NEC Multifamily Dwelling service conductor size

[Kartracer087]

I am writing to ask why do the majority of the calculations that I have seen for multifamily dwellings do not take into account 230.42 (A) 1 and 2 when sizing the service entrance conductors:

(A) General. The ampacity of the service-entrance conductors
before the application of any adjustment or correction
factors shall not be less than either (A)(1) or (A)(2).
Loads shall be determined in accordance with Part III, IV,
or V of Article 220, as applicable. Ampacity shall be determined
from 310.15. The maximum allowable current of
busways shall be that value for which the busway has been
listed or labeled.
(1) The sum of the noncontinuous loads plus 125 percent
of continuous loads
(2) The sum of the noncontinuous load plus the continuous
load if the service-entrance conductors terminate in an
overcurrent device where both the overcurrent device
and its assembly are listed for operation at 100 percent
of their rating

Most of the time you see the selection of the service entrance cable be based directly on your article 220 load calculations, but I never see the noncontinuous/continuous load factors applied to this calculation like you would have to per 230.42 (A) 1. I would just assume the worst case scenario would be you take your demand load from article 220 and multiply it by 1.25, but I've never ever seen it done this way... Is there some reason why you don't have to do it like that?

Thanks.

 

[Dennis Alwon]

The loads calculation of art 220 should have all the factors built in. There is no need to look at art. 230 for calculations because art. 220 has it all. For instance the lighting oads in T. 220.12 for schools has 3 watts/sq. ft which is probably based on continuous loads. Notice that although the same for dwellings we do have to figure 180va for receptacles in schools but not for dwellings otherwise the lighting load for dwellings would be less than 3 watts/sq.ft

 

[Smart $]

The loads calculation of art 220 should have all the factors built in. ...

It don't. Nowhere in Article 220 does it mention anything about calculating at 125% for continuous loads.

If you look at Annex D examples, there is no continuous and noncontinuous loads factoring for dwellings... but there is for commercial and industrial examples. Its odd that I don't see Chapter 2's requirements making that distinction.

 

[suemarkp]

Few loads in a dwelling are continuous. I think the default mode for inspectors is dwellings are not continuous. They may have some lights on for more than 3 hours, but probably not every one.

My swimming pool pump is certainly continuous (generally 8 hours per day), but I don't believe the continuous factor is applied to motors (only a 125% for the largest one, continuous or not). Other than that, I can't think of any continuous loads I have other than dusk-to-dawn lighting.

 

[Kartracer087]

As long as it is generally agreed upon by code officials/inspectors that the total article 220 calculated load IS the service size for a multifamily dwelling or single family dwelling without any adjustment factors, then that's fine. Commercial/Industrial/Institutional, that's where I know I have to put in the 1.25 for continuous loads applied to service calcs. The examples in the Annex show that calculation for a commercial building but they do not show the 1.25 for their dwelling unit/multifamily calc, so I'm assuming that is proof enough :happyyes: