Voltage Drop Calculator
Calculates voltage decrease over wire runs based on wire gauge, length, and load current
Voltage of the electrical system
Copper wire gauge (AWG)
One-way wire run distance in feet
Current draw of the load in amperes
Enter all values to calculate voltage drop
Percentage of system voltage lost
Actual voltage delivered to the load
Pass/warning/fail based on NEC guidelines
Frequently Asked Questions
What is acceptable voltage drop?
The NEC recommends no more than 3% voltage drop for branch circuits and 5% total for feeder plus branch circuits combined. Exceeding these limits can cause equipment to malfunction, motors to overheat, and lights to dim noticeably.
How do I reduce voltage drop?
Use a larger wire gauge (lower AWG number), shorten the wire run distance, or increase the system voltage. Going from 12 AWG to 10 AWG roughly cuts voltage drop in half. Running 240V instead of 120V also halves the current and the drop.
Why does wire length matter for voltage drop?
Voltage drop is proportional to wire length because longer wires have more resistance. The formula uses round-trip distance (twice the one-way distance) because current must travel to the load and back. A 100-foot run means 200 feet of wire resistance.
Does copper or aluminum wire have less voltage drop?
Copper has lower resistance than aluminum for the same gauge, so it produces less voltage drop. Aluminum wire needs to be about two gauge sizes larger than copper to carry the same current with similar voltage drop. Copper is preferred for most residential work.
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