Sizing motor control center

[lilsparky]

I'm back with another MCC sizing question.

I'm upgrading a facility that is fed from an existing outdoor 500 kva transformer, however I don't know the aic rating of this transformer. Is this very important in sizing the MCC? By the way the MCC is going to be used as the incoming service to this facility.

 

[Jraef]

... I don't know the aic rating of this transformer. Is this very important in sizing the MCC? ...

Important for sizing? No.
Important? Absolutely! You must have bracing and interrupt capacities that meet of exceed the AFC*.

*AFC = Available Fault Current. Technically, a transformer cannot have an "AIC" rating, AIC means "Amps Interrupting Capacity" and applies ONLY to devices that interrupt current during a fault, i.e. fuses or circuit breakers. AFC is what is available at the line terminals of the MCC, and is what you base the Fault Bracing of the bus bars and the AIC of the units from. So for example, some brands of MCC have fault bracing of 4kA, 65kA or 100kAle availab, but some invividual units might be rated only 22kAIC or 35kAIC. So if you have 45kA AFC, you cannot use those buckets and in fact must use 65kA bracing.

 

[weressl]

I'm back with another MCC sizing question.

I'm upgrading a facility that is fed from an existing outdoor 500 kva transformer, however I don't know the aic rating of this transformer. Is this very important in sizing the MCC? By the way the MCC is going to be used as the incoming service to this facility.

A 500kVA transformer is not likely be a problem as most of the MCC's and individual units produced nowdays have a fault rating that will exceed the available fault current even at infinite Utility Max SCA. Nevertheless actual numbers must be put to the case and documented.

 

[lilsparky]

Thanks guys, I will see if the folks on site can get me the id tag so I can call the manufacturer.

 

[Jraef]

The transformer name plate won't tell you that, it's calculated. There are a number of ways to calculate it, but does the facility have an EE? Or if you are talking about a utility transformer, the utility will give you the AFC.

But weressel is right, for the most part a 500kVA transformer isn't capable of delivering much AFC, so only the cheapest crappiest MCCs will be a problem. You didn't post anything about brands or such.