Yes — the one ground conductor running to your motor grounds the junction box too, and you never need a second run just for the box. Splice the ground through the box with a connector listed for grounding, add a bonding jumper from that splice to the box body, and keep the conductor one size from supply to motor. On a 90 A motor circuit that path stays one size everywhere, jumper included. The NEC expects exactly this arrangement: grounding conductors joined in a box with listed, securely fastened connectors, and the metal shell bonded to them so a fault has a metal path home. Below are the parts, the steps and the three readings that prove it.
Can one ground wire serve both the motor and the box?
Yes — the equipment grounding conductor that continues to the motor also grounds the box, once you splice it through with a connector listed for grounding and bond the body to it.
The NEC asks two things of a metal box in a motor run. Grounding conductors that meet in a box must be connected together there, and the splice holds only when the connector is listed for grounding and securely fastened. The shell, like every other non-current-carrying metal part, sits in the fault path as well — which is the jumper's whole job.
Leave the soil out of it. NEC 250.4(A)(5) names the fault return the effective ground-fault current path, and the earth is no part of it: a fault returned through soil delivers an amp or two while the breaker waits for hundreds. If your ground reaches the box on a cord connector rather than conduit, start with grounding conductor enclosure entry: connector and bond. For the wider picture — doors, gland plates, backpanels — the enclosure grounding and bonding guide covers it.
What do you need in the box before you start?
A metal box with a grounding boss or ground bar, a connector listed and marked for grounding, a jumper the same AWG as the ground conductor, and about 6 in [150 mm] of slack on each ground end so the splice stays serviceable.
- The box. 304 stainless for a machine foot indoors, 316L where washdown or chlorides reach it. A 12 x 10 x 6 in 304 box takes a small motor's three power splices; a 25 hp motor's splices sit better in a 16 x 12 x 8 in 304 box, or the 12 x 10 x 6 in 316L box in hose-down areas. All are rated NEMA 4X / IP66 by the manufacturer — not UL listed.
- The landing point. A ground boss, ground pad or ground bar gives the jumper a proper seat on the shell. Order the box with the boss and the gland holes on the drawing; custom work is delivered in 10 business days, counted from your approval of the drawing.
- The connector. A ground bar with set-screw lugs is the shop favorite, because the next circuit lands beside the last. Whatever you pick must be listed for grounding and marked to say so.
- Test gear. A low-resistance ohmmeter that nulls its leads, plus a torque driver.
Give the wire room while you are at it: NEC 312.6 sets the bending space at terminals by wire size, and NEC 312.8 caps how much of the box splices and taps may fill. The enclosure sizing guide walks that arithmetic, and wall-mount stainless enclosures cover the sizes from junction box upward.
How do you splice the ground and bond the box?
One lockout and six steps: isolate, splice the power, join both ground ends with the listed connector and the jumper, fasten the jumper to the boss, torque, then pull-test before the door closes.
- Isolate the branch circuit at its disconnect, lock it out, and verify absence of voltage at the box before you open it.
- Land the three phase conductors, supply side to motor side, on splice terminals rated for the wire. The ground stays out of the power terminals entirely.
- Bring the supply-side and motor-side ground ends into one connector listed for grounding, with the bonding jumper as the third conductor in the same lug — or land all three on the ground bar.
- Fasten the free end of the jumper to the boss or bar with the surface cleaned to bright metal; NEC 250.12 requires paint, lacquer and enamel off grounding contact surfaces.
- Torque every ground termination to the value marked on the device. Listed connectors and ground bars all carry one.
- Pull-test each ground conductor by hand. One that moves in its lug fails the securely-fastened condition the NEC sets, so fix it before the door goes on.
What size must the ground and the jumper be?
The NEC sizes the equipment grounding conductor from the rating of the overcurrent device ahead of it, using Table 250.122: 20 A gives 12 AWG copper, 60 A gives 10 AWG, 100 A gives 8 AWG and 200 A gives 6 AWG.
Worked example, the one worth memorizing. A 25 hp, 460 V three-phase motor draws 34 A full-load. The NEC motor rules let an inverse-time breaker stand at up to 250% of full-load: 34 x 2.5 = 85 A, and the next standard size is 90 A. The row of Table 250.122 covering that device is the one for ratings not exceeding 100 A, which gives 8 AWG copper. That one size runs from the source ground bar to the box, continues to the motor, and the jumper to the shell matches it, so nothing in the run drops below it.
Two things move the number. A larger breaker steps you up a row, so 200 A of protection means 6 AWG. Aluminum conductors take the larger column, so the same 100 A device calls for 6 AWG aluminum. And when you upsize the phases for voltage drop, the ground rises with them, because the NEC keeps the two proportional.
