# A Lesson in Bonding Service Enclosures



## jar546 (Feb 22, 2019)

Today's lesson is brought to you by our own Chris Kennedy who "schooled" me today on a situation that I encountered.  He and I are back and forth trying to school each other and as of right now, he is ahead by 1.

So here is what I was looking at, a simple 120/240 single phase, 400A commercial service with a CT cabinet which allows the electric company to meter the usage without putting a meter in series.
(click to make the image larger)



When I first go there I noticed there was a #3 awg copper as an equipment ground coming from the service disconnect panel back to the line/supply side CT cabinet.  I quickly looked up table 250.122 to confirm that, yes, indeed, 400A only requires an EGC (equipment grounding conductor) of #3.  After all, the sole purpose is to bond the pass through enclosure, right?

BUT, I was wrong.  The CT cabinet is on the supply side of the service and even though it is just a pass through CT (some are not) it is part of the service on the supply side and it, along with the service disconnect cabinet equals 2 enclosures.  Chris pointed out to me that NEC 250.28(D)(2) covers bonding jumpers for a service with more than 1 enclosure.  This reads:

*(2) Main Bonding Jumper for Service with More Than One*
_*Enclosure.* Where a service consists of more than a single
enclosure as permitted in 230.71(A), the main bonding jumper
for each enclosure shall be sized in accordance with
250.28(D)(1) based on the largest ungrounded service conductor
serving that enclosure._

So since this service has 2 enclosures the main bonding jumper SHALL be sized in accordance with NEC 250.28(D)(1) which reads:

_*(1) General.* Main bonding jumpers and system bonding
jumpers shall not be smaller than specified in Table
250.102(C)(1)._

This means that NEC 250.122 which is used for equipment grounding conductors no longer applies and that we must use NEC 250.102(C)(1) and that is very similar to NEC 250.66 which sizes the GEC (grounding electrode conductor).

In this case, the 400A service was fed with 500MCM and with that being said, per NEC 250.102(C)(1):
(click to enlarge)



With 500MCM wire, a minimum of a 1/0 must be used to bond the CT cabinet. 

Thank you Chris Kennedy.  Till next time.............


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## TheCommish (Feb 23, 2019)

is that a stainless steel meter box?


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## jar546 (Feb 23, 2019)

TheCommish said:


> is that a stainless steel meter box?



Aluminum


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