What Wire Size for 4.25 Amps at 400 Feet?

8 AWG copper is a typical pick for 4.25A at 400 feet on 120V under a 3% drop target. It balances NEC branch-circuit ampacity and voltage drop over distance. Drop with 8 AWG at these inputs: 2.65V (2.2%). Real-world sizing also depends on insulation temperature rating, cable type, and install conditions.

4.25A at 400ft · 120V single-phase / DC · 3% drop target
8 AWG copper
On a 240V circuit (copper)12 AWG
Voltage drop (120V, copper)2.65V (2.2%)

No aluminum row: every aluminum size in our reference table sits past the 3% drop target at 400 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.

check_circle Within the 3% branch and 5% feeder+branch total drop targets
8 AWG Cu

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

8 AWG branch-circuit OCP (50A) ≥ 4.25A ✓

The conductor needs to carry at least 4.25A 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), 8 AWG sits at a branch-circuit OCP of 50A. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (8 AWG NM-B = 40A), 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)

(2 × 400 × 4.25 × 0.778) ÷ (1000 × 120) × 100 = 2.2%

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 10 AWG (one size thinner) at these inputs gives a voltage drop of 4.22V (3.51% on 120V), and its branch-circuit OCP cap under typical conditions is 30A.

Limiting factor here: voltage drop, not ampacity. 10 AWG is still above the 4.25A load at its 30A branch-circuit OCP cap, so the conductor temperature margin is fine for this run. What pushes it off this page's pick is the 3.51% 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 6 AWG (one size thicker) would reduce voltage drop to 1.67V (1.39% on 120V). More expensive wire but better performance and more headroom for future load increases.

Wattage at This Amperage

4.25A at 120V delivers 510 watts (DC / resistive load). See conversion.

Related Calculations

trending_down V-drop: 8 AWG, 4.25A, 400ft electric_bolt 4.25A to watts at 120V functions Wire sizing formula

Frequently Asked Questions

4.25A at 400ft on 120V is commonly served by 8 AWG 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 800ft 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 4.25A continuous load that points the sizing math at the 5.31A 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 4.25A residential branch-circuit territory, copper is the default. Aluminum in 14/12/10 AWG is not typical for residential branches, both for code reasons (the NEC 240.4(D) small-conductor rule caps aluminum branch OCP tighter than copper) and because termination and anti-oxidation requirements add cost at that scale.
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.
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.
person Written by Sean Day verified Reviewed by WireResult update Last updated Apr 11, 2026