Short answer: drive the green bonding screw through the ground bus into the tapped boss in the back of the case, leave the neutral bus isolated on its plastic holder, and set the screw torque from the rating label inside the door. The screw supplied with a panelboard for the service-entrance neutral bond is the same part that bonds the ground bus in a feeder-supplied subpanel. The case has to join the equipment grounding path; the neutral has to stay off it. What follows covers which bus takes the screw, the crossbar question, the feeder ground size from NEC Table 250.122, the steps, and the three readings that prove the bond.
Why must the ground bus bond to the subpanel enclosure?
Bond it every time: the bonding screw joins the ground bus to the case so a fault to the metal returns on the feeder's copper ground and trips the breaker in milliseconds instead of leaving the enclosure at line voltage.
Two places in the NEC back this up. NEC 250.4(A)(5) defines the effective ground-fault current path and states that the earth is not part of it, so the case needs a wired way home. NEC 250.96(A) then requires metal parts serving as equipment grounding conductors — raceways, equipment, enclosures — to be bonded so they can carry any fault current likely to hit them. On most residential panelboards both buses snap into insulated plastic holders, so neither bus touches steel until you drive a screw. Leave that screw in the bag and the whole case floats above the grounded system. The enclosure-side basics sit in bonding an enclosure to a ground bus and in the grounding and bonding guide.
Which screw bonds the ground bus to the panel case?
Use the green bonding screw the panelboard ships with, driven through the ground bus's clearance hole into the tapped boss in the back of the enclosure; the maker supplies it for exactly this bond, and its green head is the grounding color the whole system shares.
The boss under the bus is already tapped and the hole in the bus already lines up with it, so the job takes a torque screwdriver and ten minutes. If the parts bag is lost, order the maker's bonding screw kit; its thread form and head are what the panel's listing was built around.
The same detail decides a control panel built into a stainless box. Bond the ground bar to bare metal, clear coating at the contact point, and give a painted door its own jumper, as control panel door bonding covers. Our custom enclosures come in 304 and 316L stainless, rated NEMA 4X / IP66 by the manufacturer; they are not UL listed, and NEC 409.110 still expects an enclosure type number marked on the finished panel, as UL 50E listing and enclosure type marking explains. Configure the box itself in the 3D enclosure builder.
Where does the bonding screw go: ground bus or neutral bus?
In a feeder-supplied subpanel the screw goes into the ground bus only; at the service disconnect it goes into the neutral bus, because that is the one place the code bonds the grounded conductor to the enclosure.
NEC 250.6 aims to keep neutral current off metal parts, which is why a subpanel's neutral rides the isolated bus while only grounds touch the case. Screw the wrong bus down and every unbalanced amp divides between the neutral and the feeder ground, warming connections and tripping breakers for no visible reason. Keeping subpanel neutral and ground apart walks the split, and removing a neutral-ground bond in an outdoor subpanel shows the repair when an earlier crew landed it wrong. A stray bond anywhere downstream pushes neutral current onto the case and the ground wires, and why a neutral-ground bond in a subpanel burns equipment traces the damage.
Can you remove the neutral crossbar for a 4-wire feeder?
Yes, when the panel's installation sheet documents it: many current residential panelboards list crossbar removal as an optional step that separates the neutral bus from the ground bus, and removing it that way keeps the listing intact.
The installation sheet in the parts bag is the maker's wiring diagram, so the strap comes out only when your sheet says so. Both buses stay on their insulated holders either way. Where the sheet gives no removal step, use the maker's documented accessory instead — an insulated bus split or a bolt-on ground bar kit — and the listing stays whole while the grounds gain their own landed bus.
What size equipment grounding conductor should the feeder carry?
Size it from the feeder breaker with NEC Table 250.122: 10 AWG copper under a 60 A breaker, 8 AWG copper under 100 A, and 6 AWG copper under 200 A, with the bonding screw and bus carrying the same path to the case.
NEC Table 250.122 sizes the equipment grounding conductor from the rating of the overcurrent device ahead of it. The common sizes:
| Feeder breaker | Copper ground | Aluminum ground |
|---|---|---|
| 60 A | 10 AWG | 8 AWG |
| 100 A | 8 AWG | 6 AWG |
| 200 A | 6 AWG | 4 AWG |
Insulated grounds 6 AWG and smaller wear green or green-with-yellow jackets under NEC 250.119, which makes a mislanded conductor easy to spot at the bus.
How do you bond the ground bus step by step?
Kill the feeder and prove it dead, land neutrals and grounds on their own buses, then drive the green screw into the tapped boss at the torque printed on the rating label; eight steps cover it from dead front to continuity check.
