Back before I knew what an intersystem bonding terminal (IBT) was properly called, I had my service entrance put in by an electrician, licensed in 3 states, who never-the-less pulled things I now know to be violations, such as stripping NM-B cable and using the unmarked conductors in conduit.

Most of the work he did was not affected by that particular stunt (just the panel light and panel outlet.) However, I did ask at the time about grounding other services, which is made inconvenient on the outside by the conduit protecting the main GEC from the meter box into the ground.

service entrance and conduits

From the left - power to the well, power from the pole, GEC (2/0) (runs to the well casing and hits 5 ground rods on the way); two communication conduits to the pole, and a golf ball on top of a section of (empty) conduit marking the first of those ground rods. From looking at "typical" IBC terminals I gather that 2/0 is a bit out of their wheelhouse anyway. The G/N bond is inside the meter box, thus, sealed. The crazy-big overkill main ground is "customer with lightning/grounding issues" rather than any fault of the electrician.

Inside, the feed from the Class 320 meter splits to two 200A panels; Ground and neutral are isolated, and the ground comes in on 4AWG green wires.

North sub-panel

The North panel is closest to the Communications ducts. The red tape was applied over the Line lugs by me, as it's fine for homeowners to do their own work in Vermont, and I'm comfortable with that, but I prefer to de-fang easily preventable stupid problem points, which those are.

His response to my inquiry was to hand me a couple of these, and while I can see that they are some sort of clamp, googling the number (G603) with ground, clamp, etc. yeilds no useful results for me thus far - and in any case, the question of WHERE to get access to attach them is unclear. side view of clamp G603 Clamp

Now, I've certainly seen the stock cable company deal of "just screw a bar to the meter box" with no copper connection, but I'm pretty sure that's a hack.

Without getting into having to break the seal on the box, my options (other than that kludge) appear to be:

  1. Run a #4 copper ground from the ground rod in front of the communications conduits to an IBT near their eventual termination. This is short, sweet, all outside, and probably as close to the actual bond point as (2) will be, but via 2/0 from the rod to the bond point in the meter box.
  2. Run a #4 ground wire out through the wall from a ground bar in the North Panel to an IBT near the eventual termination point of the communication conduits
  3. Cut access to the actual GEC through the 1" schedule 80 PVC that's protecting it, without damaging it, and clamp a #4 onto it, since I have not found a IBT that will "lay-in" on a 2/0 conductor. But now it's "not protected" at least wherever that is.
  4. Do something with the odd G603 clamps. What, I'm not clear. It's been many years and I'm unlikely to hire the same guy back after gaining proper understanding of what a blatant code violation the NM-B wires trick was.

I'm getting ready to have internet put in, and the cable company is old-fangled (they ran a brand-new coax line up the road in an era where those cost a good deal more than fiber-optic) so I'm going to need this done soon-ish.

So, The question: which of those options appears to be the best/proper approach?

Contributory questions that don't appear to merit a completely different question: Can anyone identify the "G603" clamps and their application, if any, to this problem? Does the IBT need to be as close as possible to the N/G bond point/service entrance?

  • 1
    I'm not going to post this as an answer because others might have better solutions, but IMHO I believe you are overthinking this. I just installed a IBT on my son's house and simply connected it to the ground bus bar in the main panel. I could also have connected to the ground rods (UFER in his case) and both would be OK, I think. The connection from the IBT to the ground bar passed inspection. Hope this helps. Commented Mar 14, 2020 at 4:25
  • Is that Class 320 base purely a meter socket, or is it a meter-main assembly? Commented Mar 14, 2020 at 5:26
  • Also, who's your electric utility? Commented Mar 14, 2020 at 5:32
  • Green Mountain Power - They inspected and approved the job before connecting it. The Class 320 is a meter socket only, no breaker in the box, no customer access to the box. They did not seem concerned about the (unlikely) possibility that I could possibly suck 400A from it. And yes, I might well be overthinking it - I have a history that leads me to trying to minimize the possibility of lighting-related damage, while throughly understanding that anything like a direct hit is unstoppable; but I've lived with some very poorly done systems that were prone to frying things in minor events.
    – Ecnerwal
    Commented Mar 14, 2020 at 14:24
  • 1
    @Ecnerwal Class 320 means 320A continuous, 400A max, and is what's generally used for "400A" residential service (400A commercial services are either CT metered, or use bolt-in "K-base" meters that can handle 400A continuous load) Commented Mar 14, 2020 at 22:27

