Electrical
Voltage Drop Calculator
Calculate voltage drop over a conductor run and check it against the recommended limit.
Result
Electrical work is life-safety work governed by the NEC or your local code, and must be designed and installed by a licensed electrician. Conductor and overcurrent sizing depends on ambient temperature, conductor bundling, termination ratings, continuous-load rules and more. This is an educational estimate and must never be used as the basis for an installation. The 3% branch-circuit and 5% total guideline is an NEC recommendation in informational notes, not a hard requirement, but exceeding it causes dim lights, poor motor starting and wasted energy.
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About this calculator
How much voltage drop is acceptable?
The commonly used guideline is 3% on a branch circuit and 5% total from service to load. It is advisory in the NEC rather than mandatory, but it is good practice.
Why does a long run need bigger wire?
Resistance rises with length, so drop rises proportionally. On long runs the conductor is sized by voltage drop rather than by ampacity, often two or three sizes above what the current alone would need.
The formula
single phase: VD = 2 × K × I × L ÷ circular mils (K = 12.9 copper, 21.2 aluminium)
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