Voltage drop scenario

Wire size for 100 amps at 100 feet

At 240 V single-phase, copper 4 AWG keeps a 100 amps load at about 2.57% drop over 100 feet — within the 3% design target used here. Adjust material, voltage, or phase below.

Phase

Voltage drop

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Enter a run to estimate voltage drop.

Estimated results
Voltage drop —
Percent drop —
Voltage at load —
Resistance used —
NEC 3% branch NEC 5% total
Estimate only — not a substitute for a licensed electrician. Based on NEC Ch.9 Table 8 DC resistance. Does not model ampacity, temperature derating, power factor, or continuous-load factors. Verify against the current NEC and your local code / AHJ.

100 amps at 100 feet: the short answer

At 240 V single-phase over 100 feet, a 100 amps load needs copper 4 AWG to stay near 2.57% — inside the 3% design target used for this comparison. For comparison, copper 12 AWG would drop about 38.60 V (16.08%) on the same run.

Before you pull wire

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.

Where this 100 amps, 100 feet scenario fits

A 100 A, 100-foot run is usually feeder or subpanel territory rather than an ordinary branch circuit. It may serve a detached garage, workshop, addition, or other distribution point, which brings grounding, disconnect, load calculation, and feeder rules beyond simple voltage drop.

This page deliberately uses a conservative 3% design target so conductor choices can be compared on equal terms. NEC voltage-drop language is generally informational, and feeder-plus-branch performance must be considered together. Treat the result as one input to a feeder design, not as a complete 100 A feeder specification.

Distance changes the conductor decision

Copper recommendations for a 100 amps load at 240 V single-phase, using the same 3% target
One-way distance Suggested copper size Calculated drop
50 ft 6 AWG 2.05%
100 ft 4 AWG 2.57%
150 ft 2 AWG 2.43%
200 ft 1 AWG 2.57%

Checks specific to this project

  • Complete a load calculation instead of assuming the feeder will continuously carry 100 A.
  • Check feeder conductor, neutral, equipment-grounding conductor, and disconnect requirements separately.
  • Review whether dwelling-service or feeder allowances apply; this calculator does not apply them.
  • Have the conductor and equipment terminations checked for material, temperature rating, and physical size.

FAQ

What wire size do I need for 100 amps at 100 feet?

For voltage drop at 240 V single-phase, copper 4 AWG keeps a 100 amps load near 2.57% 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.

How much voltage drop does 100 amps over 100 feet have?

On a baseline copper 12 AWG at 240 V single-phase, 100 amps over 100 feet drops about 38.60 V (16.08%). Larger conductors drop less; the calculator shows the exact figure for any size.

Is this voltage-drop sizing the same as ampacity sizing?

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).

Sources & verification

Source links and page assumptions reviewed .

Wire size for 20 amps at 100 feetWire size for 30 amps at 150 feetWire size for 50 amps at 100 feetWire size for 50 amps at 200 feetWire size for 60 amps at 100 feet All voltage drop scenarios How to calculate voltage drop

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.