Updated 480 volt machine wiring

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ADub said:
All this for an air compressor? Hopefully this isn't something they needed in service immediately.


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It is a lot for a compressor (mind you its quite a complicated one.) The unit is in place and working as we type but it has a failing / melting lug in the disconnect. When I went there to look at a simple disco swap out, I noticed it was only rated for 240 Volts. There are many other issues as well. I am tiring to re-figure everything because the original installer did not have a clue, and, do it before that disco fails. The unit runs all the air driven machines in this large manufacturing facility. Time is of the essence because if the disco fails, everything stops. I have a shut down planned for this Saturday to do the work.
Any and all input is appreciated.
 
kwired said:
It is typical to need to pay attention to series ratings at service equipment for the main device and branch devices. But at same time series ratings are needed throughout the entire system, but also remember that available fault current is not same at the end of a conductor as it is at the beginnning of a conductor and even the resistance in a run of only 25 feet can significantly reduce the amount of available fault current and make the difference between needing say a 25kA instead of a 35kA rated device. Once the available fault current has been naturally reduced because of circuit impedance, those series ratings are not as important as they are when the circuit impedance doesn't have any impact on available current.
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Thanks kwired,
I will calc the fault current but would be very supervised if it was low enough with out changing something. The service is 2 feet away. I went to the site today but I could not find SCCR ratings on or in the equipment. Unfortunately for me, the power co will take days to get back to me and I must get the non stock fuses coming for work Saturday. I think the calc will prove to under 5K for both units after wire and CL fuses. I kinda felt that from the start. I think what others are tiring to get through my thick head is that the fuses are not tested as part of the equipment (raise SCCR) or part of the service (lower AIC) and therefore it falls into a non category. I do not think they are saying they will not work, just that it cannot be tested and certified to work. For now I have no other plan. The unit is there connected and running, I need to make it as safe as I can as fast as I can.
 
petersonra said:
At this point all you may be able to do is to replace the existing disconnect switches with appropriate ones.

that is a start anyway.
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Thanks Bob, That is my start but they lack OCPDs sized to each unit. So the fuses were to fix that as well. Everything was double lugged, without any bonding, and the 100A breaker was the OCPD for everything.
 
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florida-sparkey said:
Thanks kwired,
I will calc the fault current but would be very supervised if it was low enough with out changing something. The service is 2 feet away. I went to the site today but I could not find SCCR ratings on or in the equipment. Unfortunately for me, the power co will take days to get back to me and I must get the non stock fuses coming for work Saturday. I think the calc will prove to under 5K for both units after wire and CL fuses. I kinda felt that from the start. I think what others are tiring to get through my thick head is that the fuses are not tested as part of the equipment (raise SCCR) or part of the service (lower AIC) and therefore it falls into a non category. I do not think they are saying they will not work, just that it cannot be tested and certified to work. For now I have no other plan. The unit is there connected and running, I need to make it as safe as I can as fast as I can.
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Do you know what kVA rating of the POCO transformer is? You can use that as a starting point to at least get a "worst case" estimate of available fault current. You would also need to know transformer impedance, but one can also use some industry averages for a similar sized unit for estimate purposes. You can then find SCC at the transformer terminals with an assumed infinite supply ability from the primary side, and then next need to know conductor size, type, length, number of parallel sets... and figure how much the SCC drops between the transformer and service equipment, then repeat for each segment up to the component you are interested in if there are additional feeders/branch circuits between your equipment and the source.

Mike Holt does have a spreadsheet on his free stuff portion of his main site that is great for doing these calculations, just plug in details and read the output. You can also plug in different numbers to see how a certain change (like different conductor size or length) may effect available current at a specific point.
 
florida-sparkey said:
It is a lot for a compressor (mind you its quite a complicated one.) The unit is in place and working as we type but it has a failing / melting lug in the disconnect. When I went there to look at a simple disco swap out, I noticed it was only rated for 240 Volts. There are many other issues as well. I am tiring to re-figure everything because the original installer did not have a clue, and, do it before that disco fails. The unit runs all the air driven machines in this large manufacturing facility. Time is of the essence because if the disco fails, everything stops. I have a shut down planned for this Saturday to do the work.
Any and all input is appreciated.
Click to expand...

I understand. We run 4 100hp compressors in one of our facilities and when one goes down it's mayhem. I like to park a tow behind unit near the door to the compressor room whenever we're working on the units in case anything happens


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Follow up

Follow up

I got the AFC at the bushings of xformer and did a conservative calculation. Lots was estimated but I ended up around 3.5K at the small machine due to the smaller wire. I could not get it below 7K for the larger machine. I played with adding wire in the program, but it got to the point that as I added so much wire, I had to go up for voltage drop and that brought the fault current back up. A cycle.
I would think if a real calculation were made, taking into account every connection, busbar and so on that it might be real close. What else could I do, inductor is not an option on the motor and the unit is already there so having the SCCR redone is no option. I feel it is safe and do not want to alert the plant manager about what I think is now a minor issue.
Thanks so much for the input!
 
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