First time short circuit study - doing this right?

[dec_primary]

Hi All,

I'm working with a 480/277V 2000A switchgear which has no labels on the cover or the breakers on the AIC rating. I'm adding a stepdown 112.5kVA transformer and a 400A panel downstream for EV chargers. The building department wants me to show the available fault current and the short circuit rating at each