Grounding a mounting plate inside a metal enclosure

Bond the mounting plate to the enclosure wall. Fit a copper jumper no smaller than one AWG size under the incoming equipment grounding conductor — 12 AWG against a 10 AWG ground wire. Scrape both contact faces to bare metal and clamp them under star washers. Then measure your own joint: under 0.1 ohm from wall to plate with a lead-nulled multimeter. Rubber washers and a paint film under the plate studs are assembly detail, not a grounding plan. A floating plate carries the PE terminal block, the DIN rail and every fault that lands on the gear bolted to it. NFPA 79 builds the machine's protective bonding circuit on the equipment grounding conductor. NEC 250.96(A) requires metal parts serving as grounding conductors to be bonded so they carry fault current safely. Half an hour and one jumper close that path.

Why the plate is part of the fault path

NFPA 79, the standard for the electrical equipment of industrial machinery, makes the equipment grounding conductor the backbone of the machine's protective bonding circuit and lists what may serve on it. The plate your drives, DIN rail and PE terminal block sit on is a non-current-carrying metal part, and it belongs on that circuit. NEC 250.96(A) says the same for enclosures: metal parts intended to serve as grounding conductors get bonded together to carry the fault current likely to hit them. That includes the raceways, the equipment and the enclosures themselves. NEC 250.4(A)(5) calls that path the effective ground-fault current path and rules the earth out of it.

The wire size comes from the breaker. NEC Table 250.122 sizes the equipment grounding conductor from the rating of the overcurrent device ahead of it: 10 AWG copper on a 30 A branch circuit, 8 AWG on 60 A. The plate jumper works beside that wire, so the shop rule is simple: no smaller than one AWG size under it. A 10 AWG feed takes a 12 AWG jumper, or the same 10 AWG if the scrap bin has it.

Paint is the quiet failure. 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 bite through the coating on its own. An external-tooth star washer that pierces to metal on both faces meets that intent; a flat washer resting on the factory film leaves you measuring paint instead of metal. A galvanized plate bonds through its zinc once cleaned; the paint over the zinc is what blocks the joint. Sizing tables and the whole path from service to terminal sit in the grounding and bonding guide if you want the full picture.

What you need

• A copper jumper one AWG size under the incoming equipment grounding conductor: 12 AWG for a 10 AWG feed, cut about 20 in [500 mm] for a mid-size plate. Green or green-with-yellow-stripe insulation on any jumper 6 AWG or smaller, as NEC 250.119 requires for insulated conductors that size.
• Two external-tooth star washers sized to the bolt, and flat washers to spread the load behind them.
• Ring lugs crimped to both ends, or bare strap you clamp directly.
• A file or abrasive pad to bring both faces to bright metal.
• Touch-up paint when the enclosure is painted carbon steel. Bare 304 or 316L stainless needs nothing, which is one more reason to order a custom stainless enclosure with the mounting and cutouts settled up front.
• A multimeter that reads ohms, with a zero or relative function for nulling the leads.

Bonding the plate step by step

1. Lock out the feed and prove the panel dead at its terminals before the first bolt comes out.
2. Pick the two bond points: a wall stud or ground boss on the enclosure, and the plate's ground stud or PE bar. In most panels the PE terminal block bolts to the plate, so the plate anchors the whole bonding circuit; where that bar belongs is its own decision, covered in ground bar placement: enclosure wall vs mounting plate.
3. Cut the jumper to length and crimp ring lugs on both ends. Keep the run short and direct; every extra metre of 12 AWG copper adds 5.2 mΩ.
4. Scrape or abrade both contact faces to bright metal, a ring wider than the washer footprint on each side.
5. Build the stack steel to steel: star washer against the scraped face, jumper lug under it, flat washer behind. Rubber, nylon and paint stay out of it.
6. Tighten until the star washer bites and the hardware cannot be turned by hand, then stop.
7. Land the far end on the ground boss or straight onto the PE bar beside the incoming equipment grounding conductor; connector and entry details are in grounding conductor enclosure entry: connector and bond.
8. Measure wall to plate with the leads nulled and write the reading on the inside of the door.

Worked example: a 240 V, 30 A plasma panel

Swap in your own breaker size and plate dimensions; the arithmetic follows the same way.

The rule in words: bond resistance is the jumper wire plus the two joints, and it passes under 0.1 ohm once you subtract the meter leads.

The arithmetic:

• Wire at 20 C: 5.2 mΩ/m × 0.5 m = 2.6 mΩ.
• Heat: 0.39 %/K × 40 K = 15.6 % rise, so 2.6 mΩ × 1.156 = 3.0 mΩ.
• Joints: 2 × 5 mΩ = 10 mΩ.
• Total: 3.0 mΩ + 10 mΩ = 13 mΩ, or 0.013 ohm.

The result: about 0.013 ohm, an eighth of the 0.1 ohm target, with 0.087 ohm of headroom.

The decision: under 0.1 ohm passes, and above it the wire is not the culprit. A 20 in jumper adds only 3 mΩ here, so a failed reading points at a face you left painted or at leads you forgot to null; a paint film reads tens of ohms, or open, at a multimeter's microamp test current. Scrape again, re-test, and write the number on the door when it lands.

Proving the bond when you are done

• Null the leads first: touch the tips and zero the meter. A handheld lead set alone can add 0.1 to 0.5 ohm, more than the whole bond you just built.
• Wall to plate: under 0.1 ohm.
• Wall to PE bar: at the meter's floor, milliohms or a tenth of one.
• Press the plate with your palm and watch the display. A steady number means the washers are biting metal; a wandering one means a film you missed.
• Wiggle the jumper at each end: the reading holds.
• Re-measure after every plate removal. Plenty of builds lift the plate out for bench wiring, and a removable enclosure mounting plate for bench wiring only stays bonded if you re-torque and re-measure each refit.

When a floating plate is deliberate

A plasma table is the case that starts this argument. The cutter sprays high-frequency noise into everything metal near it, and a floating plate looks like an easy way to starve the noise of a loop. Bond the plate anyway: the safety bond carries fault current, and the noise fix lives in routing. Land every cable shield at one point on the PE bar, keep encoder and torch-height leads out of the mains bundle, and let the cutter return its own current through its own work lead. When a drive or motion card manual calls for a floating signal ground, float that one signal at one point and nothing else. The trade-off is worked through in ground to chassis or float: tying PCB ground to a metal enclosure.

Paint costs you here too. Metal-to-metal contact through a film spoils the shielding you bought the steel box for, which is why paint and EMI shielding in metal enclosures belong in the same conversation.

So: bond the plate, isolate the signal, and only at one point when a manual asks for it.

Mistakes that cost the bond

• Rubber, nylon or fibre washers left in the plate stack. They are assembly shims; the bond needs external-tooth star washers biting bare metal on both sides.
• Paint left under washer heads. NEC 250.12 asks coatings stripped at grounding contact surfaces; four scraped rings cost ten minutes.
• Trusting the door hinge to bond the door. Hinges open and close on paint-on-paint contact, so a door carrying metal switches or an HMI cutout gets its own jumper.
• Reading through un-nulled leads and chasing a phantom 0.3 ohm that the leads contribute.
• Scraped bare patches left unpainted on a carbon steel box; prime and paint them the same day. Stainless stays bright.
• Bolting the plate back after bench wiring without a re-test. The number goes on the door for exactly this reason.

Bond it, scrape it, measure it, and write the number down.

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