Hey, guys. I've been reading a number of previous posts on the subject and have reviewed the code book, but still am unclear about a few things. When (under what circumstances) am I able to use the 250% rule for primary protection? When can I use primary overcurrent protection only to protect both the primary and secondary of the transformer? Table 450.3(B) indicates that secondary protection is not required for transformers rated 9 or more amps with up to 125% protection of the primary, yet secondary protection of conductors is almost always necessary according to 240.21(C). Just hoping for some clarification on this, as I'm studying for my master's exam. Thanks.
Transformer Overcurrent Protection Confusion
- Thread starter A-1Sparky
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As you noted, when dealing with transformer installations yiu must keep both Art 450 and 240 in mind.
In addressing the transformer protection (Art 450), you can elect to have primary and secondary protection or primary protection only in which case the overcurrent protection device can not exceed 125% (for transformers over 9 amps) of the transformer current (next larger permitted). If you have both primary and secondary protection, you primnary can be as high as 250%.
Once you have protected the transformer you need to address the conductor protection. For that you use Art 240.21(C).
Any transformer other than a two-wire primary and secondary, or a delta-delta 3 phase, will require secondary conductor protection in accordance with 240.21(C)(2) thru (C)(6).
If you have a two wire primary and secondary or delta-delta, if you address the transformer ration and assure your conductors are sized accordingly, the secondary conductors may be protected by the primary overcurrtent device
That help any ?
In addressing the transformer protection (Art 450), you can elect to have primary and secondary protection or primary protection only in which case the overcurrent protection device can not exceed 125% (for transformers over 9 amps) of the transformer current (next larger permitted). If you have both primary and secondary protection, you primnary can be as high as 250%.
Once you have protected the transformer you need to address the conductor protection. For that you use Art 240.21(C).
Any transformer other than a two-wire primary and secondary, or a delta-delta 3 phase, will require secondary conductor protection in accordance with 240.21(C)(2) thru (C)(6).
If you have a two wire primary and secondary or delta-delta, if you address the transformer ration and assure your conductors are sized accordingly, the secondary conductors may be protected by the primary overcurrtent device
That help any ?
Definitely helps. Thanks, Augie. I'm assuming the 250% is for inrush current associated with delta-Y transformers? I also see that note #3 in Table 450.3(B) allows up to six times the rated current of the transformer as long as there is manufacturer equipped overload protection. So for a 75KVA delta-Y transformer with a 480v primary, you could use a 500Amp breaker? It actually ends up being about 540 amps, so could you use a 600amp breaker, as that would be the next higher standard size? And would the conductors feeding the primary now need to be sized based on the breaker size?
Correct,,,
(never seen a 600v transformer above 1 kva that had thermal protection)
The 250% also allows you to have more than one transformer or a transformer and other equipment on one feeder.
See it occasionally in industrial applications.
(never seen a 600v transformer above 1 kva that had thermal protection)
The 250% also allows you to have more than one transformer or a transformer and other equipment on one feeder.
See it occasionally in industrial applications.
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You cannot use the next higher standard size when following note #3. The note says "not more than 6 times" the primary current, so you'd be limited to the 500A breaker.
I have always thought of it this way, Whenever you have a neutral on the secondary it is always possible to have a L-N overload that will not be seen by the pri OCPD. As such transformers with 1ph3w and 3ph4w secondaries will require not only a pri OCPD but also a sec OCPD.
Gotcha...thanks, David.
Ok...thanks. I think I've got my head around this now.
Welcome to the club. It took me a bit before I understood why myself. With a sec. OCPD when sized correctly it provide over current of overload protection for the transformer.
Now, with that taken care of the primary OCPD isn't really providing OC protection because that?s covered and as such the pri OCPD can be looked at as a short circuit protective device. Selecting one that is 250% puts the magnetics high enough out of range to eliminate the possibility of it tripping instantaneously when the transformer is energized. Remember that the transformer is already protected from overload. The Pri OLPD then becomes extremely impotent in taking the transformer off line if if the transformer fails preventing upstream devices from being taken out.
It's becomes sort of like a combination motor starter consisting on an mag only MCP, contactor and OLR. The OLR is sized to protect the motor form an over load, the MCP (motor circuit protector) with a mag adjustment just above the inrush current of the motor protects the motor circuit should the motor have a winding failure. It trips taking the motor off line isolating it from the upstream distribution system as would the pri OCPD for a transformer.
So essentially the Pri OCPD for a transformer with a sec OCPD does not really use its thermal but relies on its magnetics as an MCP does.
The only thing that was hard for me to understand is when you go for the 250% and there is cable sizing involved where can the breaker be located and how would you size the cable?
I raised this question a couple of months ago and one of our moderators provided a very good explaination which I failed to copy and file.
If you are depending on the primary OCP to provide protection for the secondary conductors, they would have to have a minimum size based on the transformer ratio as noted in 240.21(C)(1).
The NEC Art you are referring to is perfectly understood with a D-D with a 3ph3w secondary or a 1ph transformer with a 1ph2w secondary. But that's not what's being referred to. What is being referred to is the common installation to a D-Y or a D-D with a 3ph4w secondary or a 1ph transformer with a 1ph3w secondary where a secondary OCPD is required and you can apply up to a 250% pri OCPD. With a 250% device is the OCPD allowed to be located and what wire size if allowed?
A aforementioned one of our moderators provided an excellent explanation of the application a few months back but I failed to copy and file it.
You may have to wait on that moderator to repeat his excellent explanation, but until then, for other than delta-deta or two wire, I can only refer you to the tap rules of 240.21(C). The size of the secondary conductors is regulated by applicable rule (10 ft, 25 ft, etc).
For example, there is no minimum size for an outside tap other than the size of the OCP at the termination, whereas with other lengths you have a minimum based on the ratio and a multiplier.
This thread may help you on the 25 ft tap rule:
http://forums.mikeholt.com/showthread.php?t=139838&highlight=ratio
For example, there is no minimum size for an outside tap other than the size of the OCP at the termination, whereas with other lengths you have a minimum based on the ratio and a multiplier.
This thread may help you on the 25 ft tap rule:
http://forums.mikeholt.com/showthread.php?t=139838&highlight=ratio
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That was my thought also, that the 25' tap ruke would apply. But as I recall that response I refered to seemed to have been different. I tried to search for the response but it's like trying to find a needle in a haystack.
Moderator???
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