Need help with calcs

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sjaniga

Member
Ive been asked to calculate the total load of a laundry mat we are going to be doing. the service is 120/208 3-phase, and I have all of the loads for the equipment, lighting, and hvac.
do I just add the full load draws for the 3-phase equipment, and for the single phase loads do I add, then divide by 3? Also for the hvac I just need the larger of the heating or cooling?
and for convienece recepts it is 180 va?
any suggestions would be helpfull, thanks.
 

bphgravity

Senior Member
Location
Florida
Re: Need help with calcs

1. Use Table 220.3(A) and multiply total lighting load to total square footage. (Maybe stores?)
(Continuous loads at 125%)
2. No lighting demand per 220.11
3. Compare heat to A/C, omit smaller (220.11)
4. Heavy-duty lampholders at 600va. each (220.3(B)(5).Other outlets at 180va each (220.3(B)(11) multioutlet assemblies each 5-foot at 180va each (22.3(B)(8) Show window lighting each linear foot at 200va each (220.3(B)(7).
5. Demand for receptacle loads over 10kva. (220.13)
6. Demand for kitchen equipment (if any) (Table 220.20)
7. Increrase largest motor by 25% (220.14)
8. Size the service by dividing the total va by the applied voltage x 1.732. Use Table 310.16 to find conductor size under 75 degree colum. Use Table 250.66 to size grounding conductor. (It cannot be smaller than required grounded conductor.) (250.24(B)(1).

I am not presuming that you needed all this information, but I thought I would cover the whole gamut for anyone that could benefit. Good Luck! :)

[ May 02, 2003, 05:21 PM: Message edited by: bphgravity ]
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: Need help with calcs

I suggest that you go through NEC section 220 in detail. It tells you how to account for lighting loads, dedicated circuits for specific applications, general receptacles, and even dryers. It takes you through the process that Bryan has described. But you need to take particular note of the paragraphs that talk about demand factors for dwelling units. One key example is in 220.18. These demand factors will not apply to your facility, since it is likely that all dryers will be running at the same time (not as likely in an apartment building). And please note that nowhere (or I should say rarely) in the mathematics of 3 phase systems do you divide by 3. You may frequently come across the use of the square root of three, when converting phase-to-phase quantities against phase-to-neutral quantities. But stick with units of VA when you are adding loads.
 

sjaniga

Member
Re: Need help with calcs

Bryan, Thanks
yet one ? is it VA divided by (V x 1.732)
or (VA divided by V) x 1.732
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
Re: Need help with calcs

Sjaniga,
? is it VA divided by (V x 1.732)
or (VA divided by V) x 1.732
Your first fomula is correct and
your last formula would be correct if it was this way,
(VA divided by V) / 1.732, then either way would be the practically the same.


Roger

[ May 02, 2003, 06:43 PM: Message edited by: roger ]
 

lady sparks lover

Senior Member
Re: Need help with calcs

i'd be happy to help, for a price for 110 dollars an hour plus overhead.

just kidding, you know I had to bring in a price...i'm surprised that the inspector didn't ask for drawing. wow...that's odd...

I agree with charlie b...the code goes through all those calcs in section 220.


Lady :)
 
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