residential breakers and panel lugs are rated for 75°C.
| Wire Size (AWG/kcmil) | Material | 75°C Ampacity (Termination Limit) | 90°C Ampacity (Derating Only) | Passes 100A? |
|---|---|---|---|---|
| 4 AWG | Copper | 85A | 95A | NO |
| 3 AWG | Copper | 100A | 115A | YES (Minimum) |
| 2 AWG | Copper | 115A | 130A | YES (Upsized) |
| 2 AWG | Aluminum | 90A | 100A | NO |
| 1 AWG | Aluminum | 100A | 115A | YES (Minimum) |
If you use 4 AWG copper, the 75°C termination limit is 85A. Even though the wire's insulation won't melt until it hits 90°C limits, the breaker lugs will overheat and degrade, creating a fire hazard. Therefore, 3 AWG copper is the absolute legal minimum for a 100A feeder.
Voltage Drop: When Distance Forces an Upsize
Ampacity tells you what size wire prevents a fire. Voltage drop tells you what size wire actually delivers usable power to the load. NEC Chapter 3 recommends a maximum 3% voltage drop on feeders. For a 240V circuit, 3% equals 7.2 volts.
Let's run the math using the standard voltage drop formula: VD = (2 × K × I × D) / CM, where K is 12.9 for copper, I is 100A, D is one-way distance, and CM is the circular mil area of the wire (52,620 for 3 AWG).
- At 50 feet: Voltage drop is 2.45V (1.02%). 3 AWG Copper is perfect.
- At 100 feet: Voltage drop is 4.9V (2.04%). 3 AWG Copper still passes the 3% rule.
- At 150 feet: Voltage drop is 7.35V (3.06%). 3 AWG Copper fails.
If your subpanel is 150 feet away from the main breaker, you must upsize to 2 AWG copper (CM = 66,360), which drops the loss to 5.8V (2.4%), safely back under the 3% threshold. You can verify these figures using the Southwire Voltage Drop Calculator for your specific run.
Copper vs. Aluminum: The Cost and Sizing Trade-Off
Aluminum wire is significantly cheaper than copper, making it the default choice for utility drops and long residential feeders. However, it has lower conductivity, meaning you must use a thicker wire to carry the same 100A load. Furthermore, aluminum requires specific installation practices to prevent arcing and overheating.
| Scenario | Recommended Pick | Why? |
|---|---|---|
| Short run (< 60 ft), tight conduit space | 3 AWG Copper (THHN) | Fits easily in 1-inch conduit. No anti-oxidant paste required. Easier to bend in tight panel gutters. |
| Medium run (60 - 120 ft), budget conscious | 1/0 AWG Aluminum (XHHW) | While 1 AWG Al is the minimum, 1/0 Al handles voltage drop better over distance and costs roughly 40% less than equivalent copper. |
| Long run (> 120 ft) or direct burial | 1/0 AWG Aluminum (USE-2/XHHW) | Copper at this distance becomes prohibitively expensive. Aluminum keeps material costs manageable while maintaining the 3% drop limit. |
- Anti-Oxidant Paste: You MUST apply a conductor-approved anti-oxidant compound (like Noalox) to stripped aluminum wire before terminating. Aluminum oxidizes rapidly in air, creating a resistive layer that causes lugs to overheat.
- Torque to Spec: Aluminum exhibits 'cold flow' (creep). It will compress under pressure over time, loosening the connection. Use a calibrated torque screwdriver or torque wrench set exactly to the breaker manufacturer's specification (usually printed on the breaker label, typically 45-60 in-lbs for this wire range).
- Lug Compatibility: Ensure your breaker or panel lugs are explicitly rated for aluminum (marked AL or AL/CU). Never terminate aluminum wire on lugs rated only for copper.
Derating Factors That Invalidate the Baseline
The 3 AWG copper / 1 AWG aluminum answer assumes perfect conditions. Real-world jobsites rarely cooperate. If any of the following conditions apply, your wire size must increase.
1. High Ambient Temperatures
If your conduit runs through an attic that reaches 110°F (43°C) in the summer, the wire's ability to shed heat drops. According to NEC Table 310.15(B)(1), you must apply a correction factor. For a 43°C ambient environment, the correction factor for 90°C-rated THHN is 0.87. You calculate derating using the 90°C column (3 AWG = 115A). 115A × 0.87 = 100.05A. It barely passes, but if the attic hits 122°F (50°C), the factor drops to 0.82, resulting in 94.3A. You must upsize to 2 AWG copper.
2. Conductor Bundling
If you pull multiple circuits through the same conduit, the wires heat each other up. If you have 4 to 6 current-carrying conductors in a single raceway, NEC Table 310.15(C)(1) requires an 80% derating factor.
Calculation: 3 AWG THHN (115A at 90°C) × 0.80 = 92A. This is below your 100A breaker. You must upsize to 2 AWG copper to maintain code compliance when sharing conduit.
Continuous Loads and AHJ Sign-Off
There is a massive distinction in the NEC between a standard load and a continuous load. A continuous load is defined as any load where the maximum current is expected to continue for 3 hours or more (NEC Article 100). Examples include EV chargers, server racks, or heavy commercial lighting.
If your 100A circuit will power a continuous load, NEC 215.2 requires you to size the conductors at 125% of the load.
100A × 1.25 = 125A.
Looking back at our 75°C ampacity table, 3 AWG copper is only rated for 100A. To safely carry 125A, you must jump all the way up to 1 AWG copper (rated 130A at 75°C) or 1/0 AWG aluminum (rated 120A at 75°C). Furthermore, the breaker itself must be rated for 125A, or you must use a 125A breaker with 100A continuous trip settings if utilizing specialized equipment.
When to Defer to an Engineer or the AHJ
While this guide provides NEC-style guidance, your local Authority Having Jurisdiction (AHJ) has the final say. You must pull a permit and have an inspector verify your work if:
- This is a Service Entrance (the main feed from the utility meter to your first disconnect). Service entrance calculations require a full Article 220 load calculation, and utilities often have their own specific wire and conduit mandates.
- You are mixing solar PV backfeed into the panel, which invokes the 120% busbar rule (NEC 705.12) and may require an engineer's stamp depending on your local utility's interconnection agreement.
- Your run exceeds 300 feet, where voltage drop mitigation might require parallel conductors or transformer step-ups, pushing the design out of standard residential DIY territory.
The Final Verdict: For a standard, non-continuous 100A subpanel feeder under 100 feet in a normal climate, buy 3 AWG Copper THHN (or 4-wire SER cable if running through framing). If your run pushes past 120 feet, or you want to save money on a long trench, switch to 1/0 AWG Aluminum XHHW, apply anti-oxidant paste, and torque the lugs to the manufacturer's exact specification.
