Bonding a service wireway: size the jumper to the largest conductor

Short answer: bond the service wireway to the grounded service conductor with a copper supply-side bonding jumper sized from the largest ungrounded conductor. A landing on 4/0 copper takes #2 copper; the #4 guess and the #6 electrode lead both come up short on copper area. The #6 run from the ground rods keeps its job as the electrode conductor, and it lands on the neutral, never on the enclosure wall alone. The grounding and bonding guide covers the fundamentals; the work here is the service side, where no breaker sits upstream to size from.

Size the jumper from the largest ungrounded conductor

NEC Article 250 sizes the supply-side bonding jumper from the largest ungrounded service-entrance conductor, through the same table that sizes grounding electrode conductors. No overcurrent device sits ahead of the service landing, so there is no breaker rating to read; the conductor area sets the band. One 4/0 copper phase measures 211.6 kcmil (1 kcmil is 1,000 circular mils), which puts the job in the over-3/0-through-350 kcmil band and calls for #2 copper.

Sizes come from the NEC's grounding-electrode-conductor table, which Article 250 applies to supply-side bonding jumpers as well.

Two inputs move the band. Paralleled conductors in one raceway size from the combined area, so two 4/0 per phase is 423.2 kcmil and jumps a band to 1/0 copper. An aluminum service reads the aluminum column: 250 kcmil aluminum takes 1/0 aluminum where the same band takes #2 copper.

Past the main disconnect the logic flips. NEC Table 250.122 sizes an equipment grounding conductor from the rating of the overcurrent device ahead of it; the EGC run from a service disconnect to a subpanel follows that rule, and so does the bonding jumper sized inside a metal junction box. The service landing is the one place where the conductor, rather than a device, sets the size.

The ground rods answer a different question. NEC 250.4(A)(5) treats the earth as no part of the effective ground-fault current path, so a rod-to-box bond cannot clear a line-to-case fault. The copper jumper to the neutral clears it, through metal the whole way.

The wireway takes the splice work too. The NEC caps conductor fill at 20 percent of the interior cross-section and allows splices and taps to fill up to 75 percent at any point. A 6 x 6 in interior gives 36 in², so 7.2 in² of conductor room and a 27 in² splice zone.

What you need

• #2 copper jumper stock, one length from the wireway wall to the neutral splice plus 6 in of slack for retermination
• a two-hole lug and two bolts for the wall landing
• insulated multitap connectors with a port rated for every neutral tail: 4/0 for the utility neutral, 3/0 for the three-phase meter, 1/0 for the meter feeds (common inline units span 2/0 to 6 AWG)
• the existing #6 copper electrode run, kept in place, plus a #6 bond between the two ground rods
• a torque driver set to the value the connector maker prints on the body
• joint compound for any aluminum tail
• a wire brush or file for coated boxes: NEC 250.12 has nonconductive coatings removed from bonding contact surfaces unless the fitting is designed to bite through
• a standard multimeter for the continuity check

Sizing the jumper for a 6 x 6 x 36 in wireway on 4/0 copper

Inputs. The wireway measures 6 x 6 x 36 in, so the interior cross-section is 6 x 6 = 36 in². The largest ungrounded conductor is 4/0 copper, 211.6 kcmil, one per phase: three legs plus a 4/0 neutral from the utility. Landing beside it are 3/0 copper neutrals (167.8 kcmil) for the three-phase meter and 1/0 copper (105.6 kcmil) feeding the two-gang 100 A meter. The two candidates in question are #4 copper at 41.7 kcmil and #6 copper at 26.2 kcmil.

Rule. Find the band that contains the largest ungrounded conductor and read the copper column.

Arithmetic. The band above 3/0 runs from 167.8 to 350 kcmil. 211.6 > 167.8 and 211.6 < 350, so 4/0 lands there and the jumper is #2 copper, 66.4 kcmil of area. The #6 comes up short: 26.2 ÷ 66.4 = 0.395, about 40 percent of the copper the job calls for. The #4 does better and still fails: 41.7 ÷ 66.4 = 0.63, which leaves 37 percent of the area missing.

Result. Run #2 copper from the wireway wall to the grounded conductor.

