What Wire Size for 122.55 Amps at 400 Feet?
For 122.55 amps at 400 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 4/0 AWG, 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.
Use this citation when referencing this page.
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
The conductor needs to carry at least 122.55A 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 = (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 300 kcmil (one size thinner) at these inputs gives a voltage drop of 4.21V (3.5% 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 122.55A 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 3.5% 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 2.53V (2.11% on 120V). More expensive wire but better performance and more headroom for future load increases.
Wattage at This Amperage
122.55A at 120V delivers 14,706 watts (DC / resistive load). See conversion.