What Wire Size for 291.67 Amps at 75 Feet?

For 291.67 amps at 75 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 350 kcmil, 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.

291.67A at 75ft · 120V single-phase / DC · 3% drop target

350 kcmil copper

Aluminum option500 kcmil

On a 240V circuit (copper)350 kcmil

Voltage drop (120V, copper)1.61V (1.34%)

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

Amps

Feet

Gauge (Cu / Al)

350 kcmil Cu / 500 kcmil Al

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

350 kcmil branch-circuit OCP (310A) ≥ 291.67A ✓

The conductor needs to carry at least 291.67A 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)

(2 × 75 × 291.67 × 0.0367) ÷ (1000 × 120) × 100 = 1.34%

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 1.88V (1.56% 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 291.67A 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 1.13V (0.9406% on 120V). More expensive wire but better performance and more headroom for future load increases.

Wattage at This Amperage

291.67A at 120V delivers 35,000.4 watts (DC / resistive load). See conversion.

Related Calculations

Frequently Asked Questions

What wire size for 291.67 amps at 75 feet on 120V?expand_more

291.67A at 75ft 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.

What is the NEC voltage drop recommendation?expand_more

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.

Can I run 291.67A on a longer distance?expand_more

Yes, but you may need thicker wire. At 150ft on 120V, check the wire size calculator. You may need to go up one or two gauges.

What if I use a thinner wire than recommended?expand_more

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.

How much does 350 kcmil wire cost?expand_more

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.