Derating using B.310.11
- Thread starter electo17
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- Location
- New Jersey
- Occupation
- Journeyman Electrician
Where are you finding table B310.11 in annex B?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
How are you going to show that the load diversity is 50% or lower, given that nobody knows what the NEC means by the term "load diversity"? If you plan on using all six circuits, all twelve conductors, all at the same time, then I am reasonably certain that this table will not be applicable. But what it would take to make the table applicable is not entirely clear to me.
The circuits are for a dorm setup. 3 circuits per room. One for the GFI and 2 for each room. We would like to pick up 2 rooms with one conduit. I do understand that I could run with number 10's and increase to 1" although our conduits are run with the design of shared nuetrals and now the college wants to use seperate nuetrals. This project is in Albany, NY and I am from MA which would normally allow us to use this chart but the NEC calls for diversity. Diversity I believe states that "all not loads are on at the same time". Whats you're take on it?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
My take is that unless you have some type of interlock that absolutely precludes any possibility of the loads being on at the same time, then you do not have the "load diversity" necessary to take advantage of that table. But you have no such interlock. It is easily possible for all six circuits to be carrying load at the same time. In that case, it would not matter how lightly loaded any given circuit might be. If it carrys current, any current, at the same time as all the others are carrying current, then you do not have load diversity. Sorry. I know that is not what you were hoping to hear.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
For the record Annex B is not part of the code and provided for informational purposes only. Look at 90.3. Having said that I'm unsure how you would actually apply that table to your installation.
For what it's worth separate neutrals for these circuits are IMO a waste of money, especially if there's an up cost associated with larger conduits and conductors. Maybe all #10 conductors with shared neutrals would be a good compromise.
For what it's worth separate neutrals for these circuits are IMO a waste of money, especially if there's an up cost associated with larger conduits and conductors. Maybe all #10 conductors with shared neutrals would be a good compromise.
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One more question and I will leave it alone. Looking at the formula in B310.11 under FPN. would a conduit with 6 hots 6 nuetrals and 1 grnd. used for the formula "N" use 12 or 13 as a number. Note example 1 and 2 both use 24. Is that the number of total conductors including the grnd. or just hots and nuetrals? thank you for your help. :-?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
T'aint there. That's part of the reason that this table is in the Annex. As I understand the history (it having been posted by another member not all that long ago), there was a great debate amongst the code making panel members. They could not agree on a clear definition, so they decided to make the section "non-enforceable," by moving it to the Annex.electo17 said:P.S. Can you tell me where the definition of Load diversity is in the code? I can't seem to find it.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
It does not include the ground. In your example, you would use 12, not 13.electo17 said:
- Location
- Massachusetts
electo17 said:
To even begin considering using Table B.310.11 you have to be an engineer. See section B.310.15(B)(1).
However according to your profile your from MA like I am and if the job is in MA you need to use the MA amendments.
If you look at MA amendment 310.15(B)(2)(a) you will see MA has amended the NEC derating table, as it happens the MA derating table is in fact the same as Table B.310.11. :smile:
All that aside I would rework the circuits as multiwire branch circuits with 6 hots and 3 neutrals if a single phase system or 6 hots and 2 neutrals if a 3 phase system and forget about derating altogether.
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