malachi constant
Senior Member
- Location
- Minneapolis
I am not missing that fact at all. 220.14(C) tells us to calculate the outlet load for motors in accordance with 430.22 and 430.24. That means that the 25% factor is already part of the "connected" load. The same number is carried through to the "demand" load in Part III.
For instance, lets take a look at a panel with nothing but motors. Say a panel that had four branch circuits feeding four 5HP, 208V, 3ph motors. Per 220.14(C), each motor branch circuit has a load of 7,521VA (as calculated per 430.22.) The total connected load on the panel (the sum of all the branch circuit loads) is 4*7,521VA=30,084VA.
Now lets apply the demand factors for the panel feeder, from Article 220 Part III. 220.50 directs us to 430.24, which tells us that the load would be (16.7A*1.25 + 3*16.7A) * 208V * 1.732 = 25,570VA. The load after the demand factors of Article 220 Part III are applied is smaller than the load calculated for the individual branch circuits per Part II of Article 220.
Good example, in that it points to where we disagree.
You are correct that an individual branch circuit would have a load of 7521VA. But you are calling the loads this:
CONNECTED: 4*(16.7A*1.25*208*1.732)=30,084VA.
DEMAND: (16.7A*1.25 + 3*16.7A) * 208V * 1.732 = 25,570VA.
Whereas I would call them this:
CONNECTED: 4*(16.7A*208*1.732)=24,065VA.
DEMAND: (16.7A*1.25 + 3*16.7A) * 208V * 1.732 = 25,570VA.
Or again, maybe it is semantics, because we get the same "demand" load at the end of the day. I would never show a connected load of 30kVA in this scenario.
In the above example, the way my panel spreadsheet is set up, for one of those motors you would enter a load of 6016VA (2005VA per each of three phases), the spreadsheet figures out the amperage and multiplies it by 1.25 to get a breaker size. (For simplicity's sake it does this regardless of whether it is a motor load, lighting load, receptacle load, non-continuous load, etc. I don't think this conservative approach affects anything in practice.) Then I would enter the load into three other breakers. The "connected load" in my spreadsheet would be 6,016*4=24,065VA (as shown above). Then my spreadsheet figures out what the largest motor load on the panel is (whether it is actually on the panel or subfed from a different panel), multiplies that number by 0.25, and adds it in. So I would end up with a "demand load" on that panel of 25,570VA.