I am in school right now and the instructor is arguing that a transformer is not considered utilization equipment so the primary and secondary conductors do not need to be sized at 125% of the amp rating. Can someone shine some light on this. I don't see how a transformer can not be considered continuous.
Sizing secondary conductors.
- Thread starter rhovee
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That is the current needed to *magnetize* the core. The last thing you want to do is saturate it. 
It's continuous... but only its magnetizing current... and load adds to that, which is typically and substantially greater.
You technically could have a 3MVA transformer supplied with a 20A 12AWG circuit (provided secondary load doesn't exceed the capacity of the 20A primary circuit). It'd probably trip on energizing, but Code permits it.
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If the load on the secondary is continuous, though, how would this affect the sizing of the primary and secondary attachment wires (not the rating of the transformer itself)?
That is the current needed to *magnetize* the core. The last thing you want to do is saturate it.
opps
The primary conductors are typically a feeder, so they'll have to comply with 215.2(A)(1)... which factors the continuous load by 125%.
Secondary conductors do not have to be factored 125% for continuous loads... but the ratings required for equipment and conductors at and after the end of secondary conductors would end up carrying over 125% for continuous load to them.
Yet all that does not mean the conductors must be sized for 125% of the transformer rated current.
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:? Why wouldn't the secondary conductors also have to comply with 215.2(A)(1)? They are feeders as well.
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They are taps not feeders until they reach the first OCPD.
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I don't think that that is correct, per the NEC's definitions. In fact, they are not "taps" at all, they are "transformer secondary conductors."
And both "feeder taps" and "transformer secondary conductors" (unless they are a branch circuit) are, by definition, "feeders."
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Valid point of contention... but does it really matter? If one follows all the requirements of 240.21(C), therein as appropriate, and other requirements for equipment ratings at the end of the secondary conductors, one ends up including 125% for continuous loads anyway.
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Interesting as the NEC also says this.
From 240.2
In my opinion transformer secondary conductors are taps until they reach an overcurrent device or are supplied by a transformer in a way that the primary OCD protects the secondaries.
Not taking sides, but just because a conductor meets two definitions doesn't make it any less one than the other.Interesting as the NEC also says this.
From 240.2
In my opinion transformer secondary conductors are taps until they reach an overcurrent device or are supplied by a transformer in a way that the primary OCD protects the secondaries.
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I can't argue with that. There are many examples of that in the NEC, 'Equipment' & 'Device' for instance.
And considering my tap conductor definition only applies to article 240 David has some solid footing when citing articles 100 & 215.
That is why I said interesting.
Suffice it to say it is both a particular instance of a feeder and a particular instance of a tap...I can't argue with that. There are many examples of that in the NEC, 'Equipment' & 'Device' for instance.
And considering my tap conductor definition only applies to article 240 David has some solid footing when citing articles 100 & 215.
That is why I said interesting.
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I think the important part of the definition of "tap conductors" is that they have overcurrent protection ahead of their point supply, but transformer secondary conductors have no overcurrent ahead of their point of supply. I would guess that is why there are different sections for feeder taps and transformer secondary conductors, even though many of the rules are the same.
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I agree. For most practical purposes following the requirements of 240.21(C) would address the requirement of 215.2.
Though, in a theoretical sense or even in very unusual real world situations, applying the ambient temperature correction factors for low ambient temperatures could allow for a smaller conductor from the secondary to the OCPD than required by 215.2.
Hmm... did a brief review of 240.21(C) from that perspective and I'm not seeing it. With the exception of (4) and (5) [hurting my brain too much to make those considerations
], the others require the conductor ampacity to equal or exceed the OCPD rating, or sum thereof... and OCPD's would still have to be sized using 125% of its continuous load... meaning the 125% is already included and 110.14(C) is still covered for the secondary conductor terminations [which is technically the only portion of 215.2(A)(1) that 125% continuous load factoring applies to].
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They may be feeders, they may be secondary conductors but they are still taps.
The reason this becomes important is when someone wants to tap these raps.
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Hmm... did a brief review of 240.21(C) from that perspective and I'm not seeing it. With the exception of (4) and (5) [hurting my brain too much to make those considerations
Consider a 175A continuous load supplied by a 75kVA transformer (208V). Applying 215.2 to the OCPD and the downstream feeder, you would need an OCPD that is not less than 219A, so you use a 225A c/b, and you would need a feeder conductor that has a minimum SIZE that has an ampacity not less than 219A before the application of any adjustment/correction factors, so you use 4/0.
Now look at the transformer secondary conductors applying only 240.21(C). You will need the secondary conductors to have an ampacity that is not less than 225A. What is the ampacity of 3/0 THWN at say 55degF (maybe a refrigerated warehouse)...It would be 230A, and meet the requirements of 240.21(C).
So you could have 3/0 from the transformer to breaker, and require 4/0 downstream from the breaker.
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