ROOF TOP UNIT SCCR

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PE (always learning)

PE (always learning)

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Professional Engineer
Hey everyone,

I have a project where I am providing a short circuit evaluation and there are two roof top units that each have a short circuit current rating (SCCR) of 5000 amps. The available fault current at the roof top units is about 8000 amps at both units. The roof top units obviously do not pass the evaluation, but the contractor is asking me to help him find solutions because they already ordered the units and decided to do the study after the fact... Outside of increasing the feeder lengths, would it be acceptable to put fusible disconnects a head of the equipment? The roof top units that they ordered came with non fusible disconnects, but could they just provide their own fusible disconnects ahead of these? I was reading through Trane's literature and they provided a note for what they call their "High Short Circuit Current Rating" for equipment. This note reads "Unit shall be provided with electrical subsystem that will withstand fault currents up to 65kA (208/ 230, 460 VAC) or 25kA (575 VAC) compliant with UL 1995 and NEC 440.4 (B). Each compressor circuit and the indoor fan shall have dedicated overcurrent protection. Three phase motors shall be protected by Class J time delay fuses. Single phase motors shall be protected by Class CC time delay fuses. All transformers shall also be protected with Class CC time delay fuses. Contactors shall be din rail mounted." It seems to me they are providing fusible disconnects to achieve a high SCCR anyways. Please let me know what you all think.
 
One solution is add a Isolation transformer, that would reduce the impedance thus lowering the available fault current.
the other is installing class "J" fuses..
I would do the class "J" fuses, If available. What amperage is required?
 
Have the contractor talk to the manufacturer. If the units haven't been shipped yet, they may be able to swap components in the factory to increase the SCCR. If the units have already shipped, the manufacturer may be able to provide a field modification to the equipment to increase the SCCR.
 
The SCCR is what it is at this point, the time to think about this was when writing purchase specifications, REQUIRING that the SCCR of any electrical equipment provided must meet or exceeds the conditions. You cannot "fix" it in the field. keep that in mind for next time.

The only options now are to lower the available fault current of the system, either by adding a transformer as mentioned, a current limiting reactor, or increasing the conductor length to add impedance/resistance. The issue of "adding fuses" is often misrepresented. That could have been done BY the manufacturer BEFORE calculating the SCCR, using an engineering process of evaluating the let-through current of the fuses and the withstand rating of the worst-case component. But most likely they did nothing, instead option for the "courtesy" 5kA rating that UL allows people to apply without taking any steps to do better, and that is what you are now stuck with. It is NOT at all difficult to attain a higher SCCR, it just takes some thought and attention to details. Your supplier did neither.
 
Jraef said:
The SCCR is what it is at this point, the time to think about this was when writing purchase specifications, REQUIRING that the SCCR of any electrical equipment provided must meet or exceeds the conditions. You cannot "fix" it in the field. keep that in mind for next time.

The only options now are to lower the available fault current of the system, either by adding a transformer as mentioned, a current limiting reactor, or increasing the conductor length to add impedance/resistance. The issue of "adding fuses" is often misrepresented. That could have been done BY the manufacturer BEFORE calculating the SCCR, using an engineering process of evaluating the let-through current of the fuses and the withstand rating of the worst-case component. But most likely they did nothing, instead option for the "courtesy" 5kA rating that UL allows people to apply without taking any steps to do better, and that is what you are now stuck with. It is NOT at all difficult to attain a higher SCCR, it just takes some thought and attention to details. Your supplier did neither.
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Yea, this was a failure on the supplier and the contractors part. The engineer even specified higher rated equipment on their drawings. They ordered the equipment before they even asked me to do this study, so this was not anything missed on my part. The supplier of the Roof Top Unit was Trane and I'll be honest, they have terrible customer service in my opinion.

I'm a little confused on why I still can't add current limiting fuses after the fact. Shouldn't the manufacturer have literature on whether specific fuses that were tested with their equipment will work or not. Also, is it wrong to assume that a current limiting fuse that dropped the current below 5kA is acceptable? If I were to supply current limiting fuses in a separate disconnect why wouldn't this accomplish the same goal?
 
My apologies on my incomplete response. I did intend to indicate the fuses should be installed internal to the control panel to raise the SCCR. By contractor I was thinking manufacture not installer.

But now that I think about it I remember about 12 to 15 years had similar problem don't actually remember what it was so, it could have been the same problem, but the solution was to add verbiage on the input wiring on the drawing making it someone else’s problem. We resolved it with Labeling. That always seems the solution “Labeling” so potential option could be the Manufacture mite be able to evaluate to a higher SCCR with External Class "J" fuses if they label as such. {SCCR = 10k when class "J" fuses are used.}
 
Macbeth said:
One solution is add a Isolation transformer, that would reduce the impedance thus lowering the available fault current.
the other is installing class "J" fuses..
I would do the class "J" fuses, If available. What amperage is required?
Click to expand...