You need the green bonding screw from the parts bag, a torque screwdriver, a multimeter, a scraper or small file, and lockout means for the feeder. The job runs about half an hour on a hung panel:
- Shut the feeder off, lock it, and test for absence of voltage at the line lugs before touching a bus.
- Confirm the feeder is four wires — two hots, a white neutral, a bare or green ground — and note the breaker rating; the worked example below uses a 100 A feeder.
- Pull the dead front and read the installation sheet, which points to the rating label for tightening torque and states whether crossbar removal is a documented option.
- Remove the bus strap or crossbar where the sheet documents it, leaving both buses on their insulated holders.
- Land every white neutral under the neutral bus and every bare or green conductor under the ground bus.
- Scrape paint, plating or oxide from the tapped boss and the bus hole; NEC 250.12 requires nonconductive coatings cleared from threads and contact surfaces.
- Start the green screw through the ground bus hole into the boss and run it to the torque on the rating label.
- Tug-test the landed conductors, fill every unused opening, and refit the dead front before the readings below.
What checks prove the bond before you close the panel?
Three readings close the job: continuity from ground bus to case reads a fraction of an ohm, neutral bus to case reads open, and every neutral sits under a neutral lug with the feeder restored and the case within millivolts of the grounded slab beside it.
- Ground bus to case, feeder off: a fraction of an ohm, the same reading you get with the probes touched together.
- Neutral bus to case: an open reading; any continuity here means a stray bond or a neutral landed on the wrong bus.
- Feeder on with one load running: case to a known grounded surface reads millivolts; volts on this reading mean load current is hunting for a way home.
- A lug that spins or a screw that bottoms out points to thread trouble; grounding screw threads and lug fit covers the fix.
What happens when the case is left unbonded?
The breaker never trips: a fault to a floating case returns through about 25 Ω of earth, which limits 120 V to roughly 4.8 A, far below any feeder breaker's trip point and 300 times the 16 mA let-go threshold.
Worked example: one 100 A feeder run 30 ft [9 m] in 8 AWG copper, fault from an ungrounded conductor to the case at 120 V. Bonded: 8 AWG copper measures about 0.63 Ω per 1,000 ft at 20 °C, so the wire alone is 0.63 × 0.03 = 0.019 Ω. Add 0.05 Ω for the screw joint, the lugs and the bus, and the path sits near 0.07 Ω. Then 120 V ÷ 0.07 Ω ≈ 1,700 A. A thermal-magnetic breaker picks up magnetically around 5-10 times its rating, so 500-1,000 A already trips it, and 1,700 A clears in well under 50 ms at 60 Hz, where one cycle is 16.7 ms. Floating: the same fault returns through electrodes and earth, and the NEC caps a single driven electrode at 25 Ω, so 120 V ÷ 25 Ω = 4.8 A. The breaker holds indefinitely, the case sits near line potential, and 4.8 A is 300 times the roughly 10-16 mA let-go range in IEC 60479-1.
The mistakes that produce exactly that state: the screw still taped inside the dead front; the screw landed on the subpanel's neutral bus; coating left under the screw head; the strap pulled on a panel whose sheet documents no removal step; a ground rod counted on to carry the fault path alone.
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
Do you bond the neutral bus to the case in a subpanel?
Only once per system, at the service disconnect: NEC 250.6 aims to keep neutral current off metal parts, so a four-wire subpanel lands neutrals on the isolated bus and grounds on the bonded bus. A second bond splits every unbalanced amp between the neutral and the feeder ground, heating connections long before a breaker complains.
Does a ground rod at the subpanel replace the bonding screw?
It supplements the bond and never replaces it: NEC 250.54 calls a rod at a remote panel an auxiliary electrode connected to, rather than instead of, the equipment grounding conductor. Earth's 25 Ω passes about 4.8 A at 120 V, while the bonded 8 AWG copper path delivers well over 1,000 A and trips the breaker instantly.
Can the bonding screw bite through paint or powder coat?
Clear the coating first: NEC 250.12 requires paint, lacquer and enamel removed from threads and contact surfaces, so chase the tapped boss and scrape the bus hole bright before the screw goes in. Our 304 and 316L boxes arrive bare or brushed, while the powder-coated carbon steel line needs this scraping at every bonding point.
What if the panelboard's parts bag has no green screw?
Order the maker's bonding screw kit or fit a listed bonding jumper from the ground bus lug to a bonding point on the case, sized from NEC Table 250.122: 8 AWG copper under a 100 A feeder breaker. A random hardware-store screw into an untapped hole leaves threads that strip and a bond the installation sheet never documented.
