The ampacity of 4 AWG aluminum wire is 65 amps in the 75°C column (the standard for modern breakers and terminals), 55 amps in the 60°C column, and 75 amps in the 90°C column. However, the 90°C value is strictly a starting point for derating calculations; your final overcurrent protection device (breaker) size is almost always capped by the 75°C or 60°C rating of your equipment lugs.
Before pulling wire, clarify your terminology: "4 wire" usually means a single 4 AWG conductor. If you actually meant 4/0 AWG (four-aught, a massive feeder cable) or a 4-conductor cable, the ampacity changes drastically. The chart below covers the exact values for 4 AWG, alongside adjacent sizes and 4/0 AWG to prevent dangerous misapplications.
How to Read the NEC 310.16 Aluminum Ampacity Table
The National Fire Protection Association (NFPA) publishes the National Electrical Code (NEC), which governs wire sizing in the US. Table 310.16 is the master reference for conductor ampacity. To use it correctly, you must match your wire's insulation type to the correct temperature column. The 60°C column applies to older equipment and NM-B (Romex) cables. The 75°C column applies to most modern THHN/THWN-2 wire in conduit terminated in standard panels. The 90°C column represents the thermal limit of the wire's insulation itself, used exclusively for calculating environmental derating before you apply the termination limits.
| AWG Size | 60°C (140°F) | 75°C (167°F) | 90°C (194°F) | Common Insulation |
|---|---|---|---|---|
| 6 AWG | 40A | 50A | 55A | THW, THHN, XHHW |
| 4 AWG (Target) | 55A | 65A | 75A | THW, THHN, XHHW-2 |
| 3 AWG | 65A | 75A | 85A | THW, THHN, XHHW |
| 2 AWG | 75A | 90A | 100A | THW, THHN, XHHW-2 |
| 1 AWG | 85A | 100A | 115A | THW, THHN, XHHW |
| 4/0 AWG (Four-Aught) | 155A | 180A | 205A | THW, THHN, XHHW-2 |
Which Column Applies to Your Installation?
The most common mistake DIYers and junior apprentices make is looking at the 90°C column because THHN wire is rated for 90°C, and then sizing a 75A breaker for 4 AWG aluminum. This violates NEC 110.14(C), which dictates that the circuit ampacity is limited by the lowest temperature rating of any connected component, termination, or conductor.
The 60°C Column (55 Amps)
Use this column if you are running NM-B (Romex) cable, which is strictly limited to the 60°C column regardless of the wire's actual insulation. You must also use this column if your breaker or panel lugs are not explicitly marked with a temperature rating, or if they are marked 60°C. For 4 AWG aluminum, this caps your breaker at 50A (the next standard breaker size down from 55A).
The 75°C Column (65 Amps)
This is the default for modern THHN/THWN-2 or XHHW-2 individual conductors pulled through conduit into a modern load center. Almost all breakers and panel lugs manufactured after 1995 are rated for 75°C. Because the table lists 65A, and 65A is a standard breaker size per NEC 240.6, you can legally protect 4 AWG aluminum in conduit with a 60A breaker (using the next size down rule for continuous loads, or a 60A breaker directly if the load is non-continuous and exactly 65A, though 60A is the standard practical choice).
The 90°C Column (75 Amps)
You cannot terminate a circuit on a 75A breaker using the 90°C column unless the breaker lugs, the wire, and the downstream equipment are all explicitly rated for 90°C (which is exceptionally rare in residential and light commercial work). The 90°C column exists solely to give you a higher baseline number to apply mathematical derating penalties before you drop back down to the 75°C termination limit.
How Derating Modifies the Base 4 AWG Value
Ampacity tables assume ideal conditions: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors bundled together. When you deviate from these conditions, you must apply correction factors outlined in NEC 310.15. This is where the 90°C column saves your installation.
Worked Example: 4 AWG Aluminum in a Hot Attic
Imagine you are pulling four current-carrying conductors (e.g., two hots, a neutral carrying unbalanced non-linear load, and a fourth wire for a multi-wire branch circuit) through an attic that reaches 113°F (45°C) in the summer.
- Start with the 90°C base: 4 AWG Aluminum THHN = 75A.
- Apply Temperature Correction: At 113°F, the NEC Table 310.15(B)(1) mandates an 87% multiplier for 90°C insulation. (75A × 0.87 = 65.25A).
- Apply Bundling Adjustment: Four current-carrying conductors require an 80% multiplier per NEC 310.15(C)(1). (65.25A × 0.80 = 52.2A).
- Compare to Termination Limit: Your final derated ampacity is 52.2A. You now compare this to the 75°C termination limit (65A). Because 52.2A is lower than 65A, the derated value governs.
The Result: Your 4 AWG aluminum wire is now only good for 52.2 amps. You can no longer use a 60A breaker. You must drop down to a 50A breaker, or you must upsize your wire to 2 AWG aluminum to maintain the 60A circuit capacity.
What the Ampacity Table Cannot Tell You
Relying solely on Table 310.16 leaves three critical blind spots that can cause voltage drop, physical installation failures, or code violations.
1. Voltage Drop Over Distance
The NEC ampacity tables do not account for distance. A 4 AWG aluminum wire carrying 60A on a 240V circuit will experience a voltage drop of roughly 2.3% over a 150-foot one-way run (well within the NEC informational recommendation of 3% for branch circuits). However, if that same wire is carrying 60A on a 120V circuit over 150 feet, the voltage drop doubles to 4.6%, which exceeds the 3% guideline and can cause motors to overheat or electronics to brown out. For long 120V runs, you must upsize to 2 AWG or 1 AWG aluminum, regardless of the breaker size.
2. Physical Lug Sizing
Ampacity tells you the electrical limit, not the mechanical one. Many 50A and 60A breakers have lug tunnels designed specifically for copper wire dimensions. While 4 AWG aluminum and 4 AWG copper share the same AWG designation, stranded aluminum wire sometimes has a slightly larger overall diameter due to the stranding geometry required to achieve equivalent conductivity. Always verify the breaker's datasheet or the physical lug stamping to ensure it accepts "Al" or "Cu/Al" and physically accommodates 4 AWG stranded conductors without fraying the strands to force them into the tunnel.
3. Local AHJ Overrides and Anti-Oxidant Paste
The NEC is a baseline standard; your local Authority Having Jurisdiction (AHJ) has final authority. Some municipalities strictly prohibit aluminum branch circuit wiring entirely, limiting it to feeders and service entrances. Furthermore, when terminating 4 AWG aluminum wire, NEC 110.14 requires you to use an approved anti-oxidant compound (like Noalox or Penetrox) on the conductor strands unless the termination is explicitly tested and listed for use without it. Failing to apply this paste can lead to galvanic corrosion, increased resistance, and eventual thermal failure at the lug.
