What Wire Size for 285.28 Amps at 25 Feet?

For a 285.28-amp circuit running 25 feet on 120V, 350 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 285.28A. A shorter run of 25 feet at the same voltage often allows 350 kcmil. Treat this as an estimate, not an install spec.

285.28A at 25ft · 120V three-phase L-L · 3% drop target

350 kcmil copper

Aluminum option500 kcmil

On a 240V circuit (copper)350 kcmil

Voltage drop (120V, copper)0.4533V (0.3778%)

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

Amps

Feet

Gauge (Cu / Al)

350 kcmil Cu / 500 kcmil Al

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

350 kcmil branch-circuit OCP (310A) ≥ 285.28A ✓

The conductor needs to carry at least 285.28A 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), 350 kcmil sits at a branch-circuit OCP of 310A. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (350 kcmil NM-B = 260A), 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 × 25 × 285.28 × 0.0367) ÷ (1000 × 120) × 100 = 0.3778%

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 300 kcmil (one size thinner) at these inputs gives a voltage drop of 0.5299V (0.4416% on 120V), and its branch-circuit OCP cap under typical conditions is 285A.

Limiting factor here: branch-circuit ampacity. 300 kcmil has a branch-circuit OCP cap of 285A under the typical 75°C-termination assumptions used here, which is below the 285.28A 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 500 kcmil (one size thicker) would reduce voltage drop to 0.3187V (0.2656% on 120V). More expensive wire but better performance and more headroom for future load increases.

Wattage at This Amperage

285.28A at 120V delivers 34,233.6 watts (DC / resistive load). See conversion.

Related Calculations

Frequently Asked Questions

What wire size for 285.28 amps at 25 feet on 120V?expand_more

285.28A at 25ft on 120V is commonly served by 350 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.

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 285.28A continuous load that points the sizing math at the 356.6A 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.

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 285.28A on 120V, a 25ft run is often served by 350 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.

Can I run 285.28A on a longer distance?expand_more

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.

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.