Grounding and bonding an electrical enclosure

Grounding vs bonding: what each one does

An automation wiring guideline from a major controls maker puts it simply: bonding is connecting together the metal parts of chassis, assemblies, frames, shields and enclosures; grounding is the connection to the grounding electrode system, which places equipment at earth potential. Both matter, but for safety they do different jobs.

In US installations, NEC Article 250 covers both. Article 250's bonding requirements establish a low-impedance fault current path back to the source of supply so that the overcurrent device opens when a ground fault occurs. NEC 250.4(A)(5) calls this the effective ground-fault current path, and the earth is not considered part of it. A training guide to the 2020 NEC explains why: the contact resistance between an electrode and the earth is so high that very little fault current returns through it, so the breaker would not open and the metal parts would stay energized.

The practical result for an enclosure: the equipment grounding conductor that arrives with the supply circuit is what clears a fault. A ground rod driven next to a pole-mounted enclosure is an auxiliary electrode under NEC 250.54; it may be connected to the equipment grounding conductor, but it is never a substitute for it. Check the edition your authority having jurisdiction enforces for every code reference in this guide.

What has to be bonded

NEC 250.96(A) (2020 edition, as explained in an NEC training guide) requires metal parts intended to serve as equipment grounding conductors, including raceways, cables, equipment and enclosures, to be bonded together so they can safely conduct any fault current likely to be imposed on them. For a typical enclosure that means:

• the enclosure body, through the incoming equipment grounding conductor;
• the mounting panel and everything grounded through it;
• a door or cover that carries electrical devices;
• metal conduit and fittings entering the enclosure;
• the metal frames of devices that have their own ground terminal.

The equipment grounding conductor is sized from the rating of the overcurrent device ahead of it, using NEC Table 250.122. Some rows from the 2020 table for copper conductors:

The conductor is not required to be larger than the circuit conductors, and NEC 250.122(B) requires it to be increased in proportion if the phase conductors are upsized, for example for voltage drop. Insulated equipment grounding conductors of 6 AWG and smaller must be green or green with one or more yellow stripes (NEC 250.119).

Ground studs, the ground bus and how to stack lugs

Land the incoming equipment grounding conductor on a ground stud on the enclosure body or on a ground bus bar mounted to the mounting panel, then take a separate conductor from each device or chassis to its own terminal on the bus. The automation guideline above recommends exactly that: a central ground bus on the back panel, with each chassis connected to an individual bolt rather than daisy-chained.

How the conductor is fastened matters too. NEC 250.8(A) (2017 edition, as summarized by EC&M) lists the permitted methods for equipment grounding conductors and bonding jumpers, including listed pressure connectors, terminal bars, exothermic welding, machine screws that engage at least two threads or are secured with a nut, thread-forming machine screws that engage at least two threads, and connections that are part of a listed assembly. Under 250.8(B), connections that depend solely on solder are not allowed.

Where two lugs share a stud, the same guideline describes sandwiching the first lug between a star washer and a nut with a captive star washer, then the second lug between that nut and a second captive-star-washer nut, so each lug is clamped independently. Every NEMA4 Enclosure wall-mount enclosure has ground studs for these connections.

Bonding the mounting panel

The mounting panel carries most of the devices, so it has to be a solid part of the fault path. The automation guideline asks for a good electrical connection between each chassis, the back panel and the enclosure through every mounting bolt or stud, with paint or other nonconductive finish removed from around studs and tapped holes. In addition to those connections, it recommends 1 in copper braid or at least 8 AWG stranded copper wire from each chassis, the enclosure and the central ground bus on the back panel.

Our wall-mount enclosures have a removable galvanized mounting panel on welded studs. Because the panel comes out, treat the studs and nuts as a mechanical fixing and add a bonding conductor from the panel to a ground stud, so the bond does not depend on how tightly someone refits the nuts after a rebuild.

Bonding the door

A hinge is a poor conductor: it is a moving joint and may be painted. The automation guideline is direct about it: do not rely on the hinge for electrical contact between the door and the enclosure; install a bonding wire.

For industrial machinery, NFPA 79 adds a requirement. Section 8.2.3.6.1 of the 2007 edition, as quoted by practitioners (check the current edition), reads: "Where electrical devices are mounted on conductive doors or covers, an equipment (protective) bonding jumper shall be installed." The jumper connects the door or cover to the enclosure or to a protective bonding terminal inside it. Clause numbers move between editions, so check the edition that applies to your machine.

