SCCR?

[deetz]

This is just a generic schematic that I just modified to show what this machine that came in has. My question is; what would the SCCR be now with that transfomer in the power circuit? Or would it still be based on the lowest down stream component? I would think it would change. Hopefully you can open the file.

 

[waross]

Or would it still be based on the lowest down stream component?

Unless the *ASCC of the new transformer is less than the SCCR of the lowest rated component.
* ASCC Available Short Circuit Current

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

deetz (Electrical)(OP)2 Aug 23 13:00
"....This is just a generic schematic that I just modified to show what this machine that came in has. My question is; what would the SCCR be now with that transfomer in the power circuit? Or would it still be based on the lowest down stream component? I would think it would change....".
1. Your modified schematic is excellent as an illustration " what would be the SCCR after the introduction of the 28kVA 480/415V transformer". Although there is a 480V to 415V voltage difference, but that is irrelevant to the question.
2. It is "understood" or to presume that the ASCR of the 480V power source at that point is >> contactor 5kA and MCCB 14kA .
3. Now with the 28kVA primary is protected up the ASCR, the secondary output SCCR would be very much lower than the source ASCR. How much would be the SCCR is dependent on the transformer kVA rating and the percentage impedance.
4. Based on kVA = 28kVA and say 3% impedance, the SCCR at the output would be:
Full load current I= kVA/1.732 x kV = 28kVA/1.732 x 0.415kV = 38.95A
SC current = I/%Z = 38.95A x 100%/3% = 1298.33A about 1.3kA.
4.1 With the 28kVA transformer, the SC current is reduced to < 1.3kA.
4.2 Therefore, the contactor rated 5kA and MCCB rated 14kA are >> 1.3kA; have NO issue.
5. The contactor and MCCB have NO issue with the Peck asymmetrical short-circuit current and has nothing to do with the power source 480V ASCR.
6. Attention: The 28kVA transformer shall be adequately sized for the motor loads during starting etc....
Che Kuan Yau (Singapore)

 

[LionelHutz]

UL508A has a section on how to figure that out.

 

[deetz]

Thanks for the input. Yeah I'm just trying to figure out, if and how, to get the supply from the building to a lower level to pass inspection. One thought was to maybe move this transformer to the supply side, before the machine disconnect. Or just get another one for that. It's hard to get any assistance on getting the SCCRs of machines up to a decent level. Or help from the purchasers to require a higher SCCR when they order the machine. I thought about piping and wiring one so that the pipe and wire go throughout the plant for as long as I need it to get the level low enough and see what they say when I tell them I had to spend this much for wire and pipe just to do that. Ha.