Bond them. AC grounds and DC common belong on one reference point in your enclosure: a copper ground bar bolted to bare metal, with the DC common carried to that bar on its own conductor — 6 AWG [13.3 mm²] insulated copper on a panel fed through a 100 A breaker. Keep the conductor runs separate and the reference single. An insulated DC ground bar on insulating standoffs earns its place only when your control system vendor's grounding document calls for one, and even then it bonds to the master ground point. The sections below sort out the three grounds people mix up, size the conductors with real ohm math, and end with what to put on the order.
The three grounds people mix up
Your cabinet runs three circuits that all answer to the word ground, and each does a different job. The 480 V AC feed needs a fault path back to its source. The 24 V DC bus needs a stable 0 V reference for the controller carriers, the network switches and the analog cards. The cable shields need a quiet place to drain. Panels that merge the jobs into one shared conductor pick up hum on 4-20 mA loops; panels that split them into separate islands get a DC bus that drifts for want of a reference.
| Ground | The job | Typical conductor | Where it lands |
|---|---|---|---|
| Protective earth (AC) | Carries fault current from the 480 V supply | Sized from the breaker ahead of it (NEC Table 250.122) | Ground bar bolted to enclosure metal |
| DC common (0 V) | Sets the reference for 24 V I/O and analog loops | Insulated copper, sized on impedance | The same bar, on its own run |
| Shield drain | Drains induced noise off cable shields | Each cable's drain wire | The same bar, one end only |
One copper bar carries all three, and that is the design in one line: a single reference point, three separate conductor runs into it. The code fundamentals sit in the grounding and bonding guide; the rest of this piece stays inside the box.
Why one shared reference beats two islands
Two code sentences set the floor. NEC 250.96(A) calls for metal parts serving as grounding conductors — raceways, cable grounds, enclosures — to be bonded together so they can carry any fault current likely to hit them. NEC 250.4(A)(5) rules the soil out of that path: the fault returns through permanently bonded, low-impedance copper back to the source. It is the same reasoning that keeps a welding return on its own low-resistance bond instead of the building frame.
The costs you feel are touch and noise. Bonding the supply chassis and the DC common to one bar holds the voltage between them near zero during a fault, so a hand spanning the two reads millivolts instead of tens of volts. The bond also anchors the DC reference. A floating 0 V bus drifts with leakage and coupling, and the drift lands on your analog inputs: 0.1 V of offset on a 250 Ω input is 0.4 mA of error, 2.5 % of a 4-20 mA span, or 2.5 °C on a 0-100 °C loop.
One bond makes a reference; two bonds between the same cabinets make a loop. The loop turns earth-potential difference into circulating current, and the current rides your shields. Run the ground loop checks for a control cabinet before you add a second bond, and follow the single-ended-signal plan when the loop crosses between cabinets: grounding between enclosures with single-ended signals.
When an insulated DC ground bar earns its place
Distributed control system vendors publish a power and grounding document, and some of them call for the DC ground bar to sit on insulating standoffs, clear of the backplane. If yours calls for it, follow it. Pull the current document from the vendor's support portal before you lay out the bar, because grounding guidance gets revised and the revision changes the arrangement.
The bar still gets a conductor: its own insulated copper run, green or green with a yellow stripe, run to the master ground point. The standoffs and the bond do different jobs. The standoffs keep AC fault current off the DC reference; the bond gives the 24 V bus one defined place to sit. A rail on plastic bushings with no conductor simply floats, and the first ground fault then pulls the bus to a potential nobody chose.
Sizing the bonds in a 100 A panel with two 24 V supplies
Every input below is a typical cabinet assumption — swap in your breaker size and supply ratings before you order copper.
- Feeder: 480 V AC, three-phase, through a 100 A breaker — the example cabinet used throughout.
- DC side: two 24 V supplies, each at 20 A output at current limit, a typical rating for controller carriers plus I/O.
- Bond run: 3 m [10 ft] from the DC bar to the master ground point, a typical wall-mount build.
- Copper: 1.72 × 10⁻⁸ Ω·m at 20 °C, the accepted value for annealed copper; 6 AWG is 13.3 mm², 12 AWG 3.31 mm², 4 AWG 21.2 mm².
The rule in words: the AC protective earth is sized from the breaker ahead of it, and NEC Table 250.122 puts 8 AWG copper behind a 100 A device. The DC reference bond gets its own insulated 6 AWG [13.3 mm²] run, judged on impedance rather than ampacity.
