3 Phase Panelboards with Single phase loads?

[gcross6]

Hi,

I have been giving the duty of designing the electrical system for our new company building and I have no experience with a commericial electrical so I was hoping someone out there could give me a hand.

I am trying to size breakers for panelboards in a new commercial building for our company. We have a 347/600V 800A main service coming into our buidling and then will have various transformer and panels in order to provide our desired voltages around the building and shop. We will be using all 3 phase panels and I am wondering how I go about making sure that the main breaker of each panel is sized appropriately.

For example one panelboard is 120/208V with about 45 circuits at 15A per circuit. Since this is a 3 phase panel can we assume that the main breaker for that panel would be 45*15/3 = 225A, therefore the breaker that is equal to or a bit greater the 225A?

As well as sizing the transformers I would just be:
S=root3(225A)(208V) correct?

Thanks in advance for your help and suggestions

 

[X49]

You should size your main breaker for a panel based on the connected load, not the branch circuit breaker ratings. For instance, if you have a lighting circuit that will draw 7 amps, use this figure instead of the breaker rating.

There are code requirements regarding the minimum loading you must assume for general use receptacles. There is also a minimum loading you must assume for an entire facility based on the type of facility and the square footage. Keep in mind that you can only load a breaker to 80% of its rating if the load is considered continous (180A for a 225A breaker.) Be sure to allow some room for future loads.

Let us know what standards apply in your jurisdiction (NEC? CEC?) and you may get more specific answers. You should become very familiar with the codes that apply to your design.

 

[gcross6]

Thanks for the reply. I am using the CEC for my jurisdiction. I was not aware that the main breaker also needs to be sized at 80% of its load. Thanks again for the reply

 

[PHovnanian]

gcross6, the issue of single phase loads in the subject line is also something to consider (and not yet addressed). The connected loads on each phase must be added separately and the breaker feeding that sized to the highest of the phase loads.

Generally, one will attempt to balance the single phase loads on each panel and feeder for reasons other than breaker sizing. Occasionally, one phase ends up with a slightly higher connected load and that may push you to the next wire and/or breaker size.

 

[dpc]

The 80% loading criteria is for continuous loads (three hours or more). I don't know about CEC, but the NEC requires taking continuous loads at 125% of the actual load for sizing circuits so this covers the 80% continuous load issue. You don't have to do it twice.



David Castor

http://www.cvoes.com

 

[X49]

You should become familiar with the following CEC Sections at a minimum:

8-104 - Maximum Circuit Loading (80%/125% loading rule is covered here)
8-106 - Use of demand factors
8-210 - Other types of occupancy (and Table 14) (minimum service size based on square footage)

For transformers, see 26-240. If it is an office space with a large number of PC's or other harmonic generating loads, you should consider using a K-rated transformer.

 

[wareagle]

Gcross
This project is far too complex for you to attempt to design. you need someone who has more experience to assist you.

 

[waross]

X49, is that the California Electric Code or the Canadian Electrical Code?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

Those sections are from the Canadian Electrical Code. CSA C22.1