For a 312.5-amp circuit running 175 feet on 120V, 500 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 312.5A. A shorter run of 87.5 feet at the same voltage often allows 500 kcmil. Treat this as an estimate, not an install spec.
312.5A at 175ft · 120V single-phase / DC · 3% drop target
500 kcmil copper
Aluminum option750 kcmil
On a 240V circuit (copper)500 kcmil
Voltage drop (120V, copper)2.82V (2.35%)
Use this citation when referencing this page.
check_circle
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
500 kcmil branch-circuit OCP (380A) ≥ 312.5A ✓
The conductor needs to carry at least 312.5A 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), 500 kcmil sits at a branch-circuit OCP of 380A. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (500 kcmil NM-B = 320A), 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 350 kcmil (one size thinner) at these inputs gives a voltage drop of 4.01V (3.35% on 120V), and its branch-circuit OCP cap under typical conditions is 310A.
Limiting factor here: branch-circuit ampacity. 350 kcmil has a branch-circuit OCP cap of 310A under the typical 75°C-termination assumptions used here, which is below the 312.5A 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?
Using 750 kcmil (one size thicker) would reduce voltage drop to 1.87V (1.56% on 120V). More expensive wire but better performance and more headroom for future load increases.
Wattage at This Amperage
312.5A at 120V delivers 37,500 watts (DC / resistive load). See conversion.
312.5A at 175ft on 120V is commonly served by 500 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.
Voltage drop scales linearly with distance: doubling the one-way run length doubles the drop in volts. At 312.5A on 120V, a 175ft run is often served by 500 kcmil to land under the 3% drop target, a run half that length can sometimes use one gauge thinner, and a run double that length usually needs one or two gauges thicker. Ampacity is set by the conductor itself (Table 310.16 at the applicable termination temperature), so the binding constraint is ampacity on short runs and voltage drop on long runs.
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.
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 312.5A continuous load that points the sizing math at the 390.63A 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.
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.
