Errant 2 amps in Annex D example D(1)(b)?

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DimlyLit

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I have been racking my brain, what small part I have remaining after studying calculations, with the second part of the first load calculation example from Annex D.

There is a table showing the method of getting to the least unbalanced load of the neutral via the list of loads added to this structure for the second part of the example. After a lot of study, I think I understand all of it except the bottom row. The 3 amps, I assume, is the 25% motor load addition from NEC 430.24 derived from the 12 Amp AC unit as the largest motor load? But there is a 2 amp load shown on the lower right corner in the same row. I assume this is also a 25% load addition from the 8 amp disposal. But wasn't this 25% already taken with the 3 amp addition as the largest motor load? This adds amps to the phase line load and could potentially change the calculated load. Why was the 25% taken twice??
 
I beleive the (2) is the 25% of the dishwasher load. (The 3 amps is on the A/C Phase A, the 2 amps on the dishwasher phase B)
 
I don't think the 2 amp, Line B load is from the dishwasher, for two reasons;

First, that would mean the 25% increase in its rating is the second load to receive this increase, whereas it should only be increased on the one, highest rated motor load [via 430.24].

Second, If it was the the added 25% increase it should be larger.
[25% of 10 amps = 2.5 amps, rounded up equals 3 amps, via 220.5(B) ]
 
I thought so too. But if it is the disposer it means that the 25% single largest motor load has already been taken by the AC unit, which is the 3 amps on Line A and the Neutral, leaving the disposer 25% as a secondary increase not recognized by 430.24. I am thinking that the 2 amp load is erroneous and should not be listed at all?
 
IMO, if you were doing a load calculation (total load in kw) then the 25% of the largest motor would apply but looking at by phase you would need to look at the load per phase and since those two loads are on different phases that's how they looked at it.
 
I guess I don't understand why the Annex, which is giving load calculation examples, switches to phase balancing. I can't find anything about phase balancing in the NEC other than this example. If this second half of the example is done via the normal load calculation rules for the standard method, which was the first half of this example, I get 99.27 amps. How does one know, especially when being tested with the Calculations portions of exams, which method to use. It seems arbitrary. [I am talking from my frustration at having a clear load method at this point.]
 
The 2A entry in (2017) D(1)(a) is based on the idea that the extra 25% of the largest motor only applies to the A/C, and not to the disposer or the dishwasher. I guess on the presumption that those appliances already have the extra 25% incorporated into their ratings, but the A/C ratings given do not?

I think the idea of D(1)(b) is that in D(1)(a) you assume the code specified loads (3 VA/sq ft, laundry and SABCs, etc) are evenly divided between legs, but that when you have discrete additional loads, it makes sense to consider how closely you can get them to be balanced.

As to the 2A, it's a matter of applying the 25% of the largest A/C on a per conductor basis, although it's in error. Line A has on it a 6A A/C and a 12A A/C. The largest A/C is 12A, and 25% is 3A. The neutral has on it only a 12A A/C, and 25% of that is 3A. And line B has on it only a 6A A/C, 25% of which is 1.5A, which gets rounded up to 2A.

Cheers, Wayne
 
DimlyLit said:
I guess I don't understand why the Annex, which is giving load calculation examples, switches to phase balancing. I can't find anything about phase balancing in the NEC other than this example. If this second half of the example is done via the normal load calculation rules for the standard method, which was the first half of this example, I get 99.27 amps. How does one know, especially when being tested with the Calculations portions of exams, which method to use. It seems arbitrary. [I am talking from my frustration at having a clear load method at this point.]
Click to expand...
210.11(b) talks about load balancing.
 
wwhitney said:
The 2A entry in (2017) D(1)(a) is based on the idea that the extra 25% of the largest motor only applies to the A/C, and not to the disposer or the dishwasher. I guess on the presumption that those appliances already have the extra 25% incorporated into their ratings, but the A/C ratings given do not?
Click to expand...
25% of 8A disposer is 2A. It's the largest motor load on Line B as Gus noted in post 6.
 
david luchini said:
25% of 8A disposer is 2A. It's the largest motor load on Line B as Gus noted in post 6.
Click to expand...
Good point. I was thinking of the disposer as an appliance like the dishwasher, but it's basically just a motor. So a couple questions:

1) If a disposer is described as a "8A disposer" then we take that as the full load current? Vs the MCA, which would already include the extra 25%.

2) I looked at some disposer nameplate images on the web. One says, e.g. 120V 9.5A 3/4 HP. So do we treat the full load current as 9.5A, or the value of 13.8A from Table 430.248? (If the latter, it would unusual to have an "8A disposer" as the table values jump from 7.2A at 1/3 HP to 9.8A at 1/2 HP).

Thanks,
Wayne
 
Oh, and same question for window air conditioners--I was looking at a few nameplate images, and they just say "Amps" or "Rated Amps". So should we treat that as the branch circuit selection current, or the MCA?

Cheers, Wayne
 
After reading all these replies [thank you], I wish I could say these comments have answered my questions, but I am still unaware as to how it can be noted on the Annex line at the bottom of the load balancing table, that the 25% load is taken via 430.24, but substitutes both motor increases when it only stipulates the largest load to have 25% added.
 
DimlyLit said:
After reading all these replies [thank you], I wish I could say these comments have answered my questions, but I am still unaware as to how it can be noted on the Annex line at the bottom of the load balancing table, that the 25% load is taken via 430.24, but substitutes both motor increases when it only stipulates the largest load to have 25% added.
Click to expand...
430.24 starts off "Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity . . ." Implicitly that means when different conductors are supplying different sets of loads, you do the computation on a conductor by conductor basis. When the largest motor load is supplied by all the feeder conductors, it makes no difference, but when you have a 2-wire motor load on a 3 or more wire feeder, it does.

Cheers, Wayne
 
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