The correct wire for a 100 amp subpanel is 3 AWG copper or 1 AWG aluminum THHN/THWN-2, protected by a 100A double-pole breaker. This assumes a 240V feed, 75°C termination ratings, and a 30°C (86°F) ambient temperature. Here is the exact NEC math, voltage drop limits, and upsizing rules.
Baseline Assumptions for This Sizing
Wire sizing is not a one-size-fits-all lookup. The recommendations in this guide are based on the following strict parameters. If your installation deviates from these, you must adjust your wire size accordingly:
- Conductor Material: Copper (primary recommendation) or Aluminum (secondary).
- Insulation Type: THHN/THWN-2 routed in EMT, PVC, or flexible metal conduit.
- Temperature Column: 75°C column (NEC Table 310.16), matching modern breaker and panel lug ratings.
- Ambient Temperature: 30°C (86°F) or lower.
- Conductor Count: 3 current-carrying conductors (L1, L2, Neutral) plus 1 Equipment Grounding Conductor (EGC).
The Baseline Sizing: Why 3 AWG and Not 4 AWG?
A common point of confusion on jobsites and DIY forums is the use of 4 AWG copper for a 100A feeder. According to the NFPA 70 (National Electrical Code) Table 310.16, 4 AWG copper in the 75°C column is rated for 85A.
Many will point to NEC Article 240.4(B), the "next standard size up" rule, which allows you to protect an 85A conductor with a 100A breaker if the calculated load does not exceed 85A. However, if you are sizing a feeder specifically for a 100 amp subpanel, the intent is to provide 100A of capacity to that panel's busbar. If you use 4 AWG, your legal continuous load limit is restricted, and you cannot fully utilize the panel's rating.
3 AWG copper is rated exactly 100A at 75°C. It provides the full, unambiguous capacity required for a 100A subpanel feed without relying on the 240.4(B) round-up loophole. For aluminum, 1 AWG is the exact 100A match in the 75°C column.
| Material | AWG Size | Insulation | Ampacity (75°C) | Max Continuous Load (80% Rule) |
|---|---|---|---|---|
| Copper | 4 AWG | THHN/THWN-2 | 85A | 68A |
| Copper | 3 AWG | THHN/THWN-2 | 100A | 80A |
| Copper | 2 AWG | THHN/THWN-2 | 115A | 92A |
| Aluminum | 2 AWG | THHN/THWN-2 | 90A | 72A |
| Aluminum | 1 AWG | THHN/THWN-2 | 100A | 80A |
Voltage Drop & Distance: The Decision Tree
Ampacity dictates the minimum wire size to prevent a fire; voltage drop dictates the wire size needed to ensure your equipment actually runs. The NEC recommends a maximum 3% voltage drop on feeders. For a 240V system, 3% equals a 7.2V drop.
Using the Southwire Voltage Drop Calculator logic (K=12.9 for copper, K=21.2 for aluminum), we calculate the drop based on an 80A continuous load (the practical maximum for a 100A panel under NEC 210.20(A) continuous load rules). At 100 feet, 3 AWG copper yields a 1.6% drop. But as distance increases, resistance accumulates, and you must upsize.
| One-Way Run Distance | Copper AWG Required | Aluminum AWG Required | Estimated Voltage Drop (Cu) | Action |
|---|---|---|---|---|
| Up to 110 ft | 3 AWG | 1 AWG | < 1.8% | Use baseline size. |
| 111 ft to 160 ft | 2 AWG | 1/0 AWG | 1.8% - 2.6% | Upsize one step. |
| 161 ft to 210 ft | 1 AWG | 2/0 AWG | 2.6% - 3.0% | Upsize two steps. |
| Over 210 ft | 1/0 AWG or larger | 3/0 AWG or larger | > 3.0% | Consult engineer / upsize further. |
Derating Factors: What Changes the Answer?
