Short answer: yes — a bolted steel back panel is a sufficient path between two ground bars, and the wire joining them is optional. The NEC's list of permitted equipment grounding conductor types runs to fourteen entries, and the metal enclosure and raceway you already have are part of that system. Bolt both bars to bare steel and the panel carries ground-fault current the way a conduit run does. Fit the short jumper anyway: size it to the largest EGC landing on either bar — 3 AWG copper behind a 400 A fuse in the worked panel below — and it costs one pair of lugs and ten minutes to settle the question before an inspector asks it.
What the code counts as the path
The NEC keeps a list of what may serve as an equipment grounding conductor, and it runs to fourteen types. Copper wire is one entry. Rigid metal conduit, EMT, metal-clad and mineral-insulated cable sheaths and other listed metal assemblies sit on the same list. That is why a feeder in steel conduit needs no wire EGC of its own (metal raceway as equipment grounding conductor: when it counts). A bolted steel back panel is one more bonded steel part of that same wiring system.
NEC 250.96(A) is the rule that puts the panel in the circuit. Metal parts serving as grounding conductors — raceways, cables, equipment, enclosures — must be bonded so they can safely conduct any fault current imposed on them. The bond gives fault current what NEC 250.4(A)(5) calls the effective ground-fault current path. It is a low-impedance metal route that lets the overcurrent device clear, and the earth carries none of it. Land the feeder's raceway or EGC on the enclosure, and the wall, the back panel and both bars act as one body. It is the same trick a panelboard's enclosure uses to join its own parts (panelboard enclosure as an equipment bonding jumper).
One line the panel cannot cross: it carries fault current, not load current. The grounded conductor lands on an isolated neutral terminal, because NEC 250.6's whole aim is keeping neutral current off metal parts. The fundamentals behind all three rules sit in the grounding and bonding guide.
Bare metal under every lug
NEC 250.12 takes the coating out of the circuit: paint, lacquer and enamel come off threads and contact surfaces used for grounding, unless the fitting is designed to cut through them. On a bare 304 or 316L back panel there is nothing to remove; the lug lands on brushed steel. On painted or powder-coated carbon steel, strip each bar footprint to bare metal or bite through with external-tooth lock washers. A lug sitting on coating can read milliohms or an open circuit, because the paint has become the conductor.
Bolt each bar down with two fasteners on bare metal, torque every lug to the value stamped on its body, and re-torque after the first heat cycle. A clean joint on bare steel measures a few tenths of a milliohm; that is the working figure the example uses. De-energize the panel and prove absence of voltage before you open a lug or clamp a meter — everything after this point assumes the panel is dead and locked out.
Milliohms: the panel against the jumper
Here is the panel this question usually arrives with: 480 V, three-phase, a 400 A fused disconnect on the line side and seven 60 A resistive circuits on the load side. NEC Table 250.122 sizes each EGC from the overcurrent device ahead of it, so the line-side EGC is 3 AWG copper and the seven load-side EGCs are 10 AWG. The line-side bar sits at the top right of a 24 × 20 in [610 × 510 mm] back panel. The load-side bar sits at bottom middle, beside its seven load terminals, so nobody strips 6 ft [1.8 m] of jacket off a three-conductor cable to reach a bar at the far corner.
The working figures, all of them assumptions you confirm with the meter:
- 304 stainless resists about 0.7 μΩ·m, roughly 40 times copper's 0.017 μΩ·m (handbook values at room temperature).
- Path length is about 0.6 m [24 in], corner bar to bar, through a 300 mm [12 in] wide strip of the 2 mm (0.079 in) sheet: 600 mm² of cross-section.
- Steel: 0.7 × 10⁻⁶ Ω·m × 0.6 m ÷ 6 × 10⁻⁴ m² = 0.7 milliohm.
- Joints: two bar-to-panel joints at 0.2 milliohm each add 0.4, so the panel path lands near 1.1 milliohm.
- Jumper: 150 mm [6 in] of 3 AWG copper at about 0.65 milliohm per metre (standard copper resistance data) adds 0.1, and its two lugged joints add about 0.4, for roughly 0.5 milliohm.
At a 5,000 A ground fault the panel path drops 5,000 A × 1.1 mΩ = 5.5 V, and the jumper path drops 2.5 V. Both are small beside the 20 milliohms the line-side EGC itself shows over a 30 m [100 ft] run back to the source. The panel is a valid path; the jumper is cheap insurance.
