HVAC Short Circuit Withstand Rating

[steve66]

So I have a chiller, and like almost all HVAC Equipment, it has a short circuit withstand rating of 5KA.

It has a MCA of about 500 amps, so I'm obviously going to supply it with a system that has a much greater available fault current than 5KA, which is a code violation.

I can add a fused disconnect with current limiting fuses to the units supply. But my understanding is that this doesn't necessarily make everything code compliant. Is that correct? If so, is there any code compliant way to install this short of looping coils of 500KCM wire around the building until the fault current is below 5KA? (My voltage drop along the feeder will probably be much more than 5% by the time I do that, so now it will cause other problems.)

The last time this happened, we just required the manufacturer to provided a higher rated short circuit withstand rating. And after a lot of back and forth, all they did was to add the same fused disconnect I would have added, except they charged a lot more for it.

So what does anyone do in this situation? Its just crazy to have a 5KA rating on something that needs 500 amps or more to run.

 

[dkidd]

There is a good article at http://ecmweb.com/design/sccr-liability-trap

An option is to add an isolation transformer to increase the impedance of the supply.

 

[ron]

Adding a fused switch does not make it ok, unless the manufacturer has a tested assembly that says it is ok for the needed SCCR.
Find out what the manuf would do to provide the proper SCCR and evaluate from there.
The listed assembly is important from a liability standpoint.
We have used isolation transformers, looped extra conductor, added a reactor, and also made the equip manuf upgrade the equip to the proper SCCR.

 

[Jraef]

Adding a fused switch does not make it ok, unless the manufacturer has a tested assembly that says it is ok for the needed SCCR.
Find out what the manuf would do to provide the proper SCCR and evaluate from there.
The listed assembly is important from a liability standpoint.
We have used isolation transformers, looped extra conductor, added a reactor, and also made the equip manuf upgrade the equip to the proper SCCR.

All correct. YOU cannot change the SCCR of the panel, YOU cannot change the Available Fault Current. You CAN in some jurisdictions hire a registered PE to select a current limiting fuse system that will limit your AFC because it requires a lot of knowledge about what the system is feeding the circuit and how that affects the clearing time of the selected fuse. But getting down to 5kA on a machine needing 500A is going to be damned near impossible...

This issue is the direct result of people buying equipment without consulting an electrician or EE ahead of time to consult with them on the installation requirements. SCCR is a problem biting people in the butt every day since the concept was added to the NEC, I think in 2011.

The lazy equipment suppliers are taking advantage of them. It really is not that hard to attain a higher SCCR on a system, but you do have to try and it generally means you have to use components from a single manufacturer, rather than buying the cheapest junk they can get for each individual part. So another option is to have your customer go back to the HVAC equipment supplier and insist that they provide equipment suitable for use in their system. It might require that someone scrap and rebuild the control panel. If they refuse, option 3 might be to pay a UL508A panel shop to duplicate the control panel with an adequate SCCR listing.

 

[ron]

. You CAN in some jurisdictions hire a registered PE to select a current limiting fuse system that will limit your AFC because it requires a lot of knowledge about what the system is feeding the circuit and how that affects the clearing time of the selected fuse.

I believe this is only permited by code for existing equipment, and I don't know many folks that do this considering the liability (not me)

 

[Jraef]

I believe this is only permited by code for existing equipment, and I don't know many folks that do this considering the liability (not me)

Yes, I agree on both counts. I do not know any PEs who will do it either, but I have heard of it being done.

 

[steve66]

Ok, I think I understand it a little better now. Easy enough to require a higher withstand rating on the Chiller.

And I guess the same will probably apply to almost all the HVAC equipment on this project. So now I wonder where it ends.

It sounds like its the control panel that is the real issue. I assume any chiller will have a control panel to start and stop the fans and compressors - so I would have to ask for a higher rating on almost any chiller.

Likewise, most Roof Top Units and packaged units will have a control panel. So they need higher ratings also.

