Voltage Drop Calculator
Calculate voltage drop in volts and percent for copper or aluminum wire on single-phase, DC or three-phase circuits, check it against the NEC 3% and 5% recommendations, find the smallest wire that passes, and see the AWG gauge wiring chart.
Voltage drop calculator
Voltage drop formula
Single-phase or DC: VD = 2 × L × I × R ÷ 1,000; three-phase: VD = 1.732 × L × I × R ÷ 1,000, where L is the one-way length in feet, I the current in amps and R the conductor resistance in ohms per 1,000 ft (NEC Chapter 9, Table 8, at 75 °C). 10 AWG copper (1.24 Ω/kft) carrying 20 A over 100 ft on 120 V drops 2 × 100 × 20 × 1.24 ÷ 1,000 = 4.96 V (4.13%); 8 AWG drops 3.11 V (2.59%).
The NEC's informational notes recommend no more than 3% drop on a branch circuit and 5% combined for feeder plus branch circuit. It's a recommendation for efficiency and equipment performance, not a mandatory rule (sensitive electronic equipment and fire pumps have specific requirements). This DC-resistance method is accurate for small wires; for large AC conductors, NEC Table 9 (effective impedance) gives slightly different results.
Gauge wiring chart: ampacity and resistance
| Size | Copper 60 °C (A) | Copper 75 °C | Copper 90 °C | Copper Ω/kft | Aluminum Ω/kft |
|---|---|---|---|---|---|
| 14 AWG | 15 | 20 | 25 | 3.14 | 5.17 |
| 12 AWG | 20 | 25 | 30 | 1.98 | 3.25 |
| 10 AWG | 30 | 35 | 40 | 1.24 | 2.04 |
| 8 AWG | 40 | 50 | 55 | 0.778 | 1.28 |
| 6 AWG | 55 | 65 | 75 | 0.491 | 0.808 |
| 4 AWG | 70 | 85 | 95 | 0.308 | 0.508 |
| 3 AWG | 85 | 100 | 115 | 0.245 | 0.403 |
| 2 AWG | 95 | 115 | 130 | 0.194 | 0.319 |
| 1 AWG | 110 | 130 | 145 | 0.154 | 0.253 |
| 1/0 AWG | 125 | 150 | 170 | 0.122 | 0.201 |
| 2/0 AWG | 145 | 175 | 195 | 0.0967 | 0.159 |
| 3/0 AWG | 165 | 200 | 225 | 0.0766 | 0.126 |
| 4/0 AWG | 195 | 230 | 260 | 0.0608 | 0.1 |
| 250 kcmil | 215 | 255 | 290 | 0.0515 | 0.0847 |
| 300 kcmil | 240 | 285 | 320 | 0.0429 | 0.0707 |
| 350 kcmil | 260 | 310 | 350 | 0.0367 | 0.0605 |
| 400 kcmil | 280 | 335 | 380 | 0.0321 | 0.0529 |
| 500 kcmil | 320 | 380 | 430 | 0.0258 | 0.0424 |
Ampacity: NEC Table 310.16 (not more than three current-carrying conductors, 30 °C ambient). Small conductors are limited by 240.4(D): 14 AWG copper to 15 A, 12 AWG to 20 A, 10 AWG to 30 A. Most residential terminations are rated 60 °C or 75 °C; size to the lowest rating in the circuit. Resistance: NEC Chapter 9 Table 8, stranded, uncoated copper and aluminum at 75 °C.
Common questions
How do I calculate voltage drop?
VD = 2 × length × current × resistance per 1,000 ft ÷ 1,000 (single-phase). 10 AWG, 20 A, 100 ft: 4.96 V.
What is the maximum voltage drop allowed?
The NEC recommends 3% on a branch circuit and 5% total; it's a recommendation, not a requirement.
What size wire for 20 amps at 100 feet?
10 AWG copper drops 4.1% on 120 V; 8 AWG keeps it under 3%.
How many amps can 12 gauge wire carry?
20 A for 12 AWG copper (240.4(D) limit), though Table 310.16 lists 25 A at 75 °C.
Does three-phase have less voltage drop?
Yes: the factor is 1.732 instead of 2 for the same current and length.
Sources
- NFPA 70 (NEC 2023): Table 310.16 and Chapter 9 Table 8 (reference summary)
- EC&M: How to use Chapter 9 in your next design
- ExpertCE: Conductor resistance, NEC Chapter 9 Table 8
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General information, not financial, tax or legal advice. Check important numbers with the official source before acting on them.