is it necessary to do a short circuit for this?

[Isaiah]

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.

Nice feedback. This particular unit is an industrial type HVAC with MCA of 70A at 480V 3 phase. They ran number 3/C w/Grd, 4AWG, XHHW-2 for a distance of around 100’, which looks good from an ampacity and VD standpoint but wasn’t sure how to calculate the SC when asked


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[kwired]

Nice feedback. This particular unit is an industrial type HVAC with MCA of 70A at 480V 3 phase. They ran number 3/C w/Grd, 4AWG, XHHW-2 for a distance of around 100’, which looks good from an ampacity and VD standpoint but wasn’t sure how to calculate the SC when asked


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Mike Holt's main website under free stuff has (at least used to) a fault current calculator that is an Excel spreadsheet you can download.

You need to know what fault current is at your starting point to plug into the sheet, or it can calculate starting from the source but you need to know kVA and impedance of the source - it assumes an infinite primary which is ok for determining withstand ability like you are doing.

I'd guess with 100' of 4 AWG your result will be lower than rating of the equipment though. If you went through mentioned spread sheet starting with source and going to the final load you would likely see one the bigger drops of AFC in that final branch circuit segment, and it could surprise you just how much it dropped over that length and size of conductor.

 

[augie47]

Do you know the SCCR of the unit ?
As can been seen in this earlier thread, HVAC units are notorious for low SCCR ratings.

HVAC and SCCR requirements

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...

forums.mikeholt.com

With that in mind, 100' of #4 would lower a 65k AIC to below 10k but the devil is in the details.

 

[kwired]

As can been seen in this earlier thread, HVAC units are notorious for low SCCR ratings.

My understanding is 5kA is kind of a default rating if not tested for higher ratings. Many HVAC likely can withstand higher but if they don't test it it defaults to 5kA.

I've also heard stories on forums like this of getting higher rating labels from manufacturer (for a price) but they never evaluated the individual unit the label will apply to. This tells me they already have tested them just won't label them unless requested.

 

[Isaiah]

My understanding is 5kA is kind of a default rating if not tested for higher ratings. Many HVAC likely can withstand higher but if they don't test it it defaults to 5kA.

I've also heard stories on forums like this of getting higher rating labels from manufacturer (for a price) but they never evaluated the individual unit the label will apply to. This tells me they already have tested them just won't label them unless requested.

I just checked nameplate SCCR: 5kA symmetrical
So we should be good right? But I will go thru the calc as you and Augie have noted
Thank you very much Kwired!


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[electrofelon]

HVAC units are notorious for low SCCR ratings.

If you saw over in that thread about the wireway where I said If I were King I would round up pre punched ones up and throw them in the Mariana trench. Well I would also do that for 5K HVAC units (the people who made them 5k)

 

[augie47]

That would be a red flag for me and can see why it was questioned.
Just playing the numbers, even if the SCA at the supply end was as low as 10k you would still exceed the 5k SCCR of the unit

 

[Isaiah]

That would be a red flag for me and can see why it was questioned.
Just playing the numbers, even if the SCA at the supply end was as low as 10k you would still exceed the 5k SCCR of the unit

The source is in fact 10kA


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[augie47]

If the 10kA is the actual available at the source of your feeder then if I was the EI, someone one would "have some 'splainin' to do".
Without taking any motor contribution into account the SCA at the unit seems to be 5,300+

 

[Isaiah]

If the 10kA is the actual available at the source of your feeder then if I was the EI, someone one would "have some 'splainin' to do".
Without taking any motor contribution into account the SCA at the unit seems to be 5,300+

Augie would you mind posting how you derived the 5.3kA?


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[Fred B]

Nice feedback. This particular unit is an industrial type HVAC with MCA of 70A at 480V 3 phase. They ran number 3/C w/Grd, 4AWG, XHHW-2 for a distance of around 100’, which looks good from an ampacity and VD standpoint but wasn’t sure how to calculate the SC when asked


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I know was suggested the Mike Holt stuff. There is also an app from Bussman that is a fairly comprehensive tool to calculate the Available fault current. If you have a calculated starting point you can use that to go to the equipment or can start all the way back to the POCO. Starting at the POCO transformer KVA, secondary voltage and impedance and plug in figures. Add conductors other transformers, motor loads contribution etc. and you can get all the way to a piece of equipment.

 

[jim dungar]

The source is in fact 10kA


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Probably a made up number.
Most available short circuit values are expressed in actual amps to 3 or 4 significant digits.

 

[augie47]

Augie would you mind posting how you derived the 5.3kA?


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I used the Bussmann FC2 app.

 

[Isaiah]

I know was suggested the Mike Holt stuff. There is also an app from Bussman that is a fairly comprehensive tool to calculate the Available fault current. If you have a calculated starting point you can use that to go to the equipment or can start all the way back to the POCO. Starting at the POCO transformer KVA, secondary voltage and impedance and plug in figures. Add conductors other transformers, motor loads contribution etc. and you can get all the way to a piece of equipment.

If I’m only concerned about sizing a branch circuit off a three phase LV Panelboard to a HAVC unit do I have to go all the way back to the POCO XFMR to get the cable size per short circuit requirement?


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[jim dungar]

If I’m only concerned about sizing a branch circuit off a three phase LV Panelboard to a HAVC unit do I have to go all the way back to the POCO XFMR to get the cable size per short circuit requirement?


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Yes.
Short circuit values are dependent on what comes from the source.
This is one reason recent code versions have added SCA labeling requirements for the service equipment.

 

[Fred B]

If I’m only concerned about sizing a branch circuit off a three phase LV Panelboard to a HAVC unit do I have to go all the way back to the POCO XFMR to get the cable size per short circuit requirement?


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Yes, unless it was already calculated and recorded at the equipment you are feeding from. But that is usually only at a commercial/industrial facility.

 

[Isaiah]

Yes, unless it was already calculated and recorded at the equipment you are feeding from. But that is usually only at a commercial/industrial facility.

This is a very large, heavy-industrial facility with big MVA transformers in a client-owned utility SS. They feed several 15kV SWGR which in turn feed numerous 13.8kV - 480V XFMRs
They in turn feed 480V SWGR which has feeders run to local bus bar enclosures that tap down into small dry type XMFR that feed 480-480/277V and 480-208/120V panelboard that feed small loads such as the HVAC unit.
I assume the large XFMRs and SWGR have already been calculated and I should focus on the small XFMRs to Panelboard to HVAC for short circuit cable sizing.
Correct me if I’m wrong here.


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[augie47]

IMO you could get a fairly accurate number starting at the transformer feeding your panelboard. You will need the impedance of that transformer to establish a starting number. The Bussmann app will guide u through

 

[Isaiah]

IMO you could get a fairly accurate number starting at the transformer feeding your panelboard. You will need the impedance of that transformer to establish a starting number

I think you’re right on this. I believe the impedance is 5%


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[Isaiah]

Yes.
Short circuit values are dependent on what comes from the source.
This is one reason recent code versions have added SCA labeling requirements for the service equipment.

I disagree - that would require running ETAP each time you sized a branch circuit - that’s excessive to say the least

if you have a separately derived system, such as small dry type transformer you can get a very close-
also this is not a service


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