4.4 Equipment Grounding & Bonding (0304)

NEC Article 250 — Equipment Grounding & Bonding

A reliable equipment grounding and bonding system ensures that fault current has a low-impedance path back to the source, clearing OCPDs quickly and keeping non-current-carrying metal at the same potential. Domain 0304 tests EGC sizing (Table 250.122), installation methods, and bonding rules for raceways and services.

EGC Types & Sizing (250.122)

250.122 requires the equipment grounding conductor (EGC) to be sized by the rating of the OCPD protecting the circuit conductors (not by the load current). The EGC may be a wire-type conductor, a metal raceway, cable armor, or a combination. 250.122(B) requires the EGC to be increased in size proportionally whenever the ungrounded circuit conductors are increased in size for voltage drop, so the fault path impedance stays low.

Table 250.122 Excerpt

There is no 30 A or 40 A row: read the left column as "not exceeding," so a 30 A or 40 A device uses the 60 A row (10 AWG Cu). Two limits bound the result — 250.122(A) sets an absolute floor of 14 AWG copper, and the EGC is never required to be larger than the circuit conductors supplying the equipment.

EGC Installation (250.120, 250.134)

250.120 governs how EGCs are run: wire-type EGCs in raceways must be installed with the circuit conductors (or within the same cable). 250.134 lists the permitted installation methods — the EGC must be run with, or enclose, the circuit conductors (in a raceway, cable, or wireway) or be part of a listed cable assembly. A lone EGC pulled separately from the circuit conductors is not permitted because it raises the impedance of the fault path and increases clearing time.

Bonding of Raceways & Enclosures (250.96, 250.97)

250.96(A) requires metal raceways, cable armor, enclosures, frames, fittings, and other metal non-current-carrying parts to be bonded together so they are electrically continuous and able to carry fault current, with any nonconductive paint, enamel, or coating removed at threads, contact points, and contact surfaces. (250.96(B) is a different rule — it permits isolating a raceway to reduce electromagnetic interference on an isolated-ground circuit.)

250.97 is the over-250-volt rule: for circuits of over 250 volts to ground, the metal raceway or cable termination at a box or enclosure with concentric or eccentric knockouts must be made with one of the listed bonding methods — threadless couplings and connectors made up tight, two locknuts (one inside and one outside) on rigid or intermediate metal conduit, fittings with shoulders seating firmly against the box, listed fittings, or bonding jumpers with bonding-type locknuts, bushings, or listed devices. Standard locknuts alone will not do, because a punched concentric knockout leaves rings of sheet metal in the fault path.

Bonding Jumpers (250.28, 250.102)

• The main bonding jumper connects the grounded (neutral) conductor to the equipment grounding conductor and enclosure at the service disconnect; the system bonding jumper makes the same connection at a separately derived system such as a transformer or generator. Both are covered by 250.28 for material and by 250.28(D) for size — not smaller than the sizes in Table 250.102(C)(1) based on the largest ungrounded supply conductor. Supply-side bonding jumpers (on the line side of the service disconnect) are also sized from Table 250.102(C)(1).
• Equipment bonding jumpers on the load side (for example, between a panel and a metal raceway) are sized per 250.122 based on the OCPD rating protecting that portion of the circuit.

Bonding at the Service (250.92, 250.94)

250.92 requires all metallic service-entrance raceways, cable armor, and enclosures to be bonded together and to the service equipment, forming a single bonded service. 250.94 requires an intersystem bonding termination (IBT) — an accessible external point with at least three terminals for connecting the bonding conductors of communications, CATV, satellite, and similar systems to the building grounding electrode system. Section 9.4 covers the communications side of that connection.

Effective Fault-Current Path (250.4(A)(5))

250.4(A)(5) requires electrical equipment and wiring to be installed so that an effective ground-fault current path exists — one that is permanent, electrically continuous, capable of safely carrying the maximum fault current, and of sufficiently low impedance to clear the OCPD. This is the performance objective behind every rule in 250.96 through 250.122.

Separately Derived Systems (250.30)

A separately derived system (for example, a transformer or generator with no direct connection to the utility grounded conductor) requires its own grounding electrode and system bonding jumper per 250.30. The system bonding jumper ties the derived neutral to the EGC and the building grounding electrode system at, or ahead of, the first disconnect.

Worked Example — EGC Sizing with Voltage Drop Adjustment

A 200 A feeder uses 4/0 AWG copper ungrounded conductors. The designer increases the ungrounded conductors to 250 kcmil copper to limit voltage drop.

Step 1 — Base EGC: From Table 250.122, a 200 A OCPD requires a minimum 6 AWG copper EGC, which has a cross-sectional area of 26,240 circular mils (cmil).

Step 2 — Voltage-drop adjustment (250.122(B)): Because the ungrounded conductors were upsized, the EGC must be increased proportionally:

• Ratio = 250,000 cmil (250 kcmil) / 211,600 cmil (4/0 AWG) = 1.181
• Adjusted EGC area = 26,240 x 1.181 = 31,000 cmil

Step 3 — Select the next larger standard size: The smallest standard conductor with an area at or above 31,000 cmil is 4 AWG copper (41,740 cmil).

Result: The EGC must be increased from 6 AWG to 4 AWG copper to satisfy 250.122(B), keeping the fault path impedance proportional to the upsized ungrounded conductors.