Size every enclosure bonding conductor from the overcurrent device directly ahead of it. NEC Table 250.122 sets the wire: 14 AWG copper at 15 A, 12 AWG at 20 A, 10 AWG at 60 A, 8 AWG at 100 A and 6 AWG at 200 A. The grounding electrode conductor at the utility service has its own table and its own job, so its size never follows the circuit into your machine panel. A machine fed from a 60 A breaker in a wall disconnect gets a 10 AWG bond even where the service ground is much heavier copper. Bond the enclosure walls, the door, the backplate and the wireway, land every lug on bare metal, and the fault path is sized to clear.
Two conductors, two rules
The word ground hides two different wires. The grounding electrode conductor runs from the service disconnect to the electrode in the earth, and the NEC sizes it from the service-entrance conductor area, so it grows with the service while your machine bond does not. Its job is lightning and utility-side surges. The equipment grounding conductor and the bonding jumpers past the service disconnect do a different job: they carry fault current from the metal back to the breaker so the breaker clears. NEC 250.4(A)(5) calls that the effective ground-fault current path and states that the earth is not considered part of it.
Service conductors stop at the service disconnect; everything after is a feeder or a branch circuit. Downstream, every wire-type equipment grounding conductor and bonding jumper is sized from the rating of the overcurrent device ahead of it, using NEC Table 250.122. A control panel is only rarely the service equipment, so a machine hung on a wall disconnect almost always lands here. NFPA 79, the standard for industrial machinery, takes the same approach inside the machine: its table sets equipment grounding conductors and bonding jumpers from the upstream device, so one lookup answers both documents.
If the machine ships as a listed industrial control panel, UL 508A governs the assembly and NEC 409.110 requires the panel to carry an enclosure type number on its marking. The bonding scheme inside still keys to the device ahead of each circuit. For the wider picture, from earth electrodes to ground bars, see the enclosure grounding and bonding guide.
What you need
- The one-line diagram or panel schedule, so you know the breaker or fuse rating directly ahead of each enclosure run
- Your NEC edition, open to Table 250.122
- Copper wire and compression lugs sized to the conductors
- A crimper and a torque tool, with the lug maker's torque marking at hand
- A meter that reads fractions of an ohm, for the continuity check
- Green, or green with a yellow stripe, insulation for any insulated equipment grounding conductor 6 AWG and smaller (NEC 250.119)
Size each bond in five steps
- Find the device ahead of the run. The bond is sized from the rating or setting of the breaker or fuse that protects the conductors feeding the enclosure. On a machine fed from a wall disconnect, that is the disconnect's breaker. On a 115 V control circuit from a fused transformer secondary, it is that fuse; the device-side detail is in grounding a 120 V AC device inside an aluminum enclosure.
- Read the table row. NEC Table 250.122 gives the minimum wire-type equipment grounding conductor in copper. The rows machine feeders meet most:
| Overcurrent device ahead | Minimum copper bond |
|---|---|
| 15 A | 14 AWG |
| 20 A | 12 AWG |
| 30-60 A | 10 AWG |
| 100 A | 8 AWG |
| 150-200 A | 6 AWG |
- Scale the bond if voltage drop upsized the phases. When a long run pushes the phase conductors above the ampacity minimum, the equipment grounding conductor grows by the same conductor-area ratio. The worked example below runs the multiplication.
- Bond every metal part on that supply, each from its own device. NEC 250.96(A) calls for raceways, enclosures and other metal parts serving as grounding conductors to be bonded so they can carry the fault current likely to be imposed. Removable doors and panels each get their own conductor. Tie the bonds at a ground bar or terminal strip rather than chaining enclosure to enclosure. Where the bar sits — enclosure wall or mounting plate — is covered in ground bar placement in enclosures; the bus tie-in is in bonding an enclosure to a ground bus.
- Terminate on bare metal and mark the wire. NEC 250.12 requires paint, lacquer and enamel to be removed from threads and contact surfaces at grounding connections, unless the fitting is designed to make removal unnecessary. Insulated conductors 6 AWG and smaller wear green, or green with yellow stripes.
Sizing the bond for a 24 x 20 x 8 in enclosure on a 60 A feeder
The case: a machine enclosure built on the 24 x 20 x 8 in 304 stainless wall-mount enclosure from our wall-mount range, hung 45 ft from a wall disconnect and fed at 240 V through the disconnect's breaker. Treat the breaker rating, the 240 V and the 45 ft as example values; keep the method and drop in your own numbers.
Inputs:
- Enclosure: 24 x 20 x 8 in, 304 stainless, hinged door (catalog size)
- Supply: 240 V, 60 A breaker in the wall disconnect (example value)
- Run: 45 ft of conduit (example value)
- Phases: copper sized for the trip — from the NEC's copper ampacity table, the next size down carries 50 A at 75 C and falls short of the 60 A trip, so the feeder runs the size above it, which carries 65 A
- Metal to bond: enclosure body, door, backplate, wireway, conduit hub
The rule in words: NEC Table 250.122 sets the wire-type equipment grounding conductor from the rating of the overcurrent device ahead of the run. The 60 A row calls for 10 AWG copper, whatever the phases are.
