Voltage drop is caused by resistance in the conductor or connections leading to the electrical load. There are many causes of resistance in the conductor path. There are four fundamental causes of voltage drop:

1. Material - Copper is a better conductor than aluminum and will have less voltage drop than aluminum for a given length and wire size.

2. Wire Size - Larger wire sizes (diameter) will have less voltage drop than smaller wire sizes (diameters) of the same length.

3. Wire Length - Shorter wires will have less voltage drop than longer wires for the same wire size (diameter).

4. Current Being Carried - Voltage drop increases on a wire with an increase in the current flowing through the wire.

Voltage Drop Limits
The National Electric Code, Section 210-19(a), recommends limiting the voltage drop to 3% on a branch circuit to the farthest output for power, heating or lighting. The fine-print note to NEC Section 215-2(b) recommends limiting voltage drop on feeder conductors and the branch circuit to the farthest outlet should not exceed 5%.