Keep voltage drop in check

Why this rule exists

Wire has resistance, and current through resistance drops voltage along the run, delivering less to the load than the panel supplies. Long runs and small conductors make it worse. Excess drop starves motors, which then draw more current and overheat, dims lights, and makes electronics misbehave, and the lost voltage becomes heat in the wire. The NEC recommends keeping branch-circuit drop under 3 percent, and total under 5 percent, as good practice. It is not only performance: a wire dropping significant voltage is dissipating significant power along its length.

In practice

For long runs, calculate voltage drop and upsize the conductor if it exceeds about 3 percent. Approximate single-phase drop as 2 times length times current times the wire's resistance per foot. If it is too high, go up a wire size, which roughly halves resistance every two AWG steps. Watch runs to detached garages, well pumps, and outdoor equipment, where distances add up. Remember the current flows out and back, so use twice the one-way length. Size for the far end, not the panel.

Example

120 V circuit, 16 A, 100 ft one-way
12 AWG copper: ~1.98 ohms per 1000 ft
Vdrop = 2 x 100 x 16 x (1.98/1000)
Vdrop = ~6.3 V = 5.3% (too high)
Go to 10 AWG: ~1.24 ohms per 1000 ft
Vdrop = ~4.0 V = 3.3% (acceptable)

When it doesn't apply

Voltage drop is a performance recommendation, not a safety mandate, but it interacts with safety when undersized motors overheat or when compensating by overfusing. Very short runs rarely need the calculation. When a run is long enough that drop matters, do the arithmetic rather than guessing; the wire size that fixes it is cheaper than the failure it prevents.