Bonding a cable tray to an enclosure: jumper when it is required

Yes: the cable tray needs its own bonding jumper to the enclosure, sized from the breaker ahead of the run. NEC Table 250.122 gives 1 AWG copper for a 600 A breaker. Say your tray and multiconductor cables run in parallel into a distribution enclosure, 350 kcmil feeders on a 600 V-class breaker, and every cable carries its own equipment ground. The tray still needs its own jumper. A tie from the tray to the building ground grid holds the tray at earth potential, but NEC 250.4(A)(5) keeps the earth out of the fault-current path, so the grid tie carries almost none of a fault. NEC 250.96(A) requires raceways, cables, equipment and enclosures to be bonded together so they can carry the fault current likely to hit them. The sections below size the jumper, land both ends, and prove the bond with a meter.

Why the ground-grid tie does not bond the tray

Fault current is a loop. It leaves the source, finds the fault, and must return over metal fast enough to trip the breaker. Soil is a poor return: a single driven ground rod commonly sits anywhere up to 25 Ω to earth — the long-standing benchmark for a made electrode — while 1 m of the worked example's copper jumper measures about 0.0004 Ω. That is roughly 60,000 times less resistance, so a fault takes the copper path almost entirely and the earth path barely at all. NEC 250.4(A)(5) says the same thing: the earth is not an effective ground-fault current path.

The bonding rule still applies. NEC 250.96(A) requires raceways, cables, equipment and enclosures serving as equipment grounding conductors to be bonded together so they can safely carry any fault current likely to hit them. A cable tray is one of those metal parts. Each cable's own ground bonds that cable to the equipment it feeds; the tray is a separate structure and needs its own path into the enclosure. The test you apply is continuity of metal from tray rail to enclosure wall, not whether the tray looks grounded.

Keep the grid tie after the jumper goes in. It drains lightning and standing leakage and holds touch voltage down, and an electrode connection is an auxiliary path under NEC 250.54: you may add it, and it never takes the place of the conductor that returns fault current. The same split applies when you bond a disconnect enclosure to a building ground ring. The fundamentals behind all of this sit in the grounding and bonding guide.

What you need

• A copper jumper sized from NEC Table 250.122 — the table in the next section; at the worked example's trip rating, that is 1 AWG.
• Two listed connectors rated for the conductor and for the surfaces they land on: tray rail steel on one end, enclosure wall or ground bar on the other.
• A bonding point on the enclosure: a ground bar or a spare lug on bare metal.
• A torque wrench, set to the value printed on the connector.
• Surface prep: bare brushed stainless wipes clean; on a powder-coated box, clear the contact patch to bare metal, which NEC 250.12 demands at grounding connections.
• A lockout kit, a voltage tester and a low-ohmmeter for the checks that follow.

Size the jumper from the breaker, not the conductors

NEC Table 250.122 keys the equipment grounding conductor to the rating or trip of the overcurrent device ahead of it, and the NEC sizes bonding jumpers to the same table. Pick the first row at or above your breaker's rating: a 450 A trip uses the 500 A row, 2 AWG copper. If several feeders share one tray, size the tray-to-enclosure jumper to the largest breaker among them. The 350 kcmil feeder size sets ampacity; it sets nothing about the bond. The table also sizes for fault clearing rather than load, which is why the jumper — 130 A continuous at 75 °C — carries far less than the trip it serves.

Rows from NEC Table 250.122. Run aluminum and take the aluminum column, with connectors marked AL/CU at both ends.

Worked example: a 350 kcmil tray run on a 600 A breaker

Inputs, and where each comes from:

• Circuit: 350 kcmil conductors on a 600 V-class breaker, tray and cables in parallel into a distribution enclosure — the job's data.
• Trip rating: taken as 600 A for the arithmetic. Read it off the device face before you size anything; this is the input you replace.
• Hardware: copper jumper, landing on a 316L stainless enclosure with a ground bar on the mounting panel.

Rule in words: the bonding jumper is sized from the overcurrent device ahead of the run, using NEC Table 250.122, first row at or above 600 A.

