Bond an enclosure to a motor casing: when you need the wire

Short answer: bolt the box for structure and bond it with a wire — a visible jumper sized from the overcurrent device ahead of the circuit, 10 AWG copper for a 60 A device. In a Class I Zone 1 area the NEC accepts only the bonding methods on its own list, and a structural bolt through a painted joint is not one of them. Outside classified areas a bare-metal bolted joint can carry the fault current, yet the jumper costs about ten minutes and settles the question for every inspector and plant spec. A 0.2 Ω reading proves contact at meter current; it says little about the same joint at fault current.

Why four bolts are not a bond

Start with the general rule. NEC 250.96(A) requires metal parts intended to serve as equipment grounding conductors — raceways, cables, equipment, enclosures — to be bonded together so they can safely conduct any fault current likely to be imposed on them. The grounding and bonding guide walks that fault path end to end; this joint is one link in it. The NEC also names the connections it accepts: listed pressure connectors, welds and similar means, made up on bare metal. NEC 250.12 asks for paint, lacquer and enamel removed from threads and contact surfaces at grounding connections, unless the fitting is designed to make removal unnecessary.

Four structural bolts through a painted back meet none of that cleanly. A continuity meter passes milliamps, and at meter current a joint through a paint film reads 0.2 Ω and looks healthy. Fault current is where the film decides. A motor adds vibration and heat cycling that work the joint loose year after year.

The casing side is already sorted: the motor frame carries its own supply conductor, and grounding a motor junction box with the motor ground wire is its own task. The jumper makes box and casing one conductor, so a fault inside the box rides the casing's path back to the source.

Worked example — treat the inputs as illustrative and swap in the figures from your own system: a 460 V motor circuit, 10 kA of available fault current, and a box bonded only through a joint that measures 0.2 Ω. Ohm's law caps that fault near 2,300 A (460 V ÷ 0.2 Ω).

At that current, nearly the full 460 V sits across one bolted interface — exactly where it arcs. Add the jumper in parallel and the picture flips. That wire size runs about 1 Ω per 1000 ft, so a 1 ft [300 mm] run is about 1 mΩ, and a 10 kA fault drops 10 V across it (10,000 A × 0.001 Ω). The fault stays on the conductor and the breaker sees the current it was set to clear.

What a classified area demands

In a hazardous (classified) location the NEC goes a step further. Its bonding rule for classified areas requires electrical continuity of all non-current-carrying metal parts — equipment, raceways, enclosures — by one of the bonding methods on its list, whether or not a wire-type equipment grounding conductor is installed. The same rule routes you around impaired joints: a bonding jumper or listed fitting bridges concentric or eccentric knockouts and reducing washers, because those sheet-metal gaps are not dependable conductors.

A mounting bolt appears on no such list, in any area classification. In a Class I Zone 1 area the jumper is therefore the method that satisfies the rule, not a belt-and-suspenders extra. Zone areas add equipment rules on top of bonding; the certification side runs under IEC 60079, and what the NEMA enclosure types cover sorts the environmental rating from the certification question. Oil-and-gas and offshore specs go further still and ask for a green wire on nearly every skid-mounted device, transmitter to post. The NEC stops short of that outside classified areas, and the plant spec usually wins.

Size the jumper

Size from the rating of the overcurrent device ahead of the circuit, using NEC Table 250.122. The copper rows that cover most motor circuits:

A box that carries more than one circuit takes the largest row. Length changes nothing about the minimum, though each foot of 10 AWG adds about 1 mΩ, so a tidy short run stays tidy electrically. Keep insulated jumpers green: NEC 250.119 requires insulated equipment grounding conductors 6 AWG and smaller to be green or green with one or more yellow stripes. Jumpers that run between boxes on a backplate follow the same table; the backplate and DIN rail layout guide works that case in detail.

What you need

• green stranded copper of the table's size for your circuit — for a 60 A device, 16 in [400 mm] covers a 12 in [300 mm] run plus both lugs
• two ring-tongue compression connectors sized to the wire and the stud
• an abrasive pad or wire brush, to bare both contact faces
• stainless hardware: bolts, toothed lock washers, nuts
• a torque wrench set to the connector maker's printed value
• a four-wire (Kelvin) milliohm meter for the check at the end

Install the jumper in six steps

One safety line first: lock out the motor circuit and check for absence of voltage before you crimp or lug anything.

1. Read the overcurrent device ahead of the circuit and pick the copper size from NEC Table 250.122 — 60 A takes 10 AWG, 100 A takes 8 AWG.
2. Pick bond points outside the sealed volume: an outside mounting boss or bolt on the enclosure, and a tapped boss or casing bolt on the motor. The green wire then never crosses the gasket.
3. Bare both faces back to shine, as NEC 250.12 asks, on flat areas the lugs cover fully.
4. Crimp both lugs with a full-cycle compression tool, land them over toothed lock washers, and torque the studs to the printed value.
5. Route the run short and clear of the shaft, coupling and belt guard, with a 2 in [50 mm] slack loop at the motor end so the box carries none of the motor's alignment load.
6. Label the wire at both ends so the next person reads it as the bond.

Checks when done

• Measure the jumper alone with the four-wire meter: about 1 mΩ per foot at room temperature for the wire size in the 60 A row. Across the whole box-to-casing path, single-digit milliohms is a sound joint; more than about 10 mΩ means a lug sits on paint or a washer has crept.
• Tug each lug, then re-torque after the first 24 h of running — motor vibration is what kills bolted joints.
• Confirm the circuit's own equipment grounding conductor still lands on the enclosure ground terminal; the jumper bonds box to casing and replaces nothing.
• Keep the rating intact: keep every bond outside the enclosure, and if a bond must cross the wall, put the hole on the drawing per the cutouts, glands and windows guide. A manufacturer-rated NEMA 4X / IP66 box, not UL listed, keeps washdown water out; the jumper carries fault current. Listed or manufacturer-rated explains the difference when a project asks for a listing.

Mistakes that cost the bond

• Counting the mounting bolts as the bond in a classified area. The rule wants a method from its list; a structural bolt is on none of them.
• Lugging onto paint, anodize or a nameplate. The meter shows green; the fault sees an insulator. NEC 250.12 exists for exactly this.
• Bonding only to building steel. The earth is never part of the effective ground-fault current path under NEC 250.4(A)(5), and building steel adds a bolted joint at every column between motor and source.
• Letting the bond run through the door or a hinge — a door needs its own bonding jumper on its own account.
• Undersizing the jumper for the largest circuit that passes through the box.
• Drilling the enclosure on site and leaving the hole unsealed. That ends the washdown rating long before the bond earns its keep.

The decision by area classification

Classified — Class I Zone 1, a Division area, any zone system: the jumper goes on, sized by NEC Table 250.122 from the largest overcurrent device in the box, because the NEC asks for a bonding method from its list and a bolted joint is not one.

Ordinary industrial location: a bare-metal bolted joint that can carry the fault current can serve under NEC 250.96(A). Fit the jumper anyway; it is the ten-minute answer that passes the inspector, the plant spec and the next shutdown.

Either way, keep the green wire outside the sealed volume. When a bond, a gland or anything else must cross the wall, put the cutout on the drawing: our custom enclosures are built to your material, size and cutout list from one piece, delivered in 10 business days anywhere in the world, counted from your approval of the drawing. If you are choosing a box for the job first, the enclosure selector sorts by environment, size and listing need.

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.

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