What Wire Size for 366.67 Amps at 125 Feet?

500 kcmil copper is a typical pick for 366.67A at 125 feet on 120V under a 3% drop target. It balances NEC branch-circuit ampacity and voltage drop over distance. Drop with 500 kcmil at these inputs: 2.37V (1.97%). Real-world sizing also depends on insulation temperature rating, cable type, and install conditions.

366.67A at 125ft · 120V single-phase / DC · 3% drop target

500 kcmil copper

Aluminum option750 kcmil

On a 240V circuit (copper)500 kcmil

Voltage drop (120V, copper)2.37V (1.97%)

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

Amps

Feet

Gauge (Cu / Al)

500 kcmil Cu / 750 kcmil Al

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

500 kcmil branch-circuit OCP (380A) ≥ 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), 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 = (2 × L × I × R) ÷ (1000 × V) × 100 (single-phase / DC; round-trip factor of 2)

(2 × 125 × 366.67 × 0.0258) ÷ (1000 × 120) × 100 = 1.97%

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 3.36V (2.8% 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 366.67A 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 1.57V (1.31% on 120V). More expensive wire but better performance and more headroom for future load increases.

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 125 feet on 120V?expand_more

366.67A at 125ft 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.

Can I run 366.67A on a longer distance?expand_more

Yes, but you may need thicker wire. At 250ft on 120V, check the wire size calculator. You may need to go up one or two gauges.

Do I need to upsize wire for continuous loads?expand_more

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 366.67A continuous load that points the sizing math at the 458.34A 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.

How much does 500 kcmil wire cost?expand_more

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.

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.