Short answer: bond the metal box to the largest EGC in it — the feeder's equipment grounding conductor — and run the branch through with its green EGC untouched. PVC is an insulator, so the raceway bond you would get from steel conduit does not exist. The EGC connection is the only bond the box gets. The NEC requires a termination point for the EGC of every circuit that terminates or splices at the box, plus one connection between the EGCs and the box itself. A circuit that passes straight through, uncut, owes the box nothing. The worked example below takes one 20 A branch and one feeder through a single metal enclosure fed by PVC.
Which EGC bonds a metal enclosure when circuits pass through?
Bond the enclosure to the largest EGC in the box — the feeder's — and leave a branch that passes straight through with an intact EGC unbonded.
Size and future both point at the feeder. The 12 AWG branch EGC runs about 2.5 times the resistance of the feeder EGC — 1.59 against 0.628 ohms per 1,000 ft of copper — so the feeder gives you the lowest-impedance bond. Pick the circuit you expect to keep, because the bond dies with the circuit that carries it. The feeder wins on both counts.
Why one bond instead of one per circuit? The NEC asks for a connection between the EGCs in the box and the box itself, not a terminal per pass-through conductor. Terminate every circuit that ends or splices here, add the single box bond, and the job is done — two circuits or thirty, the split is the same.
The bond to the box is an equipment grounding job, not an earth job. NEC 250.4(A)(5) calls the low-impedance metal path back to the source the effective ground-fault current path and rules the earth out of it. The grounding and bonding guide walks that path from box to panel.
What does the NEC require at a multi-circuit box?
The NEC requires a termination point for the EGC of every circuit that feeds equipment grounded at that box. Each EGC lands on its own pressure terminal, solderless lug or listed clamp.
Read it as a count of terminations, made by the connection methods the NEC allows for grounding: pressure terminals, solderless lugs, listed clamps, exothermic welds. Each EGC gets its own position, so nothing lands on tape, on an unmarked shared screw, or under another lug. In a panel that means a ground bar with a position per EGC, or each lug on its own listed terminal.
Continuity rules cover the EGCs once they are inside: they interconnect with each other, and a metal box may serve as the interconnection through ground screws and a bonding bar. Wherever conductors are spliced or terminated, the EGCs of those circuits join that network — the pigtail-and-bar habit every panel builder knows. A splice is the event that turns a pass-through into a termination, one case at a time.
Does a pass-through EGC have to touch the box?
A pass-through circuit keeps its EGC intact and unbonded: with no splice and no termination at the box, the NEC gives that 12 AWG green conductor no connection to make.
Uncut is the test. Conductors that enter and leave without a joint owe the box no terminal, so pull the branch through with the EGC continuous and the insulation unbroken. NEC 250.119 wants insulated EGCs of 6 AWG and smaller green or green with one or more yellow stripes, and that jacket stays on for the whole ride.
The box still takes its one bond, and the feeder supplies it: bond the enclosure to the feeder's EGC and the branch stays untouched. Landing the branch EGC on the bar anyway is allowed and costs one lug — a choice, not a requirement — and leaving it out keeps that circuit easy to pull later.
| Situation at the box | What the NEC needs | What you do |
|---|---|---|
| Feeder passes through, no splice | One bond to the metal box | Bond the box to the feeder's EGC |
| Branch passes through, no splice | Nothing — straight pass-through | Leave the branch EGC continuous, jacket intact |
| Branch splices at the box | A termination for its EGC | Pigtail the EGC and land it on the bar or a listed screw |
| The circuit carrying the box bond may leave | A bond that survives the change | Bolt a bonding bar to the box and land every EGC there |
| Circuits arrive in steel conduit | The raceway bonds the box through its listed fittings | Land EGCs per the rows above; the conduit makes the box bond |
How big should the bond to the box be?
Size the bond for the largest EGC in the enclosure — the feeder's conductor here. Use a bonding bar or backplate screw and lug marked for that conductor range, and pigtails that keep each conductor's original size.
NEC Table 250.122 sets the EGC from the overcurrent device ahead of the circuit, and it is the table to read for both circuits in this box. A splice keeps conductor size, so a 12 AWG pigtail extends a 12 AWG EGC. The bar-to-box bond takes the biggest conductor in the box. The backplate and DIN rail layout guide shows where the bar and the rail share space.
Metal contact is part of the sizing. On a powder-coated carbon steel box, scrape the coating off the contact face before the lug lands: NEC 250.12 requires paint, lacquer and enamel removed from threads and contact surfaces at grounding connections. Brushed 304 or 316L stainless needs no scraping — a listed grounding screw with a serrated washer bites bare metal, torqued to the terminal's marking.
The bar-to-box jumper follows the same rule as any bonding jumper: size it from the overcurrent device ahead of the largest circuit it serves.
What carries the fault when only the largest EGC touches the box?
The fault rides the bonded feeder EGC back to the panel, and the branch breaker still clears. At 2,000 A of fault current — 100 times the branch rating — it opens in a cycle or less, 16.7 ms at 60 Hz.
