Short answer: make the system bonding jumper at one single point between the transformer and the first disconnecting means — at the transformer, or at one of the two disconnects, never at both. A transformer that feeds two separate sets of secondary conductors out to two disconnect enclosures is legal under the NEC's secondary-conductor rules, and the grounding rules still allow one bond anywhere in that zone. The trouble starts when both disconnect cabinets bond neutral to ground: the steel raceway between them becomes a parallel neutral and carries a share of every unbalanced amp. In the worked example below, with illustrative impedances, 39 A of a 90 A unbalanced load ends up on an 18 in [450 mm] steel nipple, heating one painted thread at about 15 W.
Where does the system bonding jumper go with two disconnects?
At one point only: the transformer, or one of the two disconnects, never both. The NEC permits the bond at any single point between the source and the first disconnecting means, so a bond in each enclosure creates a parallel neutral through the raceway between them.
You have three legal spots for that one bond: the transformer lugs, disconnect A, or disconnect B. Pick it, print it on the drawing, and land every other neutral on an isolated bar. A second bond anywhere in the group — at the transformer as well as a disconnect, or in the second cabinet — turns the metal between the enclosures into a current-carrying conductor. The single-point logic runs through the whole grounding scheme, and the grounding and bonding guide covers the electrode connection, the equipment grounding conductors and the raceway bonding around it.
What does the NEC single-point rule actually say?
The NEC requires the system bonding jumper at any single point on the separately derived system between the source and the first system disconnecting means or overcurrent device — one location, chosen freely within that zone, with the grounding electrode conductor landing at the same point.
The word that does the work is single. With two secondary conductor sets, each disconnect is a first disconnecting means for its own set, so the legal zone covers three enclosures: the transformer and both cabinets. One jumper somewhere in that zone; the other two enclosures stay unbonded.
The two-set layout itself comes from the NEC's transformer secondary rules: each set leaves the transformer and terminates in one breaker or a single set of fuses at its disconnect, with conductor size and length meeting those rules. The bonding rule reads the same whether the secondary is 208Y/120 V or 480Y/277 V.
Why is a bond in both disconnect enclosures a problem?
Two bonds tie both neutral bars to the enclosure metal, and the raceway between the cabinets closes the loop. Neutral current then divides between copper and steel, and in the illustrative example below 39 A of a 90 A unbalance ends up on an 18 in nipple.
Trace the loop before the numbers. A's neutral bar bonds to A's cabinet; B's neutral bar bonds to B's cabinet. The steel nipple between the cabinets now connects the two bars in metal as well as copper, so part of A's neutral return travels through B's neutral conductor on its way back to the transformer.
Worked example, illustrative values — replace them with your conductor sizes, lengths and conduit type before judging a real panel:
- Transformer: 45 kVA, 480 V to 208Y/120 V; two secondary sets, each 25 ft [7.6 m] of 4/0 copper in 2 in steel conduit; cabinets joined by an 18 in [450 mm] steel nipple.
- Unbalanced neutral load at disconnect A: 90 A.
- 4/0 copper at 20 °C: 0.049 Ω per 1,000 ft, so each 25 ft neutral is 0.049 × 25 / 1,000 = 1.2 mΩ.
- Assumed nipple impedance: 0.4 mΩ; steel raceway runs several times copper's impedance per equal length, and the nipple is short.
Return path 1 is A's own neutral conductor: 1.2 mΩ. Return path 2 is the nipple plus B's neutral: 0.4 + 1.2 = 1.6 mΩ. The 90 A splits inversely with resistance: 90 × 1.2 / (1.2 + 1.6) = 39 A through the nipple and B's neutral, and 90 × 1.6 / 2.8 = 51 A through A's own conductor.
Give that 39 A one poor joint. A painted thread with 10 mΩ of contact resistance dissipates 39 × 39 × 0.010 = 15 W continuously — 15 × 8,760 gives about 130 kWh a year, concentrated on one fitting. The exact split moves with conductor size, length and conduit material; it only reaches zero when the second bond comes out. This is the same current the parallel neutral current in metal raceway between panels article measures.
Where should the bond go when two disconnects are both first?
Put it at the transformer. Either disconnect is legal as the bond point, but the transformer gives you one short grounding electrode conductor run, both neutral bars isolated downstream, and no ambiguity about which cabinet holds the bond.
Bonding inside one disconnect is the fallback when the transformer is sealed, shipped or out of reach. Then that cabinet carries the jumper and the grounding electrode connection, and the other disconnect is wired like any subpanel: neutral on the isolated bar, grounds on the bonded bar. The separation rules that subpanels follow stay unchanged; subpanel neutral and ground bonding covers them.
| Bond location | Allowed indoors | Nipple between the cabinets reads |
|---|---|---|
| Transformer only | Yes | Near zero |
| Disconnect A only, or B only | Yes | Near zero |
| Both disconnects | No | Neutral current, tens of amps in the example |
| Transformer plus one disconnect | No, indoors | Neutral current on the loop |
Outdoors, the last row changes only under the exception in the next section.
When does the outdoor exception allow two bonds?
