Wire Size Calculator
Find the conductor size a run needs to stay within your voltage drop limit.
Conductor sizing must also satisfy NEC ampacity tables, temperature correction and conduit fill. This calculator addresses voltage drop only. Have all work verified by a licensed electrician.
How this calculator works
circular mils = 2 × K × amps × distance ÷ allowable voltage drop
Compare the result against standard sizes: 14 AWG=4110, 12=6530, 10=10380, 8=16510, 6=26240, 4=41740, 3=52620, 2=66360, 1=83690, 1/0=105600. Round up to the next size — never down.
Worked example
With load current 20 , one-way distance 150 ft, system voltage 120 , maximum voltage drop 3 , conductor material k 12.9 , this calculator returns 21500 .
Frequently asked questions
What wire size do I need for a 20 amp circuit?
Twelve AWG copper meets ampacity for 20 amps, but voltage drop governs on long runs. At 150 ft on 120 V you need 21,500 circular mils — that is 6 AWG, three sizes larger than ampacity alone would suggest.
Does ampacity or voltage drop determine wire size?
Whichever demands the larger conductor. Ampacity is the code requirement and sets the minimum; voltage drop is a performance recommendation that frequently governs on runs over 100 ft.
Can I use aluminium instead of copper?
Yes for feeders and service conductors, and it costs considerably less. Aluminium carries about 61% of copper conductivity, so you need roughly two sizes larger, plus antioxidant compound and rated terminations.
What size wire for a detached garage?
It depends entirely on the load and distance — this is exactly the case where voltage drop governs. A 60 amp subpanel 150 ft away typically needs 2 AWG copper or 1/0 aluminium, not the 6 AWG ampacity alone would allow.