Voltage Drop Calculator
Free voltage drop calculator for 12V–240V circuits. Uses NEC Table 8 resistance (12 AWG = 1.93 Ω/1,000 ft) to check any run against the 3% guideline.
Enter four things — system voltage (12-volt DC through 240-volt AC), wire gauge from 14 AWG to 4/0 in copper or aluminum, one-way run length in feet, and load current in amps — and the calculator returns the voltage lost in the wire, the percent drop, the voltage that actually reaches your load, and a pass/fail verdict against the 3 percent guideline. Under the hood it runs Vdrop = 2 × I × R × L ÷ 1,000 with resistance values straight from NEC Chapter 9, Table 8 (12 AWG copper = 1.93 ohms per 1,000 feet). The tool exists because voltage drop is invisible until something misbehaves: a winch that stalls, an LED strip that shifts red at the far end, a fridge that resets. At 12 volts the margin is tiny — just 0.36 volts of drop eats the whole 3 percent budget — so a wire that would be fine in a house can fail in a van. Check before you buy copper.
How do you calculate voltage drop?
Multiply 2 × current (amps) × wire resistance (ohms per 1,000 feet) × one-way length (feet), then divide by 1,000. The 2 accounts for the round trip out and back. Example: 20 feet of 12 AWG copper (1.93 ohms per 1,000 feet from NEC Table 8) carrying 15 amps drops 2 × 15 × 1.93 × 20 ÷ 1,000 = 1.16 volts — 9.6 percent of a 12-volt system, but under 1 percent at 120 volts.
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Voltage Drop Calculator
Key Information
| Parameter | Details |
|---|---|
| Voltage drop formula | Vdrop = 2 × I × R × L ÷ 1,000 |
| 12 AWG copper resistance (NEC Ch. 9, Table 8) | 1.93 Ω per 1,000 ft |
| NEC recommended limit (Informational Note) | 3% branch / 5% total |
| Aluminum vs copper resistance | ~1.65× higher per gauge |
Frequently Asked Questions
How do you calculate voltage drop?
Multiply 2 × current (amps) × wire resistance (ohms per 1,000 feet) × one-way length (feet), then divide by 1,000. The 2 accounts for the round trip out and back. Example: 20 feet of 12 AWG copper (1.93 ohms per 1,000 feet from NEC Table 8) carrying 15 amps drops 2 × 15 × 1.93 × 20 ÷ 1,000 = 1.16 volts — 9.6 percent of a 12-volt system, but under 1 percent at 120 volts.
What is an acceptable voltage drop?
Keep it under 3 percent on a branch circuit and 5 percent total including the feeder — the NEC’s recommendation in Informational Note No. 4 to 210.19(A). Those figures are advisory, not enforceable, though some local codes adopt them as law. Sensitive-electronics circuits under NEC Article 647 face a mandatory 1.5 percent limit. For 12-volt DC gear, 3 percent means losing no more than 0.36 volts.
What size wire do I need for 30 amps at 12 volts?
For a 15-foot one-way run, 4 AWG copper — it drops 0.28 volts (2.3 percent), while 6 AWG fails at 3.7 percent. Stretch the run to 25 feet and you need 2 AWG (0.29 volts, 2.4 percent), because 4 AWG would drop 3.9 percent. Distance matters as much as current at 12 volts, so run your exact length through the calculator above before buying cable.
Are these calculators free to use?
Yes, all calculators on CalcCorp are completely free — no registration, no login, no hidden charges. Results are calculated instantly in your browser and we do not store any of your data.
How accurate are these calculations?
Our calculators use standard financial formulas updated with the latest tax rates, interest rates, and government policies for 2026. Results are accurate for planning purposes but should be verified with a professional for final decisions.
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Last updated: March 2026