This section was very clear in the 1987 code. At that time the rule was found in Note 3 to Tables 310-16 through 310-31. The note said that the reduced conductor sizes could be used for a "... feeder that carries the total current supplied by the service". Very clear and concise code writing.
In the 1990 code the wording for Note 3 was changed to read; " ... single phase service entrance conductors and feeder conductors ...." There was no restriction on the use of the reduced conductor sizes for feeder conductors in the 1990 code. The proposal (TCR89A 6-83) stated that "Note 3 only applies to feeders of 100 amps or more, so the diversity contemplated in the rule is going to be present whether some additional load is taken off ahead of the feeder or not". The submitter also stated that often a second feeder is used for loads such as swimming pools or air conditioners. The panel accepted the proposal in principle and deleted the words "that carries the total current supplied by the service".
In the 1993 code the panel reversed itself. Panel action on comment (92TCD 6-49) restored wording that required a "feeder to supply the total load to the dwelling unit" in order to use the reduced conductor sizes in the table. This comment suggested use of the words "that serve as the main power supply to a dwelling unit", but the panel replaced those words with the words shown above.
In the 1996 code the panel introduced the words "main power feeder". This code cycle had a number of proposals to change Note 3, and the panel combined them into their own proposal (95ROP 6-73a). The panel comments indicated that they intend that a "main power feeder" supply the total dwelling unit load, however they also added wording that said a feeder would never have to use conductors larger than the service conductors. They added this to permit an outside service panel to feed loads such as air conditioning and still permit a feeder to use the reduced size conductors. There was a comment (95ROC 6-31) that recommended that the words "feeder conductors that serve as the main power feeder to a dwelling unit", be replaced by the words "feeder conductors that carry the load to the dwelling unit". The commentator suggested that the words "main power feeder" would lead to interpretation problems and confusion as to what is a "main power feeder'. The panel rejected the comment with a panel statement that read: "The use of Note 3 is based on load diversity and if the user does not have load diversity, Note 3 should not be used. The panel does not believe that the recommended wording provides additional clarification."
In the 1999 code the Notes were moved into code text as a part of 310-15(b)(6). The "definition" of main power feeder was added in this code cycle. The substantiation said the change was to clarify that the "main power feeder" is/are the feeder(s) that carries diversified loads. The panel accepted in principal. I really don't see how this wording clarifies the section. Why can't it just say a "feeder that carries the total dwelling unit load"? There would be no question as to the meaning then.
In the 2002 code cycle there was an attempt to clarify this section with words that would have clearly permitted the reduced size conductors to be used for any feeder that supplies at least 40% of the line to neutral load of the dwelling unit. This was a proposal that would have permitted a single 400 amp service to have two 200 amp "main power feeders". The proposal was rejected with a panel statement that said: "The present wording adequately describes the requirement." Like one of my favorite code instructors used to say, "boy, I'd like a puff of what they have been smoking". (not Mike Holt, you would have to be an old guy like me to know who often said this at his code classes)
As I said previously in this thread, I know what the intent of the rule is, I just don't agree that the current wording fully supports that intent. After researching the ROPs and ROCs tonight, I find that I am in good company with that thought. Some of the heavy hitters in the NEC world have made proposals and comments to clear up the wording. We need to go back to the 1987 wording as that is CMP 6's stated intent of the section. This would prohibit the common practice of using two 200 amp "main power
feeders" on a 400 amp service.
Don