Nec 240.21(C) history

[hhsting]

When did NEC 240.21(C) transformer secondary phase tap conductors rule of ampacity greater than or equal to secondary breaker or equipment rating came? What year?

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[don_resqcapt19]

When did NEC 240.21(C) transformer secondary phase tap conductors rule of ampacity greater than or equal to secondary breaker or equipment rating came? What year?

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The language that says

The provisions of 240.4(B) shall not be permitted for transformer secondary conductors

was added to 240.21(C) in the 2005 code cycle, but that was not a code change. The rule has always specified that the conductor have a specific ampacity. Often that ampacity was related to the rating of the OCPD on the load end of the transformer secondary conductors.

The additional language was added because too many code users thought that you could use the provisions of 240.4(B), the "round-up" rule. They failed to understand that the rule in 240.4(B) does not change the ampacity of the conductor, it only permits the use of an OCPD with a rating that exceeds the ampacity of the conductor. Adding the new language was not a code change, it was just a way to get installers to comply with code rules.

 

[jaggedben]

Don, where were you for this thread? Seems like maybe you could shed some light on my last couple posts over there.

I don't really get what you said about code users misinterpreting the conductor ampacity requirement. Perhaps you could describe an example.

 

[hhsting]

I have an installation from 1996 transformer 75kva, 480v to 208/120v, secondary main breaker panel is 400A and secondary phase conductor size is 500 kcmil. Owner refuses to change it says AHJ approved it in 1996 and has been like that since. I just dont see how secondary main breaker is protecting secondary conductors. Its like having no secondary conductor protection and Nec does not indicate anything about existing install have to change. We are working on that seconary panel adding few branch circuit breakers.

 

[GoldDigger]

I have an installation from 1996 transformer 75kva, 480v to 208/120v, secondary main breaker panel is 400A and secondary phase conductor size is 500 kcmil. Owner refuses to change it says AHJ approved it in 1996 and has been like that since. I just dont see how secondary main breaker is protecting secondary conductors. Its like having no secondary conductor protection and Nec does not indicate anything about existing install have to change. We are working on that seconary panel adding few branch circuit breakers.

What is the size of the primary OCPD? Is this, as it appears from your description, a delta-wye transformer?

 

[hhsting]

What is the size of the primary OCPD? Is this, as it appears from your description, a delta-wye transformer?

Delta to wye primary is 200 A. Xfmr is 112.5kvA not 75kva. However from what I inderstand thats transformer protection and primary conductor not secondary conductor protection.

 

[hhsting]

I have an installation from 1996 transformer 112.5kva, 480v to 208/120v, secondary main breaker panel is 400A and secondary phase conductor size is 500 kcmil. Owner refuses to change it says AHJ approved it in 1996 and has been like that since. I just dont see how secondary main breaker is protecting secondary conductors. Its like having no secondary conductor protection and Nec does not indicate anything about existing install have to change. We are working on that seconary panel adding few branch circuit breakers.

Edit:

 

[augie47]

I am certainly not surprised at the owners viewpoint. Its been operating like that for 23 years with no problem so his lack of concern is understandable. As you are aware it is a violation but, as don mentioned, the 2005 Code change likely came about due to the frequency of the violation.
Albeit it a violation, it is not a problem unless the conductors are overloaded and that somewhat the monkey on your back as you are adding the circuits. I would try to determine the load per phase after you add your circuits and if you are not
close to an overload situation, check it again in 23 more years.

 

[hhsting]

I am certainly not surprised at the owners viewpoint. Its been operating like that for 23 years with no problem so his lack of concern is understandable. As you are aware it is a violation but, as don mentioned, the 2005 Code change likely came about due to the frequency of the violation.
Albeit it a violation, it is not a problem unless the conductors are overloaded and that somewhat the monkey on your back as you are adding the circuits. I would try to determine the load per phase after you add your circuits and if you are not
close to an overload situation, check it again in 23 more years.

Transformer secondary 120/208v three phase, I have per phase connected load with addition of new branch circuits as follows for the secondary panel with 400A main breaker:

Connected Phase A: 37700vA, 314A
Connected Phase B: 37855vA, 315A
Connected Phase C: 36780vA, 306A
Total connected load: 112335va, 311A

However, total demand load is: 96389va, 268A

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[don_resqcapt19]

Don, where were you for this thread? Seems like maybe you could shed some light on my last couple posts over there.

I don't really get what you said about code users misinterpreting the conductor ampacity requirement. Perhaps you could describe an example.

An example would be the secondary conductors connected to a 400 amp OCPD and using 500 kcmil copper conductors. Many code users were under the impression that you could use the provisions of 240.4(B) and land those 500 kcmil conductors on that 400 amp device. The code language requires that the conductors have an ampacity of at least 400 amps. The 500's only have an ampacity of 380 amps, creating a violation. This was a very common practice, but was a violation. The required conductor size for this application is 600 kcmil copper.

Adding the language saying 240.4(B) does not apply to 240.21(C) was done to help prevent these violations.

Prior to the code adding that language some code classes were teaching the rule correctly but others were not.

 

[jaggedben]

...

Prior to the code adding that language some code classes were teaching the rule correctly but others were not.

I reviewed the same material again that I did in the last thread, and was reminded that the only reason that 240.21(B) prohibits using 240.4(B) is because 'that's what the code said'. What you considered 'correct' turned on a narrow legalistic interpretation of the wording. To me it's not at all surprising that, lacking an explicit prohibition, other teachers followed common sense and electrical theory. What's not clear to me is that anyone who wrote the original language in 240.21(B) or (C) ever intended to prohibit the application of 240.4(B). Rather, for all I know, they wrote the language without thinking about it, and inadvertently created the implication of a new rule.

IMO, the CMP should have judged this on the technical merits and deleted the word 'not' from "The provisions of 240.4(B) shall not be permitted...", in the original proposal back in 2002. It's really a shame considering all the extra material electricians have been forced to install in the meantime.

 

[don_resqcapt19]

I reviewed the same material again that I did in the last thread, and was reminded that the only reason that 240.21(B) prohibits using 240.4(B) is because 'that's what the code said'. What you considered 'correct' turned on a narrow legalistic interpretation of the wording. To me it's not at all surprising that, lacking an explicit prohibition, other teachers followed common sense and electrical theory. What's not clear to me is that anyone who wrote the original language in 240.21(B) or (C) ever intended to prohibit the application of 240.4(B). Rather, for all I know, they wrote the language without thinking about it, and inadvertently created the implication of a new rule.

IMO, the CMP should have judged this on the technical merits and deleted the word 'not' from "The provisions of 240.4(B) shall not be permitted...", in the original proposal back in 2002. It's really a shame considering all the extra material electricians have been forced to install in the meantime.

It is a simple concept...where ever the code require a conductor with a specific conductor ampacity, you are required to use a conductor with that ampacity. 240.4)(B) does not change the conductors ampacity, it only permits you to use an OCPD with a rating that exceeds the conductor ampacity.

I am sure the writers of the code rules for taps and transformer secondary conductors knew the definition of ampacity.

 

[jaggedben]

It is a simple concept...where ever the code require a conductor with a specific conductor ampacity, you are required to use a conductor with that ampacity. 240.4)(B) does not change the conductors ampacity, it only permits you to use an OCPD with a rating that exceeds the conductor ampacity.

I am sure the writers of the code rules for taps and transformer secondary conductors knew the definition of ampacity.

It's not a question of how to determine conductor ampacity rating. It's a question of how to determine the OCPD rating.

I don't think it was unreasonable for people before 2005 to interpret 'overcurrent device that limits the load to the ampacity of the conductors' as allowing for the provisions of 204.4(B). You would have had to really think about the wording to realize that it could call for a different result than, say, 'protects the conductor from overload'?

What we are left with is a rule that has no technical justification. At least the CMP never said there was one.

 

[don_resqcapt19]

It's not a question of how to determine conductor ampacity rating. It's a question of how to determine the OCPD rating.

I don't think it was unreasonable for people before 2005 to interpret 'overcurrent device that limits the load to the ampacity of the conductors' as allowing for the provisions of 204.4(B). You would have had to really think about the wording to realize that it could call for a different result than, say, 'protects the conductor from overload'?

What we are left with is a rule that has no technical justification. At least the CMP never said there was one.

You would have to go back to the original rule that called for the conductor to have an ampacity equal to or greater than the load end OCPD to find the substantiation.

It is very simply a conductor ampacity rule setting a minimum conductor size, and no different from the rules that require the ampacity of the conductor to be equal to 125% of the continuous load plus the non-continuous load. You can't use 240.4(B) for those applications either. They are minimum conductor ampacity rules.

 

[jaggedben]

You would have to go back to the original rule that called for the conductor to have an ampacity equal to or greater than the load end OCPD to find the substantiation.

Yes, I agree with that, although as I note below, that's not the exact language. I think that you're assuming there was a technical substantiation for that particular wording. One reason I doubt that we'll find it is that to my knowledge such a technical substantiation would make no technical sense. Also, my experience looking back through ROCs and ROPs is that you can't always tell.

It's interesting that the wording was/is:
'that limit the load to the ampacity of the tap conductors'
instead of
'with a rating not less than the ampacity of the conductors'.
not to mention the possibility of
'that protect the tap conductors according to 240.4(B) thru G'.

For what it's worth, in that last thread I mentioned, I got back as far a the 1999 cycle. Now I've looked at 1996 and still no idea.

 

[don_resqcapt19]

...
For what it's worth, in that last thread I mentioned, I got back as far a the 1999 cycle. Now I've looked at 1996 and still no idea.

I think the tap rule and secondary conductor rules are way older than that.

However there are a number of "arbitrary" rules in the code and it could be one of those, but even if it was, it will be almost impossible to change with out a solid technical substantiation.

 

[jaggedben]

I think the tap rule and secondary conductor rules are way older than that.

That's why I asked the guys over in the thread about old books! :lol:

However there are a number of "arbitrary" rules in the code and it could be one of those, but even if it was, it will be almost impossible to change with out a solid technical substantiation.

Yeah, so I found this in the 1993 Report on Proposals:



The panel statement is pretty much 'because we said so' if you ask me. I'm not up for going back further in time tonight. Maybe another time.