Proposal to Change 2011 NEC
6-49a Log #CP604 NEC-P06 Final Action: Accept
(310.15(B)(7))
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TCC Action: The Correlating Committee directs that the panel clarify their action on this proposal.
The Correlating Committee also directs the panel to revise the Informational Note as it contains permissive language, i.e. the word “may”.
This action will be considered as a public comment.
Submitter: Code-Making Panel 6,
Recommendation: Delete Table 310.15(B)(7) and replace 310.15(B)(7) with the following:
(7) 120/240 Volt, Single-Phase Dwelling Services and Feeders. For service and feeder conductors of 120/240-volt, single-phase, individual dwelling unit one-family, two-family, and multifamily service ratings from 100 through 400 amperes, an adjustment factor of 0.83 of the service ampere rating shall be permitted to be used to determine the size of the ungrounded conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided that the requirements of 215.2, 220.61, and 230.42 are met.
Informational No. 1: The conductor ampacity may require other correction or adjustment factors applicable to the conductor installation.
Informational No. 2: See example DXXX in Annex D.
Substantiation: It was determined that during the 1956 Proceedings of the Sixteenth NFPA Annual Meeting that 84 percent was used to establish the aluminum residential service conductor size. However, if the panel used 84 percent in the changed language, it would have resulted in larger sizes for some of the conductors, compared to the sizes in the 2011 NEC. Since the panel had no technical substantiation to justify these changes, 83 percent was used to maintain consistency with the sizes in the 2011 Table 310.15(B)(7).
In order to address the various proposals submitted suggesting changes to 310.15(B)(7), the panel analyzed the existing language and determined that the conductor sizes in Table 310.15(B)(7) are equivalent to those that would be used if a 0.83 multiplier was applied to each service ampere rating. The resulting conductor size will be the same as existing text in Table 310.15(B)(7), if the same conductor types and installation conditions are applied.
The informational note was added to make it clear that adjustment and correction factors apply depending on conditions of use. This action no longer requires the definition of a “main power feeder” in 310.15(B)(7).
Panel Meeting Action: Accept
Number Eligible to Vote: 10
Ballot Results: Affirmative: 9 Negative: 1
Explanation of Negative:
WALL, C.: Removal of the table does not add clarity or usability to the NEC.
Comment on Affirmative:
CLINE, S.: This proposal is the result of many, many hours of panel member time over many code cycles. It is intended to clearly and easily express the ongoing intent of the panel over these many code cycles of misinterpretation. This wording gives a simple “duty cycle” type adjustment which, through a simple mathematic multiplication, yields a minimum ampacity requirement for conductor sizing. Hopefully the twelve submitters who also spent their time trying to resolve the misunderstandings will be satisfied with this result.
It recognizes the long-known diversity of load for this exact class of load. It recognizes that conditions of installation may also affect the ampacity of the conductor. It recognizes that feeders, only if sized in relation to the service rating, may safely be allowed the same diversity adjustment since they are either carrying 100% of the diversified load, OR only loads too small to change the effective diversity have been removed ahead of the feeder, OR large enough loads have been removed ahead of the feeder to make the 17% adjusted ampacity a moot point. The concerns about increased dwelling loads in general is addressed in 230.79 where the service rating amperage itself is determined.
It should now be clear that while feeders may also use the diversity adjustment, it must be based on the 230.79 service rating, not the size of the OCPD for the feeder. If you run a 200 amp feeder from a 200 amp rated service, you get to use the adjustment, start with a 166 minimum ampacity conductor, apply any other required adjustments, and choose your conductor. If you run a 100 amp feeder from a 200 amp rated service, you still get use the adjustment, but of course the 166 minimum ampacity conductor then required might not be an advantage over the normal 100 amp conductor. You may not use 100 amps (the feeder OCPD size) to apply the adjustment to - you must use the service rating.
Separate issue No. 1:
Please note: I believe that the following editorial changes to the new wording recommendation of 6-49a need to be made for the publications of the ROP and Draft.
