Refrigeration Rack with 10K SCCR and actual Available Fault current of 39KAIC

[Pullnwire]

I have a project where the refrigeration contractor is installing a self contained 500HP refrigeration rack. It has an SCCR rating of 10K. My available fault current is 39K. The rack has a main disconnect built in, but its not fusible. Can I install a 1000amp fusible disconnect to protect the equipment, as the fuses have a much higher AIC rating? If not, what are some options? The rack is already ordered and in production and I'm still in design phase.

Brett

 

[augie47]

It's a tale we see often on this site. At that amperage it's doubtful the length of your run is going to help.
You are pretty much at the mercy of the manufacturer to come up with a solution. It should have been addressed when the equipment was ordered.
You could run it by a PE and see if there was a solution such as reactors

 

[tom baker]

Yes you could install a fuseable disconnect, and calculate the let thru current.

 

[MyCleveland]

This thread is golden for why mtr contribution needs to be taken into account.
FLA of 500HP…in multiples of smaller motors has to be large, what percentage of 10k is remaining for utility and other mtrs in facility ?

 

[Pullnwire]

This thread is golden for why mtr contribution needs to be taken into account.
FLA of 500HP…in multiples of smaller motors has to be large, what percentage of 10k is remaining for utility and other mtrs in facility ?

Fed from this 2000amp service There are 8 compressors in the rack totaling 495 HP. The rack also powers 21 fan coils with 42 motors totaling 235 HP ( sized between 1.5 HP and 9HP) Cooling tower at (3) 7.5HP fans and 10HP pump. The distance between 2000amp servce and the 1600amp switchboard feeding these loads is 550' 6 sets of 500KCMIL AL. Rack is another 100' from switchboard. the fan coils are spread out in a 100,000 SF building.
Im pretty good with ohms law, but this is over my head. Also, this is a design build project, so I need to figure it out myself or hire somone to do so.

 

[tom baker]

Yes you could install a fuseable disconnect, and calculate the let thru current.

And this becomes a series rated system, you will start with the feeder ocpd and find a downstream device they have tested

 

[petersonra]

Yes you could install a fuseable disconnect, and calculate the let thru current.

How would that help?

 

[petersonra]

I have a hard time believing any competent engineer would design a 500 horsepower system with a 10k short circuit current rating. Not in this day and age anyway. As a practical matter you would need almost 10k of short circuit current just to trip the main disconnect reliably in case of a ground fault.

 

[Pullnwire]

I have a hard time believing any competent engineer would design a 500 horsepower system with a 10k short circuit current rating. Not in this day and age anyway. As a practical matter you would need almost 10k of short circuit current just to trip the main disconnect reliably in case of a ground fault.

Since this is a design build Project, I was given a bill of material from the refrigeration company. They told me the rack was 10K SCCR. And its already in production.

 

[petersonra]

Since this is a design build Project, I was given a bill of material from the refrigeration company. They told me the rack was 10K SCCR. And its already in production.

The only real answer is to fix the control panel they are providing. There is no other fix. This is not something you can fix. The control panel has to be designed for the correct short circuit current rating.

 

[Pullnwire]

The only real answer is to fix the control panel they are providing. There is no other fix. This is not something you can fix. The control panel has to be designed for the correct short circuit current rating.

that's the conclusion I have come to, after consulting the code book and my favorite armchair experts here. I was just hoping there was a semi simple work around.

 

[ron]

You can run the feeder around the room to reduce fault current down to 10kA, you can add an isolation transformer (same voltage primary as secondary) which will reduce fault current, you can add a current limiting reactor which will reduce fault current, OR you can have them provide the correct rated downstream equipment. The ole up-over-down method of see the let-through of a current limiting fuse is not valid outside of a control panel when determining SCCR's. Dynamic impedance of the components within the downstream equipment makes it no bueno. The downstream equipment manufacturer can do that however and they can put their listing on it.

 

[petersonra]

The ole up-over-down method of see the let-through of a current limiting fuse is not valid outside of a control panel when determining SCCR's.

It is not valid in a control panel either. You have to use the charts UL provides. You cannot make any calculations yourself.

 

[petersonra]

You can run the feeder around the room to reduce fault current down to 10kA, you can add an isolation transformer (same voltage primary as secondary) which will reduce fault current, you can add a current limiting reactor which will reduce fault current, OR you can have them provide the correct rated downstream equipment. The ole up-over-down method of see the let-through of a current limiting fuse is not valid outside of a control panel when determining SCCR's. Dynamic impedance of the components within the downstream equipment makes it no bueno. The downstream equipment manufacturer can do that however and they can put their listing on it.

