Yes — rigid metal conduit can be the only equipment grounding conductor back to the panelboard, and the NEC lists it for that job. What the conduit cannot do is carry the bond through a nonmetallic box by itself. At every polycarbonate or fiberglass NEMA 4X box, grounding bushings and bonding jumpers bridge the plastic. That jumper is 8 AWG copper behind a 100 A feeder and 6 AWG behind 200 A, both off NEC Table 250.122. A copper wire pulled with the feeder is optional; treat runs past about 100 ft [30 m] and washdown plants as the point to pull it anyway. The worked numbers below assume a 480 V, 100 A feeder in 2 in rigid steel over a 250 ft run.
Can rigid conduit carry the ground back to the panelboard?
Yes. The NEC's grounding article lists rigid metal conduit among the conductors that may serve as the equipment grounding conductor. Threaded steel can carry fault current back to the panelboard on its own, with no wire pulled alongside the phase conductors.
The same list names intermediate metal conduit and EMT, so the choice of raceway rarely decides the question. One equipment grounding path per circuit is all the NEC asks for, and the raceway, the jumpers and the panelboard bond together are that path. The physics holds up: a 2 in Schedule 40 conduit wall carries about 700 mm² of steel, roughly 80 times the copper area inside the jumper wire. Steel conducts worse than copper by nearly an order of magnitude, and the area still wins by a wide margin, provided every joint on the run is made up tight.
A loose coupling on a conduit-only run is an open ground, not a code footnote. Thread engagement carries the current from wall to wall, so wrench-tight every coupling. Keep paint and heavy dope out of the joints you expect to carry fault current. For the rest of the scheme — bus bars, mounting plates, earth — the enclosure grounding and bonding guide holds the fundamentals while this piece stays on the raceway.
How do you bond the conduit through a plastic box?
A polycarbonate body carries no current, so the bond has to bypass the box. Fit a grounding bushing with a lug on every conduit entry and join the lugs with a bonding jumper, which keeps the two conduit sections one continuous conductor.
NEC 314.3 allows a metal raceway into a nonmetallic box only where the metal parts are bonded together inside the box, and the bushing-and-jumper pair is that bond. A grounding bushing is an insulated fitting with a bonding lug. It caps the freshly cut conduit end and gives the jumper a lugged land, so the two conduit sections stay one conductor through the box. The plastic locknuts hold the fittings mechanically and add nothing electrically; the jumper is the only continuity across the body.
Join bushing lug to bushing lug across the box, or land every lug on a copper bond bar screwed to the side wall. Cut the entry for the bushing body and its gasket, not for the pipe: 2 in rigid conduit measures 2.38 in [60 mm] across the threads, and a grounding bushing adds roughly half an inch [13 mm] more all round. When the box also lands a wire-type grounding conductor from a cable, the grounding and bonding guide covers how that conductor should land.
What changes with a stainless steel box?
A metal box joins the path instead of blocking it. NEC 250.96(A) requires raceways, enclosures and other metal parts acting as equipment grounding conductors to be bonded together, so the jumper lands on the enclosure's ground terminal and the box bonds every device inside it.
Our 304 and 316L wall-mount and HMI boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Their stainless walls conduct, so the bonding bushing jumper lands on the ground stud in the back wall, and the enclosure itself becomes part of the grounding path. The mounting plate bonds to the box through its own connection, and devices bolted to the plate share it — the grounding guide's mounting-plate section covers the stud and washer detail.
NEC 250.12 wants paint, lacquer and enamel off the threads and contact surfaces at bonding connections. Stainless usually ships bare, so the land bites clean; scrape any coating back to bare metal at every lug before you torque it. Bond the door with a strap across the hinge when it carries equipment, because a gasketed door is not a bond. If the raceway lands on a gland plate instead of the wall, the cutouts and glands guide sets the hole and hub sizing.
How big should the bonding jumpers be?
Size every jumper from NEC Table 250.122, which sets the grounding conductor from the rating of the overcurrent device ahead of the circuit. Read it off the breaker or fuse ahead of the run, never off the wire size in the conduit. Buy jumpers with the lug factory-crimped for the bushing; a field crimp with the wrong die is the joint that fails first.
| Breaker or fuse ahead | Bonding jumper, copper (NEC Table 250.122) |
|---|---|
| 20 A | 12 AWG |
| 60 A | 10 AWG |
| 100 A | 8 AWG |
| 200 A | 6 AWG |
For the run in the introduction — 480 V behind the example breaker, 250 ft of 2 in rigid steel, one polycarbonate pull box with two grounding bushings and one jumper across the box — the numbers come out like this:
- Jumper size: the table row above, straight off NEC Table 250.122 for the overcurrent device ahead.
- Jumper drop: that copper size runs about 0.63 Ω per 1000 ft at 20 °C (standard copper tables; copper rises about 0.39 % per °C, so about 0.76 Ω per 1000 ft at 75 °C). The 2 ft bridge is 2 ÷ 1000 × 0.63 = 0.0013 Ω, a rounding error next to the run.
