2 AWG copper is a typical pick for 31.25A at 250 feet on 120V under a 3% drop target. It balances NEC branch-circuit ampacity and voltage drop over distance. Drop with 2 AWG at these inputs: 3.03V (2.53%). Real-world sizing also depends on insulation temperature rating, cable type, and install conditions.
31.25A at 250ft · 120V single-phase / DC · 3% drop target
2 AWG copper
Aluminum option1 AWG
On a 240V circuit (copper)4 AWG
Voltage drop (120V, copper)3.03V (2.53%)
Use this citation when referencing this page.
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Within the 3% branch and 5% feeder+branch total drop targets
Assumes a 120V source on a single-phase / DC 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 three-phase L-L →
How Wire Size Is Determined
Step 1: NEC Branch-Circuit Ampacity
2 AWG branch-circuit OCP (115A) ≥ 31.25A ✓
The conductor needs to carry at least 31.25A 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), 2 AWG sits at a branch-circuit OCP of 115A. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (2 AWG NM-B = 95A), 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 = (2 × L × I × R) ÷ (1000 × V) × 100 (single-phase / DC; round-trip factor of 2)
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 3 AWG (one size thinner) at these inputs gives a voltage drop of 3.83V (3.19% on 120V), and its branch-circuit OCP cap under typical conditions is 100A.
Limiting factor here: voltage drop, not ampacity. 3 AWG is still above the 31.25A load at its 100A branch-circuit OCP cap, so the conductor temperature margin is fine for this run. What pushes it off this page's pick is the 3.19% 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 1 AWG (one size thicker) would reduce voltage drop to 2.41V (2.01% on 120V). More expensive wire but better performance and more headroom for future load increases.
Wattage at This Amperage
31.25A at 120V delivers 3,750 watts (DC / resistive load). See conversion.
31.25A at 250ft on 120V is commonly served by 2 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 and aluminum are picked independently against the same drop target on this site; neither pick implies ampacity equivalence with the other. At 31.25A, both materials are valid. Copper is common for branch circuits at this range; AA-8000 series aluminum shows up for longer runs and sub-panel feeders where the cost savings outweigh the gauge upsize. Aluminum has lower conductivity than copper, so when each material is run through the drop-target pick independently, the aluminum result typically lands one to two gauges larger than the copper result for the same duty. That gap is the result of running both picks against the same drop-target constraint, not an ampacity-equivalence rule.
Yes, but you may need thicker wire. At 500ft on 120V, check the wire size calculator. You may need to go up one or two gauges.
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) (branch circuits) and 215.3 (feeders) size the conductor and overcurrent device at not less than 125% of the continuous load plus 100% of any non-continuous load. For a 31.25A continuous load that points the sizing math at the 39.06A figure, but the actual conductor and breaker pick still depends on termination temperature rating, cable type, bundling and ambient conditions, and any 240.4(D) or 240.4(B) provisions. Treat this as the input to a sizing decision, not the output.
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.
