Grounding and bonding conductor sizing

The wire-type equipment grounding conductor (EGC) must carry fault current long enough for the breaker or fuse to open, so it is sized from the device ahead of it, not from the load.

Where to look
Article 250, 250.122 and Table 250.122. Each row is an overcurrent device rating "not exceeding" that value; read the copper or the aluminum/copper-clad aluminum column. 250.122(A): the EGC never has to be larger than the circuit conductors. 250.102(D): an equipment bonding jumper on the load side of a device is sized the same way.

Procedure

1. Find the rating of the breaker or fuse protecting the circuit (calculate it first if needed).
2. Go down the first column to the first row equal to or greater than that rating.
3. Read the copper column unless the question says aluminum.

Worked example
A feeder is protected by a 150-A breaker. Copper EGC?

• 150 A is more than 100 and not more than 200, so the 200-A row applies.
• Copper column: 6 AWG.

Most common mistake
Sizing from the load current or the conductor ampacity instead of the device rating. Using the supply-side table (Table 250.102(C)(1), based on conductor size) for an EGC is the other.

The grounding electrode conductor (GEC) connects the service to the earth electrodes. It is sized from the service conductors, not from the load.

Where to look
Article 250 Part III, 250.66 and Table 250.66. Find the row for the largest ungrounded service conductor (copper or aluminum column) and read the GEC size. Note 1: for parallel sets, add the areas of one conductor from each set. Then the limits: (A) a GEC that goes only to rods, pipes or plates need not be larger than 6 AWG copper; (B) only to concrete-encased electrodes, not larger than 4 AWG copper; (C) to a ground ring, not larger than the ring conductor.

Procedure

1. Identify the largest ungrounded conductor, or add the parallel sets.
2. Read the table row in the right metal column.
3. If the GEC runs only to one electrode type with a limit, apply it.

Worked example
Two sets of 250 kcmil copper; GEC to the water pipe.

• 2 × 250 = 500 kcmil → "over 350 through 600" → 1/0 AWG copper.
• No limit applies to a water pipe.

Most common mistake
Sizing from one conductor of a parallel set, or forgetting that a GEC running only to ground rods stops at 6 AWG copper.

Bonding jumpers on the supply side must carry fault current back to the source, so they are sized from the supply conductors.

Where to look
Article 250: 250.28(D) for main and system bonding jumpers and 250.102(C) for supply-side jumpers. Both use Table 250.102(C)(1). Its rows look like the GEC table up to 1100 kcmil copper (1750 kcmil aluminum). Above that, Note 1 applies: the jumper must be at least 12.5 percent of the area of the largest conductor or the parallel equivalent. Note 2: it never needs to be larger than the ungrounded conductors. For parallel runs in separate raceways (250.102(C)(2)), size one jumper per raceway from the conductors in it, or one jumper for all raceways from the sum of the areas. Chapter 9 Table 8 gives the areas of AWG sizes.

Procedure

1. Find the largest conductor, or the sum of areas for parallel sets.
2. Up to 1100 kcmil Cu: read the table.
3. Above: multiply by 0.125 and pick the smallest size with at least that area.

Worked example
Four sets of 500 kcmil Cu, one jumper.

• 4 × 500 = 2000 kcmil; 12.5 percent = 250 kcmil.
• Jumper: 250 kcmil Cu.

Most common mistake
Using the 3/0 maximum from the GEC table for a large service. Bonding jumpers keep growing with the 12.5 percent rule.