dual lugs on secondary of ordinary dry type

Status
Not open for further replies.
mshields

mshields

Senior Member
Location
Boston, MA
I've got a 75kVA 480 to 208Y/120V transformer. The primary is protected by a 125% of FLA rated CB. The design calls for a 250A MCB panelboard on the secondary of the transformer. This panel feeds a 100A panel.

The contractor wants to double lug the transformer, keep the 250A feeder to the panelboard and provide a 100A feeder to the 100A panel via the second set of lugs.

Since the primary is protected at 125%, no secondary protection is required and therefore I assume I don't have to worry about the sum of the OCP's adding up to 125%. But what I am worried about is utilizing a 100A feeder off the second set of lugs.

I don't see anything in Article 450 that talks about the cable on the secondary. Did I miss it? Is this some how a tap rule related issue? In short, is this permissible?

My intuition says it's ok because if you had a transformer protected by a 250% rated OCP you could have 6 OCP's on the secondary with their cable matching the size of each of the OCP's. And the OCP's would have to be relatively small since they'd have to sum to something equal to 125% of the FLA. Right?


thanks,

Mike
 
table 450.3 B (2005) appears to allow it, but I am confused by the 250 Amps. I calculate that you have 208 amps, next size breaker is 225 for the max panel ?
 
I think the 250A is ok

I think the 250A is ok

My reasoning is that no breaker is required on the secondary since the primary is kept to 125%.

If their was an applicable rule for the secondary breaker namely if the primary were larger than 125% (but smaller than 250%) then you'd take 125% of that 208A and get 260A, and a 250A would still be ok.

No?

Appreciate the feedback very much!

Mike
 
Delta to Wye transformers require primary and secondary protection per 240.4(F)
 
Mike,

I don't see a problem with another set of conductors off the secondary side of the transformer, my concern is the secondary overcurrent protection.
Assuming the 100 amp conductors are terminating on a 100 amp overcurrent device, and the other set of conductors is terminating on a 250 amp overcurrent device, would'nt this be an issue regarding 450.3(B) note 2. I think the 250 amp overcurrent device would have to drop down to 200. This would give a total of 300 amps of overcurrent protection on the secondary side. 125 percent of a 75kva transformer is about 260 amps. As per 450.3(B), note 1 the next standard overcurrent device permitted is 300 amps.

What do you think?

Joe Villani
 
tkb said:
Delta to Wye transformers require primary and secondary protection per 240.4(F)
Click to expand...
That is not correct, the end result is but the statement is not. The rules in Article 240 have nothing to do with the protection of the transformer itself, they only apply to the protection of the conductors. The rules in Article 450 cover the protection of the transformer and there are many cases where the primary OCPD can be used to protect the secondary of the transformer.
I just want to make this clear as the application of these two articles is often confused.

The new wording in the beginning of 240.21(C) makes it clear that you can run multiple sets of conductors from the transformer secondary and protect each of them per the rules in that section.
 
Reply to Joe

Reply to Joe

450,3 B notes 1 and 2 do not apply as per this table no secondary OCP device is required given that my primary OCP is at 125%.
 
Last edited:
For some cases, in supervised electrical installations, xfm secondary conductors are considered protected by the primary OCPD. No secondary OCPD is required. (2005) 240.92.D

carl
 
Status
Not open for further replies.
Top