AmpereTallyWhether 100 amps is enough, worked out the way an inspector does it.

What size aluminum wire for a 50 amp breaker 125 feet away?

50 A, 125 ft, aluminum
4 AWG
ampacity alone allows 6 AWG, but at 125 ft that drops 4.2% — voltage drop sets the size here
50 A, 125 ft, aluminum2.65 %Voltage drop at t…3 %Guideline
Wire size4 AWG
Smallest by ampacity aloneNEC 310.16, 75°C aluminum6 AWG
Voltage drop at this size6.3 V of 240 V over 125 ft2.65%
GuidelineNEC 210.19(A) informational note — a recommendation, not a rule3%
Ampacity of 4 AWGcarrying 50 A65A
Circular milsthe number the voltage-drop formula actually uses41,740

Notes

  • **Two separate questions, and only one of them is code.** Ampacity asks whether the wire survives the current — that is NEC Table 310.16 and it is mandatory. Voltage drop asks whether useful voltage reaches the far end — that is a 3% *recommendation*, not a requirement. At 125 ft the second question is the one that bites: 6 AWG is legal but arrives 4.2% low.
  • **NEC 240.4(D) overrides the ampacity table for small wire.** 14 AWG is capped at a 15 A breaker, 12 AWG at 20 A and 10 AWG at 30 A, no matter what 310.16 shows. Reading the ampacity column alone and stopping there is the single most common mistake in this whole subject.
  • **Aluminum needs roughly two sizes up on the same load.** It carries less current per gauge and has about 1.6 times the resistance, so both constraints move together. It is cheaper per foot and it is what utilities use for service entrances — but every termination has to be listed for aluminum, and antioxidant compound is not optional.
  • **Distance is the variable people leave out.** The same 50 A circuit takes 6 AWG in a panel-room subpanel and 4 AWG out to a detached garage. Measure the actual conductor path — around the joists and up the wall — not the straight line across the yard.
  • Temperature column matters too. These figures use the 75°C column, which is what the *terminations* on ordinary breakers and lugs are rated for. A 90°C cable does not let you use the 90°C column unless everything it lands on is also rated 90°C, and on residential gear it almost never is.

50 A over 125 feet in aluminum

4 AWG. Voltage drop lands at 2.65, against a 3% guideline.

Two questions, and only one of them is code

Ampacity asks whether the wire survives the current. That is NEC Table 310.16 and it is mandatory.

Voltage drop asks whether useful voltage reaches the far end. That is a 3% recommendation in an informational note — not a requirement.

Short runs are decided by the first. Long runs are decided by the second, and that is why an ampacity chart on its own gives the wrong answer past about 100 feet.

The rule that overrides the table

NEC 240.4(D) caps the breaker on small wire no matter what the ampacity column shows:

Wire Maximum breaker
14 AWG 15 A
12 AWG 20 A
10 AWG 30 A

Reading 310.16 alone and stopping there is the most common mistake in this subject.

Aluminum runs about two sizes larger

It carries less current per gauge and has roughly 1.6 times the resistance, so both constraints move the same way.

It is cheaper per foot and it is what utilities use for service entrances. But every termination has to be listed for aluminum, and antioxidant compound is not optional.

Measure the wire, not the yard

The same 50 A circuit takes one size in a panel-room subpanel and another out to a detached garage.

Measure the actual conductor path — around the joists, up the wall, along the trench — not the straight line across the lawn.

The temperature column

These figures use the 75°C column, which is what the terminations on ordinary breakers and lugs are rated for.

A 90°C cable does not let you use the 90°C column unless everything it lands on is rated 90°C too — and on residential gear it almost never is.

Nearby sizes