When is a grounding bushing required?

NEC 250.97 requires bonding type fittings or bonding jumpers where raceways for circuits over 250 volts to ground terminate at boxes or enclosures with concentric or eccentric knockouts. Clean punched openings and threaded hubs are exempt under the exception.

Does a 480 volt circuit need a bonding bushing?

A 480Y/277 volt system is 277 volts to ground, which exceeds 250 volts, so NEC 250.97 applies wherever concentric or eccentric knockouts are involved.

What size bonding jumper goes on a grounding bushing?

NEC 250.102(D) sizes a load side bonding jumper from Table 250.122 using the rating of the overcurrent device protecting the circuit in that raceway.

Do you need a grounding bushing on a 480 volt transformer feed?

On a 480Y/277 volt system, yes, in most cases. NEC 2023 Section 250.97 requires that where circuits operate over 250 volts to ground, the electrical continuity of metal raceways and cables with metal sheaths that contain any conductor other than service conductors be ensured by one of the methods in 250.92(B)(2) through (B)(4): threaded couplings and hubs made up wrenchtight, threadless couplings and connectors made up tight, or bonding jumpers with bonding type fittings such as bonding locknuts and bonding bushings.

What triggers it

The number that matters is volts to ground, not volts between conductors. A 480Y/277 volt wye system is 277 volts to ground, which is over 250, so 250.97 applies. A 480 volt corner grounded delta is 480 volts to ground on two legs, also over 250. A 240/120 volt system at 120 volts to ground is not covered. On a 480 volt transformer feed with 8/3 conductors, the primary side raceway is the one that needs the treatment if the supply is 480Y/277.

The exception nobody reads

The exception to 250.97 states that where oversized, concentric or eccentric knockouts are not encountered, or where a box or enclosure with concentric or eccentric knockouts is listed to provide a reliable bonding connection, standard locknuts and bushings are permitted. That is the practical out. A punched knockout in a panelboard that leaves a clean, full ring of metal around the connector, with no partially removed concentric rings left in place, does not need a bonding bushing. The moment a concentric knockout is only partially removed and rings remain, the metal path around the opening is compromised and a bonding jumper or bonding locknut is required. Field punched holes and threaded hubs are also fine.

Bonding jumper sizing

Section 250.102(D) sizes the bonding jumper on the load side of the overcurrent device from Table 250.122 based on the rating of the device ahead of the raceway. For a transformer primary protected at 40 amps, that is a 10 AWG copper jumper. Section 250.102(C) covers supply side bonding jumpers, sized from Table 250.102(C)(1). The jumper connects from the bonding bushing lug to the equipment grounding bus or to an approved point on the enclosure, and 250.8 lists the acceptable connection means.

Insulating versus bonding bushings

Do not confuse the two. Section 300.4(G) requires an insulated fitting or insulating bushing wherever a raceway contains conductors 4 AWG or larger entering an enclosure, to protect the conductor insulation from abrasion. With 8 AWG conductors that rule is not triggered. A grounding bushing under 250.97 is about electrical continuity, and a plastic insulating throat bushing does nothing for it. Insulated grounding bushings, which have both a lug and a plastic throat, satisfy both requirements at once and are the usual purchase.

Field takeaway

Over 250 volts to ground with a concentric or eccentric knockout means a bonding bushing or bonding locknut with a jumper sized from Table 250.122. A clean punched hole with a full metal ring and a threaded hub connection does not need one.