2 Answers 2


Not sure why 2-0 is used for residential with rod pipe and plate electrodes--#6 is all that is needed per code per 250.66.A. The IBT attached to the service equipment or disconnect is according to code not a hack but self forming screws are needed, not self tapping, or #6 to the grounding electrode conductor. 250.94.4 the length my jurisdiction uses is 20’ (see 800.100.4) except on motor homes it is allowed to be 30’. If this distance is not compliant this is when they require a 5’ ground rod (see 800.100.b.3.2). Hope that helps

  • Customer specified massive overkill, with me being the customer in question. It will be under 10 feet of cable total from IBT to GN bond point; I suspect I'll chose my option #1, barring any additional insight - it's the cleanest method by far.
    – Ecnerwal
    Commented Mar 14, 2020 at 19:11
  • 4 of the 5 ground rods are in the bottom of a 3 foot deep trench - they could just be laying there and meet code. In reality, they are driven full-depth, and at the end of the line there's 100 feet of well casing... Don't ask me how I feel about the Vermont concrete contractor who failed to provide the (required in Vermont on new concrete work since before this was done) Ufer ground to the re-bar in the footing, which would probably have reduced my overkill elsewhere, if provided.
    – Ecnerwal
    Commented Mar 14, 2020 at 19:18
  • Ufer grounds are the best but an additional rod pipe or plate electrode is required in my jurisdiction, but believe me when I built my last shop I had #4 wire tied to 20+’ in 3 places because I did not know where the power company was going to drop my 400a service , possibly they used 2-0 as a electrode I have done that a few times where we only had a few feet of top soil that sounds like it may be why so many rods. I have always thought well casings were the best but if the only grounded/grounding source I can see propblem with metallic plumbing.
    – Ed Beal
    Commented Mar 14, 2020 at 19:32
  • Thanks for the code ref. As far as I can tell, this language is for the Communications Grounds TO the IBT, rather than the IBT to the GEC or Neutral/Ground bond point. "Communications system grounding conductors must be 14 AWG or larger and be made of copper or other corrosion--resistant conducting material. They can be solid or stranded and must be insulated. Grounding conductors for communications systems should be kept short, and for one- and two-family dwelling installations, they must not exceed 20 feet." In any case I plan to keep them shorter; but investigating the acceptable.
    – Ecnerwal
    Commented Mar 17, 2020 at 13:45

Good news: there is an intersystem bonding terminal that'll fit on your existing GEC

The good news for your situation is that Arlington makes a bonding device, the GBB5250, that will fit on a 2/0 grounding electrode conductor. You'll have to cut into the conduit housing the GEC to install it, so that you can gain access to the wire inside. However, the provided cover does have conduit entry/exit points, though, so it should be possible to install it without having to sever or disconnect the GEC, or install a replacement length of conduit for that matter.

Bad news: your service configuration is no longer permitted as of the 2020 NEC

The bad news for your situation is that your overall service equipment configuration, with a Class 320 meter base providing 4-wire, 200A feeders to two 200A main breaker panels, does not comply with the new 2020 NEC requirement to provide the dwelling unit with a single emergency disconnect that's operable from the exterior of the house so that firefighters don't have to risk pulling a meter under load. Fortunately, that's fixable in your situation without rerunning any of the wiring.

Ugly news: your current configuration doesn't even comply with the 2017 NEC (or earlier editions, for that matter)

However, your electrician found a way to screw this up even under current Code. As it turns out, the green wires from the meter base to the panels are undersized! This is because the conductors from the meter to the service disconnect are service-entrance conductors, not feeders, and as a result, the normal EGC sizing rules don't apply to those green ground wires. Instead, they are a form of main bonding jumper, and thus are required by NEC 250.28(D)(1) to be sized according to NEC Table 250.102. In your case, this means that the electrician should have used 2AWG copper wires for the bonding jumpers, as the ungrounded service-entrance conductors for each service entrance are 4/0 copper.

