using load diversity for derating

Status
Not open for further replies.
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Occupation
Electrician
Table B.310.15(B)(2)(11) provides adjustment factor for more than three CCC in a cable or raceway with load diversity. What constitutes load diversity? Anyone ever used this? Could I use this table if I was feeding, say, general purpose receptacles? Does a load less than the conductor's ampacity count as diversity?
 
electrofelon said:
Table B.310.15(B)(2)(11) provides adjustment factor for more than three CCC in a cable or raceway with load diversity. What constitutes load diversity? Anyone ever used this? Could I use this table if I was feeding, say, general purpose receptacles? Does a load less than the conductor's ampacity count as diversity?
Click to expand...

Gotta be an EE to use Annex B.


B.310.15(B)(1) Equation Application Information. This informative annex provides application information for am- pacities calculated under engineering supervision.
 
jumper said:
Gotta be an EE to use Annex B.


B.310.15(B)(1) Equation Application Information. This informative annex provides application information for am- pacities calculated under engineering supervision.
Click to expand...

Ok I buy that. Its rather confusing: 310.15(A)(1) essentially says (B) for tables for us idiots, (C) for equations for engineers. Then in annex B, it has "B" all over the place and tables...... Further, at first glance B.310.15(B)(1) seems to be referring to engineering supervision only when using equations, which mirrors what is said in 310.15(A)(1).
 
The NEC does not have a definition of “load diversity,” nor for that matter “diversity factor.” These two do not represent the same concept. I have a reference that defines the later. It’s a bit complex, so let me try to explain it.

Consider a main service panel that has a meter on its incoming conductors, and meters on each of the 8 feeders that provide power to 8 distribution panels. Look at the readings for a month. Take the highest reading on each of the 8 feeders (not likely to have all happened at the same time), add them together, and call that value “X.” Find the highest reading on the service meter, and call that value “Y.” The term “diversity factor” is defined as X divided by Y. It is necessarily greater than or equal to 1.0. The NEC Annex B table speaks of a load diversity of 50%. So it cannot be talking about this concept.

Here’s my best guess.

  • Table 310.15(B)(3)(a) says that if you have 10 CCCs, you derate to 50%.
  • Table B310.15(B)(1)(11) says that if you have 10 CCCs, you only need to derate to 70%, assuming you have a “load diversity” of 50%.
  • Let’s say you have 5 single phase, 20 amp, 120 volt circuits in a conduit. That comprises 10 CCCs. You would expect that each circuit would have a maximum continuous load of 16 amps. That adds up to a total maximum continuous load of 80 amps.
  • What I believe is that instead of derating the conductors to 50%, I can derate them to 70%, if I have reason to believe the actual total continuous load is no more than 40 amps (i.e., 50% of the 80 amps).
 
With receptacles how can anyone really control the loading? People will plug in things until the breaker trips.
 
Last edited:
That table was in the code proper at one time...until they tried to define "load diversity" and were unable to agree on an acceptable definition, so they moved the table to the annex where it is not actual enforceable code.
 
don_resqcapt19 said:
That table was in the code proper at one time...until they tried to define "load diversity" and were unable to agree on an acceptable definition, so they moved the table to the annex where it is not actual enforceable code.
Click to expand...

Yeah so that is what I don't quite get: where in the code is the authority granted, even if I'm an EE, to consider diversity? 310.15 (C) discusses engineering supervision, but doesn't appear to have allowance for diversity. Diversity is only mentioned in an informational note, which references something that isn't part of the code :?:
 
electrofelon said:
Yeah so that is what I don't quite get: where in the code is the authority granted, even if I'm an EE, to consider diversity? 310.15 (C) discusses engineering supervision, but doesn't appear to have allowance for diversity. Diversity is only mentioned in an informational note, which references something that isn't part of the code :?:
Click to expand...

You just had to look didn’t you.......:D
 
electrofelon said:
I'll propose a code change: after the informational note mentioning the table in annex B for diversity, I will add another informational note saying "do not look at annex B" or "annex B is not part of this code, don't bother looking" :D
Click to expand...

Every sparky who pokes around into Annex B comes out unhappy and a bit confused.

I did once, I still have the scars.
 
charlie b said:
The term “diversity factor” is defined as X divided by Y. It is necessarily greater than or equal to 1.0. The NEC Annex B table speaks of a load diversity of 50%. So it cannot be talking about this concept.
Click to expand...
So maybe it is talking about Y / X?

Cheers, Wayne
 
Status
Not open for further replies.
Top