SCCR calculation help

I am new to this forum and am hoping that someone can help me with what is most likely a very simple SCCR rating calculation.
I have never calculated this rating and I have been reading a lot of material on how to do these Calculations as well as going thru many examples, however I am unsure of how to handle something that is present in one of the panels that I am trying to calculate the SCCR of.

I have attached a picture of a simple circuit that shows what I am looking at.

From what I can tell - we the panel rating would be 65kA based off the main disconnect, however with the disconnect switch on the right side (feeding the 5A 24V power supply) - that would limit the panel to 10kA... or at least that's how I am looking at it. I was thinking that this SCCR rating could be increased by looking at the Peak let thru of the fuse and adjusting the SCCR of the branch circuit accordingly... however I am not sure you can do this because the disconnect switch is listed as having the 10kA SCCR rating IF there is a protecting fuse in line with the switch.
I would really appreciate some advice/help with this.. If any additional information is needed or I am not explaining myself well, please let me know and I will do my best to fill in the blanks.

I don't claim to be an expert but in my understanding of how SCCRs work, the peak let through adjustment applies to feeder circuits only. Also, the SCCR on the line side of the 3A Class CC fuse is based on the BCPDs on the load side of the fuse, which you have none. So, I would think you are stuck with 10kA.

How hard would it be to make the 24VDC power supply single phase and put it in the control circuit? That would jump your SCCR to 65kA.

Also, just curious, what motor starters are you using that have a 100kA rating? Most that I see only have 5kA.

Again, I can't stress enough that I am not an expert. JMHO.

however I am not sure you can do this because the disconnect switch is listed as having the 10kA SCCR rating IF there is a protecting fuse in line with the switch.

Click to expand...

The short-circuit current at any point is dependent on ALL other points. In other words, you cannot view it as only dependent on the input current. A short circuit can also be fed (for a very short time) by other sources such as running motors (one of the largest sources), transformers, any other inductors, and the wires themselves as inductors. The calculations of the short-circuit current at any point is not an easy thing to do manually, and now most rely on computer programs to do the calculations.

Some of the things needed to calcuate the Short-Circuit Current at your "Switch" location would be the max amount of SCC available at point "480 volt Feed to Machine", the size of the motor, the voltage, size, and length of each conductor in the system, and the size of the transformer (known 1.6 KVA). A WAG would be that the short-circuit current at the "Switch" cannot be 65,000 Amps due to the small transformer and the apparently small motor, but may be more than 10,000 Amps due to the power available from the system. If you can buy a switch rated for 45, 50, 55, or 65 kAIC, then do that and forget about it.

He may still need to make the calculation as it's suppoed to be posed on the outside of panels now.

An interesting question is why the two 24 VDC power supplies are being fed from 3-phase 480 volts instead of a 1-phase 120 volt transformer? That would simplify the circuit a lot.

Lancie1 said:

An interesting question is why the two 24 VDC power supplies are being fed from 3-phase 480 volts instead of a 1-phase 120 volt transformer? That would simplify the circuit a lot.

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I don't know in the case of the OP because he has a 480 /120 xfmer in his circuit but if you have no other 120 vac loads then it is best to use 480 supplied 24 vdc power supplies. I do it a lot on systems where all controls are 24 vdc in a panel.

The only drawback to not haveing 120 vac in this case is you can't have a convenience outlet.

It may be just Me but I seem to have a higher failure rate with the 3 phase power supplies

bce123 said:

It may be just Me but I seem to have a higher failure rate with the 3 phase power supplies

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spike or larger voltage fluctuation and parasite noise is more important on the 3 phase voltage VS 120v because you got some kind of isolation/filtration thru transformer...

if you mean SCRR - short circuit current rating OR AIC - amp interrupting capacity, the you can only estimate the value.
a qualified electrical engineer is the only one who can calculate this value and place it on the machine nameplate.

that is what my company had to do on several pieces of weuipment we built.

regards,
james

From what I can tell - we the panel rating would be 65kA based off the main disconnect, however with the disconnect switch on the right side (feeding the 5A 24V power supply) - that would limit the panel to 10kA... or at least that's how I am looking at it.

Click to expand...

To beat a dead horse again, based on the given factors, the panel could be rated to handle short-circuit currents of 200,000 Amps, based on the 20 Amp 200 kA CC fuse (which really is a main fuse because all current passes through it). Therefore, if there is a short circuit, this 20 Amp fuse can interrupt any current up to 200,000 Amps, so that is the SSCR rating for this "panel", assuming that all the electrical devices shown are inside the panel under consideration.