The maximum ampacity for 3 AWG copper wire is 100 amps in the 75°C column and 115 amps in the 90°C column, per NEC Table 310.16. For 3 AWG aluminum wire, the rating is 75 amps (75°C) and 90 amps (90°C). In standard residential and commercial applications, 3 AWG copper THHN/THWN-2 is the exact size required for a 100-amp subpanel feeder when termination ratings and voltage drop permit.
The 3 AWG Wire Amp Rating: Quick Reference Chart
The following data is extracted directly from Cerrowire’s official ampacity charts, which mirror the National Electrical Code (NEC) 2023 Table 310.16. This table assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable.
| AWG Size | Copper 60°C (TW) | Copper 75°C (THW/THWN) | Copper 90°C (THHN/THWN-2) | Aluminum 60°C | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|---|
| 4 AWG | 70A | 85A | 95A | 55A | 65A | 75A |
| 3 AWG (Target) | 85A | 100A | 115A | 65A | 75A | 90A |
| 2 AWG | 95A | 115A | 130A | 75A | 90A | 100A |
| 1 AWG | 110A | 130A | 145A | 85A | 100A | 115A |
Which Temperature Column Applies to Your Installation?
The most common mistake DIYers make is looking at the 90°C column (115A for 3 AWG Copper) and assuming they can safely run 115 amps through the wire on a 115A or 125A breaker. This violates NEC Article 110.14(C), which governs terminal temperature limitations.
The ampacity of your circuit is limited by the weakest link in the chain. While THHN wire insulation is rated for 90°C, the mechanical lugs inside standard residential breakers and panelboards are typically rated for either 60°C or 75°C.
- The 60°C Column: Applies to circuits rated 100 amps or less, or wire sizes 14 AWG through 1 AWG, unless the equipment is specifically marked otherwise. However, 3 AWG is generally used for 100A feeders.
- The 75°C Column: Applies to circuits rated over 100 amps, or wire sizes larger than 1 AWG. Most modern 100A main breakers and subpanel lugs are rated for 75°C. Therefore, 100 amps is your legal maximum ampacity for 3 AWG copper in a standard 100A feeder application.
- The 90°C Column: You may only use this column to calculate derating adjustments (explained below). The final derated ampacity must still be compared against the 75°C column limit for termination.
Derating 3 AWG Wire: When Your Base Ampacity Drops
The base ampacity of 115A (90°C column) assumes you have no more than three current-carrying conductors (CCCs) in your conduit. If you pull multiple circuits through the same pipe, the wires heat each other up, and the NEC mandates an adjustment factor per Table 310.15(C)(1).
Worked Derating Example:
You are running a 120/240V multi-wire branch circuit and a subpanel feeder through the same 1.5-inch PVC conduit. You now have 4 current-carrying conductors in the pipe (2 hots for the subpanel, 1 hot for the MWBC, 1 neutral for the MWBC). Note: The subpanel neutral and all ground wires do not count as CCCs.
- Identify Base 90°C Ampacity: 3 AWG Copper THHN = 115A.
- Find Adjustment Factor: 4 to 6 CCCs requires an 80% multiplier.
- Calculate Derated Ampacity: 115A × 0.80 = 92 Amps.
Because your derated ampacity (92A) has dropped below your target breaker size (100A), you can no longer use 3 AWG wire for this specific conduit run. You must upsize to 2 AWG or 1 AWG copper to maintain a 100A circuit safely.
Decision Tree: Sizing Your Breaker for a 3 AWG Feeder
Use this decision matrix to finalize your wire and breaker purchase. This path assumes a standard 120/240V single-phase residential split-phase system.
| Installation Scenario | Conditions & Constraints | Concrete Pick (Buy This) |
|---|---|---|
| Scenario A: Standard 100A Subpanel Feeder | Distance < 100 ft. 3 CCCs in conduit. 75°C breaker lugs. | 3 AWG Copper THHN (Black, Red, White, Green) + 100A 2-Pole Breaker. |
| Scenario B: 100A Feeder, Shared Conduit | 4 to 6 CCCs in the same pipe. Derating applies. | 1 AWG Copper THHN + 100A 2-Pole Breaker. (3 AWG fails derating). |
| Scenario C: Long Distance 100A Feeder | Distance > 110 feet. Voltage drop exceeds 3% at full load. | 1 AWG Copper THHN or 2/0 Aluminum + 100A 2-Pole Breaker. |
| Scenario D: 80A EV Charger Circuit | Continuous load requires 125% sizing (100A wire ampacity needed). | 3 AWG Copper THHN + 100A 2-Pole Breaker (set to 80A continuous draw via DIP switch). |
What the Ampacity Table Cannot Tell You
While NEC Table 310.16 is the absolute authority on thermal limits (preventing the wire insulation from melting), it is blind to two critical real-world factors 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 0.248 ohms per 1,000 feet. If you run a 100-amp load through 3 AWG copper for 150 feet (300 feet total round-trip), you will experience a voltage drop of roughly 7.4 volts on a 240V circuit (about 3.1%). While the NEC recommends keeping voltage drop under 3% for feeders (Article 310.15(B) Informational Note), a 3.1% drop on a 240V line leaves your subpanel with 232V. If you are running sensitive electronics or high-inrush motors at the end of that feeder, you must upsize to 2 AWG or 1 AWG strictly to mitigate voltage drop, even though 3 AWG is thermally safe.
2. Conduit Fill Capacity
3 AWG THHN wire has an approximate cross-sectional area of 0.1158 square inches. If you are pulling four 3 AWG wires (two hots, one neutral, one ground) through a single conduit, the total wire area is 0.4632 square inches. Per NEC Chapter 9, Table 1, you cannot exceed 40% conduit fill for three or more wires. This means you must use a minimum of 1-inch Schedule 40 PVC (which offers 0.887 sq in of interior space) or 1.25-inch EMT. Trying to force four 3 AWG wires into a 3/4-inch conduit will result in jammed wires, damaged insulation, and a failed inspection.
