To feed a 100 amp subpanel, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100-amp double-pole breaker. This assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway. If your run exceeds 100 feet, you must upsize to compensate for voltage drop.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2/THWN-2)
- Termination Rating: 75°C (Standard for modern breakers and panel lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum 3 current-carrying conductors (2 hots + 1 neutral; equipment ground does not count)
- Load Type: Non-continuous (under 3 hours of maximum draw)
Note: NEC-style guidance provided here is for educational purposes. Your local Authority Having Jurisdiction (AHJ) has final authority on code compliance.
The Baseline Ampacity Table (Copper vs. Aluminum)
The National Electrical Code (NEC) dictates wire sizing based on the lowest temperature rating of any connected component. While you will likely pull THHN wire (which has a 90°C insulation rating), the breakers and panel lugs are almost universally rated for 75°C. Therefore, you must use the 75°C column in NEC Table 310.16 to determine your baseline ampacity.
| Wire Size (AWG/kcmil) | Material | 75°C Ampacity (Termination Limit) | 90°C Ampacity (For Derating Only) | Max Standard Breaker |
|---|---|---|---|---|
| 4 AWG | Copper | 85A | 95A | 90A |
| 3 AWG | Copper | 100A | 115A | 100A |
| 2 AWG | Copper | 115A | 130A | 125A |
| 2 AWG | Aluminum | 90A | 100A | 90A |
| 1 AWG | Aluminum | 100A | 115A | 100A |
| 1/0 AWG | Aluminum | 120A | 135A | 125A |
Why 3 AWG Copper and Not 4 AWG?
A common jobsite mistake is attempting to use 4 AWG copper for a 100A feeder. In the 75°C column, 4 AWG is only rated for 85 amps. While NEC 240.4(B) allows the 'next size up' rule for overcurrent protection (permitting an 85A wire to be protected by a 90A breaker), there is no standard 100A breaker that can legally protect an 85A wire. Furthermore, the subpanel busbar itself is rated for 100A; feeding it with an 85A wire creates a bottleneck that violates the panel's listing. You must step up to 3 AWG to hit the exact 100A threshold at 75°C.
When Distance Forces an Upsize (Voltage Drop)
Ampacity tables only tell half the story. They assume the wire can handle the thermal load, but they do not account for the resistance of the wire over long distances. NEC 310.15(B) (Informational Note) recommends that feeder voltage drop not exceed 3% for reasonable efficiency. On a 240V system, a 3% drop equates to a maximum loss of 7.2 volts.
To calculate voltage drop, we use the standard single-phase formula: VD = (2 × L × I × R) / 1000, where L is one-way length in feet, I is current (100A), and R is the resistance per 1,000 feet from NEC Chapter 9, Table 8 (using AC resistance values for copper in PVC conduit).
| One-Way Distance | Wire Size (Copper) | Calculated Voltage Drop | Percentage Drop (240V) | Verdict |
|---|---|---|---|---|
| 50 ft | 3 AWG | 2.45V | 1.02% | Pass |
| 100 ft | 3 AWG | 4.90V | 2.04% | Pass |
| 150 ft | 3 AWG | 7.35V | 3.06% | Fail (Upsize) |
| 150 ft | 2 AWG | 5.82V | 2.42% | Pass |
| 200 ft | 2 AWG | 7.76V | 3.23% | Fail (Upsize) |
| 200 ft | 1 AWG | 6.12V | 2.55% | Pass |
If your subpanel is located in a detached garage 150 feet away from the main service, 3 AWG copper will result in a 3.06% drop, pushing sensitive electronics and motors into brownout territory under heavy load. You must upsize to 2 AWG copper to bring the drop back under 3%. You can verify these figures using industry-standard tools like the Southwire Voltage Drop Calculator, ensuring you select the correct conduit type and power factor.
Derating, Bundling, and Aluminum Installation Realities
The baseline table assumes you are pulling exactly three current-carrying conductors (two ungrounded 'hot' legs and one grounded neutral). The equipment grounding conductor (EGC) does not count toward conduit fill derating because it only carries current during a fault.
The Bundling Derating Trap
If you decide to pull an extra circuit through the same conduit to the subpanel, you suddenly have four or more current-carrying conductors. According to NEC 310.15(C)(1), four to six conductors require an 80% derating factor. Here is where the 90°C column saves you:
- 3 AWG THHN in the 90°C column is rated 115A.
- Apply the 80% derating factor: 115A × 0.80 = 92A.
- Because 92A is less than the 100A breaker, 3 AWG is no longer legal. You must upsize to 2 AWG THHN (130A × 0.80 = 104A) to maintain compliance.
Aluminum Feeder Installation Rules
Using 1 AWG aluminum (or 1/0 AWG for longer runs) is highly cost-effective, often saving hundreds of dollars on long feeders compared to copper. However, aluminum requires strict adherence to installation physics:
- Oxidation: Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that causes heat. You must brush the wire strands and apply an anti-oxidant compound (like Noalox) immediately before terminating.
- Creep: Aluminum expands and contracts more than copper under thermal cycling, which can loosen mechanical lugs over time. You must use a calibrated torque screwdriver or wrench to tighten the panel lugs to the exact inch-pound specification printed on the panel label.
- Lug Ratings: Verify the subpanel lugs are explicitly rated 'AL/CU'. If they are marked 'CU' only, you cannot terminate aluminum wire directly; you must use a rated transition lug or stick to copper.
When an Engineer or the AHJ Must Confirm
While the 3 AWG copper / 1 AWG aluminum rule covers 90% of residential subpanel installations, specific load profiles require professional verification.
Continuous Loads (The 125% Rule)
NEC 215.2(A)(1) requires that if a feeder supplies continuous loads (defined as loads expected to run at maximum current for 3 hours or more), the conductor must be sized at 125% of the continuous load. If your subpanel is feeding a dedicated workshop with a 80A continuous load (like a large compressor or bank of heaters running continuously), you must size the wire for 100A (80 × 1.25). In this specific edge case, 3 AWG copper barely meets the 100A requirement, but many inspectors will require you to step up to 2 AWG to account for terminal heat dissipation over sustained periods. Always perform a formal NEC Article 220 load calculation to prove your continuous vs. non-continuous load split.
High Ambient Temperatures
If your feeder conduit runs through an unconditioned attic in a southern climate where ambient temperatures regularly exceed 30°C (86°F), you must apply the temperature correction factors from the bottom of NEC Table 310.16. For example, at 40°C (104°F), the correction factor for 90°C insulation is 0.91. If you are bundling wires and applying both temperature and bundling derating factors simultaneously, the math can quickly reduce a 3 AWG wire's effective ampacity below 100A. In scenarios involving high-heat attics, multiple circuits in a single raceway, or distances over 200 feet, consult a licensed electrical engineer or your local AHJ to stamp your wire sizing schedule.