As a result of this, bringing this setup to Code is going to require a bit more work than just slapping an IBT device in there and calling it a day. Since your existing service-entrance conductors (meter base to inside panels) are 4/0 copper, we can take one of three paths to fix this, since Green Mountain Power requires a lever bypass on Class 320 meters yet is fine with main bonding points in meter bases, as per their service handbook:

  1. Keep all the existing hardware, but replace the 4AWG copper bonding jumpers with 2AWG bonding jumpers to make them meet the NEC bonding jumper sizing requirements. This is the cheapest/easiest route, but doesn't provide the benefit of a single disconnecting point for all power.

  2. Keep the existing conductors and fit a single-400A-disconnect meter-main that supports a pair of 200A branch breakers, treating the existing conductors as 200A feeders. Thankfully, even with the existing oversized hot conductors, the 4AWG green wires are large enough to be EGCs, so this approach requires no wiring changes, but it does have the drawback that it's severely limiting in your choices for a meter-main.

  3. Fit a single-400A-disconnect meter main and use the existing conductors as feeder tap conductors. This provides a couple of additional meter-main options, at the cost of having to replace the existing 4AWG copper bonding jumpers with 3AWG wires (as they now need to be adequately sized for a 400A feeder). It also invokes the 10' feeder tap rule in NEC 250.24(B)(1), which is legal in this case, but has a habit of causing heartburn to home inspectors and other such residential-only folks who are unfamiliar with the feeder tap rules.

If you don't mind the current configuration...

If you have some reason to want to prefer relying on the "rule of six", you can still do that for now, although it no longer will be an option once your AHJ adopts the 2020 NEC, so your time window for fixing this is limited. The good news is that simply replacing the 4AWG bonding jumpers with 2AWG jumpers (i.e. correctly sized for the oversized service-entrance conductors) will do the trick for now.

Taking the conventional approach

While using a 400A single-disconnect meter-main with two 200A branch breakers is the most conventional solution to this issue, and requires work in only one place (replacing the meter base), it poses one major issue: equipment availability. In particular, there are only two ringless/lever-bypass meter-main combinations that can do this, namely the Siemens MC2442B1400SDL and the Square-D QUM400CL. The former is the less expensive solution of the two, uses a pair of Siemens QN2200R branch breakers, and also provides 16 spare spaces on the outside of your house, but does not series rate with the QOM2/QO breakers in your existing panels, which may be problematic, depending on the fault current available from your utility service.

The latter is costlier to begin with, requires the field fitment of a BMK2Q400 kit in addition to a pair of QBL22200 or QDL22200 (depending on interrupting rating requirements) branch breakers, and does not provide any extra breaker spaces. However, it is possible to get a full series rating on the all-Square-D solution, which allows it to handle fault current levels far in excess of what Green Mountain Power can provide. It also doesn't have the right-sized lugs for all the load wires it needs to handle, so you'll need to use a mechanical splice connector or split bolt that can handle 3 4AWG wires in order to "pigtail" the two EGCs into a single grounding conductor that can then go into the other small grounding terminal as one of the two provided will be occupied by the GEC, which is too small for the large lugs on the QUM400CL.

Tap-dancing our way around availability issues

If you cannot obtain either of the above pieces of hardware, we'll have to do some more work here. First, the 4AWG grounds in the service-entrance wiring will have to be upsized to 2AWG grounds, so that the 10' feeder tap rule from NEC 250.24(B)(1) can be applied to your situation:

(B) Feeder Taps. Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of 240.4(B) shall not be permitted for tap conductors.

(1) Taps Not over 3 m (10 ft) Long. If the length of the tap conductors does not exceed 3 m (10 ft) and the tap conductors comply with all of the following:

(1) The ampacity of the tap conductors is

a. Not less than the combined calculated loads on the circuits supplied by the tap conductors, and

b. Not less than the rating of the equipment containing an overcurrent device(s) supplied by the tap conductors or not less than the rating of the overcurrent protective device at the termination of the tap conductors.