- Wire option: a full-length copper EGC of the same size under the steel adds 250 ÷ 1000 × 0.63 = 0.16 Ω of pure copper to the return. At a 500 A ground fault that segment alone drops 500 × 0.16 = 79 V, or about 96 V with the fault heated to 75 °C.
- Trip check: the example breaker clears instantly at roughly 5 to 10 times rating, 500 to 1000 A, per its time-current curve. Every ohm taken out of the loop buys clearing speed.
So the box-to-box bond costs under a volt at 500 A — 500 × 0.0013 = 0.65 V — and the case for the wire lives in the 250 ft of run, not in the box.
When should you pull a copper wire as well?
Pull a full-length copper equipment grounding conductor with the feeder once the run passes about 100 ft [30 m] or the plant washes its conduit down. The wire costs minutes while everything is open, and it gives the fault current a return no corroded thread joint can take away.
Below that length, with tight threaded steel and a jumper at every plastic box, the conduit-only path is a normal build, and plants run it for decades. Past it, the stack of threaded joints starts to show: series resistances add, and each joint contributes its own film and contact resistance to a path that is already longer than a wire would be. Washdown service moves the switch point to zero, because chlorides and caustics corrode threads from the outside, and the joint that fails is the one you cannot see.
Pull the wire green where it is insulated: NEC 250.119 requires insulated equipment grounding conductors 6 AWG and smaller to be green or green with one or more yellow stripes. Land it on the bond bar or a lug in every box so the pulled conductor and the raceway stay one path.
Some systems take the conduit out of the design altogether: a high-resistance grounded distribution is planned around a dedicated equipment grounding conductor, and the raceway carries conductors rather than fault current.
What goes on the drawing and the order?
Send the entry schedule with the enclosure: grade 304 or 316L, outside size, and each conduit entry as a position with its trade size and bushing or hub type. The cutouts then arrive sized for the bushing bodies rather than the bare pipe.
The cutout drawing checklist on the custom stainless enclosures page is the format the shop reads fastest. The stainless size finder narrows a stock body before you commit to fully custom walls. Every box is delivered in 10 business days, anywhere in the world — counted from your order, or from your approval of the drawing on custom work. Put the bond-bar position on the drawing too, so the bar arrives where the jumpers can reach it.
Before you close the first box: lock out the feeder and verify absence of voltage. Wrench-tight every coupling on the run. Tug every jumper by hand. Measure one jumper end to end: the 2 ft bridge from the example is about 0.0013 Ω, so a jumper reading anything but near-zero on a low-ohms meter has a bad crimp or a loose lug.
The decision, case by case:
- Short threaded steel run, one or two plastic boxes, no washdown: conduit is the equipment grounding conductor. Jumpers per NEC Table 250.122 at each box, sized to the breaker ahead.
- Run past 100 ft [30 m], many boxes, or washdown service: pull the copper EGC with the feeder and land it in every box.
- Stainless boxes in the run: bond each box into the path — jumper to the ground stud, coating scraped at every land.
- High-resistance grounded distribution: dedicated equipment grounding conductor by design; the conduit carries conductors only.
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 |
| 16 × 14 × 8 in | 304 stainless | 16 x 14 x 8 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 |
Questions
Is IMC or aluminum rigid conduit as good as steel for the ground path?
Yes — the NEC list of permitted equipment grounding conductors covers intermediate metal conduit and aluminum rigid conduit alongside rigid steel. Aluminum earns its keep in washdown and coastal plants, where steel threads corrode first. Use a conductive joint compound on aluminum threads and re-tighten couplings at each shutdown, because every corroded joint adds series resistance to the fault path and steals tripping current.
Do grounding bushings still matter once a wire EGC is in the conduit?
Yes, for two reasons that outlive the wire. The bushing still pads the sharp cut end of the conduit so it cannot chafe conductor insulation. Its lug also keeps a bolted bond at every entry — a joint that beats a thread in a wet box. Keep the jumpers landed and sized from NEC Table 250.122; behind a 20 A breaker that is still 12 AWG copper.
Can a ground rod at the far box replace the conduit run?
No. Under NEC 250.54 a rod driven at the box is an auxiliary electrode: you may bond it to the equipment grounding conductor, but it never replaces that conductor. NEC 250.4(A)(5) keeps the earth out of the fault path — a made electrode is judged good at 25 Ω, and 480 V across 25 Ω is about 19 A, far under the 500 A a 100 A breaker needs to clear instantly.
How many bonding jumpers does one pull box need?
One per conduit entry is the tidy layout: a pull box with two 2 in and one 1 in entry gets three grounding bushings and three jumpers, landed lug to lug or on a copper bond bar. With a bar you can tug-test each jumper in seconds at shutdown and catch the one that worked loose.
