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#26722 - 06/19/03 01:01 AM exam question  
mj  Offline
Member
Joined: Jun 2002
Posts: 186
meriidian, ms
question; an office building has a total area of 50,000 sf, the actual number of gen-purpose receptacle outlets is unknown and the connected load equals the calculated load. how many 120 volt,2-wire,20 amp branch circuits are for the lighting and receptacles ? I missed this one on a practice exam. the options were,21,92,112,or 118. can someone could show me the calculations,.... thanks


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#26723 - 06/19/03 08:37 AM Re: exam question  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
How about 92...

50,000 sq. ft. X 3.5 VA (from Table 220.3A)=175,000 VA required.
175,000/1920 (16 amps continuous X 120 Volt)= 91.145

What did you get?


[This message has been edited by Redsy (edited 06-19-2003).]


#26724 - 06/19/03 01:23 PM Re: exam question  
mj  Offline
Member
Joined: Jun 2002
Posts: 186
meriidian, ms
thanks,redsy, I fogot to change the 20amp ckt to 1920 va.


#26725 - 06/19/03 01:27 PM Re: exam question  
mj  Offline
Member
Joined: Jun 2002
Posts: 186
meriidian, ms
do you have an example on using table:220.34 for selecting feeder/service conductors for a school, there are no examples for this in the nec handbook ?


#26726 - 06/19/03 02:42 PM Re: exam question  
dana1028  Offline
Member
Joined: Jan 2002
Posts: 163
San Carlos, CA
Look at Table 220-3(a), if you look at the table for office buildings it gives you a value of 3.5 v/a per sq. foot BUT there is a footnote 'b'...if you read this footnote it indicates you must ADD an additional 1 v/a per sq. ft. for general-purpose receptacle outlets where the actual number of receptacle outlets is UNKNOWN.

Therefore you have a total v/a per sq. ft. of 4.5 calculated as follows:

3.5 v/a x 50,000 sf. = 175,000 va x 125% (lighting is considered a continuous load) = 218,750 v/a + 50,000 v/a (1 v/a per sf for unknown receps) = 268,750 va/120v = 2239.58A /20A ckt rating = 112 circuits

Go to RKeiss' website (www.elec-toobox.com), in his 'Code Quiz' section, May, 2001, quiz question #6 - he takes you through the steps of computing the answer for the same situation you asked.

[This message has been edited by dana1028 (edited 06-19-2003).]


#26727 - 06/20/03 09:16 PM Re: exam question  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Good catch, dana.
But, I don't understand why the 125% is only added to the basic 3.5 va/sq.ft. and not the additional 1 va/sq.ft.

I still would suggest the following...

50,000 x 4.5 = 225000
225,000 / 1920(80% of 120x20) = 117.19 (118 ckts.)
OR...
50,000 x 4.5 x 125% = 281,250
281,250 / 2400 (120x 20) = 117.19 (118 ckts.)


#26728 - 06/20/03 10:03 PM Re: exam question  
dana1028  Offline
Member
Joined: Jan 2002
Posts: 163
San Carlos, CA
because they are applying the 1 va to the recep load and applying the 3-1/2 va to the lighting load...lighting loads are considered continuous loads in non-dwelling occupancies....if you read the title to the table " General Lighting Loads"

...per your calculations - 117.19 is rounded down, not up (see 220-2b)...117 is not one of the options for an answer.



[This message has been edited by dana1028 (edited 06-20-2003).]


#26729 - 06/21/03 07:44 AM Re: exam question  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Thanks for the clarification, dana.



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