Table 310-15-B-6

[ToolHound]

Table 310.15(B)(6)

What is the first thing that comes to your mind when you think of Table 310.15(B)(6) ?

And what is the 2nd thing that comes to mind ?

- - -
Me, I think....well, here's the little orphaned table over here that is crowded out of all the sunlight and glory that is all sucked up by the much larger Table 310.16.

I think of houses. When I think of Table 310.16(B)(6) I think...ok, here's this tiny but de facto nerve center for (choosing) large conductors for wiring houses.

But I also think of confusion, double-talking lawyerly electrical legaleeze. Why in the tarnation is the wording so inpentrable. Horsefeathers, can't they hire somebody to speak plain English. They use sentences some of which are openly hostile to comprehension. If we want to beat enimies in the Middle East or elsewhere, just send 'em a gift-wrapped copy of the NEC. That'd keep 'em running in circles for awhile. Ok, now that I have that out my system...

Here's a question...

Consider the statement, quote:

"The feeder conductors to a dwelling unit are not required to be larger than its service-entrance conductors."
END QUOTE

Now, this is one of the better sentences, so I'm not complaining about this setence. But I am wondering if there is some significance of it that I am issing. Why is this sentence used several times in the Code? The information of the setence, on the one hand, would seem to be common sense. But, not to be confident, I want to ask...why is it important enough to throw in that setence several places in the Code ? What are the implications of that information? And what would be the consequences if that sentence were not included in the Code ? What am I missing about that sentence, I keep wondering.

"The feeder conductors to a dwelling unit are not required to be larger than its service-entrance conductors."
END QUOTE

Thanks.

 

[kwired]

You are forgetting one thing - the feeder in question must feed the entire dwelling load. This doesn't apply to just any feeder, but one that serves the entire dwelling load. The reduced conductors in the table are all about expected load diversity. A feeder not supplying entire dwelling is not necessarily subjected to same diversity. Neither is a service conductor if it doesn't supply the entire dwelling load.

 

[GoldDigger]

You are forgetting one thing - the feeder in question must feed the entire dwelling load. This doesn't apply to just any feeder, but one that serves the entire dwelling load. The reduced conductors in the table are all about expected load diversity. A feeder not supplying entire dwelling is not necessarily subjected to same diversity. Neither is a service conductor if it doesn't supply the entire dwelling load.

Well and good when calculating the expected load and connected load, etc.

But it still seems a bit odd that "diversity" will let you use a smaller feeder to carry the entire load than you can use for either feeder if the load is split equally for that same dwelling unit. Unless you are looking only at cancellation in the neutral conductor where you use a reduced neutral, I still do not see the point.

 

[kwired]

Well and good when calculating the expected load and connected load, etc.

But it still seems a bit odd that "diversity" will let you use a smaller feeder to carry the entire load than you can use for either feeder if the load is split equally for that same dwelling unit. Unless you are looking only at cancellation in the neutral conductor where you use a reduced neutral, I still do not see the point.

The diversity is based on the entire dwelling unit load. If you start to divide the load up you no longer have same diversity. Take heating or cooling equipment for example. They may run for hours at a time. A feeder supplying only this kind of equipment has no where the same load diversity as a feeder supplying the entire dwelling.

I really don't know how to figure out load diversity, but in that example I think it is somewhat obvious the diversity is not the same.

 

[GoldDigger]

The diversity is based on the entire dwelling unit load. If you start to divide the load up you no longer have same diversity. Take heating or cooling equipment for example. They may run for hours at a time. A feeder supplying only this kind of equipment has no where the same load diversity as a feeder supplying the entire dwelling.

I really don't know how to figure out load diversity, but in that example I think it is somewhat obvious the diversity is not the same.

I agree 100% with that, but I cannot see any way in which the calculated diversity load on some of the loads could possibly be greater than the calculated diversity load for all of them. I certainly can see the sum of the two calculated loads being greater than the calculated load when they are on the same circuit. That can happen because you would not be allowing for diversity for loads on different feeders when calculating each feeder load.
But I just cannot see that diversity of (A) or diversity of (B) could be greater than diversity of (C) where C = A + B. You cannot increase the diversity calculation of a set of loads by removing one of the loads. Unless for one load you figure 100% and for two identical loads you figure them as less than 50% each, of course. That might happen, I guess.

 

[ToolHound]

Thanks.

Thanks.

You are forgetting one thing - the feeder in question must feed the entire dwelling load. This doesn't apply to just any feeder, but one that serves the entire dwelling load. The reduced conductors in the table are all about expected load diversity. A feeder not supplying entire dwelling is not necessarily subjected to same diversity. Neither is a service conductor if it doesn't supply the entire dwelling load.

kwired. That helps. Thanks.

 

[ToolHound]

See related thread and pic at:

http://forums.mikeholt.com/showthread.php?t=152411

 

[texie]

I understand there are big changes on this for the 2014 edition. As I recall, the table will be replaced with a formula. Maybe somebody can point out the proposal details that I think was accepted.

 

[jumper]

I understand there are big changes on this for the 2014 edition. As I recall, the table will be replaced with a formula. Maybe somebody can point out the proposal details that I think was accepted.

Here ya go.

This from the 2014 ROC. 6-52

Panel Meeting Action: Accept in Principle
Revise text to read as follows:

(7) 120/240 Volt, Single-Phase Dwelling Services and Feeders. For onefamily
dwellings and the individual dwelling units of two-family and
multifamily dwellings, service and feeder conductors supplied by a single
phase, 120/240-volt system shall be permitted be sized in accordance with
310.15(B)(7)(a) through (d).

(a) For a service rated 100 through 400 amperes, the service conductors
supplying the entire load associated with a one-family dwelling or the service
conductors supplying the entire load associated with an individual dwelling
unit in a two-family or multifamily dwelling shall be permitted to have an
ampacity not less than 83% of the service rating.

(b) For a feeder rated 100 through 400 amperes, the feeder conductors
supplying the entire load associated with a one-family dwelling or the feeder
conductors supplying the entire load associated with an individual dwelling
unit in a two-family or multifamily dwelling shall be permitted to have an
ampacity not less than 83% of the feeder rating.

(c) In no case shall a feeder for an individual dwelling unit be required to have
an ampacity greater than that of its 310.15(B)(7)(a) or (b) conductors.

(d) Grounded conductors shall be permitted to be sized smaller than the
ungrounded conductors provided the requirements of 220.61 and 230.42 for
service conductors or the requirements of 215.2 and 220.61 for feeder
conductors are met.

Informational Note No. 1: It is possible that the conductor ampacity will
require other correction or adjustment factors applicable to the conductor
installation.

Informational Note No. 2: See example DXXX in Annex D.