Basic calculations

[RCoiley]

I need help with a basic calculation please! I am missing something and cannot figure out what it is.. please help

A store building measures 60 ft. by 80 ft. and is supplied by a 240/120V, 3Ø, 4-wire delta system. Which of the following is the minimum allowable number of 2-wire, 20-ampere branch circuits required to supply the general lighting in this building?
(A) 6 (B) 7 (C) 8 (D) 9

 

[oldsparky52]

I suggest you review example D3 (I'm in the 2017 NEC, my newest edition) in the informative annex in the back of the book.

Follow the references.

If you give your thinking and answer, people will respond quicker than you just asking the answer.

 

[Strathead]

the supplied system is not relevant:

60*80=4800 square feet
@3va per foot *4800=14,400 total watts
20a * 120v = 2400 watts per circuit
2400*.8 = 1920 watts of continuous load (the real math is 1920 *1.25 = 2400)
14,400/1920 =7.5 so C is the answer.

That is all the steps, now, since you are obviously studying, Since I gave you the answer and the steps. I challenge you to cite the ohms law for step 3 and the code reference for steps 2 and 4

 

[augie47]

the supplied system is not relevant:


@3va per foot *4800=14,400 total watts

I'm away from my Code at the moment, but aren't offices figured at 3-1/2 watts ?

 

[oldsparky52]

I'm away from my Code at the moment, but aren't offices figured at 3-1/2 watts ?

Offices are 3.5, but stores are still 3.

 

[oldsparky52]

I suggest you review example D3 (I'm in the 2017 NEC, my newest edition) in the informative annex in the back of the book.

Follow the references.

If you give your thinking and answer, people will respond quicker than you just asking the answer.

Or not. ....

 

[RCoiley]

I suggest you review example D3 (I'm in the 2017 NEC, my newest edition) in the informative annex in the back of the book.

Follow the references.

If you give your thinking and answer, people will respond quicker than you just asking the answer.

I suggest you review example D3 (I'm in the 2017 NEC, my newest edition) in the informative annex in the back of the book.

Follow the references.

If you give your thinking and answer, people will respond quicker than you just asking the answer.

Trust me I have reviewed all the examples in the back of the code book.

 

[oldsparky52]

Trust me I have reviewed all the examples in the back of the code book.

So what answer did you come up with and how did you do it? I'm not trying to give you a hard time but this is how you learn and remember these things.

 

[RCoiley]

I'm away from my Code at the moment, but aren't offices figured at 3-1/2 watts ?

Actually it’s a store. So in the code book it says it is 1.9 va/ft2?

 

[oldsparky52]

Actually it’s a store. So in the code book it says it is 1.9 va/ft2?

Where did you get that?

I look at Table 220.12 and see 3 watts per ft2.

 

[Dennis Alwon]

Where did you get that?

I look at Table 220.12 and see 3 watts per ft2.

He is studying the 2020

 

[Dennis Alwon]

Move the heading over so that va/m^2 and Va/ft^2 are over the col. Table 220.12 General Lighting Loads by Non-Dwelling Occupancy � Type of Occupancy Unit Load Volt-amperes/ m2 Volt-amperes/ ft2 Automotive facility 16 1.5 Convention center 15 1.4 Courthouse 15 1.4 Dormitory 16 1.5 Exercise center 15 1.4 Fire station 14 1.3 Gymnasiuma 18 1.7 Health care clinic 17 1.6 Hospital 17 1.6 Hotels and motels, including apartment houses without provisions for cooking by tenantsb 18 1.7 Library 16 1.5 Manufacturing facilityc 24 2.2 Motion picture theater 17 1.6 Museum 17 1.6 Officed 14 1.3 Parking garagee 3 0.3 Penitentiary 13 1.2 Performing arts theater 16 1.5 Police station 14 1.3 Post office 17 1.6 Religious facility 24 2.2 Restaurantf 16 1.5 Retailg,h 20 1.9 School/university 33 3 Sports arena 33 3 Town hall 15 1.4 Transportation 13 1.2 Warehouse 13 1.2 Workshop 18 1.7 Note: The 125 percent multiplier for a continuous load as specified in 210.20(A) is included when using the unit loads in this table for calculating the minimum lighting load for a specified occupancy. aArmories and auditoriums are considered gymnasium-type occupancies. bLodge rooms are similar to hotels and motels. cIndustrial commercial loft buildings are considered manufacturing-type occupancies. dBanks are office-type occupancies. e Garages — commercial (storage) are considered parking garage occupancies. fClubs are considered restaurant occupancies. gBarber shops and beauty parlors are considered retail occupancies. hStores are considered retail occupancies. 220.14 Other Loads — All Occupancies

 

[RCoiley]

He is studying the 2020

Wouldn’t it fall under the retail column? 1.9?

 

[Dennis Alwon]

Wouldn’t it fall under the retail column? 1.9?

Note "H" specifically states that stores are considered retail

 

[Strathead]

I haven't had any interaction with the 2020 code yet. Are they finally adjusting the the lighting levels per the chart Dennis attached in 2020?

 

[RCoiley]

Where did you get that?

I look at Table 220.12 and see 3 watts per ft2.

 

[oldsparky52]

View attachment 2556892

Thank you for that. A change from 2017.

 

[oldsparky52]

I'll bet the question the OP posted is from a 2017 or earlier code edition and the correct answer is not an option.

 

[RCoiley]

I'll bet the question the OP posted is from a 2017 or earlier code edition and the correct answer is not an option.

Based on the 2017 code 8 would be correct. But not on the 2020, that is what was confusing me to no end. I could not get it to work out no matter how many times I worked the equation.

 

[Dennis Alwon]

Are you sure the test you are taking is for the 2020? The website still has it at 2017 and that particular question, as you know, is in the handbook online and is from the 2017 questions.

NC will not go to the 2020 Nec until Oct 1, 2021. They just passed the amendments along with the 2020 NEC this week or last week.