Technical Article

NEC 2023 Basics: Sizing Equipment Grounding Conductors

September 03, 2023 by Lorenzo Mari

Learn how to size equipment grounding conductors.

National Electrical Code (NEC) Section 250.122 rules the sizing of equipment grounding conductors. Table 250.122 displays the minimum conductor size for grounding raceways and equipment based on the ampere rating or setting of the circuit’s overcurrent protective device. The conductor must be large enough to carry the fault current.

Image used courtesy of Pixabay

National Electrical Code Section 250.122 Size of Equipment Grounding Conductors

Section 250.122(A) General

The rules for sizing wire-type equipment grounding conductors – copper, aluminum, or copper-clad aluminum – are:

Use Table 250.122 to establish the minimum size – based on the ampere rating or setting of the automatic overcurrent device protecting the circuit ahead of the raceway or equipment.
Employing a conductor size larger than the circuit conductors is unnecessary.
Install cable trays, raceways, and cable armors or sheaths to establish low-impedance paths for fault currents.
Divide the equipment grounding conductors inside multiconductor cables only if the joint cross-sectional area satisfies Table 250.122.

Figure 1 shows two equipment grounding conductors (EGC) sized per the ampere rating or setting of the overcurrent device ahead of the raceway or equipment. Size EGC1 per CB1 and EGC2 per CB2, using Table 250.122 in both cases. The equipment grounding conductors are not required to be larger than the phase conductors.

Figure 1. Size the equipment grounding conductors per the rating or setting of the overcurrent device ahead of the raceway or equipment. Image used courtesy of Lorenzo Mari

Example 1: Determine the minimum size of the equipment grounding conductor for a circuit protected by a 250 A automatic overcurrent protective device.

Solution: Enter table 250.122. 250 A is not listed. Go to the next larger size, 300 A, and select size N° 4 AWG copper or N° 2 AWG aluminum or copper-clad aluminum.

Section 250.122(B) Increase Size

There are several reasons to increase the size of the ungrounded conductors. If so, increase the wire-type equipment grounding conductor size proportionally.

This rule does not apply when the causes for increasing the ungrounded conductor size are:

Ambient temperature corrections.
To adjust for more than three current-carrying conductors in a raceway or cable.
Adjustment where single conductors or multiconductor cables not installed in raceways does not maintain spacing for a continuous length longer than 600 mm.

Increasing the size of the ungrounded conductors to correct voltage drops is a good illustration of this rule’s application.

Example 2: Determine the minimum size of a copper equipment grounding conductor for a 350 kcmil THW circuit protected by a 250 A automatic overcurrent device. According to Table 310.16, a conductor size N° 250 kcmil THW has an ampacity of 255 A. The reason for using a conductor size N° 350 kcmil is for voltage drop correction.

Solution: Table 250.122 asks for a minimum equipment grounding conductor size N° 4 AWG, copper. This conductor requires an increase in size.

From Table 8, Chapter 9, a conductor size N° 4 AWG has a cross-sectional area of 41,740 circular mils.

The increasing factor equals circular mil selected/circular mil required.

350 kcmil/250 kcmil = 1.4

41,740 x 1.4 = 58,436 circular mils

From Table 8, Chapter 9, this cross-sectional area stands between the conductor sizes N° 3 AWG and N° 2 AWG.

Select conductor size N° 2 AWG.

An exception permits a qualified person to size the equipment grounding conductor to provide an effective ground-fault current path.

Section 250.122(C) Multiple Circuits

This section permits using a single equipment grounding conductor when multiple circuits share the same raceway, trench, cable, or cable tray.

Size per Table 250.122 entering with the largest overcurrent device protecting the circuits.

Single equipment grounding conductors installed in cable trays must be insulated, covered, or bare and sized N° 4 AWG as a minimum, according to Section 392.10(B)(1)(c).

Example 3: Compute the size of a single equipment grounding conductor in Figure 2.

Solution: The largest overcurrent device rating is 60 A. From Table 250.122, use conductor size N° 10 AWG, copper, or N° 8 AWG aluminum or copper-clad aluminum.

Figure 2. Setup for Example 3. Image used courtesy of Lorenzo Mari

Section 250.122(D) Motor Circuits

250.122(D)(1) General

Determine the minimum size of the equipment grounding conductor per Table 250.122 based on the branch-circuit short-circuit and ground-fault protective device rating.

Recall that the equipment grounding conductor does not need to be larger than the circuit conductors supplying the equipment.

Example 4: Size a copper wire-type equipment grounding conductor for a squirrel cage, 7 ½ hp, 3-phase, 208 V motor.