When two motors share one box, every ground lands on that same bar and each keeps the size its own breaker calls for. The multiple motor circuits in one junction box rules sort what else the shared box needs.
Where does the ground land at the motor?
On the grounding provision the terminal housing provides — the NEC requires that means at every motor terminal housing, inside or outside — with the ground conductor under its screw or lug, torqued.
That provision exists for exactly this conductor: bare metal, sized for the fault, beside the joints you are already making. A frame bolt elsewhere on the machine carries paint and a thread that vibration loosens within a season, so treat the housing point as the landing. For the joints on the motor side of the run, motor junction box connectors that survive service compares the types.
Does metallic conduit or a gland entry change anything?
No — it strengthens the bond. With metallic conduit or bonding-type glands, the raceway and the box shell themselves join the fault path, and NEC 250.96(A) requires exactly that bonding for raceways, cables and enclosures.
Metallic entries earn it by being tight and bare: two locknuts on a hub, threads free of coating, the same bright-metal rule as step 4. A cord on a gland depends on the gland, and the cable gland bonding rules for stainless boxes say which designs bond through and which need help. With PVC conduit or a plastic gland, the wire ground is the only bond the box gets, so the jumper carries the whole job — still without a second run.
How do you prove the ground path is sound?
Lock out again, then take three low-resistance readings: source ground bar to box body, box body to motor frame, and source to motor frame — each well under 1 Ω on a meter with its leads nulled.
Clip the leads together first and zero their resistance, because at these values the leads are part of the reading. The first reading proves the jumper and boss, the second proves the motor-side splice and the housing termination, and the end-to-end figure should land near the two added together. Tens of ohms means paint under a lug or a loose locknut; clean it, re-torque, repeat. The door is a bond of its own — control panel door bonding sizes that jumper and its routing.
What mistakes break the ground path?
Five, in the order you meet them: no splice at the box, a connector not listed for grounding, coating under the bonding point, a smaller ground downstream, and a landing on a frame bolt.
| Mistake | What it costs |
|---|---|
| Ground cut at the box, never joined | The motor frame loses its path home; a phase-to-frame fault stays on the frame because the earth will not clear it (NEC 250.4(A)(5)) |
| Connector not listed for grounding | The splice misses the NEC condition of listed, securely fastened connectors; swap in a ground bar or a listed grounding connector |
| Coating under the boss or lug | A joint reading tens of ohms instead of milliohms; NEC 250.12 requires paint, lacquer and enamel off the contact surfaces |
| Motor-side ground smaller than supply side | The path chokes at fault current — on the 90 A example, 10 AWG after the box defeats the correctly sized run ahead of it |
| Ground on a frame bolt, not the housing provision | Vibration and paint leave it loose within months; the housing point is the designed termination |
So the order reads: box size from your splice count, 304 indoors and 316L in washdown, ground boss and factory-cut gland holes on the drawing, jumper cut to the ground size. The decision by feed: with cord or PVC, the spliced ground and its jumper are the entire bond; with metallic conduit or bonding glands, the shell gains a second path and you keep the wire regardless.
Enclosures in this article
| 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 |
| 12 × 10 × 6 in | 316L stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can I drive a ground rod at the junction box instead of splicing the ground through?
No. A rod next to the box counts as an auxiliary electrode under NEC 250.54: you may connect it to your equipment grounding conductor, and it still never replaces the conductor, because the earth will not carry the fault current back to the source. Splice the ground through the box and bond the shell with the same-size jumper; the splice is the path.
What size bonding jumper does a 40 A motor circuit need?
10 AWG copper. Table 250.122 sizes the equipment grounding conductor from the overcurrent device rating, and a 40 A device falls under the row for ratings not exceeding 60 A, which is 10 AWG copper — 8 AWG if the conductor is aluminum. The jumper to the box body matches that size, and any smaller ground on the motor side chokes the fault path.
Can the mounting screws through a painted wall ground the box?
No. The wall and its screws sit outside the fault path back to the source. NEC 250.12 requires paint, lacquer and enamel removed from grounding contact surfaces anyway, so a screw through painted steel starts as a bad joint. Bond the shell to the spliced ground conductor with the jumper inside the box, and let the wall carry weight, not current.
Does the bonding jumper have to be green?
If it is insulated, yes: NEC 250.119 requires insulated equipment grounding conductors of 6 AWG and smaller to be green, or green with one or more yellow stripes. The jumper on a 90 A circuit falls under that rule. A bare jumper or one bolted to the ground bar is equally good — the color rule only applies when there is insulation.
