Put the main bonding jumper in the 600 A disconnect. That enclosure is the first service disconnecting means. The bonding screw, the grounded service conductors and the 2/0 copper grounding electrode conductor all meet on its neutral bar, and nowhere else. What is installed there is already right. The faults sit downstream. The bonded lug pair in the MDP turns the equipment grounding bar into a second neutral, and every subpanel with its grounded conductor bonded to the EGC adds another. Each stray bond moves normal load current onto grounding conductors and metal that should carry current only during a fault. The repair is a day's work: break every bond after the disconnect, tie the electrodes back to the service neutral with 2/0 copper, and prove the isolation with a clamp meter and an ohmmeter.
Where the main bonding jumper goes
A service gets exactly one connection between the grounded conductor and the grounding system, and the National Electrical Code places it at the first service disconnecting means. In this layout that enclosure is the 600 A disconnect, ahead of the MDP. Three things land on its neutral bar: the grounded service conductors, the grounding electrode conductor, and the enclosure through the factory bonding screw. That screw serves as the main bonding jumper. The fundamentals sit in why the neutral-ground bond lives only at the service disconnect.
After that point the system splits in two. Grounded conductors ride on insulated lugs with no contact to any metal part. Equipment grounding conductors bond to enclosures and stay idle until a fault gives them work. Held apart this way, load current stays on the neutral, fault current stays on the EGC, and neither turns up where it is not wanted.
The electrode set the specs list — building steel, water pipe, concrete-encased footer rebar and a driven rod — connects to the grounded conductor at or ahead of that same point. That is why a bond in the MDP cannot stand in for one at the disconnect. The grounding and bonding guide walks the full chain. The earth itself is not an effective ground-fault current path under NEC 250.4(A)(5): a ground rod holds the system near earth potential, and it clears nothing.
Moving the single bond to the MDP looks tidy and fails on physics. With the neutral left unbonded at the disconnect, a phase fault to that enclosure has to return through the copper electrode conductor to the rod and then through soil. A single rod commonly reads tens of ohms; at the NEC's 25 Ω benchmark for one rod, a 480 V fault returns about 19 A. That is nowhere near the trip curve of a 600 A breaker, so the disconnect would sit ahead of the only bond, protected by dirt.
What the 600 A disconnect already gets right
The 2/0 copper terminated on the grounded conductor lug is the grounding electrode conductor, and the size holds. The NEC's sizing table calls for 2/0 copper when the largest phase conductor runs over 3/0 through 350 kcmil, and this service runs two sets of 350 kcmil per phase. The same 2/0 covers the concrete-encased electrode. A run that ended on a driven rod alone could stop at 6 AWG copper; with rebar and building steel in the set, the 2/0 stands. See landing a grounding electrode conductor on the service enclosure for the termination details.
The bonding screw is the main bonding jumper, installed exactly where it belongs. With PVC raceways on both sides there is no metallic raceway path to share the work, so the screw and the 2/0 copper carry the whole connection.
The feeders check out too: 350 kcmil copper is good for 310 A at 75 °C under the NEC's ampacity table, so the two parallel sets give 620 A against a 600 A device. Nothing at the disconnect needs to change. The electrode conductors simply have to reach this neutral bar, which the repair below arranges.
The MDP and the subpanels: the two faults
In the MDP, the neutral lugs and the EGC lugs sit on bars bonded to each other. That bond turns the first panel after the service point into a second bonded service. Break it: mount the neutral bar on insulated standoffs with no contact to the enclosure, and bond the EGC bar to the enclosure on its own hardware. On a painted steel enclosure, NEC 250.12 calls for paint lifted from the bonding screw's contact surfaces unless the fitting is designed to bite through. Bond through bare metal either way.
Every subpanel repeats the fault: each grounded conductor bonded to its EGC inside the panel. A factory bonding screw left in place does it by itself. Remove the screw or the strap, and land the neutrals on the isolated bar. Give the EGC bar its own bond to the enclosure with the supplied screw, as in bonding the ground bus to the panel enclosure. Removing a neutral-ground bond from a subpanel is the same job indoors or out, per removing a neutral-ground bond from an outdoor subpanel.
| Location | Installed now | What belongs there |
|---|---|---|
| 600 A disconnect | Neutral bar bonded to the enclosure by screw; 2/0 Cu GEC on the neutral lug | Keep both; the screw is the main bonding jumper |
| MDP | Neutral bar bonded to the EGC bar and the enclosure | Neutral bar insulated; EGC bar bonded to the enclosure; electrode bus tied to the disconnect neutral with 2/0 Cu |
| Subpanels | Every EGC bonded to the grounded conductor | Neutral bar insulated; EGC bar bonded to the enclosure; each feeder EGC sized from its own breaker |
Size each feeder EGC from its own breaker at NEC Table 250.122: 8 AWG copper at 100 A, 6 AWG at 200 A, 1 AWG at 600 A. The same table drives sizing the EGC after the service disconnect.
