I know that we have a consensus based code, but there are parts of the code that have "scientific" research behind them. Take the following examples: 1. Who is responsible for the ampacity tables in 310.16? 2. Who decided that cooking equipment could be derated on a commercial kitchen? 3. Why are the EGC's in 250.122 sized way they are? There has to be scientific study behind these sections, and others in the code. Taping the white conductor with black tape for the hot conductor in a romex is a good example of how the "consensus" part of the code works. I'm not saying that the examples that I mention above are not consensus based, but I do think that there is a difference between those, and 6" of free conductor, or having a green grounding conductor. Somewhere....someone....had to do the "leg work", the lab work, the testing, the this, the that, the whatever to come up with a good reason for all this stuff. I said all that to say this....besides the ROP's and the ROC's is there a source(s) of information out there to help me with some of the "why's" behind the code?
lotta heads in the NEC Doc, yet the history of the NEC has, to my knowledge, not been published.
I've often wondered myself of those who write the trade mag articles, as they must have some resource, or all be good researchers.
Case in point pg 70-143, 310.15(C), i think it may be McGrath-Neer (SP?) formula. I read a story on how they researched captive conductor heat, assumably leading to UL standards, whis more of this was , as you say...documented...
#12084 - 07/31/0210:35 PMRe: Science behind the NEC
Spark, I just think it would be a great study. Why? Some may ask. For the knowledge, and also because when we have the reasons behind the rules, it opens up our eyes, and we reduce the chance of violating the code because we know the principals upon which it is established.
Why does the sun make your hair lighter, but your skin darker?, Doc
The Watt Doctor Altura Cogen Channelview, TX
#12085 - 08/01/0212:17 AMRe: Science behind the NEC
My mentor was a good one for not only saying "do this" but also "and this is why" type thing...
The "why's" definitely help one to remember the "do's"...
I've always said to myself that I would train apprentices in the same manner.
BTW: Got a young man interested in taking the test, a hard-working, bright and easy-going guy... It's getting to the point that Joe and I can't handle it all! Looks like I may finally get the chance to train someone from scratch!
-Virgil Residential/Commercial Inspector 5 Star Inspections Member IAEI
#12086 - 08/01/0206:44 AMRe: Science behind the NEC
Watt, As far as grounding electrode conductors go and egcs use the same formula, It is based on a 5 second withstand rating (that is as long as any fault should last in theory), Circular mil area (from Chap 9, table 8) divided by 42.25 gives you that rating.
I keep that one on the top of my head, the ampacity tables I don't, they involve a lot more math, but basically it is from the Neher-Mcgrath, and is a compromise (it may be better to say "most often encountered) of all the conditions a cable can be placed in based on time of load, duration of load, and maximum ampacity that can be conducted for the CMA used.
Soares Grounding workbook has a lot of history of the NEC and grounding in it and can be an interesting read.
#12087 - 08/01/0207:53 AMRe: Science behind the NEC
Don, Geez don't start that. BIG bone of contention as to if that was properly considered. I think that's probably how I started with all the calcs anyway based on that very question. The NEC in the EGC area is woefully inadequate, and it's something most designers/engineers/electricians never seem to pick up on.........until it's too late anyway.
The proper answer to your question is Yes, it's supposed to be. It never seems to consider length in their calc, only cma though.
#12089 - 08/01/0211:34 AMRe: Science behind the NEC
1. Tests and studies by Sam Rosch in the 1930's 2. Utilities nationwide conducted surveys which gave us many of the demand factors in the NEC. 3. Eustace Soares in the 1960's tested and calculated much of the information on equipment grounding conductors. Suggest you join NFPA and IAEI, as a means of keeping up to date on future changes. Creighton
#12090 - 08/01/0212:33 PMRe: Science behind the NEC
Creighton, Agreed, it is definitely Eustace Soares that did the grounding, AND an explanation of it is in the book named after him. I still feel the IAEI took a lot of the meat out of his original books that should have been kept in them.
#12091 - 08/01/0204:41 PMRe: Science behind the NEC