Bond every metal box on the supply side of the service into the grounded-conductor path: the CT can, the splice box and every nipple between them. The ground rod beside the can is an auxiliary electrode that clears no fault. Size the bond from the largest ungrounded supply conductor: 6 AWG copper for a 1 AWG run, 4 AWG for 3/0 copper. The grounded conductor itself lands once, at the service disconnect. Below are the fittings that count, the arithmetic that shows why the rod never carries a fault, the line where the utility's rules end, and the splice box you order.
Should the grounded conductor bond to the CT enclosure?
Yes: bond the CT can, its nipples and every metal box on the supply side into the grounded-conductor path. Treat the ground rod as an auxiliary electrode only, because the NEC's bonding rule wants that metal to carry the fault current likely to be imposed on it.
A fault inside the can has one road home: the grounded conductor. NEC 250.96(A) asks for the metal around that conductor — raceways, cables, equipment, enclosures — to be bonded together so it can safely conduct any fault current likely to be imposed on it. A drawing that shows only a bonding screw to a driven rod leaves the can protected by luck rather than by design.
If the utility owns the can, its crews bond it under the National Electrical Safety Code and you leave the cabinet to them. If you own the metering ahead of your main disconnect, the rules below are yours to satisfy.
Why doesn't the ground rod carry the fault?
The rod never carries the fault. At 25 ohms to earth, a 120 V line-to-can fault pushes only 4.8 A through the soil. The same fault through the bonded copper loop draws about 1,200 A and clears in a few cycles.
Run the numbers for a small commercial service. Assume a 25 kVA, 120/240 V secondary at 4% impedance, 25 ft [7.6 m] of 1 AWG copper each way, and one rod that reads 25 ohms, a common field value for a single rod.
- Transformer: 240 V x 240 V / 25,000 VA = 2.304 ohms, and 4% of that is 0.092 ohms.
- Copper loop, out and back: 1.72 x 10⁻⁸ Ω·m x 15.2 m / 42.4 mm² = 0.0062 ohms at 20 °C.
- Bonded path: 120 V / 0.10 ohms with the joints counted in, so about 1,200 A flows and the main breaker clears in a few cycles.
- Rod only: 120 V / 25.1 ohms = 4.8 A, a 100 A main sits there all day, and the can stays energized.
The code says the same in words. NEC 250.4(A)(5) keeps the earth out of the effective ground-fault current path, and NEC 250.54 files a rod beside a pole-mounted enclosure as an auxiliary electrode. Bond that rod with its own copper electrode wire for voltage-equalizing work, and let the jumpers do the fault work. The grounding and bonding guide lays out the chain end to end, and connecting a grounding electrode conductor at a service enclosure covers the rod connection itself.
Which fittings count as a bond?
Threaded hubs, bonding-type locknuts, bonding bushings with a jumper and wrenchtight threadless fittings all count. A plain locknut on painted steel does not, and above 250 V to ground it drops off the accepted list entirely.
Paint is the saboteur. NEC 250.12 calls for paint, lacquer and enamel to come off threads and contact surfaces at bonding connections, unless the fitting is designed to cut through the coating. Service gear arrives coated, so bare the landing points or buy fittings that bite through.
| Fitting | Does it bond? | Where you use it |
|---|---|---|
| Threaded hub, wrench tight | Yes | CT can and box entries |
| Bonding-type locknut | Yes | Metal raceway at each enclosure |
| Bonding bushing plus jumper | Yes | Splice boxes; jumper to the grounded conductor |
| Wrenchtight threadless fitting | Yes, up to 250 V to ground | Bare, clean threads |
| Plain locknut | No, above 250 V to ground | A 480 V service: step up to bonding type |
Above 250 V to ground, the accepted methods narrow to bonding-type locknuts, bonding bushings with a jumper or a dedicated bonding jumper across the joint. Doors count as metal too; the bonding rules for covers and doors keep the panel at the same potential as the can.
How big does the supply-side bonding jumper get?
Size a supply-side bonding jumper from the largest ungrounded supply conductor, not from the main breaker. The NEC's table keys the jumper to that conductor, so 1 AWG copper conductors and a 200 A service on 3/0 copper each land on their own table row.
The utility secondary has no breaker ahead of it, so the jumper keys to the conductor rather than to a trip rating. For copper runs, the NEC's supply-side bonding jumper table reads:
| Largest ungrounded supply conductor | Minimum copper bonding jumper |
|---|---|
| 1 AWG | 6 AWG |
| 2/0 to 3/0 | 4 AWG |
| 4/0 to 350 kcmil | 2 AWG |
Aluminum conductors follow the aluminum column and step the jumper up. For the 130 A service in the rod example, where 1 AWG copper carries 130 A at 75 °C, the can and the splice box bond with the table's 6 AWG copper minimum. Load-side jumpers key to the breaker rating instead; the jumper sizing inside a metal junction box covers that table. The same conductor-keyed sizing runs up a medium-voltage entrance too — the supply-side bonding walkthrough for MV transformers works the larger values.