Arithmetic: the 600 A row gives 1 AWG copper, 42.4 mm², or 2/0 aluminum, 67.4 mm². Check what 1 m of that copper adds to the path: R = ρ × L ÷ A = (1.72 × 10⁻⁸ Ω·m × 1.00 m) ÷ (42.4 × 10⁻⁶ m²) = 4.1 × 10⁻⁴ Ω at 20 °C. Four hundred micro-ohms — the two contact points will matter more than the copper.

Result: fit at least 1 AWG copper from the tray side rail to the enclosure's bonding point.

Decision: at the example's trip, that copper size is the minimum and 2/0 aluminum works in its place. At 800 A, step up to 1/0 copper; at 400 A, 3 AWG copper is enough. Nothing on this job calls for a 350 kcmil jumper — that number belongs to the feeder, not the bond.

Install the jumper end to end

1. Lock out and tag the feeder, then test for absence of voltage at the enclosure terminals before you touch metal.
2. Pick the tray end: a side rail close to where the cables drop toward the enclosure. Keep the jumper short — under about 2 m where the layout allows — and as straight as the route permits.
3. Pick the enclosure end: the ground bar or a spare lug on bare metal. On a coated box, clear the contact patch first, per NEC 250.12.
4. Cut the jumper to length and land both ends in listed connectors rated for the conductor and the surface. Torque each to the value printed on the connector.
5. Bolt the tray end to the side rail through a clean hole or a punched slot, using a bonding fitting designed for cable tray.
6. Land the enclosure end on the bar. Keep it off the door leaf: a hinge alone is not a path, and the enclosure door bonding jumper article shows what to fit when a bond must cross the door gap. On a removable panel, bond the panel to the box as well; backplane bonding vs a ground bar covers that choice.
7. Where the cables enter through glands, give armor and shields their own path: why cable gland earthing is required walks the fault current through the gland.
8. Dress the jumper clear of sharp edges, leave a 50 mm [2 in] service loop at the tray end, and tag both ends with the size: "1 AWG CU BOND".

Checks when the jumper is in

• Measure from tray rail to enclosure wall with a low-ohmmeter. A healthy bond reads milliohms: the worked example's copper contributes 0.0004 Ω per metre, so a reading in tenths of an ohm points at a connector or a coated surface.
• Confirm the jumper size against the table row for the breaker, not the feeder size.
• Check that every cable ground still lands on the equipment ground bar; the jumper bonds the tray, the cable grounds bond the cables.
• Photograph both landed ends and note the torque values in the panel record.
• Re-check the bond after any tray work: a run re-spliced during maintenance can open the path you just proved.

Mistakes that undo the bond

• Counting the grid tie as the bond. The grid drains lightning and leakage; NEC 250.4(A)(5) keeps it out of the fault-current path.
• Habit-sized grounds: a token 12 AWG wire where the table row calls for a full size up, as the worked example shows.
• Lugs set on powder coat or anodizing. NEC 250.12 wants coatings off the contact surfaces unless the fitting is designed to cut through them.
• Treating splice plates as a bonding system. They clamp sections mechanically; fit the bonding jumpers the NEC's cable-tray article calls for across expansion joints and wherever the tray maker's continuity fittings are absent.
• Aluminum jumper on copper-only connectors. The connector must be marked for the metal it holds — AL/CU on mixed joints.

What to put on the order

Order the enclosure that receives the bond. A 316L stainless wall-mount box built to your width, height and depth gives you a removable mounting panel in bare stainless, so a ground bar or lugs land on it without scraping a coating off, and NEC 250.12 costs you nothing. On a powder-coated carbon steel box, plan to clear both contact patches during the install. The boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed, and each one is built from a single piece with your cutouts, delivered in 10 business days from your approval of the drawing, anywhere in the world. Browse stainless wall-mount enclosures for outside sizes from 6 x 4 x 4 in to 60 x 36 x 16 in, or mark the bonding points on the drawing of a custom enclosure.

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.

Questions