But I what about a simple motor (like a pump or a fan) that only has a basic motor starter? Does that still have a Short Circuit withstand rating? Aren't those usually just based on the rating of the fuse or breaker that supplies it?

And what about single motor equipment, like a condenser or an evaporator. Will they need ratings equal to the available fault current?

 

[Jraef]

60A and below HVAC equipment does not need an SCCR (440.4.B, exception 3). Everything else does.

 

[ron]

And what about single motor equipment, like a condenser or an evaporator. Will they need ratings equal to the available fault current?

Everything except single and two family dwelling equipment and cord and plug connected equipment. Granted I have not seen a motor or compressor itself have a SCCR, but the associated controller or starter portion does.

 

[powerpete69]

This is really strange. Only 5kA for 500 amps!!!???? Let me simulate this on Easypower and see if I can use some sort of fuse combination to get the short circuit rating under 5kA.
I'll assume this is a 4160V motor since 500 amps is likely to large for 480V.

 

[Jraef]

This is really strange. Only 5kA for 500 amps!!!???? Let me simulate this on Easypower and see if I can use some sort of fuse combination to get the short circuit rating under 5kA.
I'll assume this is a 4160V motor since 500 amps is likely to large for 480V.

I'd be willing to bet it is NOT Medium Voltage for HVAC equipment...

 

[powerpete69]

This is really strange. Only 5kA for 500 amps!!!???? Let me simulate this on Easypower and see if I can use some sort of fuse combination to get the short circuit rating under 5kA.
Is this 480 V or 4160V....or?

 

[ptonsparky]

We have the same problem with a certain brand of larger grain dryers. Probably all of them.

 

[ptonsparky]

We have the same problem with a certain brand of larger grain dryers. Probably all of them.

 

[powerpete69]

4160V would NOT make sense at 500 amps since that would be a massive amount of HP.​

Assuming a 500KVA transformer with a 5.75 impedance, It doesn't matter what breaker, fuse combination you use for 480V.
Trying multiple configurations, the best short circuit rating I could get was 11kA by using a fast acting 800A fuse.
One way to meet that rating would be to use a 500KVA transformer with a 12% impedance. But why on Earth would you want to do that?
This makes no sense, why would a 500 amp piece of equipment only have a 5kA short circuit rating? It's counterproductive.
The manufacturer of this piece of equipment may want to consider a new profession.

If you happen to be buying a new piece of electrical equipment, make sure the short circuit rating meets the transformer feeding it. Infinite bus rating is the most conservative. Maximum amps of secondary of transformer divided by its impedance. This is the safest way but not necessarily the cheapest way. Or you can use one of the Easypower programs or simply use Mike Holt's short circuit excel spreadsheet. If you inherit a large piece of equipment with a LOW short circuit rating, you simply inherit a problem.

 

[jim dungar]

This makes no sense, why would a 500 amp piece of equipment only have a 5kA short circuit rating? It's counterproductive.
The manufacturer of this piece of equipment may want to consider a new profession.

A 5kA rating is the default for most UL control panels built, anything higher usually requires some amount of electrical engineering, careful product selection, and modifications to the panel's 'listing sheet'.

The OEM most likely provided what was called for in the boilerplate mechanical division specifications. I seems on many jobs, the HVAC stuff is ordered before an independent short circuit analysis is even provided by the electrical contractor.

 

[powerpete69]

A 5kA rating is the default for most UL control panels built, anything higher usually requires some amount of electrical engineering, careful product selection, and modifications to the panel's 'listing sheet'.

The OEM most likely provided what was called for in the boilerplate mechanical division specifications. I seems on many jobs, the HVAC stuff is ordered before an independent short circuit analysis is even provided by the electrical contractor.

I believe you and all, but that is strange considering an average home panel breaker is rated at 10kA.
When you consider the 500 amps, 5kA is ridiculous!!

 

[Jraef]

4160V would NOT make sense at 500 amps since that would be a massive amount of HP.​

Assuming a 500KVA transformer with a 5.75 impedance, It doesn't matter what breaker, fuse combination you use for 480V.
Trying multiple configurations, the best short circuit rating I could get was 11kA by using a fast acting 800A fuse.
One way to meet that rating would be to use a 500KVA transformer with a 12% impedance. But why on Earth would you want to do that?
This makes no sense, why would a 500 amp piece of equipment only have a 5kA short circuit rating? It's counterproductive.
The manufacturer of this piece of equipment may want to consider a new profession.

If you happen to be buying a new piece of electrical equipment, make sure the short circuit rating meets the transformer feeding it. Infinite bus rating is the most conservative. Maximum amps of secondary of transformer divided by its impedance. This is the safest way but not necessarily the cheapest way. Or you can use one of the Easypower programs or simply use Mike Holt's short circuit excel spreadsheet. If you inherit a large piece of equipment with a LOW short circuit rating, you simply inherit a problem.

I think you might be misinterpreting...

He is not saying that the Available Fault Current at his site is only 5kA, he is saying that the HVAC EQUIPMENT is only rated 5kA SCCR; it's HIGHLY likely that his AFC is much much higher.

The problem is, the NEC has required that all equipment be marked with a Short Circuit Current Rating (SCCR) and that the installer is responsible for ensuring that anything connected to the line has an SCCR that is equal to or greater than the AFC. But the mental disconnect takes place when the user buys the equipment and doesn't specifically require the vendor to have an SCCR listing that is commensurate with their site conditions. So low-bid vendors will cobble together a cheap set of parts and pay zero attention to the SCCR, because UL allows them to put on a "courtesy" listing of 5kA without any testing or forethought whatsoever. So the vendor is trying to dump the problem into the hands of the electrician, when in fact it SHOULD have been their problem to begin with, because it's virtually impossible to get down to 5kA in any reasonably sized facility.

Any competent panel builder can usually attain a much much higher SCCR by simply using pre-tested and listed combinations of devices. But that then means the power circuit devices, i.e. breaker, contactor, overload relay, must be specifically tested and listed TOGETHER, which is expensive. Since it is the component manufacturer that pays for that testing, one manufacturer is not going to pay to list a competitor's product. So you are not going to see a Sq. D breaker listed with an ABB contactor and a Cutler Hammer overload relay. The components will have to be all one source to be listed together for a higher SCCR, and the listing dictates exactly which versions you can use. It's really not that big of a problem, they all make it very easy to pick out the right parts now. That's why I say that the HVAC vendor was just lazy; they didn't bother to even look into it, choosing instead to try to dump the problem into the electrician's lap.

So word to the wise: talk to your industrial and commercial customers and tell them that when they go to buy equipment, they should ask you first what the Available Fault Current is at their facility, and SPECIFY to the vendors that all equipment MUST have an SCCR listing at or above that value.

 

[steve66]

Yes, 480 volt equipment. And the 500 amps is approximate: I've actually had this on more than one project and chiller, some above 500 amps and some below 500 amps, so I just kind of used 500 amps as an example.

I believe you and all, but that is strange considering an average home panel breaker is rated at 10kA.
When you consider the 500 amps, 5kA is ridiculous!!

Yes, my point exactly!! Although 5kA should probably be the minimum rating for any HVAC equipment, nobody calls out the manufacturers and forces them to provide anything higher rated.

Worse, I believe there are probably a lot of people who think a fused disconnect fixes everything.

And no, I don't want to run 500 amps through an isolation transformer that is going to waste energy 24x7.

 

[steve66]

Everything except single and two family dwelling equipment and cord and plug connected equipment. Granted I have not seen a motor or compressor itself have a SCCR, but the associated controller or starter portion does.

Maybe this should be a new thread, but now you have me wondering if it is a violation to use residential equipment in a commercial project.