In practice: a door with pushbuttons, pilot lights, a meter or a display gets a flexible bonding jumper from a stud on the door to a stud on the body, with enough slack for the door to open fully. Our wall-mount enclosures have a continuous hinge, and the same rule applies: bond the door with a jumper when it carries devices. If you need an extra stud on the door of a custom build, specify it in your quote request.

Paint, coatings and contact surfaces

Bonding only works through bare metal. NEC 250.12 requires nonconductive coatings such as paint, lacquer and enamel on equipment to be grounded to be removed from threads and other contact surfaces, unless the fittings are designed to make removal unnecessary; 250.96(A) repeats the rule for enclosures and other metal parts serving as grounding conductors. The same training guide notes that driving a locknut tight with a screwdriver and pliers is considered sufficient to cut through paint and other nonconductive finishes on a conduit connection. That applies to conduit locknuts only: still remove paint at ground studs and every other bonding point.

• Powder-coated carbon steel: scrape or mask the coating at every ground stud, bonding point and conduit entry, or use fittings made to bite through it.
• Stainless steel: no paint to remove, but the contact faces should be clean and free of oil or sealant.
• Galvanized mounting panels: the zinc coating is metallic; keep the contact areas free of paint, labels and tape.

Watch for gaskets and sealing washers under conduit hubs and glands: they keep water out, but they can also break the metal-to-metal contact a bonding path needs. Use a bonding locknut or bushing, or a fitting with its own bonding provision, where the metal conduit is part of the grounding path.

DIN rails, shields and noise

A DIN rail is a mechanical support. IEC 60715:2017 states that the electrical function of a rail for protective earthing is covered by the relevant product standard, so a rail grounds a device only when that device's documentation says it does. Some products have no ground terminal and are grounded only through the rail; for those, the automation guideline says to run an equipment grounding conductor from the rail's mounting bolt to its own bolt on the ground bus.

Good bonding also reduces electrical noise. The same guideline recommends a steel enclosure to guard against interference, notes that a viewing window in the door should be a laminated screen or a conductive optical substrate to block it, and asks that each cable shield be grounded at one end only, at the end the product's instructions specify, and never connected to the common side of a logic circuit. Separation of power and signal cables is covered in the backplate and DIN rail layout guide.

Keep the neutral off the enclosure

Downstream of the service disconnect, the neutral (grounded conductor) and the equipment grounding conductor are kept separate. NEC 250.6 requires systems to be installed so that neutral or circuit current does not flow on metal parts; the training guide lists, as a cause of objectionable current, connecting the neutral to the metal case of a disconnect that is not part of the service disconnect, and warns of shock, fire and malfunction of protective devices. In an enclosure fed from a panel elsewhere, land neutrals on an isolated neutral terminal and grounds on the ground bus, and never bond the two together there. Separately derived systems, such as a control transformer with a grounded secondary, follow their own rules in Article 250; follow the transformer manual and the code edition in force.

Nonmetallic enclosures

Polycarbonate and fiberglass enclosures do not conduct, so nothing bonds through the wall. Every metal part inside or on the box (the mounting panel, metal conduit hubs, door hardware that carries devices) has to be connected to the equipment grounding conductor by its own bonding conductor or a fitting with a bonding provision. Compare the materials in polycarbonate vs fiberglass enclosures and enclosure materials compared.

If the finished enclosure becomes an industrial control panel, listing and inspection add their own requirements; see UL 50E, NEMA 250 and listing.

Frequently asked questions

Sources

1. Mike Holt, Understanding 2020 NEC Requirements for Bonding and Grounding: 250.96 and 250.12 (and Article 250 introduction)
2. Mike Holt, 2020 NEC: 250.54 auxiliary electrodes, earth not the effective ground-fault current path of 250.4(A)(5)
3. Mike Holt, 2020 NEC: 250.122 and Table 250.122, sizing equipment grounding conductors
4. Mike Holt, 2020 NEC: 250.119, identification of equipment grounding conductors
5. Mike Holt, 2020 NEC: 250.6, objectionable current
6. EC&M, "Code Q&A: Connection of bonding and grounding equipment" (NEC 250.8, 2017)
7. Mike Holt's Forum, "Control panel door grounding" (NFPA 79 8.2.3.6.1 quoted)
8. Rockwell Automation, Industrial Automation Wiring and Grounding Guidelines, publication 1770-4.1 (bonding, ground bus, door bonding wire, DIN rail grounding)
9. IEC 60715:2017 (protective earthing function of rails left to product standards)
10. NFPA 70, National Electrical Code (free read-only access): Article 250