Having a 240/120V 3? 4W secondary, it cannot be primary-only protected. See 240.4(F) and 240.21(C)(1)....
Per 450.3(B), Primary Only Protection would be 125A max.
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Having a 240/120V 3? 4W secondary, it cannot be primary-only protected. See 240.4(F) and 240.21(C)(1).
You are correct. It can be primary only protected per 450. But if secondary conductor protection is required, primary-only transformer protection won't be very usefulThat is not correct. The transformer can be "Primary Only" protected per Art 450. What 240.4(F) and 240.21(C) say is that the secondary conductors cannot be protected by the Primary overcurrent device. Protection of the transformer and the conductors are separate issues.
You are correct. It can be primary only protected per 450. But if secondary conductor protection is required, primary-only transformer protection won't be very useful
Also per Table 450.3(B) Secondary Protection (See Note 2.) It starts, "Where secondary overcurrent protection is required, ..." Just exactly where is it stated that secondary overcurrent protection is required?
Yes, yes... but the Table doesn't require you to use either Primary-only or Primary and Secondary Protection Method.Table 450.3(B) states that secondary overcurrect protection is required for "Primary and Secondary" Protection method.
For "Primary Only Protection" - Secondary Protection is "Not Required"
For "Primary and Secondary Protection" - Secondary Protection is required to be not greater than 125% (or next size up) or not greater than 167% (for currents less than 9A).
It would be a very rare case where the secondary conductor overcurrent protective device did not also provide the required protection for the secondary winding....
Also, when you do use P&S, is the secondary protection afforded by the secondary conductor ocpd or completely separate ocpd? ...
Exactly. And the only rare case I can think of is where there are no secondary conductors, because the secondary ocpd is connected directly to the transfomer leads.It would be a very rare case where the secondary conductor overcurrent protective device did not also provide the required protection for the secondary winding.
A single transformer feeding more than 6 sets of secondary conductors. I vaugley remember doing this when I neede to feed some 600V equipment in a test cell.It would be a very rare case where the secondary conductor overcurrent protective device did not also provide the required protection for the secondary winding.
Yes, yes... but the Table doesn't require you to use either Primary-only or Primary and Secondary Protection Method.
Also, when you do use P&S, is the secondary protection afforded by the secondary conductor ocpd or completely separate ocpd?
Additionally, when secondary conductor protection is required (elsewhere, as previously mentioned), for what reason would you use the Primary-only Method?
We're using the same words to describe two meanings, so I'll reword. The table requires using one of the methods, but which one to use is not required by the table itself.The table does require you to use either the Primary-only or the Primary and Secondary Protection Method. 450.3(B) says that transformer overcurrent protection SHALL BE provided in accordance with T450.3(B). T450.3(B) gives you two protection methods. You must use either one method or the other.
The question was to make a point... which was that it is impossible to have separate ocpds. Secondary conductors can only exist between the transformer secondary and first ocpd(s). For typical installations, secondary conductor protection must serve as transformer secondary protection when primary and secondary method is used.I'm with Don. I can't think of any reason why you would want separate OCPDs. Why provide two devices when one can serve both purposes?
Okay, I concede. This is one scenario I had not considered... probably because I'd never put three 100A panels on a secondary having a rated current of 180A. I had also not considered Jim's. Both are atypical cases IMO.Imagine that the transformer in the OP supplied three 100A MCB panels with each panel being supplied by #3 feeder. The MCB in each panel provides the secondary conductor protection for the #3 conductors as required by 240.21(C). You must use the Primary-Only method in this case.
We're using the same words to describe two meanings, so I'll reword. The table requires using one of the methods, but which one to use is not required by the table itself.
The question was to make a point... which was that it is impossible to have separate ocpds. Secondary conductors can only exist between the transformer secondary and first ocpd(s). For typical installations, secondary conductor protection must serve as transformer secondary protection when primary and secondary method is used.
Okay, I concede. This is one scenario I had not considered... probably because I'd never put three 100A panels on a secondary having a rated current of 180A. I had also not considered Jim's. Both are atypical cases IMO.
Just FYI, there was a post a few months back were a user found a transformer that was not properly protected by either the Primary Only or Primary-and-Secondary Method (45kVA, 480-208/120V xfmr, with 80A OCPD on primary and 200A OCPD on secondary.)
Changing the primary OCPD to 70A would have provided proper transformer protection (primary only) leaving the 200A OCPD to provide secondary conductor protection only. (Or he could have changed the secondary OCPD to 175A for primary-and-secondary protection.)
http://forums.mikeholt.com/showthread.php?t=147085