Exception to b: Where listed equipment, such as a surge protective device(s) [SPD(s)], is provided with specific instructions on mini‐ mum conductor sizing, the ampacity of the tap conductors supply‐ ing that equipment shall be permitted to be determined based on the manufacturer’s instructions.

(2) The tap conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply.

(3) Except at the point of connection to the feeder, the tap conductors are enclosed in a raceway, which extends from the tap to the enclosure of an enclosed switchboard, switchgear, a panelboard, or control devices, or to the back of an open switchboard.

(4) For field installations, if the tap conductors leave the enclosure or vault in which the tap is made, the ampacity of the tap conductors is not less than one-tenth of the rating of the overcurrent device protecting the feeder conductors.

Informational Note: For overcurrent protection requirements for panelboards, see 408.36.

Otherwise, the hots would be insufficiently sized as a 4/0 Al wire with 75°C terminations can only carry 180A, and 240.4(D)'s "round up" rule cannot be applied to feeder taps. Furthermore, 4AWG grounds are one size too small for a 400A circuit -- table 250.122 requires 3AWG grounds on a 400A circuit.

Once the wiring is redone, then the meter socket can be replaced with any ringless, Class 320, lever-bypass meter-main that has a single 400A service disconnect in it. This includes the Square-D QUM400CL from before, the Siemens MM0202B1400SCL or SDL, or the Milbank U6227-X-400-K3L. If you use the QUM400CL, though, you'll still need that splice and pigtail for the grounding wires, although it will need to be able to handle 3 3AWG wires in this case.

Making all the right connections

You will need to adjust the locations of the various conduits on the outside to accommodate the new meter-main if you go down that path. It should be possible to fit it in the space available; however, you'll need to either have the utility move their service to the left, or adjust the GEC conduit route to the right and have the feeder conductors run directly into a panel instead of into the T-body, with the other feeder simply passing through the panel as permitted by NEC 312.8's rules on feedthrough conductors. (You'll also need to blank off the unused T-body port if you do this.)

One more thing...

While you have the right idea with the tape (in fact, NEC 408.3(A)(2) requires insulating barriers on all ungrounded service terminals in panelboards), there is a better solution to that problem. Due to the aforementioned NEC (and UL) requirement, Square-D makes retrofit service barrier kits for their loadcenters; simply order a pair of PKSB1HA kits from your local supply house and fit them in place of the tape using the supplied instructions, with the service turned off by the utility of course!

  • Service was installed circa 2008/9, so NEC 2020 or 2017 does not really apply. DIY building can be a remarkably slow process, but I'm at the point of giving it a kick in the pants. So far as I understand, I don't have to meet a standard that was not written when it was installed... As already commented above, that is 4/0 copper, not aluminum.
    – Ecnerwal
    Commented Mar 18, 2020 at 3:44
  • @Ecnerwal -- oh! missed that in the comments re: wire material, will fix when I get a chance. I'm pretty sure NEC 250.24(A) on bonding point accessibility has been around for quite some time, though... Commented Mar 18, 2020 at 11:40
  • I think (after some further research) that you may be conflating "accessible" and "readily accessible" as applied to wiring methods, or else applying the as applied to equipment definition to wiring. From Article 100: Accessible (as applied to wiring methods). Capable of being removed or exposed without damaging the building structure or finish or not permanently closed in by the structure or finish of the building. I am pretty sure that the lack of an IBT is another ding on the licensed master who did my service, though.
    – Ecnerwal
    Commented Mar 18, 2020 at 14:14
  • @Ecnerwal I am indeed applying the "as applied to equipment" definition here; my understanding is that the "as applied to wiring methods" definition is meant to be used when working in Chapter 3 Commented Mar 18, 2020 at 22:07
  • @Ecnerwal -- HOWEVER, the 4AWG copper bonding jumpers in the current service are also a Code vio.... Commented Mar 18, 2020 at 22:20

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