What Wire Size for 12.5 Amps at 100 Feet?

For a 12.5-amp circuit running 100 feet on 120V, 10 AWG copper is the smallest gauge in our table that both stays within the 3% drop target and covers the branch-circuit OCP cap for 12.5A. A shorter run of 50 feet at the same voltage often allows 14 AWG. Treat this as an estimate, not an install spec.

12.5A at 100ft · 120V three-phase L-L · 3% drop target
10 AWG copper
On a 240V circuit (copper)14 AWG
Voltage drop (120V, copper)2.68V (2.24%)

No aluminum row: every aluminum size in our reference table sits past the 3% drop target at 100 feet on 120V, or the amperage is below the 30A residential threshold where aluminum is not a typical pick. On a higher source voltage, a shorter run, or a looser drop target, aluminum is still the standard feeder material at higher amperages.

check_circle Within the 3% branch and 5% feeder+branch total drop targets
10 AWG Cu

Assumes a 120V source on a three-phase L-L circuit and a 3% voltage-drop target. Each material is picked independently against the same target, so the copper and aluminum results are two separate recommendations, not an ampacity equivalence. Switch to single-phase / DC →

How Wire Size Is Determined

Step 1: NEC Branch-Circuit Ampacity

10 AWG branch-circuit OCP (30A under NEC 240.4(D)) ≥ 12.5A ✓

The conductor needs to carry at least 12.5A without going past its temperature rating, and the OCP protecting it needs to respect the NEC branch-circuit cap. Under the typical assumptions used in this table (copper, 75°C termination, no bundling or ambient derates), 10 AWG sits at a branch-circuit OCP of 30A because the NEC 240.4(D) small-conductor rule caps it below the 35A 75°C ampacity table value. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (10 AWG NM-B = 30A), bundling more than three current-carrying conductors requires a 310.15(C)(1) adjustment, ambient temperatures above 30°C require a 310.15(B) correction, and 60°C terminations on typical residential equipment can pull the usable value lower still. Use the nameplate and local code for the actual install value.

Step 2: Voltage Drop Check

%VD = (√3 × L × I × R) ÷ (1000 × V) × 100 (three-phase L-L; √3 factor)

(√3 × 100 × 12.5 × 1.24) ÷ (1000 × 120) × 100 = 2.24%

NEC 210.19(A) Informational Note 4 recommends ≤ 3% for branch circuits and ≤ 5% for feeder + branch total as performance targets, not hard code requirements. This run sits within the 3% target used for this calculation.

Practical Information

What If You Go One Size Smaller?

Using 12 AWG (one size thinner) at these inputs gives a voltage drop of 4.29V (3.57% on 120V), and its branch-circuit OCP cap under typical conditions is 20A.

Limiting factor here: voltage drop, not ampacity. 12 AWG is still above the 12.5A load at its 20A branch-circuit OCP cap, so the conductor temperature margin is fine for this run. What pushes it off this page's pick is the 3.57% drop sitting past the 3% target, which is a performance recommendation (NEC 210.19(A) Informational Note 4), not a code requirement. On shorter runs or at higher source voltage the same gauge would often clear the drop target too.

What If You Go One Size Larger?

Using 8 AWG (one size thicker) would reduce voltage drop to 1.68V (1.4% on 120V). More expensive wire but better performance and more headroom for future load increases.

Wattage at This Amperage

12.5A at 120V delivers 1,500 watts (DC / resistive load). See conversion.

Related Calculations

trending_down V-drop: 10 AWG, 12.5A, 100ft electric_bolt 12.5A to watts at 120V functions Wire sizing formula

Frequently Asked Questions

12.5A at 100ft on 120V is commonly served by 10 AWG copper to land under the 3% voltage-drop target, under the typical 75°C-termination assumptions used in this table. Actual install sizing also depends on conductor material, insulation and termination temperature rating, cable type, ambient and bundling conditions, and local code.
Copper wire pricing tracks the LME copper spot price and varies with insulation type, cable assembly (THHN, NM-B, MC, SE, USE), and quantity. Check current pricing with a local electrical supply house or distributor catalog; commodity-driven numbers inlined on a calculator page age quickly.
NEC 210.19(A) Informational Note 4 recommends ≤3% for branch circuits and ≤5% total (feeder + branch). These are performance recommendations, not hard code requirements.
Copper and aluminum are picked independently against the same drop target on this site; neither pick implies ampacity equivalence with the other. At 12.5A residential branch-circuit territory, copper is the default. Aluminum in 14/12/10 AWG is not typical for residential branches, both for code reasons (the NEC 240.4(D) small-conductor rule caps aluminum branch OCP tighter than copper) and because termination and anti-oxidation requirements add cost at that scale.
It depends on which factor the thinner gauge violates. If its branch-circuit ampacity is still at or above the load, the limiting factor is usually voltage drop (a performance recommendation per NEC 210.19(A) Informational Note 4, not a hard code requirement) and the symptom is dimming lights, motor startup issues, or wasted energy as I²R losses. If the thinner gauge is actually below the load's ampacity ceiling at the relevant termination temperature, that is a conductor-heating / code compliance issue, and the wire should not be used for that load. A calculator page cannot tell you which category applies to your install: verify against the conductor type, termination temperature, and install conditions.
This calculator provides estimates for reference purposes only. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before performing any electrical work.
person Written by Sean Day verified Reviewed by WireResult update Last updated Apr 11, 2026