Electrical Service Sizing

[Jimmy4645]

Hello, I need a little help understanding the right way to size an electrical service. I am hearing two ways here at my office and now i really don't know which is the correct way. I think the best way is to explain my situation is to show you the calculation i have come up with and then we can debate whats right and what not.

ELECTRICAL SERVICE LOAD CALCULATION: Fast-Food In Mall Resturant
GENERAL LIGHTS @ 125% = 5,370VA
GENERAL RECEPTACLES @ 100% = 2,280VA
KITCHEN EQUIPMENT @ 65% = 18,635VA
HVAC @ 100% = 3,500VA
VAV ELEC. HEATER @ 100% =19,000VA
NEW DUCT HEATER @ 100% = 30,000VA
LARGEST MOTOR @ 25% = 437VA

TOTAL DEMAND LOAD = 79,222VA
TOTAL DEMAND AMPS = 95.33 AMPS @ 277/480V, 3 PHASE

**Now my question is what size service & MCB do i need?*
Person 'A' at my office is telling me a 100 Amp service (MCB) (the next breaker size up)

Person 'B' at my office is telling me a 125 amp service (MCB) since can not load the circuit breaker more than 80% because 95.33 is way more than 80% you need to go with a 125 amp service....

What do you guys thing??? Thanks for all the help..

 

[Dennis Alwon]

If your calculated load is 95 amps then yes 100 amps would be code compliant. 80% is only for continuous load. If 95 amps were the calculated continuous load then I would agree you need 125amps.

That being said I believe you would be making a big mistake by putting a 100 amp services in. This leaves absolutely no room for expansion. I would do a minimum 125 amps and perhaps 200 amp just because I can.:grin:

 

[petersonra]

If your calculated load is 95 amps then yes 100 amps would be code compliant. 80% is only for continuous load. If 95 amps were the calculated continuous load then I would agree you need 125amps.

That being said I believe you would be making a big mistake by putting a 100 amp services in. This leaves absolutely no room for expansion. I would do a minimum 125 amps and perhaps 200 amp just because I can.:grin:

or leave provision to add another 100A pb in parallel.

my experience is that you will either decide to live with what you have and than need more, or add more just in case and never need it.

my point - its a crap shoot whether you will even need all of what you already have.

 

[Jimmy4645]

I understand that you should always leave room for future expansion, but this service is existing. So, pretty much all i have is a 100 amp service to work with.

Thanks

 

[satcom]

What do you guys thing?

I think you have a commercial service, and as Dennis said "I believe you would be making a big mistake by putting a 100 amp services in. This leaves absolutely no room for expansion. I would do a minimum 125 amps and perhaps 200 amp just because I can"

 

[LarryFine]

I would go with 200a because the labor is the same and the materials are barely more.

I've done services where the load increased while it was still under construction.

 

[kfenn22]

I agree with Larry on the 200A.
How offend do you see restaurants add new cooking equipment.
I noticed it all the time.. Add another fryer, oven or what ever...

It is money well spent..

 

[Jimmy4645]

My main concern is with the calcuation i provided provided am i code compliant using an existing 100 amp service...

If i was engineering from new yes a 200 amp service makes lots of sense..

 

[petersonra]

I understand that you should always leave room for future expansion, but this service is existing. So, pretty much all i have is a 100 amp service to work with.

Thanks

If thats all there is, and it is adequate, you have answered your own question.

You might want to point out the potential benefits of easier accommodation of future loads to the customer. Its his money, let him decide.

 

[dnem]

My main concern is with the calcuation i provided provided am i code compliant using an existing 100 amp service...

Maybe, maybe not
You have to look at 2 things: 1] load calc + 2] breaker rating
Looks like you already handled the load calculations. . Make sure you multiple certain loads x125%.

For the service calc, you only have to look at continuous load for certain items, see 220.14(C). . For feeders and branch circuits, you have to consider the 3 hour or more usage of everything, see 215.3 and 210.19(A)(1).

For the breaker rating, I'm looking at the '07 UL Whitebook, Molded Case Circuit Breaker Marking Guide section page 17, item 36:
100 Percent Continuous Rated - Unless otherwise marked for continuous use at 100 percent or its current rating, a circuit breaker is intended for use at no more than 80 percent of its rated current where in normal operation the load will continue for three hours or more. . A breaker with a frame size of 250 A or more, or a multi-pole breaker of any current rating greater than 250 V, may be marked to indicate it is suitable for continuous use at 100 percent of its current rating. . The marking is "Suitable for continuous operation at 100 percent of rating only if used in a circuit breaker enclosure Type __ or in a cubicle space __ by __ by __inches" or an equivalent statement. . This type of breaker may also be marked to indicate it is to be used with wire sized for a 75?C conductor with 90?C insulation and used with 90?C wire connectors.

 

[haskindm]

You have already compensated for the 80% Continuous load rating by multiplying the lighting load by 125%.
I do have a couple of questions?
1. Is this the actual lighting load or the VA per sq. foot lighting load? You will need to figure it both ways and use whichever is greater (at 125% since it is a continuous load).
2. Is there a show window? You will have to figure 200VA per foot of show window, whether or not you intend to use it.
3. You also need to figure a sign circuit.
4. Is the receptacle load based on the actual number of general use receptacles?