art. 240-4 (B)

[reddogadventures]

I need clarification on art.240-4(B). It states that in situations of 800 amps or less, the next size larger breaker can be used if the size of conductors being used doesn't match up with a breaker size.
For example, feeding a 400 amp panelboard with 500mcm's is legal even though 500mcm's are rated for 380 amps @ 75 c.
Is the intent of the law that using 600mcm's would force the installer to spend more than neccessary with the increase of pipe size and wire size?
What is the reason? It's always made me curious what the reasoning is behind this article.

 

[charlie b]

Re: art. 240-4 (B)

First of all, it is written by the NFPA, not by a legislative body. The document becomes law only after it is adopted by any legal jurisdiction. Nevertheless, the intent of the document is the intent of that organization, not the ?intent of law.?

Secondly, the authors of the NEC are not concerned in any way with the price of installation of any electrical equipment. Their only concern is safety.

But to address your question, you are not quite correct in your description. In order for a set of 500 MCM conductors (copper, THHN, 75C) to be adequately protected by a 400 amp breaker, the total load must be proven to be lower than 380 amps. In other words, if a load of 390 amps were to be passed continuously through these conductors, it may eventually cause failure, and the 400 amp breaker would not be able to prevent that failure. It is our responsibility to ensure that no conductor is loaded beyond its ampacity limits.

 

[reddogadventures]

Re: art. 240-4 (B)

Thank you for your quick response to my question. I am aware that the NFPA writes the NEC and I do realize that our code must be adopted by a legislative body inorder for it to become law. The State of MN (the location of the majority of my work) has adopted the NEC as minimum requirements for electrical installations and therefore is the law here in Minnesota, governed by the State Board of Electricity and other municipal governing bodies.
I agree with your assesment of art. 240-4. However, it is common practice to use 500mcm to feed 400 amp panelboards as well as parallel 500mcm's to feed 800 amp services and I've never seen it red tagged nor have I ever had, or heard of any inspector questioning the calculated load of said services. I see it constantly laid out this way by engineers on prints. Even manufactorers have made it common to use lugs that will only accept conductors up to size 500mcm for their parallel feeds of 800 amp services. I learned this the hard way after pulling parallel 600mcm to feed an 800 amp service only to find that when the gear showed up, the lugs were too small to accept my 600mcm's. That one really impressed my project manager who gave me particular hell for trying to do the right thing!
I realize this doesn't make it correct but it is done often. I didn't really think that the NFPA was concerned with costs for the installer but was grasping at reasons why this situation is never questioned.
Thanks again for your help.