Explaination of 220.33

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darrylb

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Can someone explain the meaning of 220.33 (220.85 for 2005 code)? It says "Where two dwelling units are supplied by a single feeder and the computed load under Part II (Part III) of this article exceeds that for three identical units computed under 220.32 (220.84), the lesser of the two loads shall be permitted to be used.
Does it mean when I calculate a duplex using the optional method, I re-calculate for three units and choose the lesser of the two loads?
I am studying for the master's exam and safe to say I should know this in case it comes up on the exam.
Thanks
 
Re: Explaination of 220.33

The optional method for multi-family dwelling units in 220.32 can only be used if there are 3 or more units. I suspect that is because the chances that everyone will be using their appliances at the same time goes down, when you add in more and more dwelling units. Therefore, the demand factors just wouldn't be reasonable for a duplex, as it is far more likely that both will be cooking or washing clothes at the same time.

So what they let you do is to pretend that the duplex is a triplex. Add up the loads as though there were three of everything. Then apply the demand factors of 220.33. If you get an answer that is lower than the answer from the "standard method," then you can use it.

Since you are talking about a test, you should be aware that it is likely they will do the calculation using both the standard and the optional way, even for a duplex. They are going to know which gave the smaller answer, and they are going to use that as the "right answer."
 
Re: Explaination of 220.33

If this were not in the code, you might hear your inspector say this:

"The service is too small for a duplex. Either increase the service size, or add a third unit and make it a triplex."

Steve
 
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