Solution:

From Table 430.250, full load current = 24.2 A

From Section 430.22, conductor ampacity = 24.2 A x 1.25 = 30.25 A

From Table 310.16, 75°C column, a copper conductor size N° 10 AWG has an ampacity of 35 A

From Table 430.52(C)(1), inverse time breaker = 24.2 A x 250% = 60.5 A

Next standard device = 70 A

From Table 250.122, EGC = size N° 8 AWG, copper

The equipment grounding conductor is not required to be larger than the circuit conductors or 10 AWG

250.122(D)(2) Instantaneous-Trip Circuit Breaker and Motor Short-Circuit Protector

Size the equipment grounding conductor per Section 250.122(A) when the overcurrent device is an instantaneous-trip circuit breaker or a motor short-circuit protector.

Use the maximum permitted rating of a dual-element time-delay fuse selected for branch-circuit, short-circuit, and ground-fault protection per Section 450.52(C)(1).

Section 450.52(C)(1) requires using Table 450.52(C)(1). Use the higher standard rating if the value does not match a standard ampere rating or setting.

Example 5: Size a copper wire-type equipment grounding conductor for a squirrel cage, 40 hp, 3-phase, 460 V motor.

Solution:

From Table 430.250, full load current = 52 A

From Section 430.22, conductor ampacity = 52 A x 1.25 = 65 A

From Table 310.16, 75°C column, a copper conductor size N° 6 AWG has an ampacity of 65 A

From Table 430.52(C)(1), dual element time-delay fuse = 52 A x 175% = 91 A

Next standard device = 100 A

From Table 250.122, EGC = size N° 8 AWG, copper

Section 250.122(F) Conductors in Parallel

Install the equipment grounding conductor in circuits of parallel conductors following sections 250.122(F)(1) or (F)(2).

250.122(F)(1) Parallel Conductors in Raceways, Auxiliary Gutters, or Cable Trays

(a) A single wire-type equipment grounding conductor – sized per Section 250.122 – may be installed if the circuit conductors are connected in parallel in a single raceway, auxiliary gutter, or cable tray.

Example 6: Size a copper wire-type equipment grounding conductor for a 2,000 A feeder containing seven parallel sets of 350 kcmil copper conductors per phase in a single cable tray.

Solution:

Enter Table 250.122 with 2,000 A and read a minimum copper conductor size of 250 kcmil.

(b) If circuit conductors are installed in parallel in multiple raceways, install and connect parallel wire-type equipment grounding conductors – sized per Section 250.122 – in each raceway.

Figure 3 shows two equipment grounding conductors, one per raceway, connected in parallel.

Figure 3. Parallel equipment grounding conductors in two raceways. Image used courtesy of Lorenzo Mari

Single wire-type equipment grounding conductors installed in cable trays must be insulated, covered, or bare and sized N° 4 AWG or larger, according to Section 392.10(B)(1)(c).

The equipment grounding conductor may be the metal raceway or auxiliary gutter, per Section 250.118, or the cable tray, per Section 392.60(B).

250.122(F)(2) Multiconductor Cables

Size the equipment grounding conductor in each multiconductor cable per Section 250.122 – except as provided in Section 250.122(F)(2)(c) for raceway or cable tray installations.

Connect the equipment grounding conductors in parallel in each cable if the circuit conductors are connected in parallel.

It is permitted to use a single equipment grounding conductor sized per Section 250.122 in combination with the equipment grounding conductors integral with multiconductor cables paralleled in the same raceway, auxiliary gutter, or cable tray. Connect all equipment grounding conductors together.

Single wire-type equipment grounding conductors installed in cable trays must be insulated, covered, or bare and sized N° 4 AWG or larger, according to Section 392.10(B)(1)(c). The equipment grounding conductor may be the metal raceway or auxiliary gutter, per Section 250.118, or the cable tray, per Section 392.60(B).

Section 250.122(G) Feeder Taps

Size the equipment grounding conductor for feeder taps from Table 250.122 based on the overcurrent device ampere rating on the supply side ahead of the tap.
The equipment grounding conductor is not required to be larger than the tap conductors.

Example 7: Size a copper wire-type equipment grounding conductor for the feeder tap shown in Figure 4.

Solution: Enter table 250.122 with 400 A and read conductor size N° 3 AWG, copper.

Figure 4. Setup for Example 7. Image used courtesy of Lorenzo Mari

Takeaways of Sizing Equipment Grounding Conductors

Section 250.122 contains the rules for sizing equipment grounding conductors.
Table 250.122 shows the minimum conductor size for grounding raceways and equipment.
The equipment grounding conductors are not required to be larger than the tap conductors.

To catch up on Lorenzo Mari’s series on National Electrical Code 2023 Basics: Grounding and Bonding, follow these links: