Nec 310.15(b)(2)fpn #1

[Nate Scott]

What loads are considered diversified loads regarding NEC 310.15(B)(2)FPN #1?How & when is this applied to table B 310.11 in Annex B.

 

[bphgravity]

See 310.15(C). Only an engineer can determine this and make calculations based on Annex B and this section.

 

[charlie b]

That process requires you to start by knowing (1) How many conductors are in the raceway, and (2) How many can be carrying current at the same time. It does not matter if one conductor is loaded to 100% of its ampacity and another conductor is loaded to 5% of its ampacity. Both are counted as carrying current. So to say that some conductors will not be carrying current, at least not at the same time as the others, requires you to have some type of interlock or lockout feature included in the design.

Look at the example that follows Table B310-11. It says that there are 24 conductors in the conduit, and that (by virtue of load diversity) only 12 will carry current at any given time. I can imagine a set of eight 3-phase motors, set up as four pairs of motors. In any given pair, the motor starters are interlocked such that when one is running, the other is blocked. That could account for there being 24 conductors, with no more than 12 carrying current at the same time.

If I were to be asked to apply Table B310-11 to a real design, I would insist on there being a physical means (i.e., not just an administrative procedure) to prevent more conductors from carrying current at the same time.

 

[iwire]

So to say that some conductors will not be carrying current, at least not at the same time as the others, requires you to have some type of interlock or lockout feature included in the design.

Where would I find that NEC requirement?

I agree it's good design and usually easily accomplished but I don't see a requirement that says they must be interlocked.

As another example of where the code is counting only on the designers word is Exception 3 to 230.90

The only thing I have to do is show calculations that prove the load is under the conductor ampacity.

FWIW here in MA local amendment we use Table B310-11 in place of Table 310.15(B)(2). :smile:

 

[iwire]

I apologize, I was thinking of load calculations and 220.60, not ampacity adjustments.

 

[charlie b]

I agree it's good design and usually easily accomplished but I don't see a requirement that says they must be interlocked.

The article in question is in the Annex, so there are no ?requirements? at all. But ?load diversity,? as that term is used in B310-11, means that not all the conductors in the raceway will be carrying current at the same time. I submit that absent a physical control, such as an interlock, ?load diversity? cannot be achieved. It is a simple matter of, if current can flow in a wire, it must be counted as a current-carrying conductor.

FWIW here in MA local amendment we use Table B310-11 in place of Table 310.15(B)(2).

How, then, do you determine whether a system has ?load diversity?? Also, what value do you use for the ?Adjustment Factor? for, let us say, 25 conductors in a raceway?

 

[iwire]

How, then, do you determine whether a system has ?load diversity??

We don't, we could determine a system does not have load diversity.

It appears the electrical board in MA assumes most circuits do have diversity and when they don't you will take appropriate steps.

A FPN was added to 'MA' 310.15(B)(2) (which is just a duplicate of B310.11)

FPN: Overheating may occur where continuous, fully loaded conductor diversity is less than 50 percent and the number of current-carrying conductors exceeds nine. See 310.10

Also, what value do you use for the ?Adjustment Factor? for, let us say, 25 conductors in a raceway?

25 to 42 gets a 60% reduction.