For a 300-amp circuit running 100 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 300A. A shorter run of 50 feet at the same voltage often allows 350 kcmil. Treat this as an estimate, not an install spec.
300A at 100ft · 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)1.91V (1.59%)
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) ≥ 300A ✓
The conductor needs to carry at least 300A 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 2.23V (1.86% 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 300A 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 1.34V (1.12% on 120V). More expensive wire but better performance and more headroom for future load increases.
Wattage at This Amperage
300A at 120V delivers 36,000 watts (DC / resistive load). See conversion.
300A at 100ft 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.
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) Informational Note 4 recommends ≤3% for branch circuits and ≤5% total (feeder + branch). These are performance recommendations, not hard code requirements.
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.
Copper and aluminum are picked independently against the same drop target on this site; neither pick implies ampacity equivalence with the other. At 300A, 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.