The arithmetic: R = ρL ÷ A = (1.72 × 10⁻⁸ Ω·m × 3 m) ÷ (13.3 × 10⁻⁶ m²) = 3.9 × 10⁻³ Ω, or 3.9 mΩ. The fault lift is V = IR = 20 A × 3.9 mΩ = 0.078 V, which is 0.078 ÷ 24 = 0.3 % of the 24 V bus.
What moves it: every metre of run adds 1.3 mΩ, and every ampere of fault current adds 3.9 mV across that 3 m. The same 3 m on 12 AWG reads 15.6 mΩ and lifts 0.31 V, 1.3 % of the bus — four times more.
The decision: keep the DC bond at 6 AWG [13.3 mm²] and hold the run at 4 m [13 ft] or shorter, which keeps the lift near 0.1 V with these supplies. A longer run steps to 4 AWG [21.2 mm²], which holds the lift to 65 mV at 4 m. The AC side stays at 8 AWG copper behind this breaker.
Where each conductor lands
Isolate the panel and confirm absence of voltage with a tested meter before you land anything. Then work down this list:
- Ground bar: bolted to the backplane near the power entry, one stud per conductor, toothed washers under the bar and every lug. NEC 250.12 wants paint, lacquer and enamel off the threads and contact surfaces at grounding connections, unless the fitting is made to bite through them.
- Incoming protective earth: lands on the bar through its own connector — grounding conductor entry: connector and bond covers the entry hardware.
- Bar position: wall-mounted or on the mounting plate changes what bonds what; ground bar placement on the wall vs the mounting plate walks the choice.
- Bar vs bare backplane: bonding everything to a bare backplane works too; the trade-offs sit in backplane bonding vs a ground bar.
- Power supplies: each protective earth terminal direct to the bar; each 0 V terminal on its own bond run sized as above.
- Neutrals: neutral on its isolated terminal, grounds on the bus — NEC 250.6 aims to keep neutral current off the metal.
- Shield drains: one end only, at the bar.
Where the bar, DIN rail and wire duct sit is backplate planning; the backplate and DIN rail layout guide sorts the zones so the bond run stays short and straight.
What to put on the order
The decision by case:
- Your vendor's grounding document is silent on isolation: one copper bar on the backplane, and the PE conductors, DC commons and shield drains all land on it. Done.
- The document calls for an insulated DC bar: order the second bar on insulating standoffs plus its own bond jumper to the master point, sized like the DC bond in the worked example.
- Several cabinets share signals and power: bond each cabinet once at its own bar, then look for loops between them.
Order the cabinet around that plan: you set the grade, the size and every cutout, and we build it from one piece — 304 stainless for indoor washdown and most outdoor sites, 316L for salt air, brine and chemical areas. The ground bar bolts to the removable mounting panel, and the wall-mount line brings the hinged gasketed door and mounting feet as standard. It is delivered in 10 business days, counted from your approval of the drawing. The boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Start from stainless wall-mount and HMI enclosures or send the layout straight into custom enclosures.
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.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does an insulated DC ground bar still need a bond to earth?
Yes. Run its own insulated copper conductor from the bar to the master ground point, sized like the DC bond in the worked example, and make it green or green with a yellow stripe, the color NEC 250.119 sets for insulated equipment grounding conductors of 6 AWG and smaller. The insulating standoffs keep AC fault current off the DC reference; the bond gives the bus one defined reference instead of a floating one.
What size conductor bonds the DC common to the ground bar?
6 AWG [13.3 mm²] insulated copper on a 100 A panel. Over a 3 m [10 ft] run it reads 3.9 mΩ, so a 20 A fault at one supply's current limit lifts the 24 V reference just 78 mV, about 0.3 % of the bus. Hold runs at 4 m [13 ft] or shorter; beyond that, 4 AWG [21.2 mm²] keeps the lift near 0.1 V. The AC protective earth is sized separately from the breaker ahead of it.
Can 24 V DC return current use the enclosure or the bond as its path?
Keep the return off the cabinet metal. A 20 A supply needs return copper rated for 20 A continuously between the loads and the supply's 0 V terminal; hinges, doors and the bond jumper are fault-duty parts, not a continuous return. NEC 250.6 applies the same idea to neutrals — keep that current off the metal — and the DC return earns the same treatment.
One ground bar or two in the same cabinet?
One, unless your control system vendor's power and grounding document says otherwise. PE conductors, DC commons and shield drains all land on that single bar. When the vendor calls for an insulated DC bar, order it on insulating standoffs plus the bond jumper described above, and count studs first: two PE conductors, two DC commons and two shield drains need eight studs with two spare.
