Office Lighting Calculation

[bigpapa400lb]

I'm trying to figure out this question.

An office building with 2500 sq has 42 general purpose receptacle outlets. What is the minimum number of two wire, 120v 20 amp branch circuits required to supply the total lighting and receptacle load for this office?

Answer key says 8 but I forgot how to calculate from table 220.12.
So how do you figure out the 8 circuits?

 

[chris kennedy]

Find the lighting load in T 220.12. Then see example D3 for some clues to the calc.

 

[Smart $]

. .

 

[Jim Shorts]

2500 x 3.5 va = 7500 x 125% = 9375 va (lighting Table 220.12 & 210.20(A))
42 x 180 va = 7560 va (receptacles 220.14(I))
9375 va + 7560 va = 16,935 va (total load) / 120 volts = 141.125 amps / 20 = 7.05 circuits (round up to 8)

 

[bigpapa400lb]

I see the calculation in D3.

They way I have it calculated is:

2,500 x 3.5 = 8750 x 1.25 = 10938 for general lighting
42 receptacles x 180v = 7560

 

[cpal]

Soooo??
How many 1P-20 BC,s????

 

[Smart $]

2500 x 3.5 va = 7500 x 125% = 9375 va (lighting Table 220.12 & 210.20(A))
42 x 180 va = 7560 va (receptacles 220.14(I))
9375 va + 7560 va = 16,935 va (total load) / 120 volts = 141.125 amps / 20 = 7.05 circuits (round up to 8)

You can't combine loads in the determination (though it may work out to the same result in the end... but it also may not).

 

[bigpapa400lb]

2500 x 3.5 va = 7500 x 125% = 9375 va (lighting Table 220.12 & 210.20(A))
42 x 180 va = 7560 va (receptacles 220.14(I))
9375 va + 7560 va = 16,935 va (total load) / 120 volts = 141.125 amps / 20 = 7.05 circuits (round up to 8)

2500 x 3.5= 8750 x 1.25= 10938 ?

 

[Jim Shorts]

You can't combine loads in the determination (though it may work out to the same result in the end... but it also may not).

Why can't you combine loads? There is no code requirement to put plugs and lights on separate circuits. Sorry about the typo I didn't multiply by 3.5, I used 3 on my calculator.

 

[Smart $]

Why can't you combine loads?

To tell you the truth, now I'm not certain you have to. Perhaps I formulated the opinion because the example calculations in Annex D always separate them when they are a separate branch-circuit calculation. In actually looking for such a stipulation, I cannot find one. Anybody know of one?

There is no code requirement to put plugs and lights on separate circuits.

True... but what does that have to do with calculating the minimum number of branch circuits, especially when you only have to install enough to serve the connected load for lighting rather than the minimum number required when the connected load is less than the calculated load (i.e. both used and unused spaces in panelboards and allowance in the service/feeder calculations cover the number requirement, not the actual number of installed circuits—making one wonder how many times one checks upon adding a load to see whether an unused space is reserved for a calculated load, don't it?).