Bonding jumper size inside a metal junction box

Short answer: the jumper from the equipment grounding conductor splice to a metal box is sized exactly like the largest EGC in that box. Read NEC Table 250.122 at the rating of the overcurrent device ahead of the circuit; the worked example below sizes a 100 A feeder at 8 AWG copper. The supply-side bonding table has nothing to do with this bond, and neither does the voltage at the box — 120/208 V and 277/480 V size it the same way. Below is the sizing rule, the splice-to-screw connection step by step, the checks that prove the bond, and the four mistakes that fail it at inspection. The grounding and bonding guide covers the fundamentals this page assumes.

What size bonding jumper goes from the EGC splice to a metal box?

Size the jumper from the EGC splice to the grounding screw exactly like the largest equipment grounding conductor in the box. The copper column of NEC Table 250.122, read at the rating of the largest overcurrent device, gives you that size in one step.

A bench rule lands in the same place: take the biggest EGC you actually pulled into the box. If a voltage-drop calculation made you run 6 AWG copper where the table allows a smaller conductor, the jumper is 6 AWG. Match the metal as well, because the copper and aluminum columns give different answers.

The reason sits in NEC 250.4(A)(5), which calls the metal fault path the effective ground-fault current path and rules the earth out of it. A splice left floating off the box breaks that path at the box wall. The walkthrough on bonding a metal junction box with spliced feeder conductors covers the splice itself; this page stays on the jumper size.

Which NEC table sizes the jumper, Table 250.122 or the supply-side table?

NEC Table 250.122 sizes this jumper, because it is keyed to the rating of the overcurrent device ahead of the circuit — the same basis as the EGCs you are joining.

The supply-side bonding table lives nearby in the NEC but does a different job. It sizes conductors that carry fault current with no overcurrent device ahead of them, and it reads across from the size of the ungrounded supply conductors. Your box bond sits on the load side of a breaker or fuse, so the device rating decides. The code states it plainly: wire-type equipment grounding conductors of copper, aluminum or copper-clad aluminum are never smaller than Table 250.122 shows.

Hold onto one contrast that stops the confusion early. The NEC ampacity tables put 8 AWG copper at only about 50 A as an insulated conductor at 75 °C, yet 8 AWG is the required EGC on the 100 A device of this example. The two tables answer different questions, and this bond follows the EGC table. Where a sub-panel feed raises the same which-table question, the two-rule walkthrough on sub-panel bonding jumper size separates the supply-side case from this one.

How do you size and build the bond step by step?

Work from the largest overcurrent device protecting anything in the box, and let the jumper follow any installed EGC that voltage drop has made larger.

What you need

• jumper wire of the size you computed, green insulated or bare copper
• listed twist-on connectors rated for the exact conductor combination
• a green grounding screw; most stamped boxes tap the ground hole for a 10-32 screw
• screwdriver, wire strippers and a multimeter

1. Isolate the box and verify absence of voltage before the cover comes off.
2. List every circuit in the box with its breaker or fuse rating and take the largest device, the feeder in the example below.
3. Read NEC Table 250.122 at that rating, then size the jumper to the largest EGC the box actually holds.
4. Cut the jumper about 6 in [150 mm] long; extra length adds resistance for nothing.
5. Twist the EGCs and the jumper tail into a listed connector named for that mix on its packaging chart.
6. Land the tail on the green screw and seat it on bare metal; strip paint or powder coat at the contact point on coated boxes under NEC 250.12.
7. Tug every conductor and measure splice-to-box continuity before the cover goes back on.

Worked example

The largest device is the feeder, so the jumper is 8 AWG copper, green insulated or bare. The splice holds one feeder-size, one 10 AWG and three 12 AWG EGCs plus the jumper tail. Six conductors fit one listed connector, or two connectors bridged by the tail. The 6 in tail lands on the green screw and the bond is done. The splice group eats box volume; the walkthrough on box size for 8 AWG three-phase splices shows how much room conductors that size really take.

When does a ground bar beat a single splice?

Put a ground bar in the box once the splice grows past about four EGCs: each conductor lands on its own lug, and a jumper sized to the largest EGC bonds the bar to the box.

Beyond that count the twist-on bundle gets clumsy fast. A bar gives every EGC its own lug and turns a fistful of tails into one tidy termination. Mount the bar to bare box metal and bond it with a single jumper of the feeder size, which covers the whole group. On powder-coated carbon steel the coating comes off the contact points under NEC 250.12, while stainless arrives bare so the lug bites clean metal. The enclosure itself can carry bonding duty when the build allows it — the walkthrough on a panelboard enclosure as an equipment bonding jumper shows when that works.

Does the voltage at the box change the jumper size?

No — a 20 A branch takes the same 12 AWG copper jumper at 120/208 V as it does at 277/480 V, because the box-bonding rule covers every voltage and the size follows the overcurrent device rating, not the volts.

The size moves only when the device rating moves or the conductor metal changes. The aluminum column of Table 250.122 runs one to two sizes larger than copper, so read the column that matches the conductors you installed. Color holds at every voltage too: insulated conductors 6 AWG and smaller stay green or green with yellow stripes under NEC 250.119.

How do you prove the bond is good before the cover goes back on?

Three checks settle it: a continuity reading of 0 Ω from splice to bare box metal, a firm tug on every conductor, and the right connection at the screw — the feeder-size tail adds only about 0.0003 Ω over 6 in.

Run the arithmetic first. Standard copper tables put 8 AWG at 0.63 Ω per 1000 ft at 20 °C, so a 6 in [150 mm] tail measures 0.63 × 0.5 ÷ 1000 = 0.0003 Ω, and any meter reads that as 0 Ω. Then work the list:

• Measure from the splice to bare box metal; on coated boxes, land on the screw itself or a filed spot.
• Tug each conductor at the splice and each lug on a bar.
• Confirm insulated jumpers 6 AWG and smaller are green or green with yellow stripes (NEC 250.119).
• Look at the screw seat: bare metal, screw tight, tail free of nicks.

What fails this bond at inspection?

Four failures turn up again and again: a jumper sized from the supply-side table, a jumper sized to a branch EGC beside a bigger feeder EGC, a grounding screw sitting on paint, and a splice connector not listed for the mix inside it.

• Wrong table: supply-side values read across from the ungrounded conductor size, while this bond follows the overcurrent device rating through Table 250.122.
• Sized to a branch: a 12 AWG jumper beside the feeder-size EGC leaves the box unbonded for the biggest fault path, so the feeder size wins.
• Coating under the screw: NEC 250.12 takes paint, lacquer and enamel off the contact surfaces at grounding connections unless the fitting is made to bite through.
• Unlisted mix: the connector's packaging names the combinations it holds, such as 12 AWG with 10 AWG; a bundle outside its chart fails the splice, not just the bond.

Does anything change inside a stainless junction box?

The sizing never changes — the same table runs the bond in stainless as in painted steel — and bare stainless helps twice: there is no paint to strip under the screw, and we cut the box to your layout and deliver it in 10 business days.

The table asks about the device and the conductors, not the box metal. Stainless removes one chore, because there is no coating to strip at the screw and no rust bleeding out of a scratched paint line. Our stainless wall-mount boxes come with a hinged gasketed door and a removable mounting panel. A hinge is not a bond, so a door that carries grounded parts gets its own jumper; the enclosure door bonding jumper walkthrough covers when a hinge stops counting. The removable back panel bonds by the same pigtail-and-bare-metal method shown for back panel bonding on a fiberglass enclosure. For a box at your own size, depth and cutouts, we build each one from a single piece. Start from custom stainless enclosures or the stainless size finder.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions