What Wire Size for 366.67 Amps at 250 Feet?

For a 366.67-amp circuit running 250 feet on 120V, 750 kcmil copper is the smallest gauge in our table that both stays within the 3% drop target and covers the branch-circuit OCP cap for 366.67A. A shorter run of 125 feet at the same voltage often allows 500 kcmil. Treat this as an estimate, not an install spec.

366.67A at 250ft · 120V single-phase / DC · 3% drop target

750 kcmil copper

On a 240V circuit (copper)500 kcmil

Voltage drop (120V, copper)3.14V (2.61%)

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

Amps

Feet

Gauge (Cu / Al)

750 kcmil Cu

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

750 kcmil branch-circuit OCP (475A) ≥ 366.67A ✓

The conductor needs to carry at least 366.67A 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), 750 kcmil sits at a branch-circuit OCP of 475A. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (750 kcmil NM-B = 400A), 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)

(2 × 250 × 366.67 × 0.0171) ÷ (1000 × 120) × 100 = 2.61%

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 500 kcmil (one size thinner) at these inputs gives a voltage drop of 4.73V (3.94% on 120V), and its branch-circuit OCP cap under typical conditions is 380A.

Limiting factor here: voltage drop, not ampacity. 500 kcmil is still above the 366.67A load at its 380A branch-circuit OCP cap, so the conductor temperature margin is fine for this run. What pushes it off this page's pick is the 3.94% 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?

The recommended gauge is already the largest available option.

Wattage at This Amperage

366.67A at 120V delivers 44,000.4 watts (DC / resistive load). See conversion.

Related Calculations

Frequently Asked Questions

What wire size for 366.67 amps at 250 feet on 120V?expand_more

366.67A at 250ft on 120V is commonly served by 750 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.

Why does distance affect wire size?expand_more

Voltage drop scales linearly with distance: doubling the one-way run length doubles the drop in volts. At 366.67A on 120V, a 250ft run is often served by 750 kcmil 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.

What if I use a thinner wire than recommended?expand_more

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.

Can I run 366.67A on a longer distance?expand_more

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.

What is the NEC voltage drop recommendation?expand_more

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.