Voltage Drop Calculator
Enter your circuit details to calculate the exact voltage drop across a wire run, the voltage at the load end, and whether it meets NEC recommendations.
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Voltage drop (V)
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Voltage drop %
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Voltage at load
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Power loss (W)
Voltage Drop Formula
For a single-phase AC or DC circuit with two conductors:
$$V_{drop} = \frac{2 \times K \times I \times L}{CM}$$
- K = resistivity constant (12.9 for copper, 21.2 for aluminum)
- I = current in amps
- L = one-way wire run length in feet
- CM = wire cross-section in circular mils (a function of AWG gauge)
For three-phase circuits, replace the factor of 2 with √3 (1.732).
The percentage voltage drop = (V_drop ÷ V_source) × 100.
NEC Voltage Drop Guidelines
| Voltage drop % | NEC guidance |
|---|---|
| ≤ 3% | Recommended for branch circuits |
| ≤ 5% | Maximum recommended (feeder + branch combined) |
| > 5% | Not recommended; equipment may malfunction |
For low-voltage DC systems (12 V, 24 V), voltage drop has an outsized effect because the source voltage is small. On a 12 V system, even a 3% drop is only 0.36 V — but that can noticeably dim LED lighting and cause unreliable relay operation.
Wire Resistance Reference (Copper, per 1,000 ft)
| AWG | Resistance (Ω/1000 ft) | Circular mils |
|---|---|---|
| 14 | 3.14 | 4,107 |
| 12 | 1.98 | 6,530 |
| 10 | 1.24 | 10,380 |
| 8 | 0.778 | 16,510 |
| 6 | 0.491 | 26,240 |
| 4 | 0.308 | 41,740 |
| 2 | 0.194 | 66,360 |
| 1/0 | 0.122 | 105,600 |
Frequently Asked Questions
Why does voltage drop matter for 12 V systems?
In a 12 V DC system (e.g., RV, boat, solar), even a 1 V drop represents an 8.3% loss. This can cause LED lights to dim noticeably, motors to run slowly, and electronic devices to behave erratically. Use a larger wire gauge or keep runs short on low-voltage systems.
How do I reduce voltage drop?
The three ways to reduce voltage drop are: (1) use a heavier wire gauge, (2) shorten the wire run, or (3) increase the source voltage. For long outdoor lighting runs, using 24 V fixtures instead of 12 V cuts voltage drop in half for the same wire size.
Does voltage drop affect AC circuits the same as DC?
The same formula applies for resistive loads. However, AC circuits with inductive loads (motors, transformers) have additional voltage drop due to reactance, especially at low power factor. This calculator covers the resistive component only.
