When electricity travels through wire, some voltage is lost along the way. The longer the run or the thinner the wire, the more you lose. NEC recommends keeping this loss under 3% for branch circuits. This calculator shows exactly how much voltage is lost and whether your wire size is big enough.
Tip: Good to use when running a circuit to a detached garage, barn, or outdoor panel — anything more than about 100 feet from your main panel.