How to Use This Calculator
Voltage drop is the loss of electrical pressure as current travels through a conductor. The longer the wire run and the higher the current, the greater the voltage drop. The National Electrical Code limits voltage drop to 3% on branch circuits and 5% on the combination of feeder and branch circuits. If your circuit exceeds these limits, the equipment won’t operate efficiently and you risk damaging motors or appliances.
To use this calculator, you need four inputs: the resistivity constant K (which depends on the type of conductor and temperature—use 12.9 for copper at 75°C, which is the standard for most installations), the load current in amps, the one-way distance from the panel to the load in feet, and the circular mil area of the conductor you’re considering. The result tells you the actual voltage drop for your circuit.
Real-world example: You’re running a 120-amp subpanel 150 feet from the main panel. Using 2 AWG copper (at 12.9 K), the voltage drop comes out to about 3.2 volts on a 240V circuit. Since your feeder can only handle 3% drop (7.2 volts), this is acceptable. If you’d used 4 AWG instead, you’d hit 5.1 volts—still legal but getting close to the limit. Always have a buffer.
Formula
Voltage Drop Formula: VD = (2 × K × I × D) / CM
Where:
- VD = voltage drop in volts
- K = resistivity constant (12.9 for copper at 75°C; 21.2 for aluminum at 75°C)
- I = current in amps
- D = one-way distance in feet
- CM = circular mils of the conductor
The factor of 2 accounts for current traveling out through the hot conductor and back through the neutral.
When to Use This
You need this calculation on every circuit run longer than about 50 feet, especially for motor circuits, electric heater circuits, and welder feeds. I’ve run into plenty of situations where a three-phase motor was undersized wire and the motor couldn’t reach full speed because of voltage drop—the owner thought it was a bad motor until we checked the wire size.
Branch circuits running to a garage 80 feet away, pool pump wiring, temporary power distribution on large jobs, and feeder calculations for additions all require checking voltage drop. The code isn’t always negotiable; inspectors will call out violations, and you’ll have to tear out wire runs to upsize them. Better to calculate it right the first time.
Code References
- NEC Article 210.19(A): Branch circuit conductors minimum sizing
- NEC Article 215.2(A): Feeder conductor sizing
- NEC Informational Note No. 4 to Article 210: Voltage drop guidance (recommends 3% max on branch circuits, 5% max on feeder + branch combined)
- NEC Chapter 9, Table 8: Conductor properties (includes circular mil area by AWG)
Frequently Asked Questions
Can I go over 3% voltage drop if the code only “recommends” it?
The code states it as guidance, not a hard requirement, but most AHJs (Authority Having Jurisdiction) will enforce it. More importantly, equipment won’t work right. Motors will run hot and burn out. Lights will dim. You’re setting the customer up for callbacks. Size it right.
Do I need to add adjustment factors for temperature and conduit fill?
This calculator applies the standard K value at 75°C. If you’re running wire in attics or hot locations where ambient temperature exceeds 86°F, or if you’re cramming 6+ wires in conduit, you need to derate the ampacity and potentially recalculate with a higher K value. Those corrections go beyond the basic formula, so run the numbers conservatively.
What’s the difference between single-phase and three-phase voltage drop?
For single-phase, use the formula as shown (with the factor of 2). For three-phase circuits, the multiplier changes: use 1.73 instead of 2. The K value stays the same. Most calculators handle this internally.
How do I find the circular mil area of a wire size?
NEC Chapter 9, Table 8 lists it by AWG. 10 AWG = 10,380 CM, 8 AWG = 16,510 CM, 6 AWG = 26,240 CM, 4 AWG = 41,740 CM, 2 AWG = 66,360 CM. Memorize the common ones or keep the table handy.
Should I use one-way or round-trip distance?
Use one-way distance from the source (panel, breaker) to the load. The formula already includes the factor of 2 to account for current returning through the neutral.