Grounding enclosure doors: bonding rules for covers

Short answer: bond an enclosure door with a green copper jumper whenever wiring crosses the hinge, and leave a bare door with no devices unbonded. Size the jumper with the circuit ground, 12 AWG copper as the working minimum for control doors (4 mm² in metric sizes), and land both ends on bare metal. Accept the bond when it measures under 0.1 Ω from door to ground bus. A hinge is not a fault path. Paint films, pivot wear and occasional metal contact fail exactly when a pushbutton faults to its bushing and puts line voltage on the door. NFPA 79 draws that line for industrial machinery, and OSHA's electrical rules expect equipment that meets it, so one jumper answers both. Below are the decision rule, the wire sizes, the install steps and the tests that prove the bond.

When does an enclosure door need a bonding jumper?

Bond the door whenever wiring crosses the hinge to door-mounted devices, whether that is one pushbutton, a bank of pilot lights, an HMI or an interlock switch, and leave a bare door with no devices unbonded because nothing on it can go live.

Metal-body pilot devices screw through the door, so their bushings clamp the door to line potential the day an insulation fault lands inside them. NEC 250.96(A) puts the duty in code words: metal parts intended to serve as equipment grounding conductors, enclosures included, must be bonded together so they safely conduct any fault current likely to be imposed on them. A door holding live bushings is exactly such a part.

NFPA 79 carries the same logic for industrial machinery: exposed conductive parts that can go live on a fault join the protective bonding circuit, and the accepted reading for doors is devices mounted, wiring crossed, bond it. The door-bonding requirements for industrial control panels walk through that standard text in detail.

The hinge does not count as the path. NEC 250.4(A)(5) calls for an effective ground-fault current path that safely carries the fault current likely to be imposed, and a pivot joint behind paint is neither permanent nor low impedance. The hinge-versus-jumper article works that argument in full, and the grounding and bonding guide holds the NEC 250 fundamentals behind it.

Does OSHA require the covers of enclosures to be grounded?

OSHA never names enclosure doors; its electrical rules demand equipment that is safe and approved, and 29 CFR 1910.303(a) with 1910.399 define approved as accepted, certified, listed, labeled or otherwise determined safe by a nationally recognized testing laboratory.

OSHA inspects outcomes rather than door hardware, and the outcome it writes up is a conductive surface holding line voltage with no low-impedance path to trip the breaker. NFPA 79 is the consensus standard inspectors recognize as the measure for industrial machinery, so a machine built and bonded to NFPA 79 answers the OSHA question before it is asked. Bond the doors that carry devices and that hazard has nowhere to happen.

Panels sold as industrial control panels add a listing path of their own. NEC 409.2 defines an industrial control panel as an assembly of two or more power circuit components, control circuit components or both. UL 508A is the standard for listing one, and NEC 409.110 wants an enclosure type number marked on the panel.

What size wire do you bond an enclosure door with?

Size the door jumper to match the largest equipment grounding conductor behind the door circuits, with 12 AWG copper as the practical floor for doors that carry only pilot devices and HMIs (4 mm² in metric sizes).

NEC Table 250.122 ties each equipment grounding conductor to the overcurrent device ahead of it. A 20 A device needs 12 AWG copper, 60 A needs 10 AWG, 100 A needs 8 AWG and 200 A needs 6 AWG. Door pilot circuits usually sit behind 5-10 A control fuses or a branch breaker up to 20 A, so that floor covers them with margin.

Step the jumper up when the door carries more than pilots: a door-mounted receptacle or control transformer takes the bond size of its own circuit, and the bond itself is green insulated copper. The jumper size, routing and testing article runs the same sizing from the panel side.

Worked example. A 316L door carries four metal-body pushbuttons, one 7 in HMI and a door interlock switch; the control wiring is fused at 10 A and the panel's branch breaker is 20 A. The numbers fall out like this:

If the hinge film holds, no fault current flows, the breaker never trips and the door sits near the full 120 V. That is the failure the 0.6 m jumper removes, because the fault finds 5 mΩ of copper instead of paint.

How do you install a door bonding jumper step by step?

Run a green copper jumper from a bare-metal point on the door to the panel's ground bus with ring lugs, slack for the full swing and no more route than the swing needs, and most doors are done in 30 minutes.

