Voltage Drop Calculator
Engineering Specification Sheet
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Generated: August 22, 2026
Voltage Drop Calculator
Free voltage drop calculator for electricians. Calculate voltage drop, wire size, and max distance per NEC 210.19(A) guidelines.
Circuit Details
1.00 for lighting/resistive loads. Motors and other inductive loads typically run 0.80–0.90 — lowering this reveals the reactance NEC Table 9 accounts for.
Voltage Drop Results
✗ Exceeds NEC Recommendation (> 5%)
NEC 2023 § 210.19(A) Informational Note No. 4
Recommendation
Consider upgrading to 8 AWG/kcmil to meet the 3% branch circuit recommendation.
Exact impedance calculation — resistance (Table 8) and reactance (Table 9) at power factor 1.00, not a resistance-only approximation.
Verified against NEC 2023 · Table 8 & Table 9
How this works
Voltage drop is what happens to power as it travels down a wire. Some of it gets lost to resistance along the way, and on a long run with too small a wire, enough goes missing that motors labor, lights dim, and electronics act up before anything ever trips a breaker.
NEC 210.19(A) doesn't set a hard limit, but it recommends keeping the drop under 3% on a branch circuit and under 5% total once you add the feeder in. This calculator runs the standard formula, VD = (2 x K x I x D) / CM, using your circuit's current, one-way distance, and wire size to tell you where you land and whether you need to size up.
Heat changes the answer. A conductor's resistance rises as it gets hotter, so a run through a hot attic or rooftop conduit drops more voltage than the same wire in a cool basement. NEC Chapter 9, Table 8 has the resistance values if you need to adjust for that.
Worked example
A 16A, 120V single-phase circuit run 100 feet one-way in copper.
Stepping up from #12 to #8 copper brings this 100-foot, 16A run from 5.17% voltage drop down to 2.10%, under the 3% recommendation.
Frequently Asked Questions
What is the acceptable voltage drop according to the NEC?↓
The NEC recommends a maximum voltage drop of 3% for the farthest outlet of power, heating, and lighting loads, or a maximum combined voltage drop of 5% for both the feeder and branch circuit.
How do you calculate voltage drop for single phase?↓
The formula for single-phase voltage drop is VD = (2 * K * I * D) / CM, where K is the specific resistance of the conductor, I is current in amps, D is distance in feet, and CM is circular mils of the wire.
Does temperature affect voltage drop?↓
Yes, conductor resistance increases with temperature. While standard calculations often use a fixed K-factor, high-heat environments may require adjusting the resistance values per NEC Chapter 9, Table 8.
Learn more
How to Choose the Right Wire Size: NEC Table 310.16 Guide
A practical guide to selecting wire size using NEC Table 310.16 ampacity ratings, temperature corrections, and conductor bundling derating factors.
Understanding Voltage Drop: NEC Requirements & How to Calculate It
How to calculate voltage drop per NEC 210.19(A) and why keeping it under 3% matters for branch circuits, with formulas and a worked example.
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