ART 450 Over Current Protection on 3-Phase transformers
- Thread starter jbolen
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What makes you think Art 450 doesn't address three-phase transformer? I don't see anything that distinguishes between single and three-phase there.
Per 450.3(B), Primary Only Protection would be 125A max.
Primary and Secondary Protection would be 225A Max (prim) and 250A Max (second)
Per 450.3(B), Primary Only Protection would be 125A max.
Primary and Secondary Protection would be 225A Max (prim) and 250A Max (second)
Having a 240/120V 3? 4W secondary, it cannot be primary-only protected. See 240.4(F) and 240.21(C)(1).
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That 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.
It is of my opinion that it is very simple.
If the transformer is 1ph and has a 2w secondary then the transformer overload protection can be provided by a single primary OLPD.
This is because any load on the secondary will be seen proportional to the primary.
It is the same with a 3ph transformer with a 3W secondary.
Should a 1ph transformer have a 3w secondary or a 3ph transformer have a 4w secondary with neutrals it is now possible to overload the transformer secondary and a primary OCPD not ever see the overload.
Thus a 1ph3w and a 3ph4w transformer must be provided with secondary protection. Then the primary OCPD becomes a device that protects the up stream distribution system should the transformer fail because of an internal fault or short circuit. With secondary protection the transformer is already provided with overload protection.
If the transformer is 1ph and has a 2w secondary then the transformer overload protection can be provided by a single primary OLPD.
This is because any load on the secondary will be seen proportional to the primary.
It is the same with a 3ph transformer with a 3W secondary.
Should a 1ph transformer have a 3w secondary or a 3ph transformer have a 4w secondary with neutrals it is now possible to overload the transformer secondary and a primary OCPD not ever see the overload.
Thus a 1ph3w and a 3ph4w transformer must be provided with secondary protection. Then the primary OCPD becomes a device that protects the up stream distribution system should the transformer fail because of an internal fault or short circuit. With secondary protection the transformer is already provided with overload protection.
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?
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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?
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).
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?
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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.
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.
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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.
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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.
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?
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.
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Yes, the code doesn't dictate which method of transformer protection must be used. The designer can chose either method.
Yes, but secondary conductor protection doesn't automatically mean transformer secondary protection (if the primary only method is used.) That was the point I was making.
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
dana1028
Senior Member
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I had not previously read this post.
Wow! Two thoughts
1. David, you and Don have the patience of Job.
2. Your and Don's explanation of the difference between the requirements of 450.3(B) and 240.21(C)(1) was great; as mentioned,these two sections get frequently confused and misunderstood. The explanation(s) you both provided made these sections so understandable. I copied the entire 40 pages of discussion and have it in a folder. I want to thank you both for being so patient and clear headed in your discussion and explanations; it has certainly added to my understanding of transformer protection.
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