Voltage drop scenario
At 240 V single-phase, copper 10 AWG keeps a 20 amps load at about 2.02% drop over 100 feet — within the 3% design target used here. Adjust material, voltage, or phase below.
Voltage drop
—Enter a run to estimate voltage drop.
| Voltage drop | — |
|---|---|
| Percent drop | — |
| Voltage at load | — |
| Resistance used | — |
At 240 V single-phase over 100 feet, a 20 amps load needs copper 10 AWG to stay near 2.02% — inside the 3% design target used for this comparison. For comparison, copper 12 AWG would drop about 7.72 V (3.22%) on the same run.
This is a voltage-drop estimate only. The conductor must also meet ampacity, temperature derating, and overcurrent rules — use the larger of the voltage-drop size and the ampacity size. Aluminum of the same size drops more, so it usually needs to go up one or two sizes. Always confirm with a licensed electrician and your local code (AHJ).
Sized for voltage drop using NEC Ch.9 Table 8 DC resistance. The 3% figure is a common design target from informational guidance, not a substitute for a code review.
This is a useful planning case for a modest 240 V load in a detached garage, workshop, shed, or addition where the panel is roughly 100 feet away. It is not a promise that a particular appliance belongs on a 20 A circuit; the equipment nameplate and local code decide that.
At this current, the practical choice is often whether to accept a small conductor upgrade now or live with more voltage loss for the life of the circuit. Compare the material cost against the harder-to-change parts of the job: trenching, conduit, wall access, and future load growth.
| One-way distance | Suggested copper size | Calculated drop |
|---|---|---|
| 50 ft | 14 AWG | 2.56% |
| 100 ft | 10 AWG | 2.02% |
| 150 ft | 8 AWG | 1.91% |
| 200 ft | 8 AWG | 2.55% |
For voltage drop at 240 V single-phase, copper 10 AWG keeps a 20 amps load near 2.02% over 100 feet, meeting the 3% design target used on this page. Ampacity, derating, equipment instructions, and local code may require a different size — confirm with a licensed electrician.
On a baseline copper 12 AWG at 240 V single-phase, 20 amps over 100 feet drops about 7.72 V (3.22%). Larger conductors drop less; the calculator shows the exact figure for any size.
No. This page sizes for voltage drop only, using NEC Chapter 9 Table 8 DC resistance. The conductor must also satisfy ampacity, temperature derating, and overcurrent-protection rules. Use the larger of the voltage-drop size and the ampacity size, and verify with your local code (AHJ).
Source links and page assumptions reviewed .
Estimate only, based on NEC Chapter 9 Table 8 DC resistance. Not a substitute for a licensed electrician or your local code (AHJ). Voltage-drop sizing does not replace ampacity, temperature derating, power factor, or continuous-load checks. Verify against the current NEC and local code before wiring. Based on NEC Ch.9 Table 8.