Substantiation behind Section 240.4(D) ? Small Conductors

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mweaver

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Forum Members,

Can anyone provide the substantiation and or reasoning behind the existence of Section 240.4(D)?

While I feel as though I have a pretty good grasp of the conductor ampacity requirements or Article 310 and the protection of those conductors within Article 240 in the 2008 NEC, I am actually quite unclear as why Section 240.4(D) actually exists within the NEC. (...I completely understand what it says... I just don't understand why it even exists... But Hey! I'm Slow... :) ...)

I would definitely appreciate some clarification on just why 240.4(D) exists with the actual wording it contains.

Why are these ?small conductors? singled out to have specific maximum overcurrent protection?

I feel as though I would understand this much better if I could see the substantiation and reasoning behind the existence of 240.4(D).

I know there are many here who are quite knowledgeable regarding the history behind much of what is in the NEC and how actually got in there.

I would appreciate your help. (...please remember... Hey! I'm Slow... :) )

mweaver
 
These conductors are used on circuits that are often worked on by unqualified people...it just gives an additional margin of error. Actually I have no idea and would be curious to know the real answer.
 
Don
Im interested my self but I am not near any of my ROP or TCR's.

I have a "84" HB and the explanatory print mentions that Table 310.16 was developed by a gentelman named Sam J Rosch and published in 1938. (rubber insulation).

I'm guessing they were working with K and T and the old cutouts and loadcenters had smaller gutter spaces. ???? I know this was some of the rational for limiting the panelboards to 42 circuits maybe there was also some concern to insure safe operating temperature for the rubber insulation??
 
The 1951 Edition of the NEC doesn't have that restriction but the ampacities were 14 AWG, 15 amperes, 12 AWG, 20 amperes, 10 AWG, 30 amperes for both rubber and thermoplastic insulation. The requirements in Section 240.4(D) were at the bottom of the tables before being relocated to this section. The 1970 Edition of the American Electrician's Handbook has no notes restricting the ampacity either.

I don't know when the notes were added to the tables but I suspect I know the reason. Overcurrent devices use the wire for a heat sink during their operation. If a small conductor were used at a higher amperage, its heat sink capability would not increase and the overcurrent device would run hot. This would cause premature opening of the circuit. :)
 
Charlie and Charlie, I have a hard time with your suggestions based on the fact we can use those conductors at their 310.16 ampacitys for certain applications.

I am left confused. :confused:
 
I remember reading an article many years back, the only problem I don?t remember the details or where I came across the article. This article went into details of small conductors having high enough impedance that during a fault condition the OCPD was very slow in opening the circuit especially if the runs were long in length.
I don?t know if this was just the author?s opinion or facts as to how 240.4(D) was created.

Sorry I don?t have detailed info for this article but it might provide you a path for research.
 
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