For a 255.21-amp circuit running 100 feet on 120V, 300 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 255.21A. A shorter run of 50 feet at the same voltage often allows 300 kcmil. Treat this as an estimate, not an install spec.
255.21A at 100ft · 120V single-phase / DC · 3% drop target
300 kcmil copper
Aluminum option500 kcmil
On a 240V circuit (copper)300 kcmil
Voltage drop (120V, copper)2.19V (1.82%)
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
300 kcmil branch-circuit OCP (285A) ≥ 255.21A ✓
The conductor needs to carry at least 255.21A 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), 300 kcmil sits at a branch-circuit OCP of 285A. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (300 kcmil NM-B = 240A), 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 250 kcmil (one size thinner) at these inputs gives a voltage drop of 2.63V (2.19% on 120V), and its branch-circuit OCP cap under typical conditions is 255A.
Limiting factor here: branch-circuit ampacity. 250 kcmil has a branch-circuit OCP cap of 255A under the typical 75°C-termination assumptions used here, which is below the 255.21A 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 350 kcmil (one size thicker) would reduce voltage drop to 1.87V (1.56% on 120V). More expensive wire but better performance and more headroom for future load increases.
Wattage at This Amperage
255.21A at 120V delivers 30,625.2 watts (DC / resistive load). See conversion.
255.21A at 100ft on 120V is commonly served by 300 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 255.21A on 120V, a 100ft run is often served by 300 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.
Yes, but you may need thicker wire. At 200ft on 120V, check the wire size calculator. You may need to go up one or two gauges.
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 255.21A continuous load that points the sizing math at the 319.01A 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 255.21A, 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.
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.
