(NEC) Two dwelling unit calculation

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MrPlasma

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Is there anyone who understands how to calculate a two unit dwelling. 220.85 is not exactly clear to me.


Is there a standard calculation and an optional? If so how exactly is it done.
 
You can use either the standard method or the optional method. That indeed is the point of 220.85. Here is what it is telling us:
  • Start by performing the standard method calculation.
  • Next, pretend that you have three units, not just two, and use the optional method of 220.84. You will note that that method applies to three or more units, but the code is giving us a way to take advantage of it anyway.
  • Use whichever of these two calculations gives you the lower result.
 
clarification

clarification

So, one thing that is still confusing is:

Do you first calculate the dwelling based on the standard method which is based on 2 units (or three units?);

then

calculate a separate calculation adding a third unit based on 220.85 (and using table 220.84 3-5 dwellings).

compare the two methods (standard and optional)

then

use the lesser of the two loads.
 
You do the standard calculation for 2 units, and the optional method for 3 units, and take the lower of the two results.
 
charlie b said:
You do the standard calculation for 2 units, and the optional method for 3 units, and take the lower of the two results.
Click to expand...
Keep in mind 220.85 only applies to a shared feeder. It does not apply to shared service conductors.

The short of 220.85 when applicable is you get to apply a 67.5% demand factor (i.e. 45% of 3 = 67.5% of 2).
 
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