On a 20-ampere branch circuit the ungrounded copper conductors are increased from 12 AWG to 8 AWG to correct voltage drop. What minimum size copper equipment grounding conductor is now required for that circuit?
- A12 AWG copper
12 AWG is the unadjusted table value. Forgetting the proportional increase entirely is the most common mistake, and it leaves the fault path with higher impedance than the circuit was designed for.
- B10 AWG copper
10 AWG reflects only a one-size increase. The proportional calculation lands above 10 AWG at 10,380 circular mils, so this conductor is still too small.
- C6 AWG copper
6 AWG at 26,240 circular mils is well beyond what the ratio requires. Increasing by more than the proportion wastes copper and can make terminations difficult.
- 8 AWG copper
Why D is correct
When ungrounded conductors are increased above the minimum size needed for ampacity, any wire-type equipment grounding conductor must be increased proportionally by circular mil area. Two AWG sizes up on the phase conductors works out to two AWG sizes up on the ground.
What this question is testing
Whether you can carry out a proportional circular mil calculation, and whether you know that enlarging conductors for voltage drop triggers a matching increase in the equipment grounding conductor. It also checks that you can look up circular mil areas rather than estimating the proportion by eye.
On the job
Long runs to outbuildings, pole lights and well pumps routinely get upsized for voltage drop, and the ground gets forgotten because nobody thinks of it as a current-carrying conductor. The reasoning is sound though: if you lowered the impedance of the supply conductors, the fault path should keep pace so the breaker still sees enough current to trip. Note the increase applies only when conductors are enlarged above the size that would have had sufficient ampacity.
Memory technique
Upsize the hots, upsize the ground by the same number of steps.
Exam tip
A shortcut that works with standard AWG steps: move the ground up the same number of sizes you moved the phase conductors.
Where to look it up
The proportional increase rule sits in the equipment grounding conductor sizing section just after the general rule. Circular mil areas are in the conductor properties table in Chapter 9.