Both 3P/4W and 3P/3P services fed from same POCO pad mount

Status
Not open for further replies.
J

jtrezon

Member
We have specified a 3P/4W service on a spec. industrial warehouse building that will ultimately have multiple, utility, metered tenants (Via separate CT meters). There will also be a 3P/3W service feeding an electric fire pump. I submitted two separate load applications since the two initial services would be metered separately (house loads & fire pump), and I just assumed the POCO would feed both from the same pad mount transformer since it is in close proximity. The POCO engineer is telling me they cannot feed 3W and 4W services from the same pad mount transformer, but he cannot tell me the reason why. I'm trying to understand in what way this is (or could be) a problem for the POCO, ground fault path, etc, but I am coming up short.

I may just have to pull a reduced neutral and land it on the Pump controller ground bar to satisfy him...seems like a waste. Can someone please help educate me if this is indeed a problem?
 
I can't say you are satisfying him so much as you are the NEC.
NEC 250.24 requires a grounding electrode at the transformer and a grounded conductor to each servicesupplied by a grounded AC supply.
If they only use one pad then it is a grounded system in order to supply your 3P/4w needs.
Once it's a grounded system, all services it supplies must comply with 250.24.
If you wanted a 3w ungrounded or corner grounded for the pump, POCO would have to use a different transformer without a grounded neutral.
 
Last edited:
jtrezon said:
We have specified a 3P/4W service on a spec. industrial warehouse building that will ultimately have multiple, utility, metered tenants (Via separate CT meters). There will also be a 3P/3W service feeding an electric fire pump. I submitted two separate load applications since the two initial services would be metered separately (house loads & fire pump), and I just assumed the POCO would feed both from the same pad mount transformer since it is in close proximity. The POCO engineer is telling me they cannot feed 3W and 4W services from the same pad mount transformer, but he cannot tell me the reason why. I'm trying to understand in what way this is (or could be) a problem for the POCO, ground fault path, etc, but I am coming up short.

I may just have to pull a reduced neutral and land it on the Pump controller ground bar to satisfy him...seems like a waste. Can someone please help educate me if this is indeed a problem?
Click to expand...


Is the 3 wire service required to be ungrounded?
 
augie47 said:
I can't say you are satisfying him so much as you are the NEC.
NEC 250.24 requires a grounding electrode at the transformer and a grounded conductor to each servicesupplied by a grounded AC supply.
If they only use one pad then it is a grounded system in order to supply your 3P/4w needs.
Once it's a grounded system, all services it supplies must comply with 250.24.
If you wanted a 3w ungrounded or corner grounded for the pump, POCO would have to use a different transformer without a grounded neutral.
Click to expand...

Thanks....250.24(C) is the answer I was looking for. In this case we have chosen to connect to the utility pad mount, which is a grounded, so 250.24(C) requires it to be bonded to the service equipment and GEC, regardless of whether or not it is used on the load side....Completes the ground fault path between the fire pump service and the utility source.
 
Status
Not open for further replies.
Top