6.3 Equipment Grounding & Bonding (Article 250 Parts V-VIII)

Why This Matters

Once you leave the service, the equipment grounding conductor (EGC) is what carries fault current back to the source to trip the breaker. Part VI (250.118–250.126) defines what can be an EGC, and 250.122 sizes it. Parts V and VII cover bonding of raceways and enclosures so the fault path stays electrically continuous. Exam questions here are heavy on Table 250.122 lookups and on the bonding rules for services vs. load-side equipment.

Equipment Grounding Conductors — 250.118 and 250.122

250.118 lists the permitted types of EGCs: a wire (copper, aluminum, copper-clad aluminum), rigid metal conduit (RMC), intermediate metal conduit (IMC), electrical metallic tubing (EMT), flexible metal conduit (FMC, with conditions), the armor of Type AC cable, the metallic sheath of Type MC cable (with conditions), and cable trays. The EGC must be electrically continuous or made continuous with listed fittings.

250.122 sizes the wire-type EGC based on the rating of the overcurrent protective device (OCPD) ahead of the circuit — not the phase conductor size. Use Table 250.122:

Key 250.122 rules:

• (A) The EGC need not be larger than the circuit conductors.
• (B) If the ungrounded conductors are increased in size for voltage drop (or any reason other than 310.15(B)/(C) adjustment), the wire-type EGC must be increased proportionately based on circular mil area. A new 2020 Exception permits a qualified person to size the EGC per 250.4(A)(5) instead.
• (C) A single EGC sized for the largest OCPD can serve multiple circuits in the same raceway.
• (D) Motor circuits — size the EGC from Table 250.122 based on the motor branch-circuit short-circuit ground-fault protective device per 430.52, not the motor FLC.
• (F) Parallel conductors in nonmetallic raceways require a full-size EGC in each raceway, sized per Table 250.122.

Worked Example — Sizing an EGC for a 100 A Circuit

A 100 A breaker feeds a 100 A subpanel through 2 AWG copper THHN in EMT. The OCPD rating is 100 A. From Table 250.122, the row "70–100 A" requires 8 AWG copper EGC (or 6 AWG aluminum). If the feeder is 2 AWG copper but the engineer upsized the phase conductors to 1/0 AWG to address a 4% voltage drop on a long run, 250.122(B) requires the EGC to be increased proportionately. The ratio of 1/0 to 2 AWG circular mil area (105,600 / 66,360) ≈ 1.59; the 8 AWG EGC (16,510 cmil) × 1.59 ≈ 26,250 cmil → round up to 6 AWG copper.

Bonding — Parts V and VII

250.92 — Bonding at services. Service raceways, cable trays, cable bus framework, service cable armor, and service equipment enclosures must all be bonded together to form the fault-current return path. Standard locknuts and bushings alone are NOT enough; you need a bonding jumper or listed bonding fitting.

250.94 — Sizing service bonding jumpers. Service bonding jumpers are sized per Table 250.102(C)(1) based on the largest ungrounded service-entrance conductor — the same table used for the main bonding jumper.

250.96 — Bonding of raceways and enclosures. General rule: metal raceways, cable armor, boxes, and enclosures containing conductors must be effectively bonded where required to ensure the fault-current path. Locknuts that are the only means of bonding at service equipment are not enough; use bonding bushings or a bonding jumper.

250.97 — Bonding for systems over 250 V to ground. For circuits over 250 V to ground, the bonding of raceways and enclosures must be positive; standard locknuts are not permitted as the sole bonding means. Listed bonding bushings, jumpers, or fittings are required.

Box and Raceway Bonding — Practical Notes

• Service equipment: bond with listed fittings or jumpers; concentric knockouts can be problematic and may require bonding bushings.
• Load-side panels: the EGC terminal bar is bonded to the enclosure; the neutral bar is isolated.
• FMC over 6 ft, liquidtight flexible metal conduit over 6 ft (or over 1¼ in.) generally cannot be the sole EGC — run a wire EGC.
• 250.120: EGCs of 6 AWG and smaller must be green or green-with-yellow-stripe; 4 AWG and larger can be re-identified at accessible points.

Traps

• "EGC is sized by phase conductor ampacity" — false; it is sized by OCPD rating (250.122).
• "A 100 A breaker needs a 6 AWG copper EGC" — false; 100 A copper EGC is 8 AWG. 6 AWG is the aluminum column for 100 A, or the copper column for 110–200 A.
• "Standard locknuts bond a service raceway" — false at services and over 250 V to ground; use bonding bushings.
• "You can downsize the EGC when using a metal raceway as the EGC" — the raceway itself must still meet 250.122 where used as EGC, and a wire EGC inside still sizes per the table.