Does the grounded conductor land in every box?
Connect the grounded conductor to metal once, at the service disconnect where its bonding jumper lands. Bond every other supply-side enclosure through jumpers or raceway fittings; a splice in the junction box needs no second neutral-to-can bond.
On the supply side that means bonding everywhere and neutral terminations only where the splice lives. The junction box may hold the grounded-conductor splice; bond the box to that splice with a jumper, or let the bonding bushings and metal raceway carry the loop. Past the disconnect, keep the neutral off the metal entirely: NEC 250.6 exists to keep neutral current off metal parts, so load current returns on the conductor alone. A stray parallel path heats connections and skews the meter. Where the neutral bond and the grounding electrode conductor land at the disconnect gets its own treatment in the service disconnect walk-through.
Whose rules apply at the service point?
Utility-owned gear answers to the National Electrical Safety Code, and the utility bonds its own cabinets. The NEC takes over at the service point, usually the transformer secondary terminals or the customer exit of the metering section.
The handoff line matters because every fitting and jumper size above is an NEC rule. Utility side of it, their crews supply, bond and seal the cabinet. Customer side, everything above applies, including the splice box and its jumper. If you own the CT can ahead of your disconnect, the NEC treats it as service metal and the same table sizes its jumper. Ask the utility before you add an entry to their gear; inside their cabinet, their spec sheet wins.
What do you order for the splice box?
Order a stainless box sized to the splices plus jumper slack. A 12 x 12 x 6 in [300 x 300 x 150 mm] 316L box with two 2 in [51 mm] bottom hubs takes the four splices of a 130 A service with its jumper loops.
Material first: 304 stainless covers most outdoor service yards, and 316L takes over within reach of salt air, brine or chemical spray. The enclosure is rated NEMA 4X / IP66 by the manufacturer, not UL listed, so phrase it that way on the submittal. Rain that pools on top calls for a decision: NEC 314.15 permits approved drainage openings of 3 mm (1/8 in) to 6 mm (1/4 in) in boxes and conduit bodies listed for damp or wet locations. A hole of that kind takes a Type 4X box down to Type 3R duty, so pick one job for the box — the 3R versus 4 comparison explains the trade.
You set width, height, depth and every hub position on the drawing, we cut from one piece, and the box lands in 10 business days from your approval of that drawing. The outdoor service pull-box spec list carries the remaining line items, and custom enclosures is where the drawing starts.
Which checks prove the bond?
De-energize, lock out and confirm absence of voltage first. Then meter from the CT can to the grounded conductor bus: 25 ft [7.6 m] of that jumper copper reads about 10 milliohms, and anything over 1 ohm means paint, corrosion or a loose bushing.
- Remake and re-torque any joint that reads over 1 ohm; the target stays in the milliohms.
- Inspect every entry: bonding-type locknut or bushing where the voltage to ground exceeds 250 V, wrench tight, no coating in the bite.
- Follow the splice-box jumper to the grounded conductor splice, not to the rod alone.
- Bond the door with its strap or jumper so the panel sits at can potential.
Sign the submittal only when the last can-to-bus reading sits in the milliohms. The 4.8 A road through the soil trips nothing, and the 1,200 A road through your jumpers clears in a few cycles.
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 × 12 × 6 in | 304 stainless | 12 x 12 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 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
Who bonds the CT can if the utility owns it?
The utility does, under the National Electrical Safety Code, and it usually owns the cabinet outright. The service point sits at the transformer secondary terminals or at the customer exit of the metering cabinet, and the NEC rules with their jumper table start on your side of that line. Ask before you add an entry to a utility cabinet; their crews supply, bond and seal their own gear.
What size supply-side bonding jumper does a 200 A service need?
Size it from the largest ungrounded supply conductor, not from the 200 A breaker. A run built on 2/0 or 3/0 copper needs a 4 AWG copper supply-side bonding jumper; a 1 AWG copper run needs 6 AWG copper. Both minimums come from the NEC's supply-side bonding jumper table, and a jumper smaller than the table value is a defect an inspector flags on sight.
Is a small bond to the ground rod legal?
Yes. A grounding electrode conductor serving only a rod electrode may stay at 6 AWG copper no matter how large the service conductors are, which is the connection utilities make at metering cabinets. Keep it in its own lane: the rod and its electrode wire equalize lightning and leakage voltage, while the bonding jumpers carry the fault current that trips the service breaker.
Can the grounded conductor splice live inside the junction box?
Yes, and that is common practice: splice it, then bond the box to the splice with a copper jumper or let the bonding bushings and metal raceway carry the bond. The one-connection principle stays intact: the grounded conductor ties to metal at the service disconnect, and the rest of the metal joins that path through jumpers and fittings, not through fresh neutral taps.
Which enclosure works for an outdoor service splice box?
A Type 4X stainless box rated by the manufacturer, not UL listed, handles rain and hose splash; pick 304 for most sites and 316L within reach of salt, brine or chemical spray. We build it to your width, height, depth and every hub cutout, from one piece, delivered in 10 business days from drawing approval.
