What Is the Smallest Wire You Can Use on a Grounding Bushing?
It depends on which side of the service or feeder overcurrent device you are on. On the load side, the bonding jumper is sized from Table 250.122 based on the rating of the overcurrent device ahead of the raceway, and the table bottoms out at 14 AWG copper. On the supply side, it is sized from Table 250.102(C)(1) based on the size of the ungrounded service conductors, and the smallest value in that table is 8 AWG copper.
Load side: 250.102(D) and Table 250.122
Section 250.102(D) covers equipment bonding jumpers on the load side of an overcurrent device. It requires the jumper to be sized in accordance with 250.122, based on the rating of the overcurrent device in the circuit ahead of the equipment. So a grounding bushing on a raceway protected by a 20 ampere breaker gets a 12 AWG copper jumper per Table 250.122. A 100 ampere feeder gets 8 AWG. A 200 ampere feeder gets 6 AWG. A 400 ampere feeder gets 3 AWG.
Where a single equipment bonding jumper bonds two or more raceways, 250.102(D) requires it to be sized in accordance with 250.122 based on the rating of the largest overcurrent device supplying those raceways.
Section 250.122(B) adds an upsizing rule that catches people. Where ungrounded conductors are increased in size for any reason, such as voltage drop, the equipment grounding conductors and by extension the bonding jumpers have to be increased in size proportionally to the increase in circular mil area of the ungrounded conductors.
Supply side: 250.102(C) and Table 250.102(C)(1)
Supply side bonding jumpers, meaning anything between the service point and the service disconnect, are sized from Table 250.102(C)(1) using the total circular mil area of the largest ungrounded service conductor per phase. A 200 ampere service on 4/0 aluminum, for example, lands on a 4 AWG copper supply side bonding jumper. The minimum value anywhere in that table is 8 AWG copper.
Section 250.102(C)(2) covers the case where the service conductors are in parallel across multiple raceways: an individual supply side bonding jumper is required in each raceway, sized from the largest ungrounded conductor in that raceway.
When a grounding bushing is required at all
Section 250.97 requires bonding for metal raceways containing circuits over 250 volts to ground. Where a raceway terminates at a ringed knockout, a concentric or eccentric knockout, or an oversized knockout, standard locknuts and bushings are not accepted as the bonding means, and you need a bonding type locknut, a bonding bushing with a jumper, or a listed fitting. There is an exception where the box or enclosure knockout is punched to the exact size of the fitting with no concentric rings remaining.
Section 250.92(B) requires bonding for service raceways regardless of voltage, and that is why grounding bushings show up on nearly every service riser and meter main nipple even at 120/240 volts.
Two field notes. The bushing lug and the jumper must be listed for the conductor material, and 250.8 governs the connection means, so no sheet metal screws. And a bonding bushing installed with the jumper omitted is one of the most common corrections written on service inspections, because the bushing looks right from across the room.