My intersystem grounding terminal bus is out of available terminals, and I have two additional communication wires to ground to it. Does NEC allow supplementary bus bars? NEC 250.94 seems to imply no, but doesn't say that definitively.

I'm in NJ, and so under the 2014 NEC.

  • What make and model is your intersystem grounding termination device? – ThreePhaseEel Dec 30 '18 at 1:04
  • @ThreePhaseEel Arlington GBB5. But I'm not opposed to replacing the device entirely if necessary to have additional terminals. – David Pfeffer Dec 30 '18 at 13:57

Nothing in 250.94 is intended to prohibit multiple IBT bars, so get another one and toss it on the same conductor the first one is attached to

The concept behind the presence of 250.94 in the NEC and its requirement for an intersystem bonding termination (IBT) is to provide installers of auxiliary systems (such as telecom, antennas, alarms, and such) a convenient, centrally-located place to land the bonding conductor that connects their ground point (ground block, protector, or auxiliary ground rod) to the electrical system ground.

As a result, and according to this ASHI Reporter article, nothing in 250.94 prohibits multiple IBTs from being present in a building. In fact, there are situations where fitting a second IBT would be encouraged to avoid telecom installers giving up on reaching an IBT simply because they are not equipped to reach the existing one from where they are at. As the article states:

The author suggests that a best-practice IBT deployment would include an interior IBT device at the service equipment location — typically the main electrical panel — and an exterior IBT device at the service entrance location. Such a plan will accommodate almost every possible grounding and bonding scenario for one- and two-family residential dwellings. The Code requires only one such device, preferably mounted on the exterior in the case of residential structures.

So, you can simply get another GBB5 (or equivalent IBT device) and attach it using the lay-in lug to the same grounding electrode conductor the existing one is attached to, then attach the unterminated communications bonding wires to the new IBT.

  • Ground busses can usually have 2-3 wires under a lug but the brand would be required to check. Can't do this with a neutral in my opinion because of NEC. Code requirements but most busses are listed for 2- 3 grounding conductors under 1 lug depending on the brand. – Ed Beal Dec 30 '18 at 22:40
  • @EdBeal -- IBT devices seem to be listed and labeled for only 1 wire under a lug (checked the docs for the Ilsco and Arlington incarnations) – ThreePhaseEel Dec 30 '18 at 22:49

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