EGC from service disconnect to subpanel: size and jumper

Short answer: every panelboard after the service disconnect is a subpanel. The bare copper running from the disconnect to each downstream board is an equipment grounding conductor, and NEC Table 250.122 sizes it from the overcurrent device ahead of it: under a 400 A breaker, that is 3 AWG copper, not 4 AWG. The system bonding jumper itself lives only in the service disconnect enclosure, tied to the grounded service conductor. Keep the neutral isolated everywhere else, bond each ground bar to its cabinet, and the installation holds up at inspection.

Is a panelboard after the service disconnect a main panel or a subpanel?

Once the grounded service conductor and the grounding electrode conductor land in the service disconnect, every panelboard downstream is a subpanel, whether it feeds a 20 A bench circuit or carries a 400 A main breaker on its door.

The name on the door decides nothing. The single point where the neutral bonds to ground decides everything, and the service disconnect is that point, once per service.

NEC 250.6 aims to keep neutral current off metal parts, so neutrals land on an isolated neutral terminal and grounds on the ground bus. Bond a neutral downstream and load current returns through the EGC, the raceway and the cabinet skin. A 60 A load then heats copper you sized for faults, and every bonded enclosure joins the return path.

That isolation is the working definition of a subpanel; the subpanel neutral and ground bonding page walks the bar-by-bar detail.

Where does the system bonding jumper go?

The system bonding jumper lands only inside the service disconnect enclosure, where it ties the grounded service conductor to the cabinet and the equipment grounding conductor; from that enclosure outward, every bare copper bonding conductor is an equipment grounding conductor.

The NEC definitions draw the line in three pieces. A main bonding jumper connects the grounded circuit conductor to the equipment grounding conductor at the service. A system bonding jumper makes the same connection to the supply-side bonding jumper or the EGC, the form it takes at a separately derived system. An equipment bonding jumper connects two portions of the equipment grounding conductor.

At a service disconnect, one bond can carry both jumper names. After it, the copper from the ground terminal to a downstream panelboard is a plain EGC. The grounding electrode conductor to the rod or building steel lands on the grounded conductor here too, sized from the service conductors rather than from the breaker rating. The enclosure-side basics, from ground bars to bonding the cabinet skin, sit in the enclosure grounding and bonding guide.

What size EGC runs from the service disconnect to a subpanel?

NEC Table 250.122 sizes the EGC from the rating or setting of the overcurrent device ahead of it: a 400 A breaker calls for 3 AWG copper or 1 AWG aluminum, and the load at the far end plays no part.

The table keys to the breaker because the EGC has one job: carry fault current long enough, and hot enough, for the device ahead to clear. The ampacity column proves the point. In the NEC conductor ampacity table at 75 °C, 3 AWG copper carries 100 A and 4 AWG carries 85 A, both far below the breaker rating, because the EGC never carries load current.

Two habits keep the sizing honest. When voltage drop pushes the phase conductors up in size, the EGC grows in the same proportion. And in a raceway, bare copper is the normal choice; an insulated EGC of 6 AWG and smaller must be green or green with one or more yellow stripes under NEC 250.119.

How the 400 A device itself relates to the two downstream panelboards is its own sizing question; the NEC subpanel ampacity vs feeder OCPD sizing page works that relationship through.

Why does bare 4 AWG copper fail on a 400 A service?

Because the overcurrent device ahead of that copper is rated 400 A and NEC Table 250.122 calls for the next size up at that rating, the installed 4 AWG is one gauge short, and an inspector will flag both feeder runs.

The worked example, with the arithmetic in the open:

1. Setup: a service disconnect rated 400 A on the outside wall, two 200 A panelboards just inside, non-metallic raceways between them, one bare copper bonding conductor in each raceway.
2. The overcurrent device ahead of that copper is the service disconnect itself, and nothing smaller sits between.
3. NEC Table 250.122 at that row: 3 AWG copper, or 1 AWG aluminum.
4. Installed: 4 AWG bare copper.
5. Cross-section: 4 AWG is 21.2 mm² of copper against 26.7 mm² for the correct size, about 21 % less metal in the fault path.
6. Verdict: undersized at both panelboards, because the disconnect is the only protection ahead of the runs.