Final Edited wording:
(7) 120/240-Volt, Single-Phase Dwelling Services and Feeders. For service and feeder conductors of 120/240-volt, single-phase, individual dwelling units of one-family, two-family, and multifamily service ratings of 100 through 400 amperes, 83 percent of the 230.79 service ampere rating shall be permitted to be used as the minimum ampacity to determine the size of the ungrounded conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided that the requirements of 215.2, 220.61, and 230.42 are met.
Informational Note No. 1: The conductor ampacity may require other correction or adjustment factors applicable to the conductor installation.
Informational Note No. 2: See example DXXX in Annex D.
Editorial changes to be considered (in order of occurrence):
(Legislative text will not paste into this comment area.)
1) change “an adjustment factor of 0.83” to “83 percent”
Examples elsewhere in the code (within text sentences as opposed to within Tables), such as 310.60©(2)(b), 430.122(A), 630.31(A)(1), etc, utilize the percentage wording. It is consistent with existing NEC usage.
2) add “230.79” in front of “service ampere rating”
Direct reference to the NEC source of the “Service Rating” amperage value.
3) add “as the minimum ampacity” before “to determine”
To proactively state the mathematically obvious result of scientific units which results from the multiplication of the service rating (amps) times 0.87 (87%), and that it is a minimum number still subject to the other adjustments of 310.15(B).
Separate issue No. 2:
I believe that an Informational Note No. 3, showing a restructured form of the 2011 NEC Table 310.15(B)(7), would be very helpful as a transition from Table to adjustment factor.
The title and title heading would be unused. Only the portion of the existing Table 310.15(B)(7) below the double line would be used, and one column heading must be edited:
“Informational Note No. 3: Partial listing of conductor AWG or kcmil sizes for 310.15(B)(7) applications, showing only 75°C (167°F) conductors, under conditions of installation which do not require any other adjustments.”
The heading of the first column would need to have “or Feeder” deleted since the relationship is always to the “Service Rating” even for feeders. It should read “Service Rating (Amperes)” - OR it should read “230.79 Service Rating (Amperes)” if the addition of “230.79” within the body of the 310.15(B)(7) text is approved.
The rest of the Table could be used as-is.
“AWG or kcmil” is used purposefully in the note to avoid the ambiguity of the word “size” as it relates to conductors. Does “size” mean “physical size” or “ampacity”?
I believe that in general the unqualified word “size” should not be used anywhere in the code in reference to conductors. “AWG or kcmil size” or “ampacity” would be unambiguous terms.
Separate issue No. 3:
I recommend another Informational Note:
“Informational Note No. 4: Section 310.15(B)(7) excludes 208Y/120-Volt supplied systems (single or three phase) due to the additional heat from the presence of a third conductor carrying current.”
I realize that the NEC is not a design manual, but this issue is so often misunderstood that it seems worth the print space to help assure that AHJs have proper and easy tools to use, and to help avoid repeated proposals and comments.
KENT, G.: This proposal is correct as a Reject, however, I disagree that permission exists in the code to allow this type of wiring.
LAIDLER, W.: I’m voting to accept the proposal. I would also recommend that the editorial changes recommended by NFPA staff be incorporated into the proposal (as stated below).
(7) 120/240-Volt, Single-Phase Dwelling Services and Feeders. For service and feeder conductors of 120/240-volt, single-phase, individual dwelling units of one-family, two-family, and multifamily service ratings of 100 through 400 amperes, 83 percent of the 230.79 service ampere rating shall be permitted to be used as the minimum ampacity to determine the size of the ungrounded conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided that the requirements of 215.2, 220.61, and 230.42 are met.
Informational Note No. 1: The conductor ampacity may require other correction or adjustment factors applicable to the conductor installation.
Informational Note No. 2: See example DXXX in Annex D.
These recommended changes will provide better clarity for the user on how to apply this new language.
