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#75532 - 03/01/07 02:56 PM Load calculation
ToHo Offline
Registered: 12/29/04
Posts: 46
Loc: N.Y.
Hello everyone....
Looked at a job today and needed to do a load calculation on the size of service this job will require. Customer has existing 200amp 3 phase 208v service. 7500 sq ft building. Built as a warehouse but being converted to a printing shop.It's been some time since the last load calculation so I want to make sure I'm not missing anything.Any hel[p in double checking my numbers is greatly appreciated.Here are my numbers w/ the proposed loads. Thanks ToHo

7500sq x 3.5 x 1.25% = 32812.5va
(AC Unit)60a x 220v = 13200va
(50 receptacles)x 180va = 9000va
TOTAL = 55012.5va
Other Outlets
50a x 220v = 11000
50a x 220v = 11000
50a x 220v = 11000
30a x 220v = 6600
30a x 220v = 6600
15a x 220v = 6600
15a x 220v = 6600
Total va = 52800
1st 10000va @100% = 10000
42800 @ 50% = 21400
60a x 220v(AC #2) x 25% = 16500
Grand Total of = 102912.5va
102912.5va / 360v = 285.8a
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#75533 - 03/02/07 07:19 AM Re: Load calculation
Jps1006 Offline
Registered: 01/22/04
Posts: 615
Loc: Northern IL
Are you sure those RTU's aren't three phase?
How about your 50a, 30a, and 15a loads? single or three phase?
#75534 - 03/02/07 08:50 AM Re: Load calculation
ITO Offline
Registered: 11/26/06
Posts: 348
Loc: Texas
1) Lighting Load- (220.14-D) I don’t think you can use the 3.5va per foot. Print Shops and other production facilities are not covered under T220.12. Take a look at 220.14-D, you will need to use the actual lighting load for this calculation. (125%)

2) Receptacles- (220-14 I & L) The 180va is correct: 50 x 180 = 9,000va

3) Demand Factors for Receptacle load- (T220-44) Since you only have 50 x 180 = 9,000va this will note apply.

4) Water heater- (220-14 A) Will there be one? (125%)

5) Outside lighting- (220-14 F & 600.5 A) Will there be any? If yes 1,200va (minimum) @ (125%)

6) Heat/AC- (220-60) The larger of the 2 (100%)
Note: You can’t use the breaker size to determine the load, you will need to look at the name plate on the equipment.

7) Equipment- (220-14 A) List each piece of equipment and again you can’t determine the load by breaker size calculation, you need to get this info from the name plate or a cutsheet, and list them out individually. (100%)

8) Sign circuit- You have to do it, per (220-14 F): 1,200va @ (125%) = 1,500VA

9) Show window- (220-14 G)

10) Largest Motor- (220-14 C)- Add 25% of the largest motor load.

11) Add it all up and divide by 360 (208 x 1.72)

Edit- Water heater is 125%

[This message has been edited by ITO (edited 03-05-2007).]
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#75535 - 03/02/07 09:21 AM Re: Load calculation
renosteinke Offline
Cat Servant
Registered: 01/22/05
Posts: 5316
Loc: Blue Collar Country
I have a little trouble following those figures. It almost looks like you're "adding up the breakers."

I would approach a load calc a bit differently.

First, I would break up the area into different "occupancies," and apply the square foot calculation for my general lighting load.

I would add up all the general-purpose receptacles, and apply that watt multiplier.

Finally, I would look at specific equipment, and use the nameplate ratings.

I would review the plans, and the operations ... looking for continuous loads, additional lighting, etc.

This brings us back to the sq. ft. multiplier you started with; always calculate your known lighting load, and compare it to the per sq. ft. figure. The sq. ft multiplier is a minimum; you may very well exceed it. That sq. ft. multiplier is pretty much a useless figure, unless the actual loads are unknown (as in a commercial rental).

Let me give a general example. Assume a 20 amp circuit with 6 duplex receptacle. I would NOT base my calculations on 20A x 120V; rather, I would use 6 receptacles x 180 watts.

Or, if my shop lighting uses 10 fixtures, that were marked 1.1A on the ballasts, my calculation would be 10 x 1.1 x 1.25 for continuous use.

Likewise, the calculations are quite different for three phase than for single phase. It is important also to use the 'nominal' voltage of the system.
#75536 - 03/02/07 10:53 AM Re: Load calculation
ToHo Offline
Registered: 12/29/04
Posts: 46
Loc: N.Y.
All the loads are single phase 220v. I assume the company is moving from a facility that had only 120/220v single phase service.So from the info I have recieved there are no 3 phase loads.The existing AC units are single phase 220v. Why I'm not sure !!! Again.. Thank you .. all your help is appreciated. I have to get some name plate info from the supplier of the equipment. ToHo
#75537 - 03/04/07 03:26 AM Re: Load calculation
jdo1942 Offline
Junior Member
Registered: 11/13/05
Posts: 31
Loc: st. louis, mo. usa
Any electrian worth his salt uses the code book for the base of his calculations. First go to chapter 9 in the code book and use the the examples that will refer you back to the 220 area of the code. Use these for your base, either the standard calculation or the optional. But you must use common sense, and remember the code book calculations are a minimum standard, if you feel that the service that you need is more than the minimum based on continious loads or other factors, you than must increase the service. An example is a motel with PTAC units, do size to a per centage or do size for the worst case, a zero degree day and the motel is full. You as the electrian have to make the decision.

Joe Ogle
#75538 - 03/04/07 11:44 AM Re: Load calculation
electure Offline

Registered: 12/24/00
Posts: 4259
Loc: Fullerton, CA USA
I believe ITO's covered it well.

Something that a whole lot of people need to keep in mind is that there is no 220V.
I see otherwise accomplished electricians talking 110 and 220 in their advertisements and everyday speech, and I just don't understand why.

From 220.5(A) Voltages.
Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal
system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.

[This message has been edited by electure (edited 03-04-2007).]

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