What you need:

• Green insulated copper at the size set above, 4 mm² minimum in metric sizes, sized up per NEC Table 250.122 when the door carries power circuits
• Two ring-tongue lugs per door, crimped with a tool that matches the lug
• Serrated lock washers and stainless-steel hardware on a stainless door
• A scraper or abrasive pad on a painted box
• A 4-wire micro-ohmmeter, or at least a multimeter with an ohms scale

The steps:

1. List every device on the door and note the largest overcurrent device behind any of them; that device sets the jumper size from NEC Table 250.122.
2. Pick a bare-metal stud or a spare bolted hole on the door near the hinge, clear of the gasket line. On a painted box, scrape a bright ring at the contact, which NEC 250.12 requires at bonding connections.
3. Cut the jumper at the bench with the door open, leaving a loop so it reaches full swing relaxed. Crimp a ring lug at each end; insulated 6 AWG and smaller jumpers are green or green-yellow under NEC 250.119.
4. Route the jumper across the hinge outside the pivot, away from sharp edges, and swing the door a few times to confirm the loop hangs free through the full 90-180° of travel.
5. Land the door end with the lug flat on bare metal and the serrated washer under the nut, tightened to the printed value; terminal labels typically print 2.3 N·m [20 lb-in] for #10 hardware.
6. Land the bus end on the enclosure's ground bus or on the landing of the incoming equipment grounding conductor, so the bond joins the same path; the EGC entry and bonding article covers that connection.
7. Measure door to ground bus with the panel still open and dead; under 0.1 Ω passes, and the tests below show how.

How do you test an enclosure door bond?

Accept the bond when a 4-wire meter reads under 0.1 Ω from door to ground bus and still reads there after five full door swings, the same 0.1 Ω limit IEC 60204-1 applies when it tests a protective bonding circuit at 10 A.

De-energize the panel, lock it out and confirm absence of voltage before you measure; the bond test is a bench job, not a live one. A 4-wire (Kelvin) reading separates the jumper's milliohms from the leads, which add 0.2-0.5 Ω on a plain handheld meter. With a handheld meter, touch the probes together first, note the lead value, then measure from the door stud to the ground bus on bare metal and accept a result close to it.

A wandering or high number means paint under a lug, a loose nut or a nicked conductor. Open and close the door five times and re-measure; the value should return unchanged. Log the reading with the date and repeat it in the annual maintenance round, plus after any repaint, hardware swap or door change. Strapped doors measure the same way, and the door bonding straps and continuity article covers the strap hardware.

Which mistakes leave a door unbonded?

Six failures account for almost every unbonded door: trusting the hinge, bonding through paint, a jumper routed through the hinge pin, zinc-plated hardware on a washdown door, a neutral on the bond stud, a hole through the gasket line.

• Trusting the hinge. The pivot is intermittent contact behind paint, and NEC 250.4(A)(5) wants a permanent path. Run the jumper; a loop of green copper costs minutes.
• Landing on paint or oxide. A lug on powder coat measures open however tight the nut is. Scrape to bright metal, and on stainless let the serrated washer bite the oxide, then measure.
• Routing through the hinge pin. Wire flexed at one point work-hardens and cracks after thousands of door cycles. Route beside the hinge with the slack loop from step 4.
• Zinc-plated hardware on a 316L washdown door. The plating rusts, seizes and lifts the lug. Use stainless hardware and re-measure after the first washdown cycle.
• Neutral or shield landed on the bond stud. NEC 250.6 keeps neutral current off metal parts. Move the neutral to its insulated terminal; the stud carries fault current only.
• A hole through the gasket line. Keep every opening outside the seal and deburr it. Our gasketed doors are rated NEMA 4X / IP66 by the manufacturer; not UL listed, so the seal is kept at installation, and the gasket and door hardware article lists what the rating asks of a door.

What should you order for a door that needs bonding?

Put the bond point on the drawing when you order a custom stainless body, keep jumper stock at the minimum size, ring lugs and stainless hardware on the shelf, and the door arrives ready for the bond instead of field-hunting a stud.

A drawing-specified bond stud lands the factory cut outside the gasket line, and the custom body is delivered in 10 business days from your approval of the drawing. The custom enclosure builder takes the material, size and every cutout from your layout, and when a standard body only needs new openings, the factory cutout page for stainless enclosures compares the fresh-cut route against a full custom body.

On the shelf for install day: 4 mm² green copper at the size the door's circuits set, two ring lugs per door, stainless hardware, serrated washers and the 4-wire meter.

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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