Conductor Resistance

To calculate voltage drop, you need to know the resistance of the conductor (wire) used in the branch circuit. Resistance is a function of wire size and length. Resistance for both wires going to the luminaire must be considered.
The following table shows wire resistance for various American Wire Gauges (AWG). Since both wires are the same size in typical circuits, the table shows “loop resistance”; thus the designer need only calculate the distance between luminaire poles.
Wire Resistance by Gauge
Wire Size
Loop Wire Resistance*
(AWG)
(ohms/foot)
(ohms/meter)
12
0.003360
0.011023
10
0.002036
0.006680
8
0.001308
0.004291
6
0.000820
0.002690
4
0.000518
0.001700
2
0.000324
0.001063
0
0.000204
0.000670
00
0.000162
0.000532
* Values shown are for uncoated copper conductors in conduit at 25°C
Loop resistance accounts for the wire run in both directions, requiring the designer to measure only the one-way distance between luminaire poles.
Larger wire sizes have lower resistances. Using larger wire is one way to reduce the voltage drop in the circuit.