The baseline 3 AWG copper / 1 AWG aluminum answer assumes ideal conditions. Real-world jobsites rarely cooperate. Here is what forces you to pull larger wire.
1. Conductor Bundling (NEC 310.15(C)(1))
The baseline assumes exactly three current-carrying conductors (L1, L2, Neutral). The ground wire does not count. If you are pulling feeders for two subpanels in the same conduit, or running a 3-phase feed, you will have 4 to 6 current-carrying conductors. This triggers an 80% derating factor.
How to calculate: You apply the 80% derating to the 90°C column of Table 310.16, then verify the final derated number is equal to or greater than your 75°C termination requirement. For 3 AWG copper, the 90°C rating is 115A. Derated by 80%, it becomes 92A. Because 92A is less than the 100A required for your 75°C terminations, you must upsize to 2 AWG copper if bundling 4-6 conductors.
2. High Ambient Temperatures
If your conduit runs through an unventilated attic in a southern climate where temperatures routinely exceed 40°C (104°F), you must apply the ambient temperature correction factors from Table 310.15(B)(1). At 41-45°C, the correction factor for 90°C insulation is 0.87. Similar to bundling, this will likely force an upsize to 2 AWG copper to maintain the 100A termination rating.
3. Aluminum Installation Realities
Aluminum is an excellent, cost-effective conductor, but it requires specific installation practices. Aluminum expands and contracts more than copper under thermal load, which can cause lugs to loosen over time (creep). If you choose 1 AWG aluminum, you must apply a UL-listed anti-oxidant compound (like Noalox) to the conductor ends before termination, and you must use a calibrated torque screwdriver to hit the exact inch-pound spec on the breaker and panel lugs.
Installation, Torque, and Termination
Sizing the wire correctly is only half the battle; terminating it properly prevents arc faults and melted lugs. A 100A feeder requires strict attention to mechanical details.
- Conduit Sizing: Three 3 AWG THHN wires plus an 8 AWG ground will fit in 1-inch EMT or PVC, but pulling it will be tight. Stepping up to 1.25-inch conduit makes the pull significantly easier and reduces the risk of damaging the insulation jacket.
- Grounding Conductor: Per NEC Table 250.122, a 100A overcurrent device requires a minimum 8 AWG copper or 6 AWG aluminum equipment grounding conductor. Do not downsize this.
- Torque Specifications: This is where most DIYers fail. Modern NEC 110.14(D) mandates that terminations be torqued to the manufacturer's specifications. For standard 100A breakers (like Eaton BR2100 or Siemens Q2100), the line and load lugs typically require 45 to 50 in-lbs of torque. Use an insulated torque screwdriver. Guessing the tightness leads to high-resistance connections that will thermally fail under continuous load.
When an Engineer or AHJ Must Confirm
While the decision tree above covers 95% of residential and light commercial subpanel feeds, you must pull permits and have a licensed professional or electrical engineer review your plans if:
- The run exceeds 250 feet: Voltage drop calculations become complex, and the physical size of the wire (e.g., 1/0 or 2/0) may require specialized pulling equipment, sweeps, and pull boxes.
- You have complex continuous loads: If the subpanel is feeding a 60A EV charger, a 40A heat pump, and a 30A welder simultaneously, the continuous load calculations require a formal Article 220 load calculation signed off by your local Authority Having Jurisdiction (AHJ).
- The environment is hazardous: Runs through chemical storage areas, high-heat boiler rooms, or wet locations require specialized insulation types (like XHHW-2) and specific conduit sealing fittings that go beyond standard Table 310.16 lookups.
The Final Verdict: For a standard, code-compliant 100 amp subpanel feed under 110 feet, buy 3 AWG copper THHN (or 1 AWG aluminum), pull it through 1.25-inch conduit with an 8 AWG ground, and torque the 100A breaker lugs to exactly 45 in-lbs. Do not rely on the 240.4(B) round-up rule with 4 AWG wire if you intend to use the full 100A capacity of the panel.