The arithmetic:
- Phase check first: 50 A (the smaller conductor) sits under the 60 A trip, so the feeder takes the next size at 65 A.
- Bond: 60 A row, 10 AWG copper. The wire carries 35 A at 75 C, about half the trip rating, and that is correct — it conducts only the seconds before the breaker clears.
- Door jumper: the same 10 AWG in flexible form, door skin to a bare boss on the box, so no strap is the weak link.
Result: 10 AWG copper for the enclosure bond and the door jumper.
Now lengthen the run to 150 ft and voltage drop takes over. For copper, take K = 12.9, the usual hand-calculation constant. The drop on the 65 A-rated phase conductor (26,240 cmil) is 2 × 12.9 × 60 A × 150 ft / 26,240 cmil = 8.8 V. That is 3.7% of 240 V, past the 3% most panel shops budget for a feeder. The phases move up to 52,620 cmil and the drop falls to 2 × 12.9 × 60 × 150 / 52,620 = 4.4 V, or 1.8%. The bond then scales by the area ratio: 10,380 cmil × (52,620 / 26,240) = 10,380 × 2.005 = 20,812 cmil. The next standard size at or above 20,812 cmil is 6 AWG, at 26,240 cmil; those areas come from the NEC's conductor-property table.
Decision: while the phases stay at the 65 A size, bond the enclosure, door, backplate and wireway at 10 AWG copper. When the voltage-drop fix moves the phases up two sizes, the bond goes to 6 AWG. Length alone never changes the bond — only a phase upsize does.
Checks when the bond is in
- Continuity, door to ground terminal. Bond wire of this size runs about 1.0 Ω per 1,000 ft (3.3 Ω per km), the standard conductor-table value, so the 45 ft bond's copper reads 45/1000 × 1.0 = 0.045 Ω and the lugs add a little. Watch the meter while you wiggle the door: a jumping number means paint or a loose jumper lug, not the copper.
- Torque at every lug. Set each to the value the lug maker marks on the part or its datasheet.
- Bare metal at every seat. Scrape the lug footprint back to shiny stainless; powder coating under a lug hides a resistance that shows up a season later.
- Marking. Every insulated bond 6 AWG and smaller wears green or green with a yellow stripe (NEC 250.119).
- The entry path. Where steel conduit forms part of the route, a listed hub installed wrench-tight carries its share of fault current; listed conduit hubs for bonding sorts out which ones count. Where the wire enters on its own connector, the landing detail is in grounding conductor entry: connector and bond.
Mistakes that break the fault path
- Sizing from the service ground. Heavy copper at the meter tempts everyone to upsize the machine bond to match. The bond answers to the breaker or fuse ahead of it; oversizing burns copper and leaves the real weak spots untouched.
- Skipping the area-ratio upsize. Phases grew for voltage drop and the bond stayed at its original size; the fault path now has its thinnest link exactly where the code scales it.
- Bonding through paint. A lug torqued onto powder coating reads fine on install day and corrodes into resistance. Scrape to bare metal or use a fitting designed to cut through coatings.
- Counting the earth as the return. NEC 250.4(A)(5) settles it: the fault path is the metal, and the earth is not considered part of it.
- Leaving the door on the hinge alone. A jumper sized like the bond keeps the door in the path through years of opening and washdowns.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
What size bond does a 20 A machine circuit need?
12 AWG copper, straight from NEC Table 250.122's 20 A row. Most machine-mounted junction boxes, door-light circuits and small control loops sit on 20 A branches, so one 12 AWG green wire covers them. Upsize only when a voltage-drop fix enlarges the phase conductors; the bond then grows by the same conductor-area ratio.
Does a 115 V control circuit from an internal transformer need its own bond size?
Bond that circuit from the fuse ahead of it, like any other. A control transformer secondary fused at 15 A or less takes the table's smallest wire, 14 AWG copper. Where the transformer counts as a separately derived system, its secondary bonding jumper follows the NEC's rules for those systems and is sized from the secondary conductor size — a different wire from the enclosure bond at the door.
Can the hinge bond a door, or does the door need its own jumper?
A bare stainless hinge in metal-to-metal contact does carry fault current, but paint, gasket compression and wear make the hinge the weak point of the path. Fit a flexible jumper sized like the enclosure bond — same table row as the feeder breaker — from the door to a bare boss on the box, and land both ends on scraped-clean metal the way NEC 250.12 requires for bonding connections.
How large is the grounding electrode conductor at the service, for comparison?
The NEC sizes it from the service-entrance conductor area, so it grows with the service; for a concrete-encased electrode, 4 AWG bare copper is the largest the code asks for at any service size. That wire's job is lightning and utility-side surges at the meter, not fault clearing at your machine, which is why its area never sets an enclosure bond.
