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By LarsonElectronics.com – March 26, 2025

What Is the Ampacity of a 4 AWG Wire?

The ampacity of a 4 AWG wire depends on several factors, including whether the conductor is copper or aluminum, the insulation type, and the installation environment. The table below provides ampacity ratings based on the 2023 NEC guidelines.

4 AWG Wire Ampacity Ratings

Conductor Type	Insulation Type	Temperature Rating	Ampacity (Free Air)	Ampacity (Conduit/Raceway)
Copper	THHN/THWN-2	90°C (194°F)	120 Amps	85 Amps
	XHHW-2	90°C (194°F)	120 Amps	85 Amps
	USE-2/RHH/RHW-2	90°C (194°F)	120 Amps	85 Amps
Aluminum	THHN/THWN-2	90°C (194°F)	95 Amps	65 Amps
	XHHW-2	90°C (194°F)	95 Amps	65 Amps
	USE-2/RHH/RHW-2	90°C (194°F)	95 Amps	65 Amps

What Factors Influence Ampacity?

The actual ampacity of a 4 AWG wire can vary based on external factors. Key considerations include:

1. Ambient Temperature

NEC guidelines specify that ampacity ratings assume an ambient temperature of 30°C (86°F). If the temperature is higher, correction factors must be applied according to NEC Table 310.15(B)(1).

2. Conductor Bundling

When multiple conductors are bundled in a conduit or raceway, heat dissipation decreases, requiring derating to prevent overheating. The NEC provides specific adjustment factors based on the number of conductors in a bundle.

3. Voltage Drop

For long wire runs, voltage drop can reduce efficiency. The NEC recommends keeping voltage drop under 3% for optimal performance. Larger conductors may be required for long-distance applications.

4. Installation Conditions

Wire installed in free air typically has a higher ampacity than wire enclosed in conduit due to improved heat dissipation. Underground or direct burial applications may also impact ampacity and require additional considerations.

When Should Ampacity Be Adjusted?

Adjustments to ampacity are necessary when environmental or installation conditions differ from NEC standard assumptions. Key adjustments include:

High ambient temperatures: Use correction factors to reduce ampacity.
Bundled conductors: Apply NEC derating factors for grouped wires.
Long cable runs: Increase conductor size to compensate for voltage drop.
Enclosed installations: Consider additional cooling or spacing to prevent overheating.

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4 AWG Wire Ampacity Rating and Key Considerations (3/26/2025)