The fix is a re-pull: the correctly sized copper in each raceway, landed on the ground bar in the disconnect and on the ground bar in each panelboard. De-energize and prove absence of voltage before you open anything; the line-side lugs of a service disconnect stay live from the utility even with the main off.

Skipping the re-pull costs the inspection, then costs the same re-pull later.

What changes inside the subpanel enclosure?

Inside the subpanel, the neutral bar sits on insulating standoffs with the factory bonding screw or strap removed, and the ground bar bonds to the cabinet; the feeder EGC lands on the ground bar, the feeder neutrals land on the neutral bar, and the two bars never touch.

Three checks settle the cabinet. First, the bonding screw or strap comes out of the neutral bar. Second, grounded conductors land only on the ground bar, and that bar bonds to the enclosure: NEC 250.96(A) requires metal parts intended to serve as equipment grounding conductors, enclosures included, to be bonded so they can conduct any fault current likely to hit them. Third, neutrals land only on the floating bar.

A non-metallic cabinet has no skin to bond, so the ground bar takes its bond straight from the EGCs; grounding a fiberglass enclosure shows the back-panel pattern. On a metal cabinet you assemble yourself, bond the door as well; the enclosure door bonding jumper page covers the cases where a hinge is not enough.

Does a ground rod at the subpanel replace the EGC?

No: under NEC 250.4(A)(5) the earth is not an effective ground-fault current path, and a driven rod that reads 25 Ω passes about four thousand times less current than 30 ft of correctly sized copper.

Run the numbers. 3 AWG copper is 0.197 Ω per 1000 ft, so the 30 ft run is about 0.006 Ω end to end. A single driven rod commonly reads 25 Ω or more to earth. In parallel, the earth takes 0.006 ÷ (25 + 0.006) ≈ 0.02 % of the fault current and the copper takes the rest, which is what turns the fault into a dead short the breaker clears in a fraction of a second.

NEC 250.54 gives the rod at the panelboard its proper name: an auxiliary electrode. Connect it to the EGC if the site wants it, and keep the EGC in the raceway, because the rod adds nothing to fault clearing.

When can the raceway carry fault current instead of a wire?

A metal raceway installed with tight listed fittings and bonded at both joints can act as the EGC itself and drop the wire; the non-metallic raceways in the worked example cannot, so the copper rides in every run.

Steel conduit with its joints made up tight carries fault current well, and the metal raceway as EGC page sets out when the code lets it stand alone. PVC and liquid-tight flexible nonmetallic contribute nothing, so a wire EGC rides along in every run.

At a stainless disconnect cabinet, land each raceway on listed conduit hubs at service equipment and leave the contact metal bare. NEC 250.12 requires paint, lacquer and enamel to be removed from threads and contact surfaces at grounding and bonding connections, unless the fitting is designed to make coating removal unnecessary.

What should the disconnect and subpanel enclosures provide?

The service disconnect enclosure holds the system bonding jumper and the grounding electrode conductor landing; each subpanel enclosure holds an isolated neutral bar, a ground bar bonded to the cabinet, wire bending space per NEC 312.6, and clear working space in front.

• Bending space. NEC 312.6 sets the minimum wire bending space at terminals in two tables: Table 312.6(A) where the conductor does not enter or leave the wall opposite its terminal, and Table 312.6(B) where it does. The enclosure sizing guide turns those tables into a workable layout.
• Working space. NEC 110.26(A) asks at least 3 ft (900 mm) of depth for 0-150 V to ground in Condition 1; a 240/120 V service sits at 120 V to ground. Width comes to at least 30 in or the equipment width, and doors open at least 90 degrees.
• Handle height. NEC 240.24(A) keeps the center of the grip of the highest operating handle, at its highest position, at 2.0 m (6 ft 7 in) above the floor or less.
• Marking. A control panel built into the cabinet carries its enclosure type number on the marking under NEC 409.110.

On a washdown or salt-air site, put the same wiring in cabinets that shrug off the water. We build custom enclosures to your material, size and cutouts; configure one in the enclosure builder or start from the custom stainless enclosures range, and a corrosive area takes 316L, rated NEMA 4X / IP66 by the manufacturer; not UL listed, delivered in 10 business days from drawing approval. The breakers and panelboards inside keep their own listing; the cabinet rating covers the water and the corrosion.

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.

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