HVAC and SCCR requirements

[johnrumsey]

I am currently working on a Power Distribution Center (PDC); these PDCs are large power houses where utility comes in and is distribution throughout a plant using switchgear and MCC. The ratings of this equipment (power panels, switchgear, MCC, VFDs) are 480V and 65kA interrupting. The HVAC units on this PDC however are rated at only 5kA which I see as a routine rating since HVAC is not normally subject to SCCR testing. We usually put current limiting fuses ahead of the HVAC to satisfy this disparity but I have not seen anything official that allows me to do that. Of course now I have a customer questioning the validity of this practice.

Has anyone else here had this issue and have you been able to satisfy the customers concerns?

 

[dkidd]

There was a good discussion a while back with a link to a good article.

HVAC Short Circuit Withstand Rating​

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...
forums.mikeholt.com

HVAC Short Circuit Withstand Rating

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...

forums.mikeholt.com

 

[petersonra]

You will probably have to go back to the manufacturer and see if they can tweak the control panel. It is almost trivial to get to 65kA SCCR on a control panel these days. But someone has to go and do it.

It's possible that they even have an option for it.

 

[Jraef]

Trivial is an understatement, so when people don’t bother even trying, it means they are just lazy, or so stuck on using cheap components that have zero coordination that they “kick the can down the road” rather than change. The problem is, as you have discovered, there is no valid solution that can be easily implemented in the field.

From a marketing standpoint, the HVAC suppliers are not going to do anything about it, for fear of losing an order to a competitor who ignores it, so they pretty much ALL ignore it. The solution, albeit difficult to implement, is to educate specifiers and buyers to INSIST on requiring a proper SCCR on the HVAC equipment. I rarely see it in specifications, leaving the HVAC suppliers to continue the practice.

 

[johnrumsey]

Trivial is an understatement, so when people don’t bother even trying, it means they are just lazy, or so stuck on using cheap components that have zero coordination that they “kick the can down the road” rather than change. The problem is, as you have discovered, there is no valid solution that can be easily implemented in the field.

From a marketing standpoint, the HVAC suppliers are not going to do anything about it, for fear of losing an order to a competitor who ignores it, so they pretty much ALL ignore it. The solution, albeit difficult to implement, is to educate specifiers and buyers to INSIST on requiring a proper SCCR on the HVAC equipment. I rarely see it in specifications, leaving the HVAC suppliers to continue the practice.

From what I have seen, even the major HVAC providers still have a 5kv rating. Even when the components used are rated at a normal 480V rating (65kA) there are going to be some components that do not come in those ratings. The only way to get the whole assemblies approved is very expensive testing (Then after testing you have to pay periodically to keep the rating). Again, we are talking major national companies and not necessarily using a cheap low ball company.
Hard to believe there is not a common sense solution to this issue since it appears so wide spread.

 

[petersonra]

From what I have seen, even the major HVAC providers still have a 5kv rating. Even when the components used are rated at a normal 480V rating (65kA) there are going to be some components that do not come in those ratings. The only way to get the whole assemblies approved is very expensive testing (Then after testing you have to pay periodically to keep the rating). Again, we are talking major national companies and not necessarily using a cheap low ball company.
Hard to believe there is not a common sense solution to this issue since it appears so wide spread.

control panels are ul508a listed. No testing required as long as you follow the simple rules they require. I would not be surprised if most designs already met the requirements. The few I have seen look like they certainly could.

I have no idea why they choose not to.

 

[johnrumsey]

control panels are ul508a listed. No testing required as long as you follow the simple rules they require. I would not be surprised if most designs already met the requirements. The few I have seen look like they certainly could.

I have no idea why they choose not to.

Yes, 508A gives you some outs as long as you follow their 'black box' rules. These are not control panels but wall mount HVAC units, 5 ton, 480V.

 

[petersonra]

Yes, 508A gives you some outs as long as you follow their 'black box' rules. These are not control panels but wall mount HVAC units, 5 ton, 480V.

The control panel is what is ul listed for an sccr. The sccr specifically does not include what is connected to it. Only what is in the control panel.

It's possible the manufacturer made some kind of custom controller. That would probably not require any special testing since it is not in a power circuit and only power circuits are considered when calculating a sccr. Now if the controller included power circuits it would need testing to get listed with a sccr. But it is going to need to get ul listed anyway in most places.

5 ton air conditioners are probably small enough that they might be ok at a 5 kA sccr. Probably a ten Amp 230 v single phase circuit?

 

[johnrumsey]

There is no control panel here and the units are 480V. I run a power circuit from a 480V, 65kA rated motor control center breaker directly to the HVAC disconnect and the from disconnect to HVAC power terminals. Was wanting to replace disconnect with current limiting fuses.

 

[ron]

Other than a control panel SCCR per UL 508A, the HVAC has to be listed fully for the available fault current. Adding current limiting fuses without a tested combination with the downstream protection is not permitted per 240.86

 

[Jpflex]

Trivial is an understatement, so when people don’t bother even trying, it means they are just lazy, or so stuck on using cheap components that have zero coordination that they “kick the can down the road” rather than change. The problem is, as you have discovered, there is no valid solution that can be easily implemented in the field.

