Backplane bonding vs ground bar in control cabinets

Short answer: yes, it is normal practice. North American panel builders land the main protective-earth conductor on the metal backplane and take every outgoing earth from that same sheet, and UL 508A — the standard for listing industrial control panels — accepts it when each bond is a machine screw in a tapped hole with at least two full threads of engagement in steel or three in aluminum. A copper ground bar is a quality upgrade, never a missing bond. Below are the thread rule with its arithmetic, the conductor sizing behind it, where a bar still wins, and what to write on the enclosure drawing.

Is bonding everything to the backplane allowed?

Yes, it is accepted North American practice: UL 508A, the standard for listing industrial control panels, lets the metal backplane carry the protective-earth job, and most control cabinets built in Canada and the US land their main earth lug straight on it.

The code basis is NEC 250.96(A). Metal parts that serve as equipment grounding conductors — raceways, cables, enclosures, backplanes — must be bonded so they can carry whatever fault current the circuit imposes on them. Nothing in that rule asks for copper. NEC 250.4(A)(5) settles the other half of the worry: the earth itself is not the effective ground-fault path, so the backplane counts as equipment, not as soil.

A cabinet whose main earth lug sits on the electro-galvanized backplane, with every outgoing earth made on that sheet, follows a recognized pattern — most machine and pump control panels in North America are wired exactly that way. What separates a good one from a defect note is fastener discipline. NEC 250.12 requires paint, lacquer and enamel off the threads and contact surfaces at grounding connections; galvanized or plated sheet is the easy case, because the plating conducts and the tapped hole exposes bare metal to the screw.

For the full path — supply, enclosure body, mounting panel, doors — read the enclosure grounding and bonding guide. The short version: the enclosure and backplane form one bonded structure with every raceway and gland entering it. A metal enclosure with no protective earth at all is a different question with its own fix.

How many threads does a grounding screw need?

Two full threads into steel, three into aluminum, with machine screws in tapped holes — sheet-metal screws never qualify, because they cut their own loose thread in a panel you will rework.

An inspector measures exactly these depths with a screw and a pitch gauge. Panel thickness is the catch. A 1.9 mm [0.075 in, 14 ga] steel backplane tapped 1/4-20 gives 1.9 / 1.27 = 1.5 threads, which fails; the same panel tapped M5 x 0.8 gives 1.9 / 0.8 = 2.4 full threads, which passes. On thin sheet, order finer pitches, weld nuts or clinch nuts. On aluminum below 3.0 mm [0.118 in], add metal, because three threads of an M6 need the full 3.0 mm.

UL 508A's Supplement SA, which covers components assembled into panels, expects a grounding terminal to be secured in place — the same discipline in other words: a screwed joint that stays tight for the life of the panel. Use a machine screw with a lock washer where vibration reaches the cabinet. On painted panels, set a toothed washer between lug and sheet so its teeth cut through to bare metal.

How do you size the bond from the supply?

From the rating of the overcurrent device ahead of it, using NEC Table 250.122: 8 AWG copper covers a 100 A feed, 6 AWG covers 200 A, and the same table sizes every branch bond in the panel.

The main earth lands on a listed lug bolted to the backplane through a tapped hole, and that lug becomes the common point. NEC 250.119 keeps the colors honest: insulated equipment grounding conductors of 6 AWG and smaller are green, or green with one or more yellow stripes. The green-yellow core common in IEC-land is therefore legal in a US panel in its striped form. PLC panel wiring color codes: North American standards maps the whole color scheme, power and control.

Worked example: 480 V panel, 100 A feed

Take a 480 V three-phase panel, 100 A fuses, a 1.9 mm galvanized steel backplane and 18 grounded devices.

1. Main bond. Table 250.122 gives 8 AWG copper (8.37 mm²) for a 100 A device. The lug bolts through an M5 tapped hole — 2.4 full threads, from the table above.
2. Sheet capacity. Current flowing along 300 mm [12 in] of sheet sees 1.9 × 300 = 570 mm² of steel. Carbon steel is roughly ten times as resistive as copper, so the sheet behaves like 570 / 10 ≈ 57 mm² of copper. That is about 1/0 AWG (53.5 mm²), five wire sizes above the 8 AWG the feed needs. Skin effect at fault frequencies raises the steel's AC resistance, which is one more reason the joints, not the sheet, decide the path.
3. Branch bonds. A 20 A branch takes 14 AWG green from the same table. Eighteen of them on a bar: 18 × 1.25 = 22.5, so order 24 positions for 25 % spares.
4. Door. The device ahead is 100 A, so the door jumper is 8 AWG green to the main lug.

