NEC Table 250.66 and Table 250.122 for the Journeyman Exam: How to Look Up GEC and EGC Sizes Step by Step

Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.

This page is a lookup drill. If you want the ideas behind grounding first, read what electrical grounding is. For the job-site errors that come from mixing these conductors up, see NEC 250 grounding and bonding: 5 mistakes apprentices make.

Step 1: which table does the question need?

Conductor in the question	Table	You look it up by
Grounding electrode conductor (service to the electrode)	Table 250.66	Largest ungrounded service conductor size, or the total area of parallel sets
Equipment grounding conductor (run with the circuit)	Table 250.122	Rating or setting of the overcurrent device ahead of the circuit
Supply-side bonding jumper and main bonding jumper	Table 250.102(C)(1)	Largest ungrounded supply conductor size
Load-side equipment bonding jumper	Table 250.122 (NEC 250.102(D))	Overcurrent device rating

A quick test: if the question gives you a wire size for the service, you are probably in Table 250.66. If it gives you a breaker or fuse rating, you are probably in Table 250.122.

Table 250.66: sizing the GEC

Read down the left column until you reach the row that contains the service conductor size, then read across to the GEC size.

• 3/0 copper service conductors: the 2/0 or 3/0 row gives a 4 AWG copper GEC.
• 250 kcmil copper: the over 3/0 through 350 kcmil row gives 2 AWG copper.
• Parallel sets: add the areas of one ungrounded conductor from each set. Three sets of 400 kcmil copper are 1,200 kcmil, which is over 1,100 kcmil, so the GEC is 3/0 copper.
• Aluminum service conductors: use the aluminum column. Three sets of 300 kcmil aluminum are 900 kcmil, in the over 500 through 900 kcmil aluminum row, so the GEC is 1/0 copper.

The electrode limits. For some electrodes, the GEC need not be larger than a set size, even when the table gives a bigger one:

GEC that is the only connection to	Need not be larger than
Rod, pipe or plate electrode (NEC 250.66(A))	6 AWG copper
Concrete-encased electrode (NEC 250.66(B))	4 AWG copper
Ground ring (NEC 250.66(C))	The ring conductor

So for the same 1,200 kcmil copper service, Table 250.66 gives a 3/0 copper GEC to a metal underground water pipe electrode. Where the GEC is the only connection to a ground rod, 6 AWG copper is enough, and where it is the only connection to a concrete-encased electrode, 4 AWG copper is enough.

Table 250.122: sizing the EGC

Find the row for the overcurrent device rating. If the rating falls between rows, use the next row up.

• 200 A breaker: 6 AWG copper.
• 60 A breaker: 10 AWG copper.
• 70 A breaker: there is no 70 A row, so use the 100 A row: 8 AWG copper.
• 800 A device: 1/0 copper.
• 1,200 A device: 3/0 copper.

Rules that change the lookup:

1. Size cap. A wire-type EGC is not required to be larger than the circuit conductors supplying the equipment (NEC 250.122(A)).
2. Upsized conductors. If the ungrounded conductors are made larger than the minimum, for example for voltage drop, the EGC goes up in proportion to their circular mil area (NEC 250.122(D)). Upsizing only to cover ampacity adjustment or correction does not trigger the increase.
3. One EGC for several circuits. Where one EGC serves several circuits in the same raceway, cable or similar wiring method, size it for the largest overcurrent device protecting those circuits (NEC 250.122(E)).
4. Motor circuits. Motor branch circuits have their own basis for the lookup (NEC 250.122(F)).
5. Parallel conductors in separate raceways. Each raceway gets its own EGC, and each one is sized from the full rating of the device ahead of the whole set, not divided by the number of raceways (NEC 250.122(H)).
6. Feeder taps. Size the EGC from the device ahead of the feeder, but it need not be larger than the tap conductors (NEC 250.122(I)).

The 2026 NEC renumbered parts of 250.122. Older study guides list the parallel rule under (F) and feeder taps under (G).

Worked example 1: proportional increase

A 1,200 A feeder runs as parallel sets in separate raceways. The minimum conductor is 600 kcmil, but it is upsized to 750 kcmil for voltage drop.

1. Table 250.122 for 1,200 A: 3/0 copper, which is 167,800 circular mils.
2. Increase ratio: 750 ÷ 600 = 1.25.
3. New area: 167,800 × 1.25 = 209,750 circular mils.
4. The next standard size that covers it is 4/0 (211,600 circular mils), in each raceway.

Worked example 2: a building fed by a feeder

A detached building is fed by 3/0 copper conductors from a 200 A breaker in the main building.

1. GEC at the building: Table 250.66 from the 3/0 supply conductors gives 4 AWG copper (NEC 250.32(B)(1)).
2. EGC run with the feeder: Table 250.122 from the 200 A breaker gives 6 AWG copper.

Same building, two different conductors, two different tables.

Check yourself

1. The service is 2/0 copper. What size copper GEC goes to the water pipe? 4 AWG.
2. A 100 A breaker protects a feeder. What is the minimum copper EGC? 8 AWG.
3. A GEC goes only to a ground rod, and Table 250.66 says 1/0. What is the most the code asks for? 6 AWG copper.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, 250.32, 250.66, 250.102, 250.122, Table 250.66, Table 250.102(C)(1) and Table 250.122