Multi-family opt. calc.

[ducks1333]

When a 3 phase 208V feeder is used to serve 3 multi-family dwelling units by circuiting one unit to A-B phase, one to A-C phase and one to B-C phase can the diversity factor of table 220.84 be used?

Thanks,
John

 

[GoldDigger]

That is a nice question.
I am not sure how the exact wording of the Code treats it, but from a logical viewpoint the diversity factor table entry used should be for 2/3 of the actual number because there will be fewer loads sharing each phase.

 

[Smart $]

Yes... wherever all three loads are combined into the same 3Ø circuit.

And the correct terminology is demand.

 

[ducks1333]

Yes... wherever all three loads are combined into the same 3Ø circuit.

Yes I can use the 45% demand factor because I have 3 units on the same feeder? The demand table doesn't allow demands to be taken on feeders only serving two units. Electrically the arrangement I have is no different from a single phase feeder serving two units in that the load on each conductor will be the same as the load on each conductor as in the 3 phase feeder. So, it doesn't make sense that I would then be allowed to basically cut the demand load by 45% just because I used a three phase feeder rather than single.


And the correct terminology is demand.

Correct but it is based on diversity.

 

[Smart $]

Correct but it is based on diversity.

No, it is not.

Diversity affects conductor ampacity. See Informational Notes of 310.15(B)(3)(a) and 400.5(A), Table 520.44, and Annex B

See the Article 100 definition of demand factor. Code does not define diversity.

 

[Smart $]

Yes I can use the 45% demand factor because I have 3 units on the same feeder? The demand table doesn't allow demands to be taken on feeders only serving two units. Electrically the arrangement I have is no different from a single phase feeder serving two units in that the load on each conductor will be the same as the load on each conductor as in the 3 phase feeder. So, it doesn't make sense that I would then be allowed to basically cut the demand load by 45% just because I used a three phase feeder rather than single.

Yes.

It may be just me but that statement doesn't make any sense.

There are instances where the demand factor can be used for two units. See 220.85.