Chiller SCC Calc, or Why Is This Even a Thing?

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JoeStillman

JoeStillman

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West Chester, PA
A 225 ton air-cooled chiller has 9 compressors and 12 fans. At 460V, the total RLA/FLA is 440 amps, or 351 kVA. The nameplate says the SCCR of its single-point-connection control panel is 5,000A.

Assuming a standard impedance of 5.75%, a transformer that is limited to 5,000A secondary fault current is around 230 kVA. So, the largest transformer that can feed my 351 kVA chiller is 230 kVA. Crazy enough yet? Just wait...

When I calculate the short circuit current on the chiller input bus, The worst case is a fault in one compressor and all the others running (i.e. contributing to the fault.) So the machine is pumping 2,800A into its own 5,000A control panel, leaving me with only 2,200A allowable from the upstream system.

My transformer has other loads on it, like a building with enough heat producing equipment to need two 225 ton chillers (did I mention there are two of these crazy things?) So I need a 2,000 kVA transformer to feed the load. With the transformer impedance and those pesky short cables, I'm only down to 25 kA at the chiller. Shall we put a line reactor ahead of the chiller to limit SC current? Sorry, if the reactor impedance is high enough to knock the fault-current down to 2,200 amps, it also knocks the operating voltage down to 380 volts. Ok then, let's look at current limiting fuses. Is there a current limiting fuse at 450A with a threshold below 5000A? Nope.

I contend that there is no power system in America that can feed one of these chillers within its SCCR. Why is this even a thing?

/rant
 
augie47

augie47

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Very valid point.
Have you looked at a Bussmann JJS fuse ?
 
T

texie

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Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
JoeStillman said:
A 225 ton air-cooled chiller has 9 compressors and 12 fans. At 460V, the total RLA/FLA is 440 amps, or 351 kVA. The nameplate says the SCCR of its single-point-connection control panel is 5,000A.

Assuming a standard impedance of 5.75%, a transformer that is limited to 5,000A secondary fault current is around 230 kVA. So, the largest transformer that can feed my 351 kVA chiller is 230 kVA. Crazy enough yet? Just wait...

When I calculate the short circuit current on the chiller input bus, The worst case is a fault in one compressor and all the others running (i.e. contributing to the fault.) So the machine is pumping 2,800A into its own 5,000A control panel, leaving me with only 2,200A allowable from the upstream system.

My transformer has other loads on it, like a building with enough heat producing equipment to need two 225 ton chillers (did I mention there are two of these crazy things?) So I need a 2,000 kVA transformer to feed the load. With the transformer impedance and those pesky short cables, I'm only down to 25 kA at the chiller. Shall we put a line reactor ahead of the chiller to limit SC current? Sorry, if the reactor impedance is high enough to knock the fault-current down to 2,200 amps, it also knocks the operating voltage down to 380 volts. Ok then, let's look at current limiting fuses. Is there a current limiting fuse at 450A with a threshold below 5000A? Nope.

I contend that there is no power system in America that can feed one of these chillers within its SCCR. Why is this even a thing?

/rant
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Interesting commentary. And I think you are being generous using 5.75% impedance so the problem likely worse in many cases. This has been a rampant problem in our area with all kinds of equipment, especially with chillers. We usually get the standard "we'll just put a fusible switch in with current limiting fuses and call it good" reply. Maybe I'm naive, but one would think that something this serious that the manufacturers would be more diligent in policing this at the sales level just to help protect themselves. There is a lot of work to be done in this area of enforcement and education.
 
augie47

augie47

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It's "not a problem" as sadly only a small percentage of contractors and inspectors will address it.
 
T

texie

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Location
Fort Collins, Colorado
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Electrician, Contractor, Inspector
petersonra said:
tell your purchasing people to stop specifying the cheapest possible thing. that's why these kind of things happen.
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Yep, this is big part of the problem. But it is also the dirty business of of bid jobs. These manufacturers play these games by knowingly submitting quotes using 5K equipment to be competitive and hope it slips through the process. We just had a 200 ton or so chiller on a job with a 5K rating and the available fault current at the terminals was about 30K plus whatever motor contribution there was. It got ugly as it had to be retrofitted in the field and be field evaluated by UL to pass inspection.
I think also contributing to this the fact that on most jobs the mechanical contractor supplies the equipment and the EC connects it. So the finger pointing starts and nobody wants to own the problem. And also the engineers, while most are careful about this in their specs, some are sloppy in approving submittals haphazardly. More finger pointing.
 
Jraef

Jraef

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texie said:
Yep, this is big part of the problem. But it is also the dirty business of of bid jobs. These manufacturers play these games by knowingly submitting quotes using 5K equipment to be competitive and hope it slips through the process. We just had a 200 ton or so chiller on a job with a 5K rating and the available fault current at the terminals was about 30K plus whatever motor contribution there was. It got ugly as it had to be retrofitted in the field and be field evaluated by UL to pass inspection.
I think also contributing to this the fact that on most jobs the mechanical contractor supplies the equipment and the EC connects it. So the finger pointing starts and nobody wants to own the problem. And also the engineers, while most are careful about this in their specs, some are sloppy in approving submittals haphazardly. More finger pointing.
Click to expand...
All it takes is these simple words added to the purchase specifications.

