grounding

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bsh

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Per NEC 250.28, separately derived systems, a connection between the transformer grounded conductor (neutral) is to be made either at the transformer or at the first disconnecting means. Per exception #2 it should not be connected at both places if there could be a parallel path for the grounded conductor.
In NEC 250.24 it specifically calls for a connection between the grounded conductor (neutral bus) and the ground bus at the first disconnecting means.
As I read it the grounding for an outdoor service transformer supply and a separately derived system are somewhat different. Am I reading this correctly?
 

ibew441dc

Senior Member
Per NEC 250.28, separately derived systems, a connection between the transformer grounded conductor (neutral) is to be made either at the transformer or at the first disconnecting means. Per exception #2 it should not be connected at both places if there could be a parallel path for the grounded conductor.
In NEC 250.24 it specifically calls for a connection between the grounded conductor (neutral bus) and the ground bus at the first disconnecting means.
As I read it the grounding for an outdoor service transformer supply and a separately derived system are somewhat different. Am I reading this correctly?


Yes they are different. Much confusion has spawned over the years from many who do not understand or respect the fact that they are different. This is still very common to my ears..."A separately derived system and a service are the same so treat them the same" WRONG!

Although grounding and bonding of conductors, on the supply side of the service, from the source of a SDS to the first disconnect, and the GEC, are sized using the same table (250.66) and appear to be similar a service and SDS are not the same.

To clear this up even more review the definitions....they always help me:smile:

Whats the difference between a service and a feeder?:smile:
 

corvalan

Member
Separately Derived System VS Service system

Separately Derived System VS Service system

I understand that both systems are different in concept. But sometimes, especially in multi-voltage conversion electrical systems (large industrial or commercial facilities) the difference between these systems becomes blurry.

It is obvious for indoor lighting transformers or isolation transformers, but when you have on large transformers (34 kV to 13.8 kV) belonging to the customer (on the load side of the service point) feeding wiring feeding another transformer (13.8 kV to 480V) which in turn feeds premises wiring systems, the difference become less clear.

The first transformer has a MV metal clad switchgear which feeds the whole plant. This is the SERVICE equipment.

The wiring system fed by the second transformer (34 kV to 13.8 kV) meets the requirement to be a SDS which are:
1. A premise wiring system
2. Derived from a source of electrical energy other than the service (the service is the first transformer (34 kV to 13.8 kV)
3. No direct connection to supply conductors originating in another system.

Being that the case, and if indeed the second transformer is the source of a SDS, the grounding of this SDS system will have to be regulated by a set of different rules. For example, the equipment that houses the main disconnect means for this system will have a "System Bonding Jumper" instead of a "Main Bonding Jumper" and several other differences with the grounding of a main SERVICE system. This does not seems right to me.

So the question is how to clearly identify (using the NEC, not a personal opinion) what is a SDS or which is a SERVICE system.

Thanks.

RECS
 

augie47

Moderator
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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I agree, Ruben.
It may be crystal clear to some, but in the situations you describe, to me, the lines are more blurred. If the transformer and primary are customer owned , then the conductors would seem to be feeders. The disconenct, however, per 225.36 would have to be listed as service. If it's service equipment it would have a MBJ. As an SDS secondary, it would not.
On the few installs I have seen like that, the engineers tended to treat the secondary from a remote transformer as a service.
 
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