For a 312.5-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 312.5A. A shorter run of 125 feet at the same voltage often allows 500 kcmil. Treat this as an estimate, not an install spec.
312.5A at 250ft · 120V single-phase / DC · 3% drop target
750 kcmil copper
On a 240V circuit (copper)500 kcmil
Voltage drop (120V, copper)2.67V (2.23%)
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.
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 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) ≥ 312.5A ✓
The conductor needs to carry at least 312.5A 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)
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 4.03V (3.36% 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 312.5A 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 3.36% 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
312.5A at 120V delivers 37,500 watts (DC / resistive load). See conversion.
312.5A 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.
Copper and aluminum are picked independently against the same drop target on this site; neither pick implies ampacity equivalence with the other. At 312.5A, 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 312.5A 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.
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 312.5A continuous load that points the sizing math at the 390.63A 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.
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.