The MOCP of the Trane Roof Top Units is 150 amps.
 
engineerintraining12 said:
I'm a little confused on why I still can't add current limiting fuses after the fact. Shouldn't the manufacturer have literature on whether specific fuses that were tested with their equipment will work or not. Also, is it wrong to assume that a current limiting fuse that dropped the current below 5kA is acceptable? If I were to supply current limiting fuses in a separate disconnect why wouldn't this accomplish the same goal?
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On your evaluation are you calculating upstream "Let through" based on the component or just inductive such as wire size and distance. I know there are several was to estimate it. One way assumes the protector does not exist, the other is based on Device "Let Through"
 
engineerintraining12 said:
Shouldn't the manufacturer have literature on whether specific fuses that were tested with their equipment will work or not.
Click to expand...

You have to ask the manufacturer, if they have this literature. Your external fuses need to be 'tested' to work in series with any overcurrent protection devices in the equipment.

IMHO, The contractor owes the customer the correct equipment.
 
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
 
Be careful before recommending that they add fuses or in any way modify components within the unit. In accordance with the NEC the HVAC must be UL Listed. Modifying the equipment will void this listing and require a field inspection to be re-listed. I recently ran into this and surprisingly enough the manufacture chose to have the installer modify the unit in the field based on my recommendations and then had a UL inspector make a field visit to re-list it.
 
As long as the equipment does not have OCPDs that will introduce dynamic impedance into the short circuit, fuses can be used to limit the current, but you must have enough short circuit current available at the location of the fuse so that the fuse is operating in its current limiting range. That may not be the case with an available short circuit current of 8,000 amps.
 
Jraef said:
The SCCR is what it is at this point, the time to think about this was when writing purchase specifications, REQUIRING that the SCCR of any electrical equipment provided must meet or exceeds the conditions. You cannot "fix" it in the field. keep that in mind for next time.

The only options now are to lower the available fault current of the system, either by adding a transformer as mentioned, a current limiting reactor, or increasing the conductor length to add impedance/resistance. The issue of "adding fuses" is often misrepresented. That could have been done BY the manufacturer BEFORE calculating the SCCR, using an engineering process of evaluating the let-through current of the fuses and the withstand rating of the worst-case component. But most likely they did nothing, instead option for the "courtesy" 5kA rating that UL allows people to apply without taking any steps to do better, and that is what you are now stuck with. It is NOT at all difficult to attain a higher SCCR, it just takes some thought and attention to details. Your supplier did neither.
Click to expand...
UL508a allows current limiting fuses to be supplied outside of the UL508a panel if they are needed to increase the SCCR. It just has to be noted on the drawings. In many cases this is all it takes to improve the default SCCR, but the manufacturer has to actually do something to make it happen.
 
engineerintraining12 said:
Yea, this was a failure on the supplier and the contractors part. The engineer even specified higher rated equipment on their drawings. They ordered the equipment before they even asked me to do this study, so this was not anything missed on my part.
Click to expand...
Tread very carefully here. I am reading between the lines a bit, but it sounds like you were contracted to do the study and not the design. I would be very hesitant to offer any advice on how to potentially remedy this situation that did not directly involve the manufacturer and original design engineer. Since what was specified was not provided, this situation has the potential to get very messy and expensive. Don't open yourself up to any liability by proposing solutions that may not be acceptable to the original engineer, owner, or manufacturer. Do not go outside your specific scope to try to help with this issue. It will bite you in the butt.
 
don_resqcapt19 said:
As long as the equipment does not have OCPDs that will introduce dynamic impedance into the short circuit, fuses can be used to limit the current, but you must have enough short circuit current available at the location of the fuse so that the fuse is operating in its current limiting range. That may not be the case with an available short circuit current of 8,000 amps.
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where does it say this in the code?
 
d0nut said:
Tread very carefully here. I am reading between the lines a bit, but it sounds like you were contracted to do the study and not the design. I would be very hesitant to offer any advice on how to potentially remedy this situation that did not directly involve the manufacturer and original design engineer. Since what was specified was not provided, this situation has the potential to get very messy and expensive. Don't open yourself up to any liability by proposing solutions that may not be acceptable to the original engineer, owner, or manufacturer. Do not go outside your specific scope to try to help with this issue. It will bite you in the butt.
Click to expand...

Totally agree with you on this. I'm just going to note that the equipment SCCR is below the available fault current and tell them to consult the manufacturer for more information on what can be done. I may say that a fusible disconnect with current limiting fuses is a possible solution as long as the manufacturer has verified this and given their blessing.
 
engineerintraining12 said:
Totally agree with you on this. I'm just going to note that the equipment SCCR is below the available fault current and tell them to consult the manufacturer for more information on what can be done. I may say that a fusible disconnect with current limiting fuses is a possible solution as long as the manufacturer has verified this and given their blessing.
Click to expand...
In my experience, if a manufacturer ships something with a 5kA SCCR, it's BECAUSE they did not want to bother with even the simplest of engineering principles it would have takne to get a higher value, such as asking the component mfr for the series listings. What I see happen is that they used XYZ brand of circuit breaker, ABC brand of contactor and FGH brand of overload relay, all because each individual component was cheapest they could find; engineering by procurement department. Competitors don't typically spend the money to have their devices series listed with competitive products, they are going to test them with their own. So all it really takes is for the mfr of the panel to chose components that have been series tested and listed for a higher SCCR. It's not in the LEAST BIT difficult. So if they didn't bother with that tiny tiny amount of effort to click some links from a supplier, they most likely are not going to give you an engineered fuse selection to go ahead of it. But good luck...
 
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