250.24(C) Exception 1 - Single Assembly?

[xguard]

I have an installation with multiple service disconnects rigidly connected by a metal wire way and rigid conduit. I don't believe this meets the requirments of being a "single assembly" mentioned in 250.24 (C) Exception 1. The grounded service conductors are currently brought to each service disconnect but not conncected to each enclosure. They may be connected to the c/t enclosure which is bonded through the metal wireway to the service disconnect enclosures.

I also do not see the connection of the grounded service conductor to the grounding electrode conductor. The only grounding electrode conductor I see goes into the current transformer box. I have not had the utility company open this enclosure yet though.

I don't think this meets the Exception Listed above, but if the grounding electrode conductor is bonded to the c/t enclosure, and the grounded service condutor is connected to it also there, and I go and connect the grounded conductor to the service disconnect enclosure I'm concerned I'm creating parallel neutral current paths on the enclosures, conduits, and metal wireways.

Questions:
1. Does this installation fall under exception one?
2. Do they normally connect the grounded conductors to the grounding electrode conductor inside the c/t cabinet or connect the grounded conductor to the c/t cabinet?

Thanks!

 

[kwired]

I have an installation with multiple service disconnects rigidly connected by a metal wire way and rigid conduit. I don't believe this meets the requirments of being a "single assembly" mentioned in 250.24 (C) Exception 1. The grounded service conductors are currently brought to each service disconnect but not conncected to each enclosure. They may be connected to the c/t enclosure which is bonded through the metal wireway to the service disconnect enclosures.

I also do not see the connection of the grounded service conductor to the grounding electrode conductor. The only grounding electrode conductor I see goes into the current transformer box. I have not had the utility company open this enclosure yet though.

I don't think this meets the Exception Listed above, but if the grounding electrode conductor is bonded to the c/t enclosure, and the grounded service condutor is connected to it also there, and I go and connect the grounded conductor to the service disconnect enclosure I'm concerned I'm creating parallel neutral current paths on the enclosures, conduits, and metal wireways.

Questions:
1. Does this installation fall under exception one?
2. Do they normally connect the grounded conductors to the grounding electrode conductor inside the c/t cabinet or connect the grounded conductor to the c/t cabinet?

Thanks!

You are correct about the service conductor needing to be bonded to each enclosure - and it needs bonded to all metallic raceways as well on the supply side of service disconnecting means.

Grounding electrode conductor can be connected at the service disconnecting means, taps made to a common GEC if there are multiple disconnecting means, or it can be connected anywhere on the supply side of the service disconnecting means - which is often easier to do with multiple disconnecting means, that supply side connection could be in a meter cabinet, at the connection point to a service drop, or in a splice box where underground service conductors connect to service entrance conductors....

Sometimes a POCO will not let you make such connection in a meter cabinet/CT cabinet because access to that connection is generally restricted to POCO only, but that is their rule not an NEC rule.

 

[texie]

I agree with kwired. I might add that you also need to have bonding incompliance with 250.92 for all the metallic items as well. The single assembly does not apply here-that would be a situation such as a MLO panelboard with up to 6 breakers being used as service disconnects.

 

[augie47]

A factory assembled modular meter center is, IMO, another location the exception applies. I agree with the others that you need to bond to the neutral at each disconnecting means.\
The same situation occurs when we have a simple arrangement of a meter feeding a loadcenter thru a metallic nipple. There is a dual path but it is one accepted by the Code

 

[Smart $]

..
The same situation occurs when we have a simple arrangement of a meter feeding a loadcenter thru a metallic nipple. There is a dual path but it is one accepted by the Code

Not specifically accepted by Code... but also not specifically prohibited. The debate centers on whether neutral current on non-current-carrying metallic parts, on the utility side of the service disconnecting means, is considered objectionable current. Without objectionable current being Code defined, the debate cannot be resolved.

 

[iwire]

Not specifically accepted by Code... but also not specifically prohibited.

Meh, I might say accepted and offer the following in red as evidence.

It shall be yada yada yada.....

250.142 Use of Grounded Circuit Conductor for
Grounding Equipment.

(A) Supply-Side Equipment. A grounded circuit conductor
shall be permitted to ground non?current-carrying metal
parts of equipment, raceways, and other enclosures at any
of the following locations:

(1) On the supply side or within the enclosure of the ac
service-disconnecting means

(2) On the supply side or within the enclosure of the main
disconnecting means for separate buildings as provided
in 250.32(B)

(3) On the supply side or within the enclosure of the main
disconnecting means or overcurrent devices of a separately
derived system where permitted by 250.30(A)(1)

(B) Load-Side Equipment. Except as permitted in
250.30(A)(1) and 250.32(B) Exception, a grounded circuit
conductor shall not be used for grounding non?
current-carrying metal parts of equipment on the load
side of the service disconnecting means or on the load
side of a separately derived system disconnecting means
or the overcurrent devices for a separately derived system
not having a main disconnecting means.

Exception No. 1: The frames of ranges, wall-mounted ovens,
counter-mounted cooking units, and clothes dryers under
the conditions permitted for existing installations by
250.140 shall be permitted to be connected to the grounded
circuit conductor.

Exception No. 2: It shall be permissible to ground meter
enclosures by connection to the grounded circuit conductor
on the load side of the service disconnect where all of the
following conditions apply:

(1) No service ground-fault protection is installed.

(2) All meter enclosures are located immediately adjacent
to the service disconnecting means.

(3) The size of the grounded circuit conductor is not
smaller than the size specified in Table 250.122 for
equipment grounding conductors.

Exception No. 3: Direct-current systems shall be permitted
to be grounded on the load side of the disconnecting means
or overcurrent device in accordance with 250.164.

Exception No. 4: Electrode-type boilers operating at over
600 volts shall be grounded as required in 490.72(E)(1)
and 490.74.

 

[Smart $]

Meh, I might say accepted and offer the following in red as evidence.

It shall be yada yada yada.....

The debate is not regarding the use of grounded circuit conductor to ground equipment on the supply side. The debate is where two such bonds create a parallel path for grounded circuit conductor current... with respect to objectionable current on normally non-current-carrying metal parts [250.6].

 

[iwire]

The debate is not regarding the use of grounded circuit conductor to ground equipment on the supply side. The debate is where two such bonds create a parallel path for grounded circuit conductor current... with respect to objectionable current on normally non-current-carrying metal parts [250.6].

I submit that the debate is only in your own mind.

Those sections allow parallel paths, period.


If they did not they would have language in them like this exception in 250.30(A)(1)

Exception No. 2: A system bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the pur-poses of this exception, connection through the earth shall not be considered as providing a parallel path.

250.6 is nothing but worthless text that will be continued to be ignored as long as the MGN distribution system exists.

 

[Smart $]

I submit that the debate is only in your own mind.

Those sections allow parallel paths, period.


If they did not they would have language in them like this exception in 250.30(A)(1)



250.6 is nothing but worthless text that will be continued to be ignored as long as the MGN distribution system exists.

Perhaps

But those sections do not 'allow' parallel paths. They just don't specifically prohibit them. So it does not a stand up as a logical point of contention.

Perception can play tricks on the mind. From another perspective, the exception to 250.30(A)(1) can be interpreted as supporting 'the other side' of the debate... :happyyes: