Voltage Drop Calculator
Size conductors per NEC Chapter 9 Table 8. Single & three-phase, copper & aluminum.
Voltage Drop
7.5V
Drop %
3.1%
Voltage at Load
232.5V
NEC Reference
Per NEC 210.19(A) Informational Note No. 4, conductors should be sized to prevent voltage drop exceeding 3% at the farthest outlet, with a maximum of 5% for combined feeder and branch circuit.
Get 1,000+ NEC Practice Questions
Master the National Electrical Code with our Electrician Prep app.
Download Electrician PrepMore free tools coming soon — HVAC load calculator, pipe sizing tool
Pro Insights
- Minimum conductor size to stay under 3%:See in app
- NEC 210.19(A) continuous load compliance check:See in app
- Estimated material cost for this wire run:See in app
Electrician Prep
Take this to the job site
Use this calculator offline + 1,000+ NEC practice questions
What Is Voltage Drop and Why Does It Matter?
Voltage drop is the reduction in voltage that occurs as electrical current travels through a conductor. All wires have resistance, and that resistance causes some voltage to be "lost" as heat before the electricity reaches the load. The longer the wire run and the more current flowing through it, the greater the voltage drop.
The NEC doesn't strictly mandate a maximum voltage drop, but Informational Note 1 to NEC 210.19(A) recommends that voltage drop on branch circuits not exceed 3%, and that the combined voltage drop of feeders and branch circuits not exceed 5%. In practice, most inspectors and engineers use the 3%/5% rule as a design standard. Exceeding it causes motors to overheat, lights to flicker, and sensitive electronics to malfunction.
This calculator uses NEC Chapter 9 Table 8 resistance values for copper and aluminum conductors. Values are given for both single-phase (multiply by 2 for the round-trip distance) and three-phase (multiply by 1.732) circuits.
Frequently Asked Questions
What is the NEC maximum allowable voltage drop?
The NEC recommends (but does not mandate) 3% on branch circuits and 5% combined on feeders and branch circuits per Informational Notes in NEC 210.19(A) and 215.2(A). Some jurisdictions and engineers adopt these as hard limits in their specifications.
Copper vs. aluminum — which has less voltage drop?
Copper has lower resistance per foot than aluminum, meaning copper conductors produce less voltage drop at the same size. Aluminum is lighter and less expensive but requires a larger AWG to carry the same current and maintain the same voltage drop percentage.
How is three-phase voltage drop different from single-phase?
For single-phase circuits, current travels out and back through two conductors, so you use 2 × one-way distance. For three-phase, the formula uses a factor of 1.732 (√3) instead of 2, which reflects the phase relationship between the three conductors — resulting in less voltage drop than a comparable single-phase run.
Does voltage drop appear on the journeyman electrician exam?
Yes — voltage drop calculations are among the most commonly tested math problems on journeyman and master electrician licensing exams. Expect 3–5 calculation questions using the standard formula or resistance values from NEC Chapter 9 Table 8 and Table 9.
What wire size should I use to reduce voltage drop?
Increasing conductor size (going from 12 AWG to 10 AWG, for example) reduces resistance and therefore reduces voltage drop. Use the Wire Size Recommender tool to find the minimum AWG that keeps voltage drop within your specified limit for a given load and distance.
Also try: NEC Wire Size Recommender
Get the minimum AWG that keeps voltage drop within NEC limits