Practice: equipment grounding conductor sizing
Each problem uses new numbers, so you practice the method rather than memorizing an answer.
Pick the answer: sizes and ratings come from your code book.
Worked solution
Formula
| Symbol | Meaning | Unit |
|---|---|---|
| P | Rating of the overcurrent device ahead of the circuit | A |
| CM | Circular mil area from Chapter 9, Table 8 | cmil |
| Ratio | Increased ungrounded conductor area ÷ normal ungrounded conductor area |
Worked example
A circuit is protected by a breaker rated 15 A. The wire-type equipment grounding conductor that runs with the circuit conductors will be copper. What is the minimum size of the EGC?
- Size the EGC from the rating of the overcurrent device ahead of the circuit (15 A), not from the load or the circuit conductors.
- Look up Table 250.122, copper column, in the first row at or above 15 A: 14 AWG.
- Minimum EGC: 14 AWG.
Answer: 14 AWG
Equipment grounding conductor sizing, explained
The equipment grounding conductor carries fault current back to the source long enough to trip the breaker. It only needs to carry that current for a moment, so it can be much smaller than the circuit conductors. Table 250.122 gives the minimum size for each breaker or fuse rating, in copper and aluminum columns.
Find the row for the rating of the overcurrent device. If the rating falls between rows, use the next larger row. A 70 A breaker uses the row for 100 A.
When the ungrounded conductors are increased above the minimum size, usually to limit voltage drop, the wire-type EGC must be increased too, in proportion to the circular mil areas. Find the ratio of the larger ungrounded size to the size that would normally be required, multiply the table EGC area by that ratio, and pick the first size in Chapter 9, Table 8 that has that much area.
For conductors run in parallel in separate raceways, each raceway gets its own EGC, sized from the full overcurrent device rating. You do not divide by the number of sets.
How to solve it on the exam
- Find the rating of the overcurrent device ahead of the circuit, not the load.
- Go to Table 250.122, find the first row at or above that rating, and read the copper or aluminum column.
- If the ungrounded conductors were upsized: look up the circular mils of the normal size and the increased size in Chapter 9, Table 8, and divide to get the ratio.
- Multiply the table EGC's circular mils by the ratio, then pick the smallest size in Chapter 9, Table 8 with at least that area.
- For parallel runs, put the full Table 250.122 size in each raceway.
Common mistakes
- Sizing the EGC from the load current or from the ungrounded conductor size.
- Using Table 250.66, which is for grounding electrode conductors.
- Not increasing the EGC when the ungrounded conductors are increased for voltage drop.
- Increasing the EGC by the same number of sizes instead of by the circular mil ratio.
- Dividing the breaker rating by the number of parallel sets.
Exam tips
- Table 250.122 sits right after the 250.122 text; tab it separately from Table 250.66 so you never grab the wrong one.
- Chapter 9, Table 8 is the only place you need for the circular mil areas in the upsizing problem.
Equipment grounding conductor sizing calculator
Questions people ask
How do you size an equipment grounding conductor?
Use Table 250.122 with the rating of the overcurrent device protecting the circuit. Read the copper or aluminum column in the first row at or above that rating.
Do I need a bigger ground wire if I upsize the conductors for voltage drop?
Yes. Under 250.122, a wire-type EGC must be increased in proportion to the increase in circular mil area of the ungrounded conductors.
What size ground wire for a 200 A feeder?
Look up 200 A in Table 250.122. The table gives the minimum copper and aluminum sizes for that rating; check it in your code book.
What is the difference between Table 250.122 and Table 250.66?
Table 250.122 sizes equipment grounding conductors from the overcurrent device rating. Table 250.66 sizes grounding electrode conductors from the size of the service conductors.
Sources
- TDLR: Electrician examination information
- Electrical Contractor: Voltage drop and sizing equipment grounding conductors
About the tables: The exam is open book. These problems cite the 2026 NEC section and table so you practice finding them in your own book. We never print NEC text or tables.