Wire size and the long run

Two circuits can carry the same 20 amps and still need different wire — because distance matters as much as load. The reason is voltage drop.

Voltage leaks out along the wire

Wire has a little resistance, and over a long run that resistance eats some of your voltage before it reaches the load. The longer the run and the higher the current, the more you lose. Lose too much and lights dim, motors run hot, and tools work harder than they should.

The 3% rule

The standard target is to keep the drop to 3% or less of the supply voltage — about 3.6 V on a 120 V circuit. The fix for a run that drops too much is simple: use a fatter wire. More copper means less resistance, which means less drop.

A 20-amp load on 12-gauge wire is fine at 50 feet. Run it 100 feet and the drop climbs past 3%, so you step up to 10-gauge. Same load, longer run, bigger wire.

Why it matters for your bid

Fatter wire costs more, and a long run is exactly where estimates go wrong. The voltage-drop calculator does the math — now you know why it sometimes tells you to upsize, and you can explain it to the customer.