The ampacity of 4 AWG wire depends entirely on the conductor material and the temperature rating of the insulation and terminals. For copper 4 AWG wire, the maximum ampacity is 85 amps (90°C column), 70 amps (75°C column), or 55 amps (60°C column). For aluminum 4 AWG wire, the maximum ampacity is 65 amps (75°C column) or 55 amps (60°C column). In most modern residential and commercial installations, you will use the 75°C column, making 4 AWG copper good for 70A and 4 AWG aluminum good for 65A.
However, quoting the base ampacity is only half the battle. To legally and safely size your breaker, you must apply National Electrical Code (NEC) rules for termination limits, ambient temperature derating, and conductor bundling. Below is the complete reference data and the decision path to finalize your build.
The Direct Answer: Ampacity of 4 AWG Wire (Quick Lookup)
The following table is extracted directly from NEC Table 310.16 (formerly 310.15(B)(16)). This table assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.
| Material | Common Insulation Types | 60°C Column (140°F) | 75°C Column (167°F) | 90°C Column (194°F) |
|---|---|---|---|---|
| Copper | TW, UF | 55 Amps | — | — |
| Copper | RHW, THHW, THW, XHHW | — | 70 Amps | — |
| Copper | THHN, THHW, THW-2, XHHW-2 | — | 70 Amps | 85 Amps |
| Aluminum | RHW, THHW, THW, XHHW | — | 65 Amps | — |
| Aluminum | THHN, THHW, THW-2, XHHW-2 | — | 65 Amps | 75 Amps |
• Running a 60A subpanel? Use 4 AWG Aluminum XHHW-2 (rated 65A).
• Running a 70A EV charger or welder? Use 4 AWG Copper THHN (rated 70A at terminals).
• Using older NM-B (Romex) cable? 4 AWG NM-B is restricted to the 60°C column (55A max).
How to Read the NEC 310.16 Ampacity Table
The most common mistake DIYers and junior electricians make is looking at the 90°C column for THHN wire and assuming they can put 85 amps through a 4 AWG copper conductor. You almost never get to use the 90°C column for final overcurrent protection sizing. Here is how the columns actually apply to your installation:
The 60°C Column
You must use the 60°C column if you are using older cable types like NM-B (Romex) or UF, or if you are terminating into equipment rated for 100 amps or less that does not explicitly state a 75°C rating on the equipment label. For 4 AWG copper NM-B, your absolute maximum breaker size is 55A (which means you must use a 50A breaker, as 55A breakers are rare/specialty).
The 75°C Column (The Industry Standard)
Under NEC 110.14(C), terminations for equipment rated 100A or less are generally assumed to be 60°C unless marked otherwise. However, virtually all modern breakers, lugs, and subpanel busbars are explicitly marked 'AL/CU 75°C'. When your wire insulation is rated 75°C or higher (like THHN or XHHW-2) and your terminals are rated 75°C, you are legally permitted to use the 75°C column. This gives you 70A for copper and 65A for aluminum.
The 90°C Column (Derating Only)
The 90°C column is a mathematical tool, not a termination limit. You use the 90°C base ampacity (85A for copper) as your starting point when calculating derating factors for high ambient temperatures or conduit fill. The final derated number must still be terminated based on the 75°C or 60°C rules.
Derating 4 AWG Wire: When the Base Value Drops
Ampacity is not a static number; it shrinks when heat cannot escape the wire. The NEC mandates derating in two primary scenarios: high ambient temperature and bundling more than three current-carrying conductors in a single conduit.
Ambient Temperature Derating
If your conduit runs through an attic in a hot climate where the ambient temperature reaches 110°F (43°C), you must apply a correction factor. For 90°C THHN wire at 110°F, the correction factor is 0.87.
- Math: 85A (90°C base) × 0.87 = 73.95A.
- Result: 73.95A is still higher than the 75°C termination limit of 70A, so you can still use a 70A breaker. But if the attic hits 122°F (50°C), the factor drops to 0.82. 85 × 0.82 = 69.7A. You must now downsize your breaker to 60A.
