How this voltage drop calculator works
Every conductor has resistance, so some of the source voltage is lost as heat along the run. The longer the wire, the higher the current and the smaller the conductor, the more voltage you lose. Too much drop makes motors run hot and stall, makes LED drivers flicker and makes heaters underperform. This calculator uses the same method most electricians and inspectors use in the field, with resistance values taken directly from NEC Chapter 9, Table 8.
The formula
Single phase: VD = 2 × I × R × L ÷ 1000
Three phase: VD = 1.732 × I × R × L ÷ 1000
VD% = VD ÷ source voltage × 100
Where I is the load current in amps, R is the conductor resistance in ohms per 1,000 feet, and L is the one-way length in feet. The factor 2 accounts for the out-and-back path on a single-phase circuit; 1.732 (√3) is used for a balanced three-phase circuit. For a deeper walkthrough, including the circular-mil (K-factor) version, see the voltage drop formula guide.
Worked example: 20 A circuit, 100 ft, 120 V
A 16 A load (80% of a 20 A circuit) is 100 feet from the panel on 12 AWG copper. Table 8 gives 1.93 Ω per 1,000 ft.
VD = 2 × 16 × 1.93 × 100 ÷ 1000 = 6.18 V
VD% = 6.18 ÷ 120 = 5.15%
That is well over the 3% recommendation. Moving to 10 AWG (1.21 Ω/kft) gives 3.87 V or 3.23%, still slightly over. 8 AWG (0.764 Ω/kft) gives 2.44 V or 2.04%, which passes. The calculator above reaches the same answer, and its "smallest size" box does the upsizing for you.
Worked example: aluminum feeder at 240 V
An 80 A subpanel feeder runs 150 ft on 2 AWG aluminum (0.319 Ω/kft) at 240 V single phase: VD = 2 × 80 × 0.319 × 150 ÷ 1000 = 7.66 V, or 3.19%. It is within the 5% total only if the branch circuits downstream add less than about 1.8%. Going to 1/0 aluminum brings the feeder to about 2%.
NEC and CEC voltage drop limits
In the US, the NEC treats voltage drop as a recommendation: informational notes to 210.19(A) (branch circuits) and 215.2 (feeders) suggest 3% for either and 5% total. Some sections are mandatory, such as 647.4(D) for sensitive electronic equipment and 695.7 for fire pumps, and energy codes may also limit drop. In Canada, CEC Rule 8-102 makes the limits a requirement: 3% on a feeder or branch circuit and 5% overall from the supply side of the consumer's service. Use our Canadian voltage drop calculator for metric lengths and 347/600 V systems.
Resistance values used (NEC Chapter 9, Table 8)
| Size | Copper Ω/1000 ft | Aluminum Ω/1000 ft |
|---|---|---|
| 14 AWG | 3.07 | 5.06 |
| 12 AWG | 1.93 | 3.18 |
| 10 AWG | 1.21 | 2 |
| 8 AWG | 0.764 | 1.26 |
| 6 AWG | 0.491 | 0.808 |
| 4 AWG | 0.308 | 0.508 |
| 2 AWG | 0.194 | 0.319 |
| 1/0 AWG | 0.122 | 0.201 |
Values are DC resistance of uncoated stranded conductors at 75 °C. For large conductors on AC systems with a low power factor, reactance matters more; an engineer may use NEC Chapter 9 Table 9 effective impedance instead.
How to reduce voltage drop
- Upsize the conductor. Each step of three AWG sizes roughly halves resistance.
- Raise the voltage. The same load at 240 V draws half the current of 120 V, and each volt lost is a smaller share of the supply.
- Shorten the run or move the panel closer to the load with a subpanel.
- Use copper instead of aluminum on long, small circuits.
- Split the load across more circuits.
Common mistakes
Entering the round-trip length (which doubles the answer), entering breaker size instead of load current, and forgetting that the 5% total includes the feeder and the branch circuit together. If you upsize ungrounded conductors for voltage drop, remember NEC 250.122(B) requires the equipment grounding conductor to be increased by the same proportion of circular mils. For three-phase feeders, use the 3 phase voltage drop calculator, and to size a conductor for both ampacity and drop at once, use the wire size calculator.