Why wire itself has resistance worth calculating
Every real wire has some resistance, even copper, and that resistance eats a small amount of voltage over the length of the run before it ever reaches the load. It's Ohm's law applied to the wire itself rather than the device at the end of it — the longer and thinner the wire, the more voltage it quietly absorbs on the way.
R = (ρ × Length) ÷ Circular mils
Voltage drop = Current × R × Circuit factor
The circuit factor accounts for the fact that current has to complete a loop: for DC and single-phase circuits it's 2, because current travels out to the load and back through a second conductor, doubling the effective length of wire the current passes through. Three-phase circuits use about 1.732 (√3) instead, reflecting how three conductors share the load.
A worked example
Take a 15A, 120V single-phase circuit run 100 feet on 12 AWG copper wire:
R = (12.9 × 100) ÷ 6,530 ≈ 0.1976 Ω
VD = 15 × 0.1976 × 2 ≈ 5.93 V
VD% = 5.93 ÷ 120 × 100 ≈ 4.9%
That's already above the 3% guideline commonly cited for branch circuits, which is exactly the kind of result that pushes a real installation toward a heavier gauge like 10 AWG, or toward shortening the run if that's an option.
What the NEC guidance actually says
The National Electrical Code's informational notes suggest keeping voltage drop to around 3% on a branch circuit and no more than about 5% total from the service entrance to the furthest load, as a target for reasonably efficient operation. These are recommendations rather than a hard code violation in most jurisdictions, but exceeding them isn't harmless — a motor running on a chronically low voltage draws more current to make up for it and runs hotter, and LED drivers and electronics can behave unpredictably outside their rated input range. Local codes and specific equipment can impose stricter limits, so always check the actual requirement for the installation in question.
Bringing voltage drop back under a target
There are really only four levers, and they all follow directly from the formula: use a larger wire gauge (lower resistance per foot), shorten the run if the layout allows it, reduce the current the circuit carries, or raise the system voltage so the same power moves at lower current. Wire size is usually the practical answer for an existing layout, since it's often easier to upsize a conductor than to relocate a panel or a load.
Related electrical calculations
This calculator is really Ohm's law applied specifically to wire resistance — for the more general V = I × R relationship, see the Ohm's law calculator, and for broader appliance power and cost calculations, the electricity calculator.