For a 12-amp circuit running 25 feet on 120V, 14 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 12A. A shorter run of 25 feet at the same voltage often allows 14 AWG. Treat this as an estimate, not an install spec.
12A at 25ft · 120V three-phase L-L · 3% drop target
14 AWG copper
On a 240V circuit (copper)14 AWG
Voltage drop (120V, copper)1.63V (1.36%)
No aluminum row: every aluminum size in our reference table sits past the 3% drop target at 25 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.
Use this citation when referencing this page.
check_circle
Within the 3% branch and 5% feeder+branch total drop targets
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
14 AWG branch-circuit OCP (15A under NEC 240.4(D)) ≥ 12A ✓
The conductor needs to carry at least 12A 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), 14 AWG sits at a branch-circuit OCP of 15A because the NEC 240.4(D) small-conductor rule caps it below the 20A 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 (14 AWG NM-B = 15A), 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)
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?
The recommended gauge is already the smallest suitable option in our table.
What If You Go One Size Larger?
Using 12 AWG (one size thicker) would reduce voltage drop to 1.03V (0.8573% on 120V). More expensive wire but better performance and more headroom for future load increases.
Wattage at This Amperage
12A at 120V delivers 1,440 watts (DC / resistive load). See conversion.
12A at 25ft on 120V is commonly served by 14 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.
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.
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.
Voltage drop scales linearly with distance: doubling the one-way run length doubles the drop in volts. At 12A on 120V, a 25ft run is often served by 14 AWG to land under the 3% drop target, a run half that length can sometimes use one gauge thinner, and a run double that length usually needs one or two gauges thicker. Ampacity is set by the conductor itself (Table 310.16 at the applicable termination temperature), so the binding constraint is ampacity on short runs and voltage drop on long runs.
Yes, but you may need thicker wire. At 50ft on 120V, check the wire size calculator. You may need to go up one or two gauges.
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.
