The Quick Answer: Ampacity of 4 AWG Wire
If you are pulling wire and need the numbers right now to size your breaker, here is the direct answer based on the 2023/2026 National Electrical Code (NEC) Table 310.16. Bookmark these quick-jump values for your most common 4 AWG scenarios:
- Copper (THHN/THWN-2) in a standard residential panel: 70 Amps (60°C column)
- Copper (THHN/THWN-2) to a 75°C rated subpanel or feeder: 85 Amps (75°C column)
- Aluminum (XHHW-2) in a standard residential panel: 55 Amps (60°C column)
- Aluminum (XHHW-2) to a 75°C rated subpanel or feeder: 65 Amps (75°C column)
Most DIYers and even some journeymen make the mistake of looking exclusively at the 90°C column (which lists 95A for copper) and sizing a 90A breaker. That is a code violation and a fire hazard in standard residential terminations. The true ampacity depends entirely on the temperature rating of the equipment lugs you are terminating on, the ambient temperature of the room, and how many wires are jammed into the conduit. Let us break down exactly how to read the chart and apply the math.
NEC Table 310.16: 4 AWG Ampacity Data Table
Before applying any numbers, you need to understand how to read the NEC ampacity tables. Table 310.16 is organized by conductor material (Copper vs. Aluminum), insulation temperature rating (60°C, 75°C, 90°C), and wire gauge. The values below assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.
| Conductor Material | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column | Common Insulation Types in this Column |
|---|---|---|---|---|
| Copper | 70A | 85A | 95A | TW, UF-B (60°C); RHW, THHW, XHHW (75°C); THHN, THWN-2, XHHW-2 (90°C) |
| Aluminum | 55A | 65A | 75A | RHW, THHW, XHHW (75°C); THHN, THWN-2, XHHW-2 (90°C) |
Note: Always verify the specific insulation printed on the wire jacket. THHN and THWN-2 are 90°C rated for derating purposes, but your termination limits will usually force you to use the 60°C or 75°C columns for final breaker sizing.
Decision Tree: Which Temperature Column Applies to You?
The NEC does not just let you pick the highest number on the chart. NEC Article 110.14(C) dictates strict rules for equipment terminations. Use this decision path to find your legal ampacity limit:
| Installation Scenario | Condition | Column to Use | Final Copper Ampacity |
|---|---|---|---|
| Standard Residential Branch Circuit (< 100A) | Breaker lugs are not marked with a temperature rating | 60°C | 70A |
| Feeder to Subpanel or HVAC Disconnect | Equipment lugs are explicitly marked '75°C' or 'AL/CU' | 75°C | 85A |
| High-Terminal Industrial Gear | Lugs marked 90°C (Extremely rare in residential) | 90°C | 95A |
Derating 4 AWG: When Your Base Ampacity Drops
The table above assumes perfect conditions: 86°F ambient air and a maximum of three current-carrying conductors (e.g., two hots and a neutral for a 240V/120V split-phase feeder) in a single conduit. When you exceed these conditions, you must apply derating factors.
How derating modifies the base value: You always start your derating math using the 90°C column (95A for Copper 4 AWG), regardless of your termination limits. After applying the math, you compare the result to your termination column (60°C or 75°C) and use the lower of the two numbers.
Worked Example: Bundling and Heat
Imagine you are pulling 4 AWG THHN copper through an attic where the ambient temperature hits 110°F (43°C), and you have 4 current-carrying conductors in the conduit (e.g., a multi-wire branch circuit or two separate 240V circuits sharing a neutral).
- Base 90°C Ampacity: 95A
- Ambient Temperature Correction (110°F / 90°C wire): 0.87 multiplier (per NEC Table 310.15(B)(1))
- Bundling Adjustment (4 conductors): 0.80 multiplier (per NEC Table 310.15(C)(1))
- Derated Ampacity: 95A × 0.87 × 0.80 = 66.12A
The Decision: Your derated wire can only safely carry 66.12A. Even if your panel lugs are rated 75°C (85A limit), the wire's derated capacity is lower. Per NEC 240.4(B), you can round up to the next standard breaker size, which is 70A. If your attic hit 122°F, the math would drop the ampacity below 60A, forcing you to use a 60A breaker or upsize to 3 AWG wire.
What the Ampacity Table Cannot Tell You
Ampacity is strictly about the wire's ability to dissipate heat without melting the insulation. It tells you absolutely nothing about power quality or physical installation realities. If you are sizing a 4 AWG feeder, you must check these three edge cases:
1. Voltage Drop Over Distance
NEC Table 310.16 does not account for voltage drop. If you are running a 4 AWG copper feeder 150 feet to a detached garage subpanel drawing 60A, you will experience a voltage drop of roughly 3.8%. While the NEC recommends keeping feeder drop under 3% (Informational Note to 310.15(B)), a 4% total drop (feeder + branch) is generally acceptable for standard loads. However, if you are running sensitive electronics or a welder, 4 AWG is too small for that distance. You would need to upsize to 3 AWG or 2 AWG copper. Use a dedicated voltage drop calculator to verify long runs.
2. Physical Lug Fitment
4 AWG wire is thick. The bare copper diameter is roughly 0.204 inches, and with THHN insulation, the overall diameter approaches 0.35 inches. Many standard 50A and 60A residential breakers are physically designed to accept a maximum of 6 AWG or 4 AWG, but bending 4 AWG wire inside a cramped 200A main panel to reach a breaker on the far left rail is a knuckle-busting nightmare. If your run is long and requires tight bends, consider using aluminum SER cable, which is more pliable, or plan your panel layout carefully.
3. Short-Circuit Withstand Rating
Ampacity handles continuous thermal loads. It does not tell you if the wire will survive a dead short before the breaker trips. For 4 AWG copper, the thermal withstand rating is generally sufficient for standard residential fault currents (up to 10,000A or 22,000A depending on the breaker series), but if you are installing this in a commercial setting with high available fault current, an engineer must verify the let-through current of the breaker against the wire's circular mil area (41,740 cmils for 4 AWG per NEC Chapter 9, Table 8).
Final Verdict: Breaker and Wire Pairings for 4 AWG
Stop guessing and use these exact, code-compliant pairings for your 4 AWG installations. No 'it depends'—just the correct hardware for the job.
- For a standard 70A EV charger or subpanel feeder (Copper): Use 4 AWG Copper THHN/THWN-2 in conduit. Terminate on a 70A breaker. (Assumes standard 60°C termination limits and < 100 feet distance).
- For an 85A continuous industrial load or 75°C rated subpanel (Copper): Use 4 AWG Copper THHN. Terminate on a 90A breaker only if the equipment lugs are explicitly marked 75°C and the load is non-continuous, or use an 80A breaker for continuous loads (125% rule).
- For a budget-friendly 60A detached garage feeder (Aluminum): Use 4 AWG Aluminum XHHW-2 or 4-4-4-6 MHF/SER cable. Terminate on a 60A breaker. This safely utilizes the 55A/65A aluminum limits while keeping material costs down (aluminum is roughly 40% cheaper than copper by weight).
Always torque your lugs to the manufacturer's exact inch-pound specification printed on the breaker label. A 4 AWG wire that is under-torqued will arc, oxidize, and fail at 40A, regardless of what the ampacity chart says. For further reading on termination torque and temperature limits, consult the National Electrical Code resources at Electrical Contractor Magazine.
