Advice for Circuit Design for Cafeteria

Because of the second 80A oven, the total connected new load is 503.2A, rather than 415.2A. However, if both ovens do not operate at the same time, or other diversity factors apply, you may be able to design for less than 503.2A.

After this is clarified, there will be some other things to recheck.

1) Consider the ampere rating of second oven

2) consider ampere rating of the spares

3) calculate total load current

4) calculate required kva of transformer based on ur sec. voltage and load current

5) choose the transformer accordingly

6) calculate primary current of transformer

7) select protective device for transformer based on tht calculation......(since its a step down transformer your primary current will be smaller thn secondary)...

thanks engeneers,

Ok if there is one oven only will be operated at 88A and we keep the total current load = 415.2A. If 40% spare will be added wich it means 166.08A spare so the total current will be 581.28A, I will choose here 630A main circuit breaker for the circuit. Moreover, with the voltage = 277V the load will equil = 581.28A X 277V= 161kVA. Accordingly, I proposed 225kVA transformer 480/277V. at this step I need to determine the CB for the primary side of transformer and install this CB in a main distribution panel which is already loaded with 750.56A and the load = 360kVA. However, I choos 800A CB but I'm not sure if it appropriate CB or not.

Please advice how can I calculate for the primary CB

i forgot to mention..........while calculating take care abt the inrush current in transformers.......i think watever u ve chosen is correct jus consider above mentioned points......if possible provide a differential protection for the transformer which has to overcome transformer inrush current............

If the load amperes you have indicated for each of the listed loads are 3 phase loads you have indeed a big group of cafeteria equipment load. I guess not all of your loads indicated are 3 phase equipment. Some are probably single phase. In that case you have to balance your loads in each of the phase in your panelboard. You will realize that if you do it correctly you will end up with a much smaller total ampere load and KVA of course). Also when calculating the transformer kva size, it is based on the total demand load which is less than the total connected load. Not all the equipment operate at 100% demand at the same time.

harmonic filter

Good points harmonicfilter, the numbers on face value imply such an enormous load that I wonder about their origin. This is not a difficult engineering task once one knows the real values involved. This leads me to two conclusions that I cannot decide which is true. Either this is a student's homework problem or somebody doesn't understand a proposed or a working design's electrical loading/budget configuration.

If I am not mistaken, NEC requires that you list and accomodate the nameplate FLA of every device in the circuit. You cannot assume that some equipment will not be operating when sizing service.

But then, we don't know where this is, and I am NOT an electrical engineer.

At the bottom of your table you list "Total Req Load @220 V @3PH 415 A"

220 V X 415 A X 1.73 = 158 KVA and not 91 KVA. If you add 40% to this figure you would have 158 KVA x 1.4 = 221 kva and 581 amps.

Thanks all,

Eng. wareagle, thanks for your correction and regarding of the new values we still can use the same rating of main CB 600A and for the transformer load we can use same chosen rating load 225kVA or we can go with 300kVA.

Moreover, Eng. SHAMITHGAMBHIR1 gave a good point which is inrush current for the transformer where I couldn't till now know how to get it, (I need help if anyone can tell me how to get it).

According to Branches CB poles I proposed 1P for all breakers.

Sorry to say the following:

You are not qualified to work this out! Please consult an electrical engineer whose job is designing power circuits to supply such installation. It is dangerous, both for your costing and safety to try and work this out withut the proper knowledge! The amount of load you mentioned is too great for just an amateur to handle!

Please be wise and not foolish to attempt this exercise: No one can help you remotely when you are confusing the amps and the phases...etc...Inrush current etc...

Good luck!?

thank you for your view, I appreciate. However, I think you have a good knowledge to share it with us better than estimating my qualification.

hope to concentrate on the main query

I spent 10 years as a commercial appliance tech. and would suggest to the owner that he purchase a LOW TEMP. dishwasher that uses a chemical based sanitizer and not hot water . This would save approx. 150 amps. The unit cost is less than half of a booster type unit and is much better all around .? Or he could lease the system like everyone else does.?

He should also go with a gas water heater .?

Dont get this .