500 kcmil copper is a typical pick for 312.5A at 200 feet on 120V under a 3% drop target. It balances NEC branch-circuit ampacity and voltage drop over distance. Drop with 500 kcmil at these inputs: 3.23V (2.69%). Real-world sizing also depends on insulation temperature rating, cable type, and install conditions.
312.5A at 200ft · 120V single-phase / DC · 3% drop target
500 kcmil copper
Aluminum option750 kcmil
On a 240V circuit (copper)500 kcmil
Voltage drop (120V, copper)3.23V (2.69%)
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
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 4.59V (3.82% 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 2.14V (1.78% 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 200ft 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.
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.
Voltage drop scales linearly with distance: doubling the one-way run length doubles the drop in volts. At 312.5A on 120V, a 200ft 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.
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 and aluminum are picked independently against the same drop target on this site; neither pick implies ampacity equivalence with the other. At 312.5A, aluminum is the industry standard for sub-panel feeders, service entrance, and utility drops. AA-8000 series aluminum is the modern feeder material; copper is still used where space is tight or terminations are copper-only. Aluminum has lower conductivity than copper, so when each material is run through the drop-target pick independently, the aluminum result typically lands one to two gauges larger than the copper result for the same duty. That gap is the result of running both picks against the same drop-target constraint, not an ampacity-equivalence rule. The install still needs anti-oxidant compound and aluminum-rated lugs.
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.
