Bonding a metal junction box with spliced feeder conductors

Splice circuit conductors in a metal junction box and the box joins the ground path with them: every wire-type equipment grounding conductor associated with those spliced conductors connects together, and to the box. Metal conduit does not take that job over, and the rule holds in every occupancy. Size the jumper from the rating of the breaker or fuse ahead of the largest circuit spliced in the box, using NEC Table 250.122: 2 AWG copper covers a 500 A feeder. If you are settling a site argument or splicing four parallel 600 kcmil feeders in a pull box, the steps below give the rule, the sizing table, a worked bond at 1600 A, and the checks that prove the shell is bonded.

When a metal box must be bonded

The NEC states it in one line: when circuit conductors are spliced in a box, every wire-type equipment grounding conductor associated with those circuit conductors connects together, and to the metal box. The grounding and bonding guide explains why the shell is in the path at all: NEC 250.4(A)(5) calls for an effective ground-fault current path, and the earth is not considered part of it. A fault into an unbonded box leaves the metal live until some other connection happens to clear the circuit. You make the shell a conductor on purpose, with a jumper or a listed ground bar, on the day you make the splice.

Metal conduit does less here than the arguments usually claim. Where no wire-type EGC is run, the raceway can carry fault current, and NEC 250.96(A) requires metal parts serving as grounding conductors to be bonded together so they safely conduct any fault current likely to be imposed on them. Where a wire-type EGC does run beside the feeder, the conduit does not replace it: when you run both, the wire lands on the box. The entry hardware decides how well that connection holds, and the article on grounding conductor entry at the enclosure wall works through connector choice and the bond itself.

Two edges keep you from over-bonding. A wire-type EGC that passes through the box without a splice need not be joined to the EGCs spliced inside; leave it unbroken. Where the circuit conductors only pass through, no bond to the box is called for at all. Cut one of those conductors and splice it, and the bond is due the same day.

The occupancy argument ends quickly: NEC Article 250 covers every occupancy, and the box-bonding text carries no residential or light-commercial exception. A splice is a splice at 120 V in a shop or 480 V in a plant, and you bond it the same way.

Size the jumper from the breaker, not the conductor

NEC Table 250.122 sets the smallest wire-type copper equipment grounding conductor from the rating or setting of the breaker or fuse ahead of the circuit. The feeder conductor size never enters the lookup. In a box with several circuits spliced, take the largest rating on the list, because that device sets the fault energy the box bond has to carry. Run the jumper to the box at the same row as the largest EGC it connects. If you inherit a box that was never bonded, this table is the whole job: read the breaker, cut the jumper, land it on bare metal.

A 450 A device falls between rows, so take the next row up and run 2 AWG copper. Aluminum and copper-clad aluminum jumpers come from the same table's second column, one to two sizes larger than the copper values shown. An insulated jumper of 6 AWG or smaller keeps its color: green, or green with one or more yellow stripes, as NEC 250.119 requires for insulated equipment grounding conductors of that size.

What you need

None of it is exotic; most of it is already in your splicing kit.

• A listed ground bar with a tapped bonding hole, or bare copper jumper wire sized from the table above.
• Listed pressure or splicing connectors rated for each EGC size; for branch-circuit sizes, the page on selecting crimp tools for 6 and 8 AWG copper conductors shows how die choice changes the joint.
• A listed bonding screw or fitting for the box wall, plus a wire brush to strip paint at the contact point.
• A torque wrench or driver, so every lug lands on the value marked on it.
• A 4-wire low-resistance ohmmeter for the continuity check at the end.

Steps to bond the box

1. Lock out and tag the circuits feeding the box, then verify absence of voltage at the line lugs before the cover comes off.
2. List what the box holds: the breaker or fuse rating ahead of each spliced circuit, and whether each raceway carries a wire-type EGC.
3. Take the largest rating on that list to NEC Table 250.122 and cut the jumper to that row.
4. Splice the EGCs together with listed connectors, or land each one on the ground bar; keep every splice and termination inside the box.
5. Bond the bar or the splice to the box with one jumper. Brush the contact point to bare metal first. NEC 250.12 requires paint, lacquer and enamel to be removed from threads and contact surfaces at grounding and bonding connections, unless the fitting is designed to make removal unnecessary.
6. Torque every lug to the marking on it, and mark any insulated jumper of 6 AWG or smaller green, or green with a yellow stripe.
7. Check continuity from the box wall to the bar and out to each EGC tail; a sound bond reads in the milliohm range, and the next section gives the numbers.

