The direct answer for 3 AWG ampacity depends on your conductor material and the temperature rating of your terminations. According to NEC Table 310.16, 3 AWG copper wire has an ampacity of 100A at 75°C and 115A at 90°C. For 3 AWG aluminum wire, the ampacity is 75A at 75°C and 85A at 90°C. In standard residential and commercial feeder installations, 3 AWG copper is the go-to size for 100-amp subpanels because it perfectly matches the 75°C temperature limit of standard breaker lugs.
3 AWG Ampacity Quick Reference Chart (NEC Table 310.16)
The table below is extracted from the National Electrical Code (NEC) Table 310.16. To read this chart correctly, locate your wire gauge in the first column, then move horizontally to the column that matches your conductor material (Copper or Aluminum) and your system's lowest temperature rating. The values below assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.
| Wire Size (AWG) | Copper (THHN/THWN, XHHW) | Aluminum (XHHW, USE) | ||||
|---|---|---|---|---|---|---|
| 60°C | 75°C | 90°C | 60°C | 75°C | 90°C | |
| 4 AWG | 70A | 85A | 95A | 55A | 65A | 75A |
| 3 AWG (Target) | 85A | 100A | 115A | 65A | 75A | 85A |
| 2 AWG | 95A | 115A | 130A | 75A | 90A | 100A |
| 1 AWG | 110A | 130A | 145A | 85A | 100A | 115A |
Which Column Applies to Your Installation?
The most common mistake DIYers and junior electricians make is looking at the 90°C column because they bought THHN wire, which is printed with a 90°C rating on the jacket. However, NEC Section 110.14(C) dictates the "weakest link" rule for terminations.
Standard circuit breakers, panelboard lugs, and disconnect switches are tested and rated for 75°C terminations. Even if your 3 AWG THHN copper wire can physically handle 115A of heat (the 90°C column), the breaker lug it lands on will overheat and degrade if pushed past its 75°C rating. Therefore, for final overcurrent protection sizing, you must use the 75°C column. This makes 3 AWG copper the exact mathematical match for a 100A breaker (100A wire ampacity = 100A breaker rating).
The 90°C column is not useless, however. It serves as your baseline for derating, which we will cover next. You are permitted to use the higher 90°C ampacity to calculate heat dissipation penalties, provided your final derated number still supports the load.
Derating 3 AWG Wire: When the Base Value Drops
The base ampacity values in the chart above assume ideal conditions: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors (CCCs) bundled together. When you stuff more wires into a conduit or run them through a hot attic, the wire cannot dissipate heat as effectively, and the NEC requires you to reduce (derate) the allowable ampacity.
Derating is calculated using the 90°C column, applying the adjustment factors from NEC Table 310.15(C)(1). Let's look at a real-world jobsite scenario:
You are pulling two separate 240V circuits (4 hot wires total, plus a shared ground) through a single 1-inch EMT conduit. You have 4 current-carrying conductors.
- Base 90°C Ampacity (3 AWG Cu): 115A
- Adjustment Factor (4 CCCs): 80% (from Table 310.15(C)(1))
- Derated Ampacity: 115A × 0.80 = 92A
Always consult the Electrical Safety Foundation International (ESFI) guidelines or your local Authority Having Jurisdiction (AHJ) when dealing with complex bundling, as local amendments sometimes restrict conduit fill or derating thresholds further than the baseline NEC.
What the Ampacity Table Cannot Tell You
While NEC Table 310.16 is the bible for heat-based ampacity, it is blind to three critical physical and electrical realities that will ruin your installation if ignored.
1. Voltage Drop Over Distance
Ampacity tables assume the wire is infinitely short. In reality, 3 AWG copper has a resistance of approximately 0.245 ohms per 1,000 feet (per NEC Chapter 9, Table 9). If you are running a 100A, 240V subpanel feeder 150 feet from the main panel, your voltage drop calculation looks like this:
VD = (2 × Length × Current × Resistance) / 1000
VD = (2 × 150 × 100 × 0.245) / 1000 = 7.35 Volts
A 7.35V drop on a 240V system is a 3.06% drop. While the NEC recommends keeping feeder drop under 3% (and total system drop under 5%), you are right on the borderline. If your run exceeds 150 feet, 3 AWG is electrically inadequate for a full 100A load, and you must upsize to 2 AWG or 1 AWG to maintain voltage stability for sensitive electronics at the subpanel.
2. Conduit Fill and Physical Pulling Tension
Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Three strands of 3 AWG THHN take up roughly 0.159 square inches. Mathematically, this fits inside 3/4-inch EMT (which allows 0.213 sq in at 40%). However, the table does not account for the sheer stiffness of 3 AWG solid or stranded copper. Pulling three sticks of 3 AWG through 3/4-inch EMT with even a single 90-degree sweep will result in jammed wires, damaged insulation, and a ruined pulling rope. For 3 AWG feeder pulls, always step up to a minimum of 1-inch EMT or 1.25-inch PVC to save your knuckles and your wire jacket.
3. Aluminum Oxidation and Termination Prep
If you opt for 3 AWG aluminum to save on material costs (remember, you must upsize to 1 AWG Al for a 100A feeder, as 3 AWG Al is only rated 75A at 75°C), the ampacity table will not warn you about aluminum oxidation. Aluminum rapidly forms a non-conductive oxide layer when exposed to air. You must strip the wire, immediately coat it in an antioxidant compound (like Noalox), and torque the lug to the manufacturer's exact inch-pound specification using a calibrated torque screwdriver. Failure to do so results in high-resistance connections that will thermally fail under load, regardless of what the ampacity chart says.