What a stray bond costs: the worked example
Take one bonded subpanel and follow the current; the same arithmetic runs through every link in this job. Say a subpanel feeds a 24 A load and its neutral is bonded to its EGC. The return current now has two parallel paths home: the neutral conductor at an assumed 0.10 Ω round trip, and the EGC-and-metal path at an assumed 0.30 Ω. Those two resistances are illustrative, picked to show a typical split between copper and metal.
Current divides inversely with resistance:
- Neutral path: 24 A × 0.30 Ω ÷ 0.40 Ω = 18 A
- EGC and metal path: 24 A × 0.10 Ω ÷ 0.40 Ω = 6 A
So 6 A flows all day through the EGC, the panel bond screws, raceway fittings and anything bonded to them. Heating follows the square of the current: 6 A × 6 A × 0.30 Ω = 10.8 W of standing heat spread across joints never sized to warm. The metalwork sits 6 A × 0.30 Ω = 1.8 V above the grounded point, which is why meters read oddly and why bonded panels hum. A loose neutral flips the split: at 0.30 Ω on the neutral and 0.10 Ω on the EGC, 18 A runs through the metal. NEC 250.6 exists to keep exactly this neutral current off metal parts, and why a neutral-ground bond in a subpanel can burn equipment traces the damage chain.
A genuine fault still trips the breaker. A phase-to-case short draws hundreds of amperes through the EGC and clears in a few cycles. The damage is the ordinary load current, working joints loose and raising touch voltage year after year.
The fix, in order, and the checks that prove it
Lock out and prove the absence of voltage before you open any of these panels; the lugs ahead of a 600 A breaker stay lethal at every hour. Then work downstream:
- Leave the disconnect exactly as built. The bonding screw stays, and the 2/0 copper stays on the grounded conductor lug.
- Break the MDP bond between the neutral bar and the EGC bar. Neutrals land on insulated standoffs; EGCs land on the bar bonded to the enclosure through bare metal.
- Tie the electrodes back to the service neutral. Run 2/0 copper from the disconnect's grounded conductor lug to an electrode bus in the MDP. Land building steel, water pipe, footer rebar and the rod on that bus, and splice only at accessible points. The spec's MDP location then stands, with the connection point still at the service disconnect.
- Clear every subpanel. Remove the neutral-ground bond or screw, land neutrals on the isolated bar, land EGCs on the enclosure-bonded bar, and size each feeder EGC from its own breaker at Table 250.122.
- Mark the bars. A label reading "isolated — bond at the service disconnect only" keeps the next hand from undoing the work.
You prove the result with two readings. Clamp the feeder EGC under normal load: a healthy EGC reads near zero, and the worked example's 6 A reading means a bond is still hiding downstream. Lift the feeder neutral at a panel under test and ohm from the neutral bus to the enclosure: an isolated bar reads open, a leftover bond reads near 0 Ω. Run both readings at the MDP and at each subpanel before the job closes.
Here are the decisions, by location. At the 600 A disconnect, keep the bonding screw and the 2/0 copper as installed. At the MDP, break the lug-bar bond, insulate the neutrals and add the 2/0 electrode tie-back. In the subpanels, remove every neutral-ground bond and size each EGC from its own breaker. One bond, at the first service disconnecting means, and every panel after it clean.
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
Where does the main bonding jumper go when the MDP sits downstream of the service disconnect?
At the disconnect, because it is the first service disconnecting means. The bonding screw joins the neutral bar to the enclosure, the 2/0 copper grounding electrode conductor lands on the same bar, and the MDP gets no bond at all: its neutral bar sits on insulated standoffs and its EGC bar bonds to the enclosure with the supplied screw. Take each feeder EGC size from its own breaker per NEC Table 250.122.
Is 2/0 copper big enough for the grounding electrode conductor on a 600 A service?
Yes, when the largest phase conductor is 350 kcmil. The NEC's grounding electrode conductor table calls for 2/0 copper over 3/0 through 350 kcmil, and 2/0 also covers the concrete-encased electrode connection. A GEC that lands on nothing but a driven rod may stop at 6 AWG copper; with building steel, water pipe and footer rebar in the set, the 2/0 stands.
Can the electrodes bond in the MDP when the specs put them there?
Yes, without moving the bond. Run a 2/0 copper grounding electrode conductor from the grounded conductor lug at the 600 A disconnect to an electrode bus in the MDP, then land building steel, water pipe, footer rebar and the rod on that bus. The electrodes connect to the grounded conductor at the service and the spec's MDP location is met. Keep the run continuous and splice only at accessible points.
How do you prove the isolation with a clamp meter?
Clamp the meter around the EGC of a working feeder: a healthy EGC carries current only during a fault, so any steady reading points at a bond. In the worked example a 24 A load with a 0.10 Ω neutral and a 0.30 Ω EGC path puts 6 A on the metal, which a clamp finds in seconds. Then lift the feeder neutral and ohm the neutral bus to the enclosure: isolated reads open, bonded reads near 0 Ω.