"Electrical equipment shall be listed and labeled for use with a Short Circuit Current Rating that corresponds to the Available Fault Current at the location where they are being installed." That puts the onus of providing a proper SCCR on the people supplying the control panels. If they go cheap and ignore it, getting the "courtesy" untested rating of 5kA (which is what that one has), they will be responsible for making it right. Even YOU, as the EC, can simply add this to your PO, or better yet, add something to your bid explaining that your price is BASED ON the fact that all equipment you will be connecting is suitable for the conditions of use.

This change took place in the 2005 NEC when Article 409 was introduced. There really is no excuse for people not paying attention to that any longer. 11 years is plenty of time to get current on the code...
 
petersonra

petersonra

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Northern illinois
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engineer
Jraef said:
All it takes is these simple words added to the purchase specifications.

"Electrical equipment shall be listed and labeled for use with a Short Circuit Current Rating that corresponds to the Available Fault Current at the location where they are being installed." That puts the onus of providing a proper SCCR on the people supplying the control panels. If they go cheap and ignore it, getting the "courtesy" untested rating of 5kA (which is what that one has), they will be responsible for making it right. Even YOU, as the EC, can simply add this to your PO, or better yet, add something to your bid explaining that your price is BASED ON the fact that all equipment you will be connecting is suitable for the conditions of use.

This change took place in the 2005 NEC when Article 409 was introduced. There really is no excuse for people not paying attention to that any longer. 11 years is plenty of time to get current on the code...
Click to expand...

That sounds at first like a good solution, but when we ask what SCCR they actually need, it is very rare that they can tell us so they tell us to make it as cheap as possible.

Personally, I resist the cheap as possible approach as best I can but some customers are so price sensitive that $100 on a $20k project can swing the deal to someone else.

I don't get real excited about it at lower current requirements but when you start to talk about 50 or 100 Amps, I start to get nervous with a 10kA SCCR (I think that is what our default is these days - no idea how that number was picked by the boss).
 
don_resqcapt19

don_resqcapt19

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Sahib said:
The chiller manufacturer is not at fault, IMO. Otherwise therec would be many lawsuits against them
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In this case I think the manufacturer is at fault. It is unreasonable to expect a feeder circuit of this size to have an available fault current of less than 5000 amps.
 
Jraef

Jraef

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JoeStillman said:
I want to dare them to show me some kind of electrical system that can actually feed this monster.

Sent from my SM-G900V using Tapatalk
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And there's the problem with the specifiers not paying attention to Article 409. All it takes is basically the same amount of effort it takes to determine the correct interrupt rating of a switchboard or panelboard. In fact, at some point you will ALREADY have that information upstream of the equipment. So by simply using that value as the AFC and requiring an SCCR at that level, you guarantee that the control panel will not need anything more when it arrives. Sure, the actual AFC is going to be slightly lower by the time you get to the control panel terminals, but that isn't a problem.

Where the issue comes in with the equipment provider being cheap (and often just lazy) is that in order to get a higher SCCR rating, the panel builder becomes more restricted on what they can use in terms of components. It's fairly easy now to attain reasonable SCCR values, even relatively high ones like 65kA, by simply choosing power circuit components that have those higher values, or are UL listed in series combinations that attain them. That however generally means that you can't buy each separate component from the lowest bidder and end up using an ABB starter behind a Square D breaker, because why would ABB test their starters in series combo with a competitor's breaker? So what they do is just "kick the can down the road" to the hapless EC who has to deal with it in the field, where it's MUCH more difficult. They are only allowed to get away with that because nobody told them they couldn't. That's where the job specs come to play here.

Like Bob Peterson, when I was a panel builder, I too would choose to take the high road and inform my customers what I was giving them. But he's right in that if nobody addresses it up front ahead of time, purchasing agents and buyers are only going to look at the bottom line costs, so the "can kicker" wins, which all but forces anyone trying to compete with them to do the same. That's why it HAS to be addressed up front.
 
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JoeStillman

JoeStillman

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Location
West Chester, PA
Sahib said:
The chiller manufacturer is not at fault, IMO. Otherwise therec would be many lawsuits against them
Click to expand...

Don was being generous when he said "unreasonable". I'm saying "impossible."

The chiller has a motor contribution of 2,800A, leaving 2,200A of the 5,000A SCCR for upstream impedance. But a 460V source needs an impedance of at least 460/2,200 or 0.21 ohms to be that low. However, with a source impedance of 0.21 ohms and an operating current of 440 amps, the voltage drop is 440 x 0.21 or 92 volts. 460-92= 368 volts, but the listed operating range is 414 to 506 volts. How is this even possible?
 
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