Electricians
Voltage Drop Calculator
Calculate voltage drop across a wire run to ensure code compliance and proper performance.
Formula: VD = (2 × K × I × L) / CM where K=12.9 (copper), I=20A, L=50ft. NEC recommends no more than 3% drop on branch circuits and 5% total from service entrance to load.
Common Questions
What is the maximum allowed voltage drop per NEC?
NEC recommends no more than 3 percent drop on branch circuits and 5 percent total from the panel to the load. These are recommendations, not hard code requirements, but inspectors and engineers expect you to follow them.
How do I fix too much voltage drop?
Use a larger wire gauge, shorten the run, or increase the voltage. Going up one wire size (like from 12 to 10 AWG) cuts the resistance by about 37 percent.
Why does voltage drop matter on a job?
High voltage drop causes motors to run hot and burn out faster, lights to dim, and equipment to fail early. Staying within 3 percent protects your customer and reduces callbacks.
How do I calculate voltage drop on a long wire run?
Use the formula: VD = (2 x K x I x L) divided by CM. K is 12.9 for copper or 21.2 for aluminum. I is the amperage, L is the one-way distance in feet, and CM is the circular mils of the wire. This calculator does the math for you.
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