What Wire Size for 278 Amps at 300 Feet?

750 kcmil copper is a typical pick for 278A at 300 feet on 120V under a 3% drop target. It balances NEC branch-circuit ampacity and voltage drop over distance. Drop with 750 kcmil at these inputs: 2.47V (2.06%). Real-world sizing also depends on insulation temperature rating, cable type, and install conditions.

278A at 300ft · 120V three-phase L-L · 3% drop target
750 kcmil copper
On a 240V circuit (copper)300 kcmil
Voltage drop (120V, copper)2.47V (2.06%)

No aluminum row: every aluminum size in our reference table sits past the 3% drop target at 300 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
750 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

750 kcmil branch-circuit OCP (475A) ≥ 278A ✓

The conductor needs to carry at least 278A 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 = (√3 × L × I × R) ÷ (1000 × V) × 100 (three-phase L-L; √3 factor)

(√3 × 300 × 278 × 0.0171) ÷ (1000 × 120) × 100 = 2.06%

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 3.73V (3.11% 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 278A 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 3.11% 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

278A at 120V delivers 33,360 watts (DC / resistive load). See conversion.

Related Calculations

trending_down V-drop: 750 kcmil, 278A, 300ft electric_bolt 278A to watts at 120V functions Wire sizing formula

Frequently Asked Questions

278A at 300ft 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.
Voltage drop scales linearly with distance: doubling the one-way run length doubles the drop in volts. At 278A on 120V, a 300ft 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.
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 278A continuous load that points the sizing math at the 347.5A 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.
Yes, but you may need thicker wire. At 600ft on 120V, check the wire size calculator. You may need to go up one or two gauges.
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.
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