is it necessary to do a short circuit for this?

I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
I was asked if I had performed a short circuit calculation for a feeder to HVAC unit. I sized the feeder based on the MCA of the unit. Why would it be necessary to do a short circuit calculation on something this simple?


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ptonsparky said:
As an EI, I would think you would be telling us why.

Was this a large roof top unit in an industrial setting or a small residence in an older neighborhood?
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The Forum is for professional commentary not your personal opinion.


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Isaiah said:
The Forum is for professional commentary not your personal opinion.


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That was a professional comment. It would have been worded differently had it been my personal opinion.

A professional comment is personally voiced when I find new equipment with 5 KVA rating on a service with well over 50 available.

As an EC I am supposed to know. Why wouldn’t it be expected of you? I brought up the same point. Is this residential or not?

The informational note to 110.24(A) has part of the reason why.
 
It is technically required to compare available fault current and the sccr of the equipment and to be sized appropriately to prevent unintended rapid disassembly of the equipment in event of a fault. You want the breakers to trip and not blowup or equipment to not blowup before the breaker trips.
So technically, if you have a resi panel rated to 10kA and the branch breaker is 10kA and equipment is only 5kA but service has a potential of 18kA you have potential of catastrophic failure on the entire system. If there is only 8kA from the service then your equipment is still at rick but a proper calculation considering factors like VD prior to the equipment could have reduced the overall AFC to the equipment. So even though the numbers on the equipment may seem to be out of specs the other factors that impact the short circuit current calculations can show it to be safe from catastrophic failure related to a fault.

Never see it being done on residential systems, but should be fairly common in commercial/industrial systems.
 
Fred B said:
It is technically required to compare available fault current and the sccr of the equipment and to be sized appropriately to prevent unintended rapid disassembly of the equipment in event of a fault. You want the breakers to trip and not blowup or equipment to not blowup before the breaker trips.
So technically, if you have a resi panel rated to 10kA and the branch breaker is 10kA and equipment is only 5kA but service has a potential of 18kA you have potential of catastrophic failure on the entire system. If there is only 8kA from the service then your equipment is still at rick but a proper calculation considering factors like VD prior to the equipment could have reduced the overall AFC to the equipment. So even though the numbers on the equipment may seem to be out of specs the other factors that impact the short circuit current calculations can show it to be safe from catastrophic failure related to a fault.

Never see it being done on residential systems, but should be fairly common in commercial/industrial systems.
Click to expand...
Is this something that may be done during a plan review by the inspection department?
 
ptonsparky said:
That was a professional comment. It would have been worded differently had it been my personal opinion.

A professional comment is personally voiced when I find new equipment with 5 KVA rating on a service with well over 50 available.

As an EC I am supposed to know. Why wouldn’t it be expected of you? I brought up the same point. Is this residential or not?

The informational note to 110.24(A) has part of the reason why.
Click to expand...

There you go again
Thankfully the Forum provides a blocking means. Goodbye….


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Fred B said:
It is technically required to compare available fault current and the sccr of the equipment and to be sized appropriately to prevent unintended rapid disassembly of the equipment in event of a fault. You want the breakers to trip and not blowup or equipment to not blowup before the breaker trips.
So technically, if you have a resi panel rated to 10kA and the branch breaker is 10kA and equipment is only 5kA but service has a potential of 18kA you have potential of catastrophic failure on the entire system. If there is only 8kA from the service then your equipment is still at rick but a proper calculation considering factors like VD prior to the equipment could have reduced the overall AFC to the equipment. So even though the numbers on the equipment may seem to be out of specs the other factors that impact the short circuit current calculations can show it to be safe from catastrophic failure related to a fault.

Never see it being done on residential systems, but should be fairly common in commercial/industrial systems.
Click to expand...

Thank you Fred for this insightful and professional commentary


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ptonsparky said:
Is this something that may be done during a plan review by the inspection department?
Click to expand...
Entirely new maybe, and hopefully plan review catching an issue, but most times it is the electrical contractor or designing engineer that would be providing the calculation and sizing recommendations. Big issue come across a lot is the POCO's unwillingness to provide a solid starting number so there would be a lot of "assumptions" and worse case scenario into the calculations, and sometimes leading to unnecessary oversizing.

See a lot of times a change to a transformer after initial installation that would cause equipment to be out of specs. So anytime that there is a change to the system a Short circuit study should be done, but not really ever seen done in residential. But have done a rough study once on a system that the POCO upgrade to a new transformer that had a potential change from what was potentially a 8kA to 20kA AFC, and potentially putting the resi panels out of spec. Didn't have the transformer actual internal specs or most of the other details that POCO wouldn't release anyway, so can't say for a certainty that the system is at risk.

