What Wire Size for 40 Amps?
40-amp circuits serve large appliances and equipment including electric ranges, large air conditioning systems, and mid-size tankless water heaters. At 240 volts, a 40-amp circuit delivers 9,600 watts. The minimum wire size is 8 AWG copper. While 50-amp circuits are more common for full-size ranges, 40-amp circuits adequately serve smaller cooktops, wall ovens, and mid-size ranges. Heat pump and air conditioning condensing units frequently require 40-amp circuits based on their nameplate minimum circuit ampacity.
Minimum Wire Size for 40 Amps (Ampacity Only)
Based solely on NEC Table 310.16 ampacity ratings at 75 degrees C (the most common termination rating), the minimum wire size for 40 amps is 8 AWG for copper and 8 AWG for aluminum. However, these minimums only satisfy the ampacity requirement. For any run longer than a few feet, you must also check voltage drop and may need to upsize the conductor. The tables below show the recommended wire gauge at each distance, accounting for both ampacity and the NEC 3% voltage drop recommendation.
40A Wire Size at 120V by Distance
Recommended wire gauge for a 40-amp, 120-volt, single-phase circuit at each distance. The copper columns show recommendations at both 3% and 5% voltage drop limits. The aluminum column shows the 3% recommendation. At 120 volts, a 3% voltage drop equals 3.6 volts.
| Distance | Copper (3%) | V-Drop | Copper (5%) | V-Drop | Aluminum (3%) | V-Drop |
|---|---|---|---|---|---|---|
| 25 ft | 8 AWG | 1.30% | 8 AWG | 1.30% | 8 AWG | 2.13% |
| 50 ft | 8 AWG | 2.59% | 8 AWG | 2.59% | 6 AWG | 2.69% |
| 75 ft | 6 AWG | 2.46% | 8 AWG | 3.89% | 4 AWG | 2.54% |
| 100 ft | 4 AWG | 2.05% | 6 AWG | 3.27% | 3 AWG | 2.69% |
| 125 ft | 4 AWG | 2.57% | 6 AWG | 4.09% | 2 AWG | 2.66% |
| 150 ft | 3 AWG | 2.45% | 6 AWG | 4.91% | 1 AWG | 2.53% |
| 200 ft | 2 AWG | 2.59% | 4 AWG | 4.11% | 1/0 AWG | 2.68% |
| 250 ft | 1 AWG | 2.57% | 3 AWG | 4.08% | 2/0 AWG | 2.65% |
| 300 ft | 1/0 AWG | 2.44% | 3 AWG | 4.90% | 3/0 AWG | 2.52% |
| 400 ft | 2/0 AWG | 2.58% | 1 AWG | 4.11% | 4/0 AWG | 2.67% |
| 500 ft | 3/0 AWG | 2.55% | 1/0 AWG | 4.07% | 250 kcmil | 2.82% |
40A Wire Size at 240V by Distance
Recommended wire gauge for a 40-amp, 240-volt, single-phase circuit at each distance. The copper columns show recommendations at both 3% and 5% voltage drop limits. The aluminum column shows the 3% recommendation. At 240 volts, a 3% voltage drop equals 7.2 volts.
| Distance | Copper (3%) | V-Drop | Copper (5%) | V-Drop | Aluminum (3%) | V-Drop |
|---|---|---|---|---|---|---|
| 25 ft | 8 AWG | 0.65% | 8 AWG | 0.65% | 8 AWG | 1.07% |
| 50 ft | 8 AWG | 1.30% | 8 AWG | 1.30% | 8 AWG | 2.13% |
| 75 ft | 8 AWG | 1.95% | 8 AWG | 1.95% | 6 AWG | 2.02% |
| 100 ft | 8 AWG | 2.59% | 8 AWG | 2.59% | 6 AWG | 2.69% |
| 125 ft | 6 AWG | 2.05% | 8 AWG | 3.24% | 4 AWG | 2.12% |
| 150 ft | 6 AWG | 2.46% | 8 AWG | 3.89% | 4 AWG | 2.54% |
| 200 ft | 4 AWG | 2.05% | 6 AWG | 3.27% | 3 AWG | 2.69% |
| 250 ft | 4 AWG | 2.57% | 6 AWG | 4.09% | 2 AWG | 2.66% |
| 300 ft | 3 AWG | 2.45% | 6 AWG | 4.91% | 1 AWG | 2.53% |
| 400 ft | 2 AWG | 2.59% | 4 AWG | 4.11% | 1/0 AWG | 2.68% |
| 500 ft | 1 AWG | 2.57% | 3 AWG | 4.08% | 2/0 AWG | 2.65% |
Common Applications for 40-Amp Circuits
40-amp circuits are used for: Mid-size electric ranges, large AC condensers, heat pump systems, mid-size tankless water heaters, spas, commercial 208V equipment. Each of these applications may have specific NEC requirements beyond basic wire sizing, including dedicated circuit requirements, GFCI or AFCI protection, disconnect requirements, and specific receptacle configurations. Visit the applications page for detailed guidance on specific circuit types.
When sizing a 40-amp circuit, remember that the NEC requires conductors and overcurrent protection devices to be sized at 125% for continuous loads (loads expected to operate for 3 hours or more). This means a 40-amp breaker can only serve 32 amps of continuous load unless the breaker is specifically rated for 100% continuous duty. If your load is continuous, you may need to upsize to the next standard breaker size, which will also require the corresponding larger wire gauge.
For motor loads on 40-amp circuits, wire sizing follows the special rules in NEC Article 430. Motor branch circuit conductors must be sized at 125% of the motor full-load current from NEC Table 430.248 (single-phase) or Table 430.250 (three-phase), not the motor nameplate current. The branch circuit overcurrent device is sized separately per NEC Table 430.52, which may allow a breaker significantly larger than the conductor ampacity to accommodate motor starting current.
Other Amperage Levels
Browse wire sizing recommendations for other standard amperages. Each page provides complete distance tables for both copper and aluminum at all standard voltages.
Tools and Training for Electricians
Quality resources for your next wiring project.
Some links above are affiliate links — if you buy through them we may earn a small commission at no extra cost to you. As an Amazon Associate we earn from qualifying purchases.