Decision. The band is a floor, and nothing smaller carries a fault the way the sizing table assumes. The meter feed plays no part in the choice: 1/0 copper carries 150 A at 75 C against a 100 A meter, per the NEC conductor ampacity table for copper at 75 C. The feed passes while the jumper still grows to #2.

Landing the jumper in the wireway

1. Coordinate the outage with the utility and pull the meter or seal, then verify absence of voltage at the lugs before opening the wireway.
2. Join the neutral tails on insulated multitap connectors: a 4/0-rated port for the utility neutral, 3/0 ports for the three-phase meter, 1/0 ports for the meter feeds. Keep the whole cluster inside the 75 percent splice allowance at its cross-section.
3. Bolt the two-hole lug to the wireway wall and run the #2 copper jumper to a spare port in the neutral cluster or a separate lug on the grounded conductor.
4. Land the lug on bare metal. Stainless needs clean contact alone; on a coated box, scrape the coating from under the lug first unless the fitting is designed to bite through, as NEC 250.12 directs.
5. Move the #6 electrode lead onto the grounded conductor, where the NEC has it land, and tie the two ground rods together with #6 copper while you are there. The rod-connected portion of that lead may stay #6 even on a 4/0 service. The rod-side walkthrough continues in bonding a grounding electrode conductor to a service enclosure.
6. Torque every set screw and lug to the printed value, then tug each tail by hand.
7. Rebuild the working space before the covers go on. NEC 110.26(A) asks 3 ft (900 mm) of clear depth in front of equipment likely to be serviced while energized, at 0-150 V to ground in Condition 1. Keep room for the door to swing 90 degrees.

Checks when the work is done

• Continuity from the wireway wall to a neutral tail reads solid and well below 1 ohm on a standard meter; a good joint reads milliohms. An open circuit means the lug is sitting on paint rather than metal.
• Conductor fill: 4 x 4/0 = 0.664 in²; 4 x 3/0 = 0.528 in²; 3 x 1/0 = 0.249 in²; the #2 jumper = 0.052 in²; the #6 lead = 0.021 in². Total 1.51 in², and 1.51 ÷ 36 = 4.2 percent of the interior, far under the 20 percent cap.
• The splice cluster and its tails stay inside the 27 in² splice zone.
• Trace the #6 from the rods to its landing: it ends on the neutral, and the rods read bonded to each other.

Mistakes that leave the bond short

• Sizing from a smaller set: the 1/0 meter-feed band lands on #6 copper, the exact under-bond this example started with, and the 3/0 band's #4 still leaves 37 percent of the area missing.
• Counting the earth as a fault path: the rods hold voltage stable, and only the copper jumper to the neutral clears a line-to-case fault.
• Landing the #6 on the enclosure alone: the electrode loses its connection to the grounded conductor and the box still has no sized bond.
• Bonding through paint or powder coat: the lug reads as an insulator until bare metal shows.
• Sizing a paralleled run from one conductor: two 4/0 per phase is 423.2 kcmil and needs 1/0 copper.
• Crowding splices at one cross-section: the 75 percent cap keeps heat and crowding out of the wireway.

When the landing is an enclosure, not a wireway

The same bond works in a stainless enclosure: two-hole lug on bare wall, #2 copper tail into the neutral connector, coating scraped where it exists. A hinged door takes its own jumper across the pivot, as the enclosure door bonding jumper page details. Conductors entering through hubs bond at the entry, per the grounding conductor entry and bond page.

Odd sizes and cutouts come from custom enclosures. The stainless wall-mount line runs from 6 x 4 x 4 in to 60 x 36 x 16 in (H x W x D), rated NEMA 4X / IP66 by the manufacturer; not UL listed.

Service work adds one listing fact: OSHA 29 CFR 1910.303(a) requires electrical equipment in US workplaces to be approved, meaning accepted, certified, listed or labeled by a nationally recognized testing laboratory. Where the installation calls for a listed enclosure, the listed box is the one that goes on the wall.

Size to the largest conductor's band and land every bond on bare metal. Then each part keeps its own job: the jumper clears faults, the rods hold the earth reference, and the neutral carries the imbalance.

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