Only when nothing conductive runs parallel to the grounded conductor. The NEC's outdoor exception permits bonds at both the source and the first disconnect where there is no parallel path; steel conduit between the two points is exactly such a path, so with metal raceway you keep one point.
The exception fits runs in nonmetallic raceway with no equipment grounding conductor or bond wire spanning the two bond points. Anything conductive that joins those points — steel conduit, a wire EGC, a jumper across a gutter — becomes part of the circuit, and the single-point rule applies again. With steel cabinets and steel conduit, plan on one bond.
Inheriting an existing outdoor system? Bonds at both ends with a PVC run and no EGC between them can stay as built. If the run is steel, pull the bond at the disconnect, land its neutral on the insulated bar, and keep the one at the source — the same move as removing a neutral-ground bond in an outdoor subpanel.
What does each disconnect enclosure need after the bond?
Each disconnect keeps a ground bar bolted to the cabinet and an isolated neutral bar: the white conductor lands on insulated standoffs everywhere except the one bond point, and each equipment grounding conductor is sized from its breaker — 8 AWG copper for a 100 A breaker.
NEC Table 250.122 sizes that conductor from the rating of the overcurrent device ahead of it: 8 AWG copper at 100 A, 6 AWG at 200 A. NEC 250.119 calls for insulated conductors 6 AWG and smaller to be green or green with one or more yellow stripes. The short jumpers that tie bars to the cabinet follow the sizing logic for a metal box, and bonding jumper size inside a metal junction box covers those.
Prep the metal for the fault path, not just the neutral. NEC 250.12 sends paint, lacquer and enamel off the threads and contact surfaces at bonding connections; on a stainless cabinet that means bare metal under the ground-bar bolts and gland locknuts. NEC 250.96(A) calls for raceways, cabinets and hubs to be bonded so they can carry the fault current likely to be imposed on them. NEC 250.4(A)(5) is why the earth is not counted as that path. System-side jumper sizing is a separate table keyed to conductor size; the transformer neutral bonding jumper sizing guide works that math.
How do you check a panel for a hidden second bond?
Clamp an ammeter around the nipple or raceway that joins the two cabinets while the panel runs. With the bond at one point, only stray capacitive and induced current should show — a fraction of an amp. Five amps or more means neutral load is riding the metal.
De-energize, lock out and confirm absence of voltage before opening either cabinet. Look for a jumper strap or a green bonding screw between the neutral bar and the cabinet in both enclosures. Remove the extra one, land that neutral on its insulated bar, and re-torque the ground connections you disturbed.
Re-energize under load and re-measure: the nipple reading should fall back toward zero. A nipple joining two enclosures is the same hardware you would use when extending a junction box off a panel with a conduit nipple, so this check covers that run too.
What do you put on the enclosure order?
Order each disconnect box with a bonded ground bar, an isolated neutral bar and a hub-ready cutout for the raceway, and mark on the drawing which single cabinet carries the bond. The two example cabinets order as 304 stainless boxes rated NEMA 4X / IP66 by the manufacturer; not UL listed.
Both cabinets come from the custom stainless enclosures line, cut from one sheet to your layout. Size the cutouts to the gland and hub threads before the sheet is cut; the cutouts, glands and windows guide gives hole dimensions and clearances. Most washdown and outdoor sites start at 304; salt air, brine or chlorides move you to 316L.
Configure both cabinets in the 3D enclosure builder with the bar kits and the nipple cutout, approve the drawing, and the pair is delivered in 10 business days from that approval. Put the bond location on the as-built drawing too, so the next technician sees at a glance why only one cabinet holds the strap.
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 |
|---|---|---|---|
| 60 × 36 × 16 in | 304 stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 48 × 36 × 16 in | 304 stainless | 48 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 12 in | 304 stainless | 60 x 36 x 12 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How large does the system bonding jumper itself need to be?
Size it from the largest ungrounded secondary conductor, using the NEC bonding-size table that is keyed to conductor size rather than to the breaker rating: 4/0 copper secondary conductors call for a 2 AWG copper jumper. Oversize the jumper if you oversize the conductors for voltage drop. This jumper is a different part from the equipment grounding conductors, which follow NEC Table 250.122 from the overcurrent rating.
Where does the grounding electrode conductor connect in a two-disconnect system?
At the same single point as the system bonding jumper, at the transformer in the layout this article recommends, or inside whichever one disconnect carries the bond. Run one grounding electrode conductor, not two; each disconnect then carries only its own equipment grounding conductors, 8 AWG copper for a 100 A breaker under NEC Table 250.122.
Can both disconnects live in one enclosure instead of two?
Yes, and it ends the problem: one cabinet, one bond on the shared backplane, no nipple between two bonded neutral bars, and the clamp meter around any internal raceway stays near zero. Order one custom box wide enough for both breakers and an isolated neutral bar; you set the width, depth, material and every cutout from a single piece.
Do the ground bars in both disconnect cabinets stay bonded to their boxes?
Yes. The single-point rule limits where the grounded neutral conductor is bonded, not the equipment grounding system. Both cabinets keep ground bars bolted to the enclosure, both raceways are bonded at both ends, and every equipment grounding conductor is sized from its breaker. Only the neutral stays insulated from the metal everywhere except the one bond point.
