What Wire Size for 320 Amps at 250 Feet?

For 320 amps at 250 feet on a 120V circuit, 500 kcmil copper is a common starting point under a 3% voltage-drop target. On a 240V circuit the same current often allows 500 kcmil, because the 3% allowable drop is a larger number of volts at higher source voltage. Actual install sizing still depends on conductor material, insulation/termination temperature, cable type, ambient and bundling conditions, and local code.

320A at 250ft · 120V three-phase L-L · 3% drop target
500 kcmil copper
On a 240V circuit (copper)500 kcmil
Voltage drop (120V, copper)3.57V (2.98%)

No aluminum row: every aluminum size in our reference table sits past the 3% drop target at 250 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
500 kcmil 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

500 kcmil branch-circuit OCP (380A) ≥ 320A ✓

The conductor needs to carry at least 320A 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), 500 kcmil sits at a branch-circuit OCP of 380A. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (500 kcmil NM-B = 320A), 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 × 250 × 320 × 0.0258) ÷ (1000 × 120) × 100 = 2.98%

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 350 kcmil (one size thinner) at these inputs gives a voltage drop of 5.09V (4.24% on 120V), and its branch-circuit OCP cap under typical conditions is 310A.

Limiting factor here: branch-circuit ampacity. 350 kcmil has a branch-circuit OCP cap of 310A under the typical 75°C-termination assumptions used here, which is below the 320A load. For this load it shouldn't be used without reassessing against the actual termination temperature, cable type, ambient conditions, and any 240.4(D) or 240.4(B) provisions.

What If You Go One Size Larger?

Using 750 kcmil (one size thicker) would reduce voltage drop to 2.37V (1.97% on 120V). More expensive wire but better performance and more headroom for future load increases.

Wattage at This Amperage

320A at 120V delivers 38,400 watts (DC / resistive load). See conversion.

Related Calculations

trending_down V-drop: 500 kcmil, 320A, 250ft electric_bolt 320A to watts at 120V functions Wire sizing formula

Frequently Asked Questions

320A at 250ft on 120V is commonly served by 500 kcmil 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.
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) (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 320A continuous load that points the sizing math at the 400A 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.
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.
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