What Size Wire for 45 Amps: AWG, Breaker Sizing, and NEC Rules

For a 45-amp non-continuous load, use 8 AWG copper wire and a 50-amp breaker. If the load runs for three or more continuous hours, upgrade to 6 AWG copper wire and a 60-amp breaker. This assumes THHN/THWN-2 insulation in conduit with 75°C rated terminations.

The Ampacity Math: Why 8 AWG and Not 10 AWG?

Sizing a circuit isn't just about matching the wire to the load; it's about matching the wire to the breaker, and the breaker to the load. According to NFPA 70 (NEC) Table 310.16, an 8 AWG copper wire with THHN insulation has an ampacity of 55A in the 90°C column. However, NEC 110.14(C) requires us to size based on the lowest temperature rating of any connected component. Since standard residential and commercial breakers and lugs are rated for 75°C, we must use the 75°C column.

In the 75°C column, 8 AWG copper is rated for 50 amps. A 10 AWG wire in the same column is only rated for 35 amps. If you attempt to pull 45 amps through a 10 AWG wire, the conductor will overheat, degrading the insulation and creating a severe fire hazard long before a 50-amp breaker trips.

Because 45 amps is not a standard breaker size that is easily sourced at retail (though it is technically listed in NEC 240.6), we apply the "next standard size up" rule from NEC 240.4(B). A 45-amp load requires a 50-amp breaker, and 8 AWG copper perfectly matches that 50-amp breaker limit.

Variables That Force a Wire Size Upgrade

The 8 AWG baseline only holds true if your installation perfectly matches the assumptions above. Real-world jobsite conditions often force you to step up to 6 AWG copper. Use this decision tree to verify your final wire size:

Installation Variable	NEC Rule / Multiplier	Required Copper Wire	Required Breaker
Continuous Load (3+ hours)	125% of load (45A × 1.25 = 56.25A)	6 AWG (65A @ 75°C)	60 Amp
40°C (104°F) Ambient Temp	0.88 correction factor (50A × 0.88 = 44A)	6 AWG (65A × 0.88 = 57.2A)	60 Amp
4-6 Conductors in Raceway	0.80 adjustment factor (50A × 0.80 = 40A)	6 AWG (65A × 0.80 = 52A)	60 Amp
Using NM-B (Romex) Cable	Restricted to 60°C column (8 AWG = 40A max)	6 AWG (55A @ 60°C)	60 Amp
Aluminum Conductors	Lower conductivity requires larger gauge	6 AWG Al (50A @ 75°C)	50 Amp

The NM-B Trap: Many DIYers buy 8 AWG NM-B (Romex) for a 45-amp circuit because they see "8 AWG" and assume it's good for 50 amps. NM-B insulation is strictly limited to the 60°C column by NEC 334.80. In the 60°C column, 8 AWG is only rated for 40 amps. If you are running Romex inside a wall for a 45-amp load, you must use 6 AWG NM-B and a 60-amp breaker.

Voltage Drop Check at 50 Feet

Ampacity tells you if the wire will melt; voltage drop tells you if the equipment will actually run. The NEC recommends a maximum 3% voltage drop on branch circuits. Let's calculate the drop for our baseline 8 AWG copper wire carrying 45 amps over a 50-foot run.

Using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM

• K (Copper resistance constant) = 12.9
• I (Current) = 45A
• D (Distance) = 50 ft
• CM (Circular mils for 8 AWG) = 16,510

VD = (2 × 12.9 × 45 × 50) / 16,510 = 3.51 Volts

If this is a 240V circuit (like a heavy-duty compressor or welder), a 3.51V drop is 1.46%. This easily passes the 3% threshold. However, if this is a 120V circuit, that same 3.51V drop represents 2.92%. While technically passing, it leaves almost zero margin for error. If your 120V run exceeds 50 feet, step up to 6 AWG copper to keep the voltage drop under 2%. You can verify your specific run parameters using the Southwire Voltage Drop Calculator.

When an Engineer or the AHJ Must Confirm

While the NEC provides the baseline rules, local Authorities Having Jurisdiction (AHJ) and professional engineers step in when edge cases arise. You must pull a permit and get professional sign-off in these scenarios:

• EV Chargers: A 45-amp Level 2 EV charger is legally considered a continuous load. The AHJ will require the 125% multiplier (56.25A), forcing you to use 6 AWG copper and a 60-amp breaker. Furthermore, many local codes now require GFCI protection for EV receptacles, which dictates specific breaker types.
• Motor Starting Currents: If your 45-amp load is a large motor, the Locked Rotor Amperage (LRA) can be 6x the Full Load Amperage (FLA). An engineer must calculate the voltage dip during startup to ensure it doesn't brown out the rest of the panel, which may require upsizing the wire to 4 AWG purely for voltage drop mitigation.
• High Ambient Environments: If you are routing conduit through an unventilated attic in a climate like Phoenix or Las Vegas, attic temperatures can exceed 50°C (122°F). At 50°C, the derating factor for THHN is 0.75. A 50A wire derates to 37.5A, meaning you would need to jump all the way to 4 AWG copper to safely carry 45 amps.