Bonding the box for four parallel 600 kcmil feeders on a 1600 A breaker

A large feeder splice is where this rule earns its keep, so here is the whole bond worked through.

The inputs

• Four parallel sets of 600 kcmil copper per phase, one raceway per set, each raceway carrying its own wire-type EGC — the usual layout for a large feeder splice.
• 420 A per 600 kcmil copper conductor at 75 °C, from the NEC ampacity table for copper conductors at 75 °C.
• A 1600 A feeder breaker, a standard rating.

Replace these three inputs with your own project values before you buy copper.

The rule

Each raceway of a parallel run carries its own equipment grounding conductor, sized from the feeder breaker, and the bar-to-box jumper follows the same table row.

The arithmetic

4 × 420 A = 1680 A of ampacity, which carries the breaker with margin. NEC Table 250.122 at this rating gives 4/0 AWG copper: one 4/0 EGC per raceway, and a 4/0 AWG copper jumper from the bar to the box. Box length follows the NEC pull-box rule for raceways 4 AWG and larger, a straight pull at 8 times the largest raceway trade size. Four 4 in conduits entering and leaving opposite walls set the box at 32 in [813 mm] along the run. Smaller splices work the same arithmetic at their scale; the article on junction box size for 8 AWG three-phase splices runs the numbers for branch circuits.

The result

Four 4/0 EGCs land on a listed bar inside the box; the bar ties to the box with a 4/0 jumper through a brushed-bare contact, torqued to the lug marking.

The decision

A splice is present, so the bond is mandatory at any box size. The jumper scales with the breaker: 2 AWG copper at 500 A, 1 AWG at 600 A, 4/0 AWG at the largest row on the table. Split the same load into one 600 kcmil per phase and the breaker drops to 450 A on the next-standard-size allowance, where the table's 500 A row gives 2 AWG copper for the bond. For the shell, the largest stainless box we list is 60 x 36 x 16 in [1524 x 914 x 406 mm] (H x W x D), which carries this 32 in straight pull; corrosive sites take 316L, and the stainless enclosure finder filters by grade and size.

Checks when the bond is done

Measure from the brushed-bare box wall to the bar, then from the bar to each EGC tail, with your 4-wire meter. A 10 ft [3 m] tail of 4/0 copper runs near 0.05 Ω per 1000 ft [0.16 Ω per km], which is about 0.0005 Ω, so a sound bond reads around half a milliohm, and the bar-to-tail readings sit in the same range. A reading above 0.1 Ω points to a lug that never reached torque, a conductor not fully seated, or a contact still under paint. Record the readings and the jumper sizes on the junction box terminal drawing, so the next crew sees the bond without pulling splices apart.

If the box carries a hinged door or a removable mounting plate with metal across the gap, check those too; hinges and paint are not conductors. The article on the enclosure door bonding jumper sizes the strap across a hinged door, and the article on grounding a mounting plate covers the plate inside.

Where these bonds go wrong

• Sizing from the feeder conductor instead of the breaker. A 600 kcmil set on a 1600 A breaker takes 4/0 AWG copper on every EGC in that box; the conductor size never sets the row.
• Counting on conduit threads through paint. NEC 250.12 calls for paint, lacquer and enamel off the contact surfaces unless the fitting is designed for it; a listed bonding bushing or a tapped bonding point does the job clean.
• Landing three of four parallel EGCs. Every equipment grounding conductor associated with a spliced circuit connects to the box, and the missing one leaves a raceway's bond to chance.
• Insulated jumpers of 6 AWG or smaller left unmarked. Green, or green with a yellow stripe, keeps the next crew from cutting a bond during rework.
• Keeping the pass-through exemption after a splice is added. The exemption ends the day a conductor in that raceway is cut.

The decision for each box

So you can close the site argument in one word: bond. Splice the conductors and the box goes on the ground path with them, at the table row the breaker sets.

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