Sub panel bonding jumper size: the two rules to apply

Short answer: size the jumper from the breaker ahead of the feeder. NEC Table 250.122, keyed to that breaker rating, gives the copper EGC size, and any ground-bar-to-wall jumper takes the same row. A jumper on the supply side of the main disconnect follows the NEC grounding electrode conductor table instead, sized from the service conductors. When your ground bar bolts straight to a metal enclosure, the screws already are the bond and no jumper is needed. This article sizes both pathways, works a 60 A example in a 304 stainless wall-mount box, and ends with the checks and the case-by-case decisions.

Which table sizes a sub panel bonding jumper?

Pick the table by the pathway the jumper serves: a jumper bonding a sub panel enclosure as an equipment grounding conductor follows NEC Table 250.122 and the feeder breaker rating, while a supply-side or main bonding jumper follows the NEC grounding electrode conductor table and the service conductor size.

A sub panel fed from a breaker in another panel sits downstream of the service. Its enclosure bond performs as an equipment grounding conductor, so the sizing key is the overcurrent device ahead of your feeder. NEC Table 250.122 does exactly that: it sets wire-type EGC sizes from the breaker's rating. The supply side works the other way. Between the service point and the main disconnect there is no feeder breaker to key from, so the NEC sizes those jumpers from the service conductor size, through its grounding electrode conductor table. The grounding and bonding guide carries the fundamentals; here you need only the two keys.

What size jumper does the feeder breaker call for?

Size it from the rating of the overcurrent device ahead of the feeder, not from the phase wire. Read the breaker rating, then take its row from NEC Table 250.122, which sizes wire-type equipment grounding conductors in copper and aluminum:

Worked example: a 60 A breaker feeds a 16 x 12 x 8 in 304 stainless wall-mount enclosure as a sub panel, with 6 AWG copper phase conductors good for 65 A at 75 C. The 60 A row sets the EGC, and any ground-bar-to-wall jumper with it, at 10 AWG copper. Suppose you upsize the phases to 4 AWG (85 A at 75 C) to hold voltage drop on a 150 ft run. The NEC then asks you to upsize the EGC in proportion: 6/4 = 1.5, so 10,380 cmil x 1.5 = 15,570 cmil, and the next standard size up is 8 AWG at 16,510 cmil. The breaker stayed at 60 A; your jumper grew one size. Read the breaker, take the row, and recompute when the phases grow.

When does the grounding electrode conductor table apply?

The grounding electrode conductor table applies when the jumper works on the supply side of the main disconnect: for a 100 A service built on 2 AWG copper service conductors, it gives an 8 AWG copper bonding jumper.

This pathway lives at the service equipment: the main bonding jumper tying the grounded conductor to the case and ground bar, and any supply-side bonding conductor between the service point and the main disconnect. No breaker sits ahead of them, so the NEC keys the size to the service conductors:

Downstream of a feeder breaker you are in Table 250.122 territory, and the shared AWG numbers between the tables are what trip people up. The decision rule follows the pathway: a jumper in the main bonding pathway takes the grounding electrode conductor table, and a jumper that bonds an enclosure as an EGC takes Table 250.122.

Do you need a jumper when the ground bar bolts to the enclosure?

No: when the ground bar bolts straight to the metal enclosure, the mounting screws are the bond, and NEC 250.96(A) asks only that metal parts serving as EGCs conduct the fault current; a jumper earns its space on insulated standoffs or a coated panel.

The bond through two screws and a bare wall is a real fault path. NEC 250.96(A) requires metal parts intended to serve as equipment grounding conductors to be bonded so they can conduct any fault current likely to be imposed on them, and a bolted bar does that. NEC 250.12 adds one condition: paint, lacquer and enamel come off threads and contact surfaces at grounding connections, unless the fitting is designed to contact through them. Bare stainless arrives with nothing to strip; the rule mostly bites coated bars and plated lugs. Where your bar sits decides the case:

• Bolted to the enclosure wall: the screws are the bond; measure it and move on.
• Bolted to the removable mounting panel: the panel bolts bond the panel; measure panel to wall.
• On plastic standoffs: it is either the isolated neutral bar, which stays unbonded, or an isolated-ground bar, which takes a Table 250.122 jumper to the EGC system.

The ground bar placement article covers wall-versus-plate mounting, and the backplane bonding comparison covers when a bonded backplate replaces a bar. Our own wall-mount line — hinged gasketed door, removable mounting panel, rated NEMA 4X / IP66 by the manufacturer and not UL listed — gives you three measurable joints per box: bar, panel and door (wall-mount enclosures).

How do you install the jumper in a stainless sub panel?

Run it as green-insulated wire at the Table 250.122 size for your feeder breaker, from the ground bar to a bare boss on the enclosure wall, following the eight steps below.

