Multi family dwelling receptacle branch calc

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Jpflex

Jpflex

Electrician big leagues
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Victorville
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Electrician commercial and residential
My national electrical code calculations for electrical exam prep book has a question with an answer that seems to be in error. The question and answer is as follows:

17) a multi family dwelling with cooking facilities provided for tenants, having 20,000 square feet of living area, is required to have at LEAST _____ 20 ampere, 120 volt general lighting and general use receptacle branch circuits?

Book answers D 25 circuits

20,000 ft x 3 VA

60,000 / 120volts X 20 amperes (24,000 VA but they meant 2400 VA obviously) = 25 circuits



Mistakes in book to my knowledge?

1) was first mistake mistype 24,000 VA instead of 2,400 VA

2) they did not include demand factors in NEC table 220.42

The answer I got was 20,000 ft x 3 VA = 60,000 VA

Then add 2x small appliance branch circuits 2 x 1,500 VA = 3,000 va

Then add 1 laundry branch circuit at 1 x 1500 VA

Add total 60,000 VA
PLUS. 3,000 VA
PLUS. 1500 VA

= total connected load 64,500 VA

FINALLY apply demand factors which I DO NOT UNDERSTAND why book did not?

64,500 VA - 3,000 VA (to be taken at 100%)

= 61,500 VA

Take 0.35 percent of this remaining figure = 21,525

ADD This remain to first 3,000 VA at 100% to get a total demand of 24,525 VA


FINALLY to obtain the number of required 20 ampere branch circuits

Divide total connected load 24,525 VA by 20 amperes x 120 volts (2,400 va)

= 10.21 or 11 circuits?

What is right answer or method? Do you use demand on multi family dwellings for general lighting and rec loads? Thanks
=
 
25 circuits seems like the correct answer to me.

The question asks about general lighting and general use receptacle circuits, so that wouldn't include laundry and small appliance circuits.

220.11 requires branch circuits to supply loads calculated in accordance with 220.10.
 
david luchini said:
25 circuits seems like the correct answer to me.

The question asks about general lighting and general use receptacle circuits, so that wouldn't include laundry and small appliance circuits.

220.11 requires branch circuits to supply loads calculated in accordance with 220.10.
Click to expand...
So if you eliminate the 2 small Branch circuits and the laundry circuits you still get 20,000 x 3va = 60k - 3k (At 100%) = 57k x .35% = 20k plus 3k = 23k / 2400 va = 9.5 circuits still

Still doesn’t add up
 
They specifically mention general purpose circuits. so I would not take S/A, laundry, etc into account and 25 would be my choice of answers.
20,000 x 3/2400
Any demand factor would apply to feeder & service calculations not to the number of circuits.
 
Jpflex said:
So if you eliminate the 2 small Branch circuits and the laundry circuits you still get 20,000 x 3va = 60k - 3k (At 100%) = 57k x .35% = 20k plus 3k = 23k / 2400 va = 9.5 circuits still

Still doesn’t add up
Click to expand...
220.42 General Lighting.
The demand factors specified in Table 220.42 shall apply to that portion of the total branch-circuit load calculated for general illumination. They shall not be applied in determining the number of branch circuits for general illumination.
 
Jpflex said:
So when determining number of branch circuits you don’t include demand factors or small appliance or laundry demand?
Click to expand...
No, for the context of the question.
Another way to do calc shown in annex d
(20,000*3)/120/20= 25 this is in line with the question.

Look in annex d multi family dwelling.

" a multi family dwelling with cooking facilities provided for tenants, having 20,000 square feet of living area, is required to have at LEAST _____ 20 ampere, 120 volt general lighting and general use receptacle branch circuits?"
 
Jpflex said:
So when determining number of branch circuits you don’t include demand factors or small appliance or laundry demand?
Click to expand...
Demand factors are for feeders and services, and work because of the load diversity on the branch circuits.
 
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