Voltage Drop Calculator
Calculate voltage drop across copper or aluminum wire based on NEC Chapter 9 Table 8 resistance values. Supports single-phase and three-phase circuits.
Frequently Asked Questions
How do I calculate voltage drop?
Use the formula: Vdrop = 2 × current × resistance × length / 1000 (single-phase). Wire resistance values come from NEC Chapter 9 Table 8. For three-phase, replace 2 with √3 (1.732). The "length" is one-way distance from panel to load.
What is an acceptable voltage drop?
NEC recommends ≤3% for branch circuits and ≤5% total (feeder + branch combined). Above 5% can cause motor overheating, light flickering, and equipment malfunction.
How do I reduce voltage drop?
Three approaches:
- Use larger wire gauge — going from 12 AWG to 10 AWG cuts resistance by ~37%
- Shorter wire runs — voltage drop is directly proportional to length
- Higher voltage — 240V circuits have half the voltage drop percentage of 120V for the same power
Does voltage drop matter for DC?
Yes, the same formula applies. Voltage drop is critical for solar panels, car audio, LED lighting, battery systems, and any low-voltage DC wiring.
Voltage Drop Formula
Where:
- I = current in amperes
- R = wire resistance in ohms per 1000 feet (from NEC Table 8)
- L = one-way wire length in feet
NEC Wire Resistance Table (75°C)
DC resistance values from NEC Chapter 9, Table 8:
| AWG | Copper (Ω/1000ft) | Aluminum (Ω/1000ft) | Max Amps (Cu, 60°C) |
|---|---|---|---|
| 14 | 3.14 | 5.02 | 15 |
| 12 | 1.98 | 3.17 | 20 |
| 10 | 1.24 | 1.98 | 30 |
| 8 | 0.778 | 1.24 | 40 |
| 6 | 0.491 | 0.786 | 55 |
| 4 | 0.308 | 0.493 | 70 |
| 3 | 0.245 | 0.392 | 85 |
| 2 | 0.194 | 0.310 | 95 |
| 1 | 0.154 | 0.246 | 110 |
| 1/0 | 0.122 | 0.195 | 125 |
| 2/0 | 0.0967 | 0.155 | 145 |
| 3/0 | 0.0766 | 0.123 | 165 |
| 4/0 | 0.0608 | 0.0973 | 195 |
NEC Voltage Drop Recommendations
| Circuit Type | Max Drop | NEC Reference |
|---|---|---|
| Branch circuit | ≤3% | NEC 210.19(A) Info Note 4 |
| Feeder | ≤3% | NEC 215.2(A) Info Note 2 |
| Total (feeder + branch) | ≤5% | NEC combined recommendation |
Note: These are recommendations (informational notes), not mandatory code requirements. However, exceeding them can cause operational problems.
Related Calculators
- Wire Size Calculator — find the right AWG wire for your circuit
- Watts / Amps / Volts Calculator — convert power to current
- Ohm's Law Calculator (V/A/W/Ω) — V = I × R
Wire resistance values from NEC (National Electrical Code) Chapter 9, Table 8 at 75°C. Voltage drop recommendations per NEC 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2.