Voltage Drop Residential
Enter wire gauge, current, run length, and material to instantly see voltage drop and NEC compliance.
Circuit Parameters
CM = circular mils cross-sectional area
Distance from panel to load — the calculator doubles it for total conductor length.
Results
Enter circuit values and click Calculate.
—
Voltage Drop
—
% Drop
—
Voltage at Load
—
Total Resistance
Recommended Minimum Gauge
Formula Used
VD = (K × I × 2L) / CM
- K = resistivity constant (10.37 copper, 17.00 aluminum)
- I = current in amps
- L = one-way run length in feet
- CM = conductor area in circular mils
- VD% = (VD / supply voltage) × 100
Summary
Enter wire gauge, current, run length, and material to instantly see voltage drop and NEC compliance.
How it works
- Select the conductor material (copper or aluminum).
- Choose the wire gauge from the AWG dropdown.
- Enter the one-way circuit length in feet.
- Enter the load current in amps.
- Select the supply voltage (120V or 240V single-phase).
- The tool computes voltage drop and flags NEC compliance (3% branch, 5% total).
Use cases
- Size wire for a long run to a garage sub-panel.
- Check NEC compliance before rough-in inspection.
- Compare copper vs aluminum conductor costs for a given run.
- Verify a 240V appliance circuit meets the 3% branch-circuit limit.
- Estimate voltage at the load end of a long outdoor circuit.
- Decide whether to upsize wire gauge to stay under the 5% combined limit.
Frequently Asked Questions
Last updated: 2026-07-01 ·
Reviewed by Nham Vu