A fault from the branch's ungrounded conductor to the box returns over the bond and the feeder EGC, not through the earth. The feeder's ungrounded conductors carry none of it; its EGC just provides the return lane. The branch breaker is the one that trips.
Worked example — one 20 A branch faults to the box, 40 ft of feeder-size copper back to the panel, 2,000 A of fault current available. Swap in your own run length and available current before you reuse the numbers:
- Resistance at 20 °C: the feeder EGC is 0.628 Ω per 1,000 ft (standard copper wire tables).
- Run resistance: 0.628 Ω × 40 ÷ 1,000 = 0.025 Ω.
- Drop along the EGC at 2,000 A: 2,000 A × 0.025 Ω = 50 V — the height the box sits above the panel until the trip.
- Heating during the fault: copper resistance rises about 0.39 % per kelvin, so at 75 °C the run is about 0.031 Ω and the drop about 61 V.
- Bond with the 12 AWG branch EGC instead: 1.59 Ω per 1,000 ft gives 0.064 Ω over 40 ft and 127 V at the same 2,000 A — about 2.5 times the touch voltage for the same clearing time.
That last line is the whole argument for bonding with the largest EGC you will keep. Sizing the box itself around the conductors you run is the enclosure sizing guide's job.
How do you make the bond inside a sealed stainless enclosure?
Make the bond inside the enclosure: land pigtails on a bar or the backplate with a grounding screw, keep the gasketed walls undrilled, and put any outside bond point on the cutout drawing. The finished enclosure is rated NEMA 4X / IP66 by the manufacturer; not UL listed.
A bonding bar bolted to the backplate gives every EGC its own position and survives circuit changes; one grounding screw from bar to box completes it. Brushed stainless has no coating to scrape, and the screw's serrations bite bare metal. The gasketed door and walls stay undrilled — the bond lives on the plate, where it cannot touch the weather seal.
PVC glands keep the water out and carry no current by design — they are insulators, which is exactly why the EGC bond matters at every entry. A metal gland plate is the exception: it conducts, so size its earth link for the largest fault current it must carry before you trust the path.
What changes with fiberglass or plastic boxes?
A fiberglass or polycarbonate box has no metal to bond: every EGC still connects to the others and to the equipment's grounding terminals, usually on a copper bus screwed to the back panel.
Drop the box-bond step and keep the rest. Circuits that splice at the box still terminate their EGCs under the NEC, the bus still ties to each circuit's grounding path, and the pass-through branch still rides through untouched. The bus, its standoffs and its terminations then carry the whole grounding job, so give each landing its own screw and torque them to the bus marking.
What goes on the drawing for a custom enclosure?
Put the bond point on the drawing like any other feature: mark the bonding bar and its mounting holes, list the gland cutouts for the PVC runs with their sizes, and choose 316L stainless where brine, salt air or chemical spray will reach the box.
We cut every opening to the drawing you approve, so a bond done on paper is a bond done once. Send the gland positions and sizes, the bonding bar or ground-screw location, and spare positions for the circuits you plan to add — four circuits today, eight bar positions is cheap insurance. Check seals, mounting and rear clearance when the approval drawing comes back. 304 stainless suits indoor washdown and most outdoor sites, and 316L takes the brine and chemical duty. Start from custom enclosures or the stainless cutout and drawing page to lay the box out.
Approve the drawing and the finished enclosure is delivered in 10 business days, anywhere in the world.
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.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can the branch circuit's EGC bond the box instead of the feeder's?
You can, and the NEC asks only for a connection, not a named conductor. The larger EGC is the better connection: 12 AWG copper runs about 1.59 ohms per 1,000 ft against 0.628 for the feeder size, roughly 2.5 times the resistance, so the box sits higher above ground for every cycle the breaker needs. Bond with the largest EGC that stays in the box.
What happens to the box bond if the circuit that carries it is removed?
The bond leaves with the circuit, which is why panel builders land every EGC on a bonding bar bolted to the enclosure. Remove the feeder and the bar is still bonded to the box, so the remaining circuits stay grounded and the next circuit lands on one lug. Size the bar-to-box bond like the largest EGC you expect to keep.
What size EGC goes with a 100 A breaker?
NEC Table 250.122 sizes the EGC from the overcurrent device ahead of the circuit: that feeder takes 8 AWG copper or 6 AWG aluminum, and a branch circuit takes 12 AWG copper or 10 AWG aluminum. When ungrounded conductors are upsized for voltage drop, the table's note asks for the EGC to grow in the same proportion.
If a 30 A branch splices into the box later, does the bonding change?
Only for that circuit: a splice makes the box part of its grounding path, so the NEC asks for a termination of its EGC at the box. A 30 A branch reads off the 60 A row of NEC Table 250.122: 10 AWG copper. Land it on the bonding bar with a single lug and the box bond you already made stays untouched.
