For 278 amps at 175 feet on a 120V circuit, 350 kcmil copper is a common starting point under a 3% voltage-drop target. On a 240V circuit the same current often allows 300 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.
278A at 175ft · 120V three-phase L-L · 3% drop target
350 kcmil copper
Aluminum option500 kcmil
On a 240V circuit (copper)300 kcmil
Voltage drop (120V, copper)3.09V (2.58%)
Use this citation when referencing this page.
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Within the 3% branch and 5% feeder+branch total drop targets
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) ≥ 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), 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)
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 3.61V (3.01% on 120V), and its branch-circuit OCP cap under typical conditions is 285A.
Limiting factor here: voltage drop, not ampacity. 300 kcmil is still above the 278A load at its 285A branch-circuit OCP cap, so the conductor temperature margin is fine for this run. What pushes it off this page's pick is the 3.01% 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?
Using 500 kcmil (one size thicker) would reduce voltage drop to 2.17V (1.81% on 120V). More expensive wire but better performance and more headroom for future load increases.
Wattage at This Amperage
278A at 120V delivers 33,360 watts (DC / resistive load). See conversion.
278A at 175ft 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.
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.
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.
Copper and aluminum are picked independently against the same drop target on this site; neither pick implies ampacity equivalence with the other. At 278A, aluminum is the industry standard for sub-panel feeders, service entrance, and utility drops. AA-8000 series aluminum is the modern feeder material; copper is still used where space is tight or terminations are copper-only. Aluminum has lower conductivity than copper, so when each material is run through the drop-target pick independently, the aluminum result typically lands one to two gauges larger than the copper result for the same duty. That gap is the result of running both picks against the same drop-target constraint, not an ampacity-equivalence rule. The install still needs anti-oxidant compound and aluminum-rated lugs.
Voltage drop scales linearly with distance: doubling the one-way run length doubles the drop in volts. At 278A on 120V, a 175ft 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.
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.
