The base ampacity for 4 AWG copper wire is 85 amps in the 75°C column and 95 amps in the 90°C column. For 4 AWG aluminum wire, the base ampacity is 65 amps at 75°C and 75 amps at 90°C. These values are dictated by NEC Table 310.16 (2023/2026 editions) for up to three current-carrying conductors in an ambient temperature of 30°C (86°F). However, the number you can actually use for your breaker sizing depends entirely on your terminal temperature ratings, conduit fill, and voltage drop over distance.
The 4 AWG Ampacity Quick-Lookup Table (NEC 310.16)
Before pulling wire, you need to know how to read the NEC ampacity tables. The table below outlines the allowable ampacities for 4 AWG conductors based on material and insulation temperature rating. Bookmark this row: For 95% of residential and light commercial subpanel feeders, you will be using the 75°C column because standard breakers and panel lugs are rated for 75°C, regardless of the 90°C insulation on the wire itself.
| Conductor Material | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|
| Copper | 70 Amps | 85 Amps (Most Common) | 95 Amps |
| Copper-Clad Aluminum | 65 Amps | 75 Amps | 85 Amps |
| Aluminum | 55 Amps | 65 Amps (Most Common) | 75 Amps |
How to Read the Ampacity Columns (Temperature Ratings)
The most common mistake DIYers and junior apprentices make is looking at the 90°C column for THHN wire and assuming they can put 95 amps on a 4 AWG copper conductor. You cannot. The National Electrical Code enforces a "weakest link" rule for termination temperatures.
According to Copper Development Association guidelines and NEC 110.14(C), the ampacity of the circuit is limited by the lowest temperature rating of any connected component. Modern residential breakers (like Square D QO or Eaton BR) and subpanel lugs are typically rated for 75°C. Therefore, even if your 4 AWG THHN wire is rated for 90°C, the termination point is only good for 75°C. You must size your overcurrent protection based on the 75°C column: 85A for copper, 65A for aluminum.
When do you use the 90°C column? Only for derating calculations (which we cover next) or when connecting to equipment explicitly marked for 90°C terminations, which is rare in residential work but common in industrial control panels.
Derating 4 AWG Wire: When the Base Value Drops
The base values in Table 310.16 assume two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. If you violate either condition, you must derate the wire. Derating is always calculated starting from the 90°C column, even if you terminate at 75°C.
Scenario: Conduit Fill Adjustment (NEC 310.15(C)(1))
Imagine you are pulling two 4 AWG copper hot wires, one 4 AWG neutral, and one 6 AWG ground through a single conduit to a subpanel. You have three current-carrying conductors (the two hots and the neutral, assuming it carries unbalanced load). Three conductors require no derating (100%). Your 90°C base is 95A, and your final termination limit is 85A (75°C column).
Now, imagine you add a second circuit to that same conduit, bringing the total to five current-carrying conductors. NEC Table 310.15(C)(1) requires an 80% adjustment factor.
- Step 1: Take the 90°C base ampacity for 4 AWG Copper = 95A.
- Step 2: Multiply by the 80% adjustment factor (0.80). 95A × 0.80 = 76 Amps.
- Step 3: Compare this derated value (76A) to the 75°C termination limit (85A).
- Result: You must use the lower number. Your 4 AWG copper wire is now legally limited to 76 Amps. You must drop your breaker size to 70A (the next standard size down) or pull larger wire.
Scenario: Ambient Temperature Correction (NEC 310.15(B)(1))
If your conduit runs across an unventilated attic in a hot climate where ambient temperatures reach 50°C (122°F), you must apply a temperature correction factor. For 90°C wire at 50°C ambient, the multiplier is 0.82.
95A (90°C base) × 0.82 = 77.9 Amps. Again, your usable ampacity drops below the standard 85A 75°C limit.
Decision Path: Which 4 AWG Wire and Breaker Should You Buy?
Stop guessing at the hardware store counter. Use this decision tree to select the exact materials for your 4 AWG feeder run.
| IF Your Installation Matches This... | THEN Buy This Exact Wire... | AND This Breaker Size |
|---|---|---|
| Scenario A: 80A subpanel feeder, run is under 100 feet, standard 75°C lugs, max 3 conductors in conduit. | 4 AWG Copper THHN/THWN-2 (Black, Red, White, Green) | 80A Double-Pole Breaker |
| Scenario B: 60A subpanel feeder, budget-conscious, run is under 100 feet, standard 75°C lugs. | 4 AWG Aluminum XHHW-2 (Black, Red, White, Green) | 60A Double-Pole Breaker |
| Scenario C: 80A feeder, but you have 4-6 current-carrying conductors in the same conduit. | Upsize to 3 AWG or 2 AWG Copper (4 AWG is derated to 76A and fails) | 80A Double-Pole Breaker |
The Default Pick: If you are wiring a standard detached garage or workshop subpanel and want the highest reliability with the least physical effort bending stiff wire, buy 4 AWG Copper THHN/THWN-2 and protect it with an 80A breaker. If you are running a long trench to a barn and want to save $150+ on copper prices, buy 4 AWG Aluminum XHHW-2, use an anti-oxidant compound (like Noalox) on the terminations, torque the lugs to spec, and protect it with a 60A breaker.
What the NEC Table Cannot Tell You (Voltage Drop & Terminal Limits)
NEC Table 310.16 only tells you the thermal limit of the wire insulation before it melts. It completely ignores voltage drop, physical conduit fill, and terminal torque. If you ignore these three factors, your installation will pass a basic inspection but fail in real-world operation.
1. Voltage Drop Over Distance
The NEC recommends (via Informational Note to 210.19(A)) keeping voltage drop under 3% for branch circuits and 5% overall. Table 310.16 does not account for distance.
Let's run the math on 4 AWG Copper carrying 80A on a 240V single-phase circuit:
- At 50 feet: Voltage drop is ~1.0V (0.4%). Excellent.
- At 100 feet: Voltage drop is ~2.0V (0.8%). Excellent.
- At 150 feet: Voltage drop is ~3.0V (1.25%). Still well within limits.
However, if you are pulling 4 AWG Aluminum (which has higher resistance) at 65A over 150 feet on a 120V single-phase leg, the drop approaches 4.5%. For long runs exceeding 150 feet, always use a dedicated voltage drop calculator and be prepared to upsize to 2 AWG or 1/0 AWG aluminum to maintain power quality for sensitive electronics and motor startups.
2. Conduit Fill Limits (NEC Chapter 9)
Four 4 AWG THHN wires (three hots/neutral + one ground) require a minimum of 1-inch PVC Schedule 40 conduit to stay under the 40% fill rule. If you try to jam them into 3/4-inch conduit, you will damage the insulation skin during the pull, creating a hidden ground fault that will trip your GFCI/AFCI breakers or cause a short.
3. Torque Specifications (NEC 110.14(D))
Aluminum wire expands and contracts at a different rate than copper or brass lugs. If you do not use a calibrated torque screwdriver to tighten the lugs to the manufacturer's exact inch-pound specification, the connection will loosen over time. A loose 4 AWG aluminum connection carrying 60A will arc, oxidize, and eventually melt the panel lug. Always apply Noalox, torque to spec, and re-check torque after the first thermal cycle.
