Voltage Drop Calculator
Common Voltage Standards
NEC Guidelines
Feeders to final outlets
Feeder + Branch combined
Service to distribution panel
Related Calculators
Understanding Voltage Drop
What is Voltage Drop?
Voltage drop is the reduction in voltage as electrical current flows through a wire. All conductors have some resistance, which causes energy loss as heat. Excessive voltage drop can cause equipment malfunction, reduced motor efficiency, and safety issues.
Voltage Drop Formula
Wire Resistance by Gauge
| AWG | Diameter (mm) | ohm/1000ft (Cu) | Max Amps (60C) |
|---|---|---|---|
| 14 | 1.63 | 2.525 | 15 |
| 12 | 2.05 | 1.588 | 20 |
| 10 | 2.59 | 0.999 | 30 |
| 8 | 3.26 | 0.628 | 40 |
| 6 | 4.11 | 0.395 | 55 |
| 4 | 5.19 | 0.249 | 70 |
| 2 | 6.54 | 0.156 | 95 |
| 1/0 | 8.25 | 0.098 | 125 |
Factors Affecting Voltage Drop
Increases Drop
- Longer wire runs
- Higher current draw
- Smaller wire gauge
- Higher temperature
- Aluminum vs copper
Reduces Drop
- Shorter wire runs
- Lower current draw
- Larger wire gauge
- Higher voltage systems
- Copper conductors
Single Phase vs Three Phase
Single Phase
Uses 2 conductors. Current flows through wire to load and returns through neutral. Factor of 2 in formula accounts for round trip.
Three Phase
Uses 3 conductors with 120 degree phase offset. Uses factor of 1.732 (sqrt of 3) instead of 2 due to phase relationships.
Important Considerations
Always follow local electrical codes (NEC in the US). Voltage drop calculations determine minimum wire size for efficiency, but ampacity tables determine minimum size for safety. Use the larger of the two requirements. Consider temperature derating, conduit fill, and ambient conditions.
Frequently Asked Questions
What is acceptable voltage drop for residential wiring?
NEC recommends max 3% drop for branch circuits and 5% total from service to final outlet. For a 120V circuit, 3% = 3.6V max drop. Sensitive electronics may need even less. Always size wire for both ampacity AND voltage drop.
How do I calculate what wire gauge I need?
Use formula: CM = (K x I x D) / Vdrop, where K=12.9 for copper, I=current in amps, D=distance in feet (one-way). Then find gauge with that CM or larger. Or use tables - for 20A at 100ft on 120V, you need 10 AWG to stay under 3%.
Why does wire length affect voltage drop?
Resistance increases with length (R = rho x L / A). Longer wire = more resistance = more voltage lost as heat. Current must travel there AND back, so total wire length is 2x the run distance. This is why long runs need larger wire.
Is aluminum wire worse for voltage drop than copper?
Yes, aluminum has 61% of copper's conductivity, causing more voltage drop. Aluminum requires going up about 2 wire sizes (e.g., use 8 AWG aluminum where you'd use 10 AWG copper). Aluminum is cheaper but requires special connections.