Conduit Fill (Bundling) Derating
When you pull more than three current-carrying conductors through a single raceway, the wires heat each other up. If you are pulling two 240V circuits (4 hot wires) plus a shared neutral for a multi-wire branch circuit, you have 5 current-carrying conductors. The NEC Table 310.15(C)(1) mandates an 80% derating factor.
- Math: 85A (90°C base for 4 AWG Copper THHN) × 0.80 = 68 Amps.
- Result: The derated ampacity is 68A. Because 68A is less than the 75°C termination limit of 70A, your wire is now legally limited to 68A. You must protect this wire with a 60A breaker.
Decision Tree: Which 4 AWG Wire and Breaker to Buy
Stop guessing at the hardware store counter. Use this decision matrix to select the exact materials for your specific project.
| Your Project Scenario | If This Is True... | Then Buy This Exact Material | Breaker Size |
|---|---|---|---|
| 60A Subpanel Feeder (Standard residential, up to 100ft) | You want the most cost-effective, code-compliant solution and have 75°C rated lugs. | 4-4-4-6 Aluminum MHF (Mobile Home Feeder) or 4 AWG Aluminum XHHW-2 in conduit. | 60A Double-Pole |
| 70A EV Charger / Welder (Hardwired, indoor conduit) | You need maximum current capacity and are pulling through standard PVC/EMT conduit. | 4 AWG Copper THHN/THWN-2 (individual conductors in conduit). | 70A Double-Pole |
| 60A Outdoor Hot Tub (Wet location, direct burial or conduit) | The wire will be exposed to moisture or buried; NM-B is strictly prohibited. | 4 AWG Copper THWN-2 or XHHW-2 in Schedule 80 PVC. (Do not use bare aluminum for direct burial without specific UF rating). | 60A Double-Pole (GFCI) |
| 50A Range/Oven Replacement (Retrofitting old cable) | You are replacing an old 3-wire setup and want to use standard non-metallic sheathed cable. | 4 AWG Copper NM-B (Romex). Remember, NM-B is locked to the 60°C column (55A max). | 50A Double-Pole |
What the Ampacity Table Cannot Tell You
NEC Table 310.16 is a thermal limit chart; it tells you when the wire insulation will melt. It does not account for electrical efficiency or equipment-specific constraints. Before finalizing your 4 AWG run, check these three edge cases:
1. Voltage Drop Over Distance
Ampacity assumes the wire can handle the heat, but it doesn't guarantee the voltage will reach the load. On a 240V circuit, a 60A load on 4 AWG copper will experience a 3% voltage drop at roughly 115 feet. If your subpanel or EV charger is 150 feet away from the main panel, 4 AWG copper will result in a ~4% drop, which can cause EV chargers to fault out or motors to overheat. The fix: If the one-way distance exceeds 120 feet on a 60A/240V circuit, upsize to 3 AWG or 2 AWG copper, or 1/0 AWG aluminum, regardless of the base ampacity table.
2. The 240V Single-Phase vs. 208V Three-Phase Trap
If you are wiring a commercial space with 208V three-phase power, the same 4 AWG wire carries the same ampacity, but the total wattage delivered is different. A 60A breaker on 208V 3-phase delivers roughly 21.6 kW, whereas on 240V single-phase it delivers 14.4 kW. Ensure your equipment nameplate MCA (Minimum Circuit Ampacity) matches the voltage topology of your panel.
3. Continuous vs. Non-Continuous Loads
The NEC defines a continuous load as one that will run for 3 hours or more (like an EV charger, a space heater, or commercial lighting). For continuous loads, you must derate the breaker by 125%. Therefore, a 4 AWG copper wire on a 70A breaker can only safely supply a 56A continuous load (70 × 0.8 = 56). If your EV charger draws 60A continuously, 4 AWG wire and a 70A breaker are illegal; you must upsize to 3 AWG copper and an 80A breaker.
Always verify your final wire and breaker selections against the specific equipment nameplate and your local Authority Having Jurisdiction (AHJ), as local amendments to the NEC can override baseline table values.