That is part of the reason in NFPA 70B A.6.3
The following are some of the conditions that might require an update of the baseline short-circuit study:
(1) A change by the utility
(2) A change in the primary or secondary system configuration within the facility
(3) A change in the transformer size (kVA) or impedance (percent Z)
(4) A change in conductor lengths or sizes
(5) A change in the motors connected to the system
 
Isaiah said:
I was asked if I had performed a short circuit calculation for a feeder to HVAC unit. I sized the feeder based on the MCA of the unit. Why would it be necessary to do a short circuit calculation on something this simple?


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Click to expand...
AIC and SCCR are code requirements. It is my opinion that an inspector can ask for this information whenever he wants. Now of course it might not be common in a residential setting, but he is within his rights to inquire if he wants.
 
As an EI I would think your responsibility was to assure this had been addressed by the designer or contractor. That said, when I saw a low SCCR I often did a quick field calculation using Bussmann FC2app, based on the feeder specifics to see if there was a potential problem
 
Last edited:
augie47 said:
As an EI I would think your responsibility was to assure this had been addressed by the designer or contractor. That said, when I saw a low SCCR I often did a quick field calculation using Bussmann FC2app, based on the feeder specifics to see if there was a potential problem
Click to expand...

It’s not the inspectors responsibility to calculate short circuit - it’s plausible however to ask for it


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I have never seen an inspector perform a short circuit calc to validate cable size, MCB size etc. But sometimes they've asked to see this info, especially if there some obvious doubts, which is part of NEC compliance.
 
Fred B said:
It is technically required to compare available fault current and the sccr of the equipment and to be sized appropriately to prevent unintended rapid disassembly of the equipment in event of a fault. You want the breakers to trip and not blowup or equipment to not blowup before the breaker trips.
So technically, if you have a resi panel rated to 10kA and the branch breaker is 10kA and equipment is only 5kA but service has a potential of 18kA you have potential of catastrophic failure on the entire system. If there is only 8kA from the service then your equipment is still at rick but a proper calculation considering factors like VD prior to the equipment could have reduced the overall AFC to the equipment. So even though the numbers on the equipment may seem to be out of specs the other factors that impact the short circuit current calculations can show it to be safe from catastrophic failure related to a fault.
Click to expand...
I love this..."prevent unintended rapid disassembly of the equipment in event of a fault".
Ok if I use it?
 
MyCleveland said:
Goodbye… ? are you a Robot?
It was a kind way of saying you should know this....code requirement to calc and LABEL equipment for years along with many other items.
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Adiós instead? Hahaha - it’s kinda fun blocking folks


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Fred B said:
Entirely new maybe, and hopefully plan review catching an issue, but most times it is the electrical contractor or designing engineer that would be providing the calculation and sizing recommendations. Big issue come across a lot is the POCO's unwillingness to provide a solid starting number so there would be a lot of "assumptions" and worse case scenario into the calculations, and sometimes leading to unnecessary oversizing.

See a lot of times a change to a transformer after initial installation that would cause equipment to be out of specs. So anytime that there is a change to the system a Short circuit study should be done, but not really ever seen done in residential. But have done a rough study once on a system that the POCO upgrade to a new transformer that had a potential change from what was potentially a 8kA to 20kA AFC, and potentially putting the resi panels out of spec. Didn't have the transformer actual internal specs or most of the other details that POCO wouldn't release anyway, so can't say for a certainty that the system is at risk.

That is part of the reason in NFPA 70B A.6.3
The following are some of the conditions that might require an update of the baseline short-circuit study:
(1) A change by the utility
(2) A change in the primary or secondary system configuration within the facility
(3) A change in the transformer size (kVA) or impedance (percent Z)
(4) A change in conductor lengths or sizes
(5) A change in the motors connected to the system
Click to expand...

You’re entirely correct - short circuit is performed upfront by the design team. Inspectors sometimes validate the info during detailed inspections but they don’t perform the calculations.
There’s always going to be a certain amount of arrogance here on the Forum from the “know it alls”.


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Isaiah said:
You’re entirely correct - short circuit is performed upfront by the design team. Inspectors sometimes validate the info during detailed inspections but they don’t perform the calculations.
There’s always going to be a certain amount of arrogance here on the Forum from the “know it alls”.


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Some of us design-build a majority of our installations, but yes such calculation should be done before selecting key items or you could end up having to change things.

After you been doing this long enough you kind of get the feel to what needs looked into for SCC and what doesn't. Smaller conductors and length of run over 25 feet will drop available current at the end of the run pretty quickly compared to large conductors at those shorter lengths. Plus you also have service and feeder conductors that factor in before you have a starting SSC value to apply to your branch circuit when installing something to existing panelboard. A lot of HVAC equipment especially "residential grade" stuff is only rated for 5kA, usually that is not a problem in dwellings or light commercial locations anyway but can be situations where it doesn't hurt to double check.
 
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