Why the jumper still earns its place
The calculation leaves the wire optional, and your panel is better with it fitted. A 150 mm tail sized to the largest EGC buys:
- a bond that survives the next repaint or a fastener left a turn loose, instead of resting on two joints;
- a connection an inspector accepts on sight, which answers the question before it is asked;
- a lower path impedance at no engineering cost: 1.1 milliohm through the panel becomes 0.5 with the jumper fitted;
- a habit that carries over to non-metallic boxes, where no panel exists and the wire between bars is the only path (grounding a fiberglass enclosure: EGC and back panel bonding).
Size the jumper like any EGC: no smaller than the largest conductor it bridges, the 3 AWG tail in the worked panel (bonding conductor size for accessible equipment enclosures). Keep it short and direct. Where the tail passes within striking distance of live terminals, a coat of brush-on insulation for terminal jumper bars keeps the crossover safe.
The door stays a separate job: hinges are not a bond, so a door-mounted bar takes its own jumper (enclosure door bonding jumper: when a hinge is not enough).
Factory bar, listed panel or your own build
A listed assembly narrows your choices. NEC 409.2 defines an industrial control panel as an assembly of two or more power or control circuit components, and UL 508A is the standard for listing one; the grounding means is part of that evaluation. Land on the factory bar where the drawing puts it. A bar you add becomes field work, so hold it to the same standard: bare-metal joints, lugs listed for the conductor size, conductors sized from Table 250.122. What listing means for an enclosure covers the enclosure side of that story.
In a generic steel box you lay out yourself, you are the panel builder, and the joints and jumper above are the whole design. Our stainless bodies come cut for your layout, with mounting holes for the second bar machined at the factory. They are rated by the manufacturer and not UL listed, so your grounding layout is the paper that stands behind the work.
What goes on the drawing and the order
| Your case | Path between the bars | Jumper between the bars |
|---|---|---|
| Bare 304/316L back panel, both bars bolted to bare steel | Panel carries it, near 1.1 milliohm in the worked panel | Fit one sized to the largest EGC |
| Painted steel panel, footprints stripped to bare metal | Panel carries it once the coatings are off | Fit it, then read the bond |
| Fiberglass or polycarbonate enclosure | None; the wire is the only path | Mandatory; size from NEC Table 250.122 |
| Listed panel with a factory ground bar | The factory bar is the grounding point | Follow the panel drawing |
For a new build, mark both bars, their mounting holes and the jumper lug points on the enclosure drawing. Order the back panel in bare 304 or 316L so there is no coating to strip. The custom stainless cutout page lists what the factory needs on the drawing; a body cut to that drawing is delivered in 10 business days from your approval.
Close the job with the meter rather than the eye. A 4-wire low-resistance reading, one lead on each bar, should land near 1 milliohm through the panel alone and near 0.5 with the jumper fitted. Tens of milliohms point to a lug sitting on paint or a fastener that never reached torque. Read it at build and again after any panel repair, and the path stays a path.
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 paint have to come off under a ground bar?
Yes. NEC 250.12 requires paint, lacquer and enamel removed from contact surfaces used for grounding unless the fitting is designed to cut through. A bare 304 or 316L back panel needs nothing removed. On coated steel, strip the lug footprint or use external-tooth lock washers, then confirm each joint with a bar-to-bar reading below about 1 milliohm.
Can the bar-to-bar jumper be aluminum?
Yes, but an aluminum jumper of the same ampacity runs longer and hotter at the lugs. Aluminum carries about 60 percent of copper's current per size, so the usual practice is one size larger than the copper answer, and the lugs must be marked for aluminum. A 150 mm copper tail is cheaper, easier to bend and about half the resistance of the equivalent aluminum tail in the same footprint.
Can the back panel carry neutral current between the two bars?
No. The back panel carries ground-fault current only, never load current. A neutral lands on an isolated terminal: NEC 250.6 exists to keep neutral current off metal parts, because steel that carries load current heats at every stray joint. In a 480 V panel, any neutral bus sits on stand-offs and only the EGCs touch the ground bars and the steel.
What should a bond test read between the two bars?
About 1 milliohm through the panel alone in the worked example — 0.7 milliohm of 304 stainless sheet plus about 0.4 for the two bolted joints — and about 0.5 milliohm once the jumper is fitted. Measure with a 4-wire low-resistance meter, one lead on each bar. A reading in the tens of milliohms means a lug is sitting on paint or a bolt never reached torque.
Does the back panel need bonding to the enclosure wall as well?
Only the joints in the fault path matter. Both bars sit on the back panel, so bar-to-bar never leaves the panel. The fault path does leave it: current returns through the raceway or EGC that lands on the enclosure wall or a ground hub. On a bare stainless box the welded studs make that contact for you; on a coated enclosure, give each panel standoff an external-tooth washer or land the supply EGC directly on the back panel.