From a marketing standpoint, the HVAC suppliers are not going to do anything about it, for fear of losing an order to a competitor who ignores it, so they pretty much ALL ignore it. The solution, albeit difficult to implement, is to educate specifiers and buyers to INSIST on requiring a proper SCCR on the HVAC equipment. I rarely see it in specifications, leaving the HVAC suppliers to continue the practice.

By SCCR do you mean available fault current specifically at first HVAC disconnect? If so you would need supply transformer specs including impedance. Finding available fault current at the transformer is easy but finishing the derating calculation for the distance of feeder and wire size seems to have so many more steps and does not appear easy.

I assume contractors are ignoring specific codes because equipment with lesser available short circuit ground fault current rating are cheaper. I understand the difficulty of following code when it means you will starve.

Kind of like in my smog tech days when I would loose jobs to repair cars legally vs a competitor who would charge to illegally pass a car without doing any work or repairs.

 

[petersonra]

Other than a control panel SCCR per UL 508A, the HVAC has to be listed fully for the available fault current. Adding current limiting fuses without a tested combination with the downstream protection is not permitted per 240.86

You can add whatever you want as long as what you add is compliant. It may or may not have any effect on the downstream sccr. Likely not.

The motor does not have a sccr so it just does not matter if you put a fused disconnect in series with the motor. It might even need to be fused because most unfused disconnects are not going to be above 10 kA sccr.

 

[JoeStillman]

I was in a bind one time where a big chiller only had 5kA SCCR. I was able to show the manufacturer that if the upstream impedance was high enough to keep the AFC below 5kA, then it would be too high to allow the machine to operate within voltage tolerances.

They upgraded it to 65kA for no charge.

 

[Jraef]

I assume contractors are ignoring specific codes because equipment with lesser available short circuit ground fault current rating are cheaper. I understand the difficulty of following code when it means you will starve.

Electrical contractors tend to know better than to order equipment that cannot be connected in a Code compliant manner. This is more of an issue with equipment suppliers and mechanical contractors not caring enough to look into it, electing instead to just “kick the can down the road” to the electrical contractor, who ends up stuck with a problem they can’t solve.

 

[JoeStillman]

@Jraef , maybe you can answer this one: When we calculate the AFC for a particular control panel that might feed a 450A chiller with 5 kA SCCR, should we take into account the short-circuit contribution of the motor? A 450A motor will act like an 1800A generator for a few cycles when there's an upstream fault. Does that only leave an allowable upstream contribution of 3.2 kA allowable for a 5 kA panel?

 

[jim dungar]

@Jraef , maybe you can answer this one: When we calculate the AFC for a particular control panel that might feed a 450A chiller with 5 kA SCCR, should we take into account the short-circuit contribution of the motor? A 450A motor will act like an 1800A generator for a few cycles when there's an upstream fault. Does that only leave an allowable upstream contribution of 3.2 kA allowable for a 5 kA panel?

Technically yes. But practically no.
Say you have a fault half way between the motor and the OCPD the only current thru the OCPD will be that from the source. But if the fault is on the line side of the OCPD, the only current through the OCPD will be that from the motor. If the fault is internal to the OCPD it is possible for there to be current from both the motor and the source.

However, the motor fault contribution should be added to any source current for the other points in the system, say you have a main distribution panel source with an available current of 21kA and a total motor contribution of 1.8kA. Your main device and the motor feed would need to be rated 18kAIC, but all of the branch devices would need to be 22kAIC (using typical breaker ratings).
But for many services, people use an infinite bus fault current so adding a couple extra kA doesn't change anything.

 

[ron]

@Jraef , maybe you can answer this one: When we calculate the AFC for a particular control panel that might feed a 450A chiller with 5 kA SCCR, should we take into account the short-circuit contribution of the motor? A 450A motor will act like an 1800A generator for a few cycles when there's an upstream fault. Does that only leave an allowable upstream contribution of 3.2 kA allowable for a 5 kA panel?

Jim gave a good description, although many of the large chillers and HVAC equipment have VFDs or soft starters, which will prevent motor contribution from flowing upstream.

 

[Jraef]

Jim gave a good description, although many of the large chillers and HVAC equipment have VFDs or soft starters, which will prevent motor contribution from flowing upstream.

VFDs will (assuming diode front-end), Soft starters will not.

Newer VFDs with Active Front Ends (AFE) that are configured for Line Regenerative Braking, will as well. Low Harmonic AFE drives will not.

 

[MyCleveland]

VFDs will (assuming diode front-end), Soft starters will not.

Newer VFDs with Active Front Ends (AFE) that are configured for Line Regenerative Braking, will as well. Low Harmonic AFE drives will not.

Jraef
Thank you for sharing this knowledge.
I hope all the younger EE's grasp the importance here. I had never as a rule during AFC calcs considered this, but it is an important item to consider. I have never studied the types available because I was not the one selecting them for a given application. Just took it as a BLACK BOX.

Do you have a summary sheet of all the different types available and noting each type that does in fact prevent motor contribution from flowing upstream ? I would like to pass your list along to my son for his next coordination meeting with ME equipment selection. If not, just steer me towards a VFD manufacturer you have used in your designs and I will take it up with them.

 

[nhee2]

However, the motor fault contribution should be added to any source current for the other points in the system, say you have a main distribution panel source with an available current of 21kA and a total motor contribution of 1.8kA. Your main device and the motor feed would need to be rated 18kAIC, but all of the branch devices would need to be 22kAIC (using typical breaker ratings).

where does the 18 KAIC value come from?