Sizing Circuit Breakers for Transformer

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Jimmy4645

Member
Can you guys confirm if I sized transformer, primary and secondary circuit breakers correct?

Primary Voltage: 480 volts, 3 Phase
Primary Load: 231.97KVA (279.1 Amps)

Secondary Voltage: 120/208 volts, 3 phase
Seconard Load: (644.36 Amps)

I selected a standard 225KVA transformer.
Primary MCB: 350 Amps
Secondard MCB: 800 Amps

I was told that my circuit breakers sizes are too high and exceeded the transformer capabilities Are my calculations correct? Thanks
 

squaredan

Senior Member
Location
Pennsylvania
Sizing Circuit Breakers for Transformer

hi jimmy,
I see that your primary load(231.97kva) is larger than what you have selected(225kva)...Is that correct?

Dan
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Your breaker sizes are fine...in fact the code would permit you to use a 600 amp on the primary with the 800 amp on the secondary.
I do have the same question as Dan, as to the load being greater than the rating of the transformer.
 

Jimmy4645

Member
Your breaker sizes are fine...in fact the code would permit you to use a 600 amp on the primary with the 800 amp on the secondary.
I do have the same question as Dan, as to the load being greater than the rating of the transformer.
Yeah my load (237.97KVA) is greater than the (225KVA) transformer. Do you guys think I should go with a 300KVA transformer?
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Have you considered demand and diversity in your load calc? I've dealt with overloading transformers in the past but it's usually to avoid replacing an old transformer - I'd suggest putting in a 300KVA specially if there's possibility of some future growth.
 

Jimmy4645

Member
the load represents the demand. I think it will be a good idea to go with the 300Kva because I'm not leaving any room for growth. What do you think the primary circuit breakers should be if i go with a 300KVA
 

kingpb

Senior Member
The easy answer would be to go with 250% on the HV side and 125% on the LV side. However, that is not necessarily the correct answer. The actual selection will depend on the protection settings available on the breakers you select.

The reason is that with a transformer you have multiple issues that the breakers need to address, e.g. FLA, Inrush, and thermal damage. The HV breaker (480V) needs to meet all these criteria simultaneously, whereas the LV side (208V) does not need to be concerned with Inrush, but does have to provide FLA and be under the thermal damage curve.

You most likely will need something with adjustable trip protection of long time, short time, instantaneous, and ground fault (LSIG). Additionally, the long time setting will need to have an adjustable band capability, not fixed, otherwise you'll have a difficult time getting under the thermal damage curve. Keep in mind that breakers with this type of trip device are 100% rated.

Type of devices that I used and was able to model and coordinate the protection where breakers using the Eaton-CH OPTIM 750/1050 SPB, I'm sure there are other manufacturers as well.
 

Mike01

Senior Member
Location
MidWest
Keep in mind that breakers with this type of trip device are 100% rated.
If this you were to use a standard molded case cirucit breaker with electronic trip settings are you sure the device is 100% rated? I believe if you want a 100% rated device as a MCCB you better make sure you specify it or denote it on your one-line as 100% rated.
 
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