1. Read the breaker rating ahead of your feeder — 60 A in the worked example — and take the copper size from the table above.
2. Run the wire green: NEC 250.119 reserves green, or green with yellow stripes, for insulated EGCs 6 AWG and smaller, and your jumper qualifies.
3. Bring the feeder in through a listed gland and land the incoming EGC on the ground bar first; the ground entry connector and bond article covers the gland and the bond together.
4. Cut a jumper length from bar to wall boss, strip it, and crimp on a compression lug rated for the wire size.
5. Prepare the wall boss: deburr the hole and keep bare metal at the contact. Stainless ships unpainted, so NEC 250.12's stripping rule costs you nothing here.
6. Bolt the lug down with a stainless flat washer and an external-tooth lock washer, and torque to the value the lug maker marks on the part; sizing bolted joints for copper and aluminum under heat explains why heat cycles loosen an under-clamped joint.
7. Refasten the mounting panel on its bare bolts so the panel-to-wall joint conducts, and close the box.
8. Close the door and read the hinge; where it reads high, fit a door jumper of the same size — the door bonding jumper article shows the loop.

How do you prove the bond before energizing?

A sound bond reads only a few milliohms: the 2 ft example jumper adds about 2 mΩ, so a four-wire reading under 10 mΩ from ground bar to wall proves the joint, and a plain DMM cannot even see it.

Use a four-wire (Kelvin) milliohm meter. A hand-held DMM cannot resolve milliohms because its own test leads read typically 0.1 to 0.5 Ω. Take three readings on the example box:

• Ground bar to enclosure wall: under 10 mΩ.
• Mounting panel to enclosure wall: under 10 mΩ.
• Door to enclosure, door closed: under 10 mΩ from a door screw head to a wall boss.

The 10 mΩ pass number has arithmetic behind it. The 10 AWG example jumper runs about 1.0 Ω per 1000 ft (10.37 Ω·cmil/ft ÷ 10,380 cmil ≈ 1.0 mΩ per foot), so the 2 ft length adds roughly 2 mΩ and each clean crimped-and-bolted joint a few more. A reading near 10 mΩ means one joint is carrying the measurement; clean it and reclamp it. Write the three readings on your panel drawing so the next technician has a baseline.

What mistakes cost you the bond?

The bond fails in predictable ways: a lug set on paint (NEC 250.12 makes you strip coatings), a jumper sized from the phase wire instead of the breaker, or a neutral bar doing double duty — each one breaks the low-impedance path the sizing table assumes.

Work the list from the top; the first two causes cover most field failures:

• A lug set on paint or anodizing: strip to bare metal per NEC 250.12 and reclamp to the marked torque.
• Sizing from the phase wire instead of the breaker: a 60 A feeder on 4 AWG phases still takes the 60 A row size, or one size up after the proportional upsizing.
• Upsized phases with the EGC forgotten: recompute the area multiplier; 1.5x means the next standard size up.
• The neutral bar doubling as the ground bar: split them; the neutral rides isolated at a sub panel and bonds to a case only at the service disconnect.
• A detached building with no electrode: the feeder EGC and a local grounding electrode system both go in — the next section gives the sizes.

Does a detached-building sub panel need more than the jumper?

A sub panel in a detached building needs two things beyond the jumper: an equipment grounding conductor run with the feeder — 8 AWG copper for a 100 A feeder per Table 250.122 — and a grounding electrode system at the building itself.

The NEC asks a detached structure's feeder for both an equipment grounding conductor and a building grounding electrode system. NEC 250.4(A)(5) draws the division of labor: the earth is not an effective ground-fault current path, so the feeder EGC carries and clears fault current, while the electrode handles lightning and utility surges. NEC 250.6 supplies the third rule, keeping neutral current off metal parts: bond the grounded conductor to the case only at the service disconnect, and let it ride isolated in every sub panel downstream.

For a 100 A feeder to a workshop sub panel you pull that 8 AWG copper EGC, land it on a ground bar bolted to the enclosure wall, and take the electrode conductor out through its own sealed pass-through; the outdoor ground wire exit article shows the sealing.

The decision, case by case:

• Feeder breaker ahead of you: jumper and EGC per Table 250.122 — 10 AWG copper at 60 A, 8 AWG at 100 A, 6 AWG at 200 A.
• Ground bar bolted to bare metal: nothing to size; measure under 10 mΩ and keep the record.
• Bar on plastic standoffs: a neutral bar stays unbonded; an isolated-ground bar takes a Table 250.122 jumper.
• Supply-side pathway: grounding electrode conductor table from the service conductors — 8 AWG copper on a 2 AWG, 100 A service.
• Detached building: feeder EGC plus a local electrode, with the neutral bonded only at the service.

Enclosures in this article

Questions