Every number above belongs on the drawing before you order hardware. For where devices and duct sit around that main lug, the backplate and DIN-rail layout guide covers spacing and wire-bending room.

When should you still fit a copper ground bar?

Fit a bar when the panel will grow or when the grounds are fine stranded wires: it gives you numbered positions at one set torque — terminal-block datasheets put 2.5 mm² screw terminals near 0.5 N·m — instead of a queue at the drill press.

A bar wins on four counts. Test time: one lead on the bar answers for every outgoing earth. Retrofit: the next device takes the next position, with no drilling and no tap. Fine wires: a 0.75 mm² [18 AWG] ground with a bootlace ferrule belongs under a terminal screw, which holds it better than a sheet-metal lug. Records: numbered positions still match your drawings ten years later.

The backplane alone wins on parts count, cost and vibration: every direct bond is one fewer terminal and one fewer crimp, and a torqued machine screw holds better in shock than a push-on link. When you fit the bar, bond it through the sheet with the same two-thread rule at both mounting feet and break the coating under them — the same discipline as bonding an enclosure to a ground bus, just at panel scale. A bar on painted standoffs gives you the look of a bus without the circuit.

Keep the two systems straight while you are in there: protective earth clears faults; ESD control bleeds static charge, and they share metal only by courtesy. The ESD grounding vs protective earth comparison shows where each termination belongs. In a plastic box there is no backplane bond to argue about at all — protective earthing in plastic enclosures is a PE terminal and a conductor to every metal part.

Does the door need its own earth bond?

Yes, bond it with a conductor: size the door jumper from NEC Table 250.122 for the overcurrent device ahead of it — 10 AWG copper at 60 A, 8 AWG at 100 A — because a hinge carries no reliable fault current.

Treat the jumper as mandatory the moment the door carries anything electrical: an HMI, a door-interlock switch, a door-mounted drive. A clean, greased hinge reads a few ohms on a multimeter on day one, and reads anything at all after a few washdown seasons. Route the jumper inside the hinge side, clear of the spray path, through a grommet wherever it passes the frame.

How do you prove the bond at inspection?

Measure it with a low-resistance ohmmeter at a test current of at least 10 A, the way IEC 60204-1 tests machinery bonding: expect tens of milliohms, and read anything near 0.1 Ω as a loose joint rather than a long path.

Walk the panel with three checks. Visual: machine screws only, in tapped holes, coating broken at every ground contact. Torque: with the panel locked out and proven dead, re-torque a sample of bonding screws to the drawing value and mark them. Measurement: from the main PE lug to the farthest corner of the backplane, to the door and to each cover, logging the milliohm readings against the panel serial number.

If a panel crosses a border without a listing label, the usual route is a field evaluation — a one-time special inspection by a certification body, offered in Canada by CSA Group among others, that examines the panel against the standard. The examiner applies exactly the three checks above, thread engagement included, so build to them from the start. Mark the enclosure type number on the panel as NEC 409.110 requires, and the rating question stays out of the bond discussion.

What should you put on the enclosure order?

Write the whole bond scheme on the cutout drawing — PE entry, a tapped point for the main lug, tapped holes at every device, a ground bar with 25 % spare positions, a door jumper — because custom work is delivered in 10 business days, counted from your approval of the drawing.

In washdown and outdoor panels the whole structure is stainless: 304 for food plants and most outdoor sites, 316L where salt air, brine or chlorides reach the box. The hinged, gasketed stainless versions are rated NEMA 4X / IP66 by the manufacturer and not UL listed, so a job that needs a listing takes the field-evaluation route above. Stainless wall-mount enclosures run from 6 x 4 x 4 in to 60 x 36 x 16 in, or start from custom enclosures, where you set the material, the three dimensions and every cutout — the tapped bond points in this list included.

Enclosures for this job

Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.

Questions