Need direction with calculation

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Rell

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I'm studying for the TX Master Electrician License Exam. Can someone help work this out? Question:

A dress shop has 5,500 sq.ft of space with 15 duplex receptacles installed. What is the total computed lighting demand in amps for the lighting and for the receptacles? Lighting is continuous duty. (3 phase system voltage: 120/208)


My answer is: 5500 sqft X 1.9 = 10450

10450VA

15 receptacles X 180 = 2700 VA


10450va
2700va
=13450 VA

13450VA/208V= 64.6 Amps
 
So divide by 120 vs 208? What if it were 480/277? Do I need to multiply by 1.25 since its continuous duty?
 
Rell said:
13450VA/208V= 64.6 Amps
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If you assume that the VA of 2-wire loads can be balanced around the 3 phases of the 208Y/120V supply, then the above computation should be VA / (sqrt(3) * 208V) or equivalently VA / (3 * 120V). I find the latter easier to remember, as there are 3 separate 120V sources available.

Cheers, Wayne
 
I would say review 220 for lighting an receptacle loads. Tulsa is correct in pointing our that you used single phase, make sure after you have the total load that you are account for it being part of a 3-phase system. Take a look at 220.44 as well (unless it changed for 2023).
 
don_resqcapt19 said:
See the note below table 220.42(A).
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Looks like that's new in the 2023 NEC, so not yet applicable to CA.

Also, my reaction to that note (which says "The 125 percent multiplier for a continuous load as specified in 210.20(A) is included") would be that it would be simplest to multiply the Table results by 1/125% and track them as a continuous load through the calculations, reapplying the 125% where required. Since basically all the rest of Article 220 is silent on the continuous/non-continuous issue, so you have to do that anyway.

Cheers, Wayne
 
Sometimes I get in a hurry and miss that stepas as well . That darn 1.732. Good luck on your test.
We're all here to help any way we can.
 
wwhitney said:
Looks like that's new in the 2023 NEC, so not yet applicable to CA.

Also, my reaction to that note (which says "The 125 percent multiplier for a continuous load as specified in 210.20(A) is included") would be that it would be simplest to multiply the Table results by 1/125% and track them as a continuous load through the calculations, reapplying the 125% where required. Since basically all the rest of Article 220 is silent on the continuous/non-continuous issue, so you have to do that anyway.

Cheers, Wayne
Click to expand...
The OP specified the 2023 code in post #3.
 
wwhitney said:
If you assume that the VA of 2-wire loads can be balanced around the 3 phases of the 208Y/120V supply, then the above computation should be VA / (sqrt(3) * 208V) or equivalently VA / (3 * 120V). I find the latter easier to remember, as there are 3 separate 120V sources available.

Cheers, Wayne
Click to expand...
I am going to share the bold text with the guys. Some struggle with it.
Maybe the VA/ (3* 120) will stick.
Thanks
 
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