SCCR of HVAC Equipment

[Charz]

The available short circuit current at the switchgear is 65kA. From the switchgear the HVAC Packaged unit is to be installed at a distance of 40 Meters. Do the SCCR of HVAC Packaged unit have to be rated at 65kA?

Can it be reduced by taking in to account the cable impedance at the terminals of the HVAC Packaged unit?

 

[bwat]

Do the SCCR of HVAC Packaged unit have to be rated at 65kA?

No

Can it be reduced by taking in to account the cable impedance at the terminals of the HVAC Packaged unit?

Yes. It has to be rated for the available fault current at the point where it is installed.

Are you sure it’s 65kA at the switchgear? That’s a common number for the AIC/SCCR rating of equipment, so someone may have mistook that to mean the available at that point.

 

[wbdvt]

What bwat said. The 65kA sounds more like the rating of the switchgear not the available fault current from the utility. I suspect it would be much lower and even lower still at the HVAC unit. You need to get the available fault current from the utility.

 

[Jraef]

1. The HVAC equipment needs to have an SCCR equal to or greater than the Available Fault Current AT THE TERMINALS OF THE EQUIPMENT.
2. Yes, you factor in the impedance of the conductors between the service entrance and the equipment.

But the others are right, it's unlikely that you actually have 65kA available. Gear is made in incremental values, like 25kA, 35kA, 42kA and 65kA (it's different for different mfrs). So if your brand offers 35kA and 65kA, but the AFC is 40kA, someone had to buy the 65kA rated equipment. And ITS rating is only relevant for ITS line terminals. If that 40kA at the Service Entrance drops to 34kA by the time it gets to your HVAC equipment, then your equipment need only be rated for at least 34kA.

Now... side bet.

Its going to have a rating of 5kA SCCR because nobody thought to make that part of the purcjhse specifications, so the HVAC supplier is just going to give you the "courtesy" untested rating of 5kA... If the gear has not been bought yet and you are asking this because you are thinking ahead, BRAVO!

 

[petersonra]

a lot of places spec 65kA panels, boards, and MCCs because the added cost is not all that much over lower values and it is rare that the SCC would exceed it. often you have to order or spec stuff long before you know what the SCC is, so picking a value that almost certainly will work makes sense.

 

[topgone]

Take note of the size of conductors used and the full load amps of your HVAC! There, you'll have the available fault at your equipment!

 

[wbdvt]

Take note of the size of conductors used and the full load amps of your HVAC! There, you'll have the available fault at your equipment!

That is simply false. The FLA is completely independent of the available fault current at the equipment. The available fault current at the equipment is a function of the available fault current the utility is capable of delivering to the service entrance. Then that fault current is reduced by the system impedance to the point of interest. Then you will have the fault current that the equipment must be rated to withstand.

 

[topgone]

That is simply false. The FLA is completely independent of the available fault current at the equipment. The available fault current at the equipment is a function of the available fault current the utility is capable of delivering to the service entrance. Then that fault current is reduced by the system impedance to the point of interest. Then you will have the fault current that the equipment must be rated to withstand.

The available fault at the equipment location will be smaller at the equipment location compared to the system available fault current at POCC owing to the impedance of the conductors used:
I(fault available at equipt loc.) = I(system fault available at POCC) X (Z system)/(Z system + Z conductor)
That is the truth. Been there, done that and got the t-shirt thing!

 

[wbdvt]

Topgone - that is true about the available fault being smaller at the equipment location and is what I said but is not what you said in your post that I quoted. You implied that all you need to get the available fault current at your equipment is the size of conductors and FLA of the HVAC, which is totally incorrect. Go back and read your posting.

 

[Jraef]

Topgone - that is true about the available fault being smaller at the equipment location and is what I said but is not what you said in your post that I quoted. You implied that all you need to get the available fault current at your equipment is the size of conductors and FLA of the HVAC, which is totally incorrect. Go back and read your posting.

I too was confused by that wording, I interpreted it the same way. You need that information but that’s not all you need to do the calculation. You need to know what you REALLY have at the panel feeder, and just knowing that the panel is rated for 65kA is not the real information.

 

[GoldDigger]

I too was confused by that wording, I interpreted it the same way. You need that information but that’s not all you need to do the calculation. You need to know what you REALLY have at the panel feeder, and just knowing that the panel is rated for 65kA is not the real information.

Are you at least justified in assuming that the available fault current is less than 65kA, or do you need to start from scratch on the calculation?

 

[petersonra]

Are you at least justified in assuming that the available fault current is less than 65kA, or do you need to start from scratch on the calculation?

Start from scratch. It is likely less than 65k but no way to know without doing the work.

 

[Jraef]

Start from scratch. It is likely less than 65k but no way to know without doing the work.

Yes. You can get caught in following the previous guy’s mistake... been there, done that, got the t-shirt and the hat. Thorough it always better.

 

[topgone]

Topgone - that is true about the available fault being smaller at the equipment location and is what I said but is not what you said in your post that I quoted. You implied that all you need to get the available fault current at your equipment is the size of conductors and FLA of the HVAC, which is totally incorrect. Go back and read your posting.

It might sound indirect, but remember that you will size the conductors with the full-load current in mind! Needless to say, you take note of the FLA of the equipment and then go from there. The point is, start from the bottom up and you'll never miss a thing doing that, IMO.

 

[wbdvt]

Topgone - other than sizing the conductors in accordance with the NEC for the load and using that impedance in the fault current calculation, your statement is not really germane to the conversation on the available fault current. You need to model the system starting with the available fault current from the utility all the way to the point of interest taking into account the impedance of the conductors and transformers (if any) to determine the fault current at that point of interest. You don't start from the bottom up to build your impedance diagram.

Have you ever built an impedance diagram to determine the fault current at a particular point in an electrical system?

 

[kwired]

It might sound indirect, but remember that you will size the conductors with the full-load current in mind! Needless to say, you take note of the FLA of the equipment and then go from there. The point is, start from the bottom up and you'll never miss a thing doing that, IMO.

If you have say 43kA available at the panel, and two identical HVAC units with identical sized supply conductors, but one is only 10 feet in length and the other is 100 feet in length, you will have different available fault current at each unit even though everything else is identical. Impedance over that extra 90 feet makes a difference.