Your suggestions might work, but I think they would be problematic cost wise compared to just fixing the real problem. I would bet you run into VD problems if you install enough wire to get the SCC down to 10 kA. Probably same problem with reactors. They also add a VD to the system.

The transformer is your best bet but that would be a pricey option.

 

[MyCleveland]

Your suggestions might work, but I think they would be problematic cost wise compared to just fixing the real problem. I would bet you run into VD problems if you install enough wire to get the SCC down to 10 kA. Probably same problem with reactors. They also add a VD to the system.

The transformer is your best bet but that would be a pricey option.

Agree on the tranny…but based on size needed It still might not get you below 10k.

 

[Jraef]

One thing to check on: have the lazy supplier (and that’s what they are because it’s not that difficult) send you a Bill of Materials of exactly what they are supplying. Then contact the manufacturer of all of the power components (contactors, overloads, power blocks, breakers etc. and see if they are series listed with specific fuses to a higher SCCR. If so, add those specific fuses upstream, even if they already have an OCPD. You might get lucky, MOST reasonable component mfrs have figured out by now that not bothering with series listings is going to cost them business. Only the cheapest of cheapskate still pretend this issue doesn’t exist.

 

[petersonra]

One thing to check on: have the lazy supplier (and that’s what they are because it’s not that difficult) send you a Bill of Materials of exactly what they are supplying. Then contact the manufacturer of all of the power components (contactors, overloads, power blocks, breakers etc. and see if they are series listed with specific fuses to a higher SCCR. If so, add those specific fuses upstream, even if they already have an OCPD. You might get lucky, MOST reasonable component mfrs have figured out by now that not bothering with series listings is going to cost them business. Only the cheapest of cheapskate still pretend this issue doesn’t exist.

That won't change the SCCR of the control panel though. Unless the manufacturer is willing to re-label it, which they might. The problem is that with the ratings of current limiting fuses you would have to use for a 500 horsepower control panel it's unlikely the let-through would be low enough to do you any good so you would have to depend on some kind of series rating which is iffy on low-end components. It's not impossible but refrigeration and HVAC people are known to use the cheapest possible components to save a few bucks and they don't really give a rat's ass about anything other than making the cheapest possible product.

In many cases smaller contactors and overloads the series rating is only good for like a 30 amp class CC or j fuse which isn't going to do you any good at 500 horsepower.

 

[Jraef]

Yeah, actually you’re right. It doesn’t change the rating of the panel. I was reaching for a solution for him. Never mind.

 

[topgone]

That won't change the SCCR of the control panel though. Unless the manufacturer is willing to re-label it, which they might. The problem is that with the ratings of current limiting fuses you would have to use for a 500 horsepower control panel it's unlikely the let-through would be low enough to do you any good so you would have to depend on some kind of series rating which is iffy on low-end components. It's not impossible but refrigeration and HVAC people are known to use the cheapest possible components to save a few bucks and they don't really give a rat's ass about anything other than making the cheapest possible product.

In many cases smaller contactors and overloads the series rating is only good for like a 30 amp class CC or j fuse which isn't going to do you any good at 500 horsepower.

Definitely, no amount of series-rated breaker combo will be applicable here as the sum of the motor load currents exceeds 1% (0.01 X 10,000 =100 A)of the lowest breaker interrupting rating.
There are only two ways one has to go: 1) try to interpose impedance between the 39 kA source and the 10 kA SCCR equipment or 2) have the panel re-designed to withstand a 39 kA available fault.

 

[Fred B]

One more time the electrician is expected to jump hoops and scramble to accommodate other trades who negligently or never consider implications of what they do on the electrical and it seems to default onto the electrician to make their improper installation work. The HVAC installer should have taken this issue into consideration before ordering the parts, and if they didn't know how the electrician should have been consulted. It sounds as if the panel was only ordered at the point where someone became aware of the issue, and the HVAC company should have fixed the order. Maybe if these other trades start paying for their screwups instead of passing off to the electrician to "just make it work", they might begin to consider the electrical parts when doing their jobs. (The "you move the panel" after they install other mechanical in the dedicated space already established by your installation mentality.)
Sometimes it feels as if electricians are the Rodney Dangerfield of the trades.