Transformer tap wire size

[charlie]

I received the following e-mail and thought you would be interested in the content.

I am interested in 240.21(C)(2) Transformer Secondary. Paragraph (1)(b) requires that the rating of the conductors be not less that the rating of the OC device. An example is a 400 ampere breaker in 400 ampere panel. Following (1)(b) the conductor should be 600 kcmil. However I think the CMP meant to allow 500 kcmil at 380 amps. Many inspectors enforce the 600 kcmil and do not allow the next size breaker as stated in 240.4(B). Will you see if this is being clarified?

 

[charlie]

Re: Transformer tap wire size

Phil Simmons sent in a proposal to add a 2nd sentence to 240.21(C) "The provisions of 240.4(B) shall not be permitted for transformer secondary conductors."

His proposal was rejected in the ROP with the panel statement "The present text in 240.21(C) meets the submitter's intent. See, for example, 240.21(C)(2)(1)b, The provisions of 240.4(B) are not permitted for transformer secondary conductors that are not considered to be protected by the primary overcurrent device."

I agreed with and voted with the CMP on this issue. The panel's opinion is that the inspectors are correct for turning down the 500 kcmil conductors. I would imagine more of them will now notice the intent of the panel and start enforcing this rule now that it has been brought out into the light.

 

[bob]

Re: Transformer tap wire size

Charlie
I wish someone could explain the
thinking of the CMP when 240.21.5.C2.1b was put into the code. No where else does the code eliminate the use of 240.4B. Take the following example 150 kva 120/208 transformer with secondary to feed a 400 amp panel with 400 amp main breaker.
The FLA of the transformer = 417 amps. I would assume that the terminal connectors are rated at least 417 amps. The 400 amps panel and main are rated 400 amps. If you install 500 mcm copper it is equal to or greater that 400 amps. I know its at 90 degrees. All the components can handle 400 amps. Why do you have to used 600 mcm copper?
The conductor is protected from overload by the main breaker. Maybe we can get input for the CMP as to what they were thinking.

[ July 23, 2003, 05:24 PM: Message edited by: bob ]

 

[don_resqcapt19]

Re: Transformer tap wire size

Bob,
240.4(B) only applies to the overcurrent protection of the conductor, and does not change the conductor ampacity. Any time the code requires a minimum conductor ampacity, then the ampacity of the conductor, after applying the rules in 310.15, must equal or exceed the required ampacity. If the ampacity requirements are met, then and only then can, you apply 240.4(B) for the overcurrent protection. For example if the required circuit ampacity is 300 amps you cannot use 300 kcmil THWN because it is only rated at 285 amps, but if the required circuit ampacity is 255 amps, you could use 250kcmil THWN with a 300 amp overcurrent protective device.
Don

 

[BAHTAH]

Re: Transformer tap wire size

As pointed out by Don, when sizing a feeder for example you first determine the load and select a conductor that will meet that ampacity. Then if that value falls between two standard size overcurrent devices you can select the higher rated device if 800amps or less. I have to agree with BOB in wanting to know what the CMP is thinking where the process is changed for transformer taps to require that the conductor now must meet or exceed the ampacity of the overcurrent device even if the conductor meets the load requirements that would apply to a normal feeder application. With conductors (THHN) that have the ability to carry such high currents and the fact that these values are allowed in deration applications and with most breaker applications only allowing for 80% continuous loading how can a conductor that will carry over 400amps connected to a breaker that will not carry over 400amps continuous be a problem? Since the breaker will start to overheat at over 320amps which is well below any insulation problem with the conductor. There must be something about transformers or separately derived systems that I am not seeing.

 

[bob]

Re: Transformer tap wire size

Don
If the load was 320 amps,240.21(C)(2) Transformer Secondary Paragraph (1)(b) would still require
600 mcm using the example I gave.

 

[charlie]

Re: Transformer tap wire size

OK Bob, this is what happened. Prior to the 1999 Code, all the taps were thrown together in one portion of 240-21. A proposal was made to the 1999 Code by Jim Pauley (I think) to separate all the tap rules into their own sections (especially feeders and transformers). Jim rewrote the sections in a manner as to be more easily understood and submitted them. The panel, including me, compared them to the original text and agreed that they were correct.

After that, we went through the comment period and we didn't get negative comments that I remember, and the proposal passed as written in the current Code. I am doing this from memory since my ROP & ROC from the 1999 Code cycle are in by bookcase at home.

Now to address your problem with the interpretation, look again at my answer to the opening post. Phil wanted to make this issue very clear as a separate statement. The panel agreed that the Code language already was clear and rejected his proposal. Since no one made a proposal to change the language to allow 240.3(B)to be used, the issue was not addressed. If this is a problem, it will have to be dealt with locally until the 2008 Code cycle.

 

[don_resqcapt19]

Re: Transformer tap wire size

Bob,
The section is question is specifying a conductor ampacity, not the overcurrent protection of the conductor. When a specific conductor ampacity is required by the code you must use a conductor that has an ampacity equal to or greater than what is specified. 240.3(B) does not change the ampacity of the conductor.
Don

 

[bob]

Re: Transformer tap wire size

Charlie
Thanks for the update.
Don
I understand your explanation. My point is that
240.21(C)(2) Transformer Secondary. Paragraph (1)(b) is unnecessary. If it was omitted you would use the rules for overcurrent protection that is applied to the rest of the code. The paragraph treats this area of the code differently for some reason.

[ July 24, 2003, 11:58 PM: Message edited by: bob ]