For a 399.5-amp circuit running 100 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 399.5A. A shorter run of 50 feet at the same voltage often allows 750 kcmil. Treat this as an estimate, not an install spec.
399.5A at 100ft · 120V single-phase / DC · 3% drop target
750 kcmil copper
On a 240V circuit (copper)750 kcmil
Voltage drop (120V, copper)1.37V (1.14%)
No aluminum row: every aluminum size in our reference table sits past the 3% drop target at 100 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) ≥ 399.5A ✓
The conductor needs to carry at least 399.5A 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 2.06V (1.72% on 120V), and its branch-circuit OCP cap under typical conditions is 380A.
Limiting factor here: branch-circuit ampacity. 500 kcmil has a branch-circuit OCP cap of 380A under the typical 75°C-termination assumptions used here, which is below the 399.5A 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?
The recommended gauge is already the largest available option.
Wattage at This Amperage
399.5A at 120V delivers 47,940 watts (DC / resistive load). See conversion.
399.5A at 100ft 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.
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 399.5A continuous load that points the sizing math at the 499.38A 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.
It depends on which factor the thinner gauge violates. If its branch-circuit ampacity is still at or above the load, the limiting factor is usually voltage drop (a performance recommendation per NEC 210.19(A) Informational Note 4, not a hard code requirement) and the symptom is dimming lights, motor startup issues, or wasted energy as I²R losses. If the thinner gauge is actually below the load's ampacity ceiling at the relevant termination temperature, that is a conductor-heating / code compliance issue, and the wire should not be used for that load. A calculator page cannot tell you which category applies to your install: verify against the conductor type, termination temperature, and install conditions.
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.
