Equipment Grounding Conductor Size: Table 250.122 Worked Examples

The equipment grounding conductor is sized by the breaker or fuse protecting the circuit, not by the size of the hot wires. Under Table 250.122 of the 2020 NEC (NFPA 70), a 20 A circuit needs at least 12 AWG copper. A 100 A circuit needs 8 AWG copper, and 200 A needs 6 AWG copper. Upsized hots and parallel runs change the answer, as shown below.

Table 250.122 equipment grounding conductor calculator

Enter the breaker or fuse rating. Table 250.122 rows mean "not exceeding," so a rating between rows uses the larger row: a 30 A, 40 A or 50 A circuit uses the 60 A row, 10 AWG copper. If you made the hots bigger, the calculator applies the 250.122(B) increase too.

The table runs to 6000 A. "Aluminum" includes copper-clad aluminum. "kcmil" means thousands of circular mils, the unit for wire cross-section.

How to find equipment grounding conductor size

1. Find the breaker or fuse rating. Ignore the load and the wire ampacity.
2. Go down Table 250.122 to the first row equal to or larger than that rating.
3. If the hots were upsized for a reason other than 310.15(B) or (C), upsize the EGC (Example 1).
4. For parallel sets, put a full-size EGC in every raceway or cable (Example 2).

Example 1: hots upsized for voltage drop (250.122(B))

A 200 A feeder would normally use 3/0 copper (200 A at 75°C, Table 310.16). For voltage drop, the designer pulls 250 kcmil copper. Areas come from Chapter 9, Table 8.

1. Table 250.122 at 200 A: 6 AWG copper, 26,240 circular mils.
2. Ratio: 250,000 ÷ 167,800 (3/0) = 1.49.
3. New area: 26,240 × 1.49 = about 39,094 circular mils.
4. 6 AWG is too small. The next size, 4 AWG, has 41,740 circular mils.
5. Answer: 4 AWG copper EGC.

Example 2: a 400 A parallel feeder (250.122(F))

A 400 A breaker feeds two sets of 3/0 copper (200 A each at 75°C), one set per PVC conduit. Parallel conductors must be 1/0 or larger (310.10(G)), so 3/0 qualifies.

1. Table 250.122 at 400 A: 3 AWG copper.
2. PVC can't be the ground path, so each conduit needs a wire-type EGC.
3. Pull a 3 AWG copper EGC in each conduit and connect them in parallel.

Common Table 250.122 exam traps

• Sizing from the wire. A 100 A breaker gets 8 AWG copper no matter what size the hots are.
• Using the 2017 trigger. In 2017, any upsizing triggered the increase. The 2020 text exempts upsizing required by 310.15(B) or (C), like hot-ambient correction or bundling.
• Rounding down. If the math lands between sizes, take the larger one.
• Halving the EGC on parallel runs. Each raceway or cable gets a full-size EGC.
• Motor circuits. Size from the motor's branch-circuit breaker or fuse, then apply the limit. 14 AWG motor conductors on a 40 A breaker need only a 14 AWG EGC. JCR's Table 430.52 guide shows how that breaker is chosen.

Some states test or enforce an older edition, such as the 2017 NEC on Tennessee's exam. Find your state's edition in NEC adoption by state or on JCR's state requirements page.

Equipment grounding conductor questions

What size ground wire do I need?

It depends on the breaker or fuse, not the hot wires. Table 250.122 calls for at least 14 AWG copper up to 15 A, 12 AWG up to 20 A, 10 AWG up to 60 A, 8 AWG up to 100 A and 6 AWG up to 200 A.

What size ground wire do I need for a 200 amp feeder?

6 AWG copper or 4 AWG aluminum, from Table 250.122. Service equipment uses different bonding rules, covered in JCR's main bonding jumper guide.

What size ground wire for a 100 amp subpanel?

At least 8 AWG copper or 6 AWG aluminum. That comes straight from the 100 A row of Table 250.122.

Do I have to upsize the ground wire when I upsize for voltage drop?

Yes. Section 250.122(B) says a wire-type EGC grows in proportion to the hots' circular-mil increase. Exceptions: upsizing for 310.15(B) or (C), or an EGC sized by a qualified person for an effective ground-fault path.

Does each conduit in a parallel run need its own ground wire?

Yes. Each raceway gets a wire-type EGC sized from the full overcurrent device rating. Metal raceways that qualify under 250.118 can serve as the EGC instead.

Can one ground wire serve several circuits in the same conduit?

Yes. Section 250.122(C) allows one EGC for several circuits in a raceway, cable, trench or cable tray. Size it for the largest breaker or fuse among them.

Related guides: Main bonding jumper size (250.28) · Voltage drop calculation · 200 amp service wire size (Table 310.12)

More worked examples: grounding electrode conductor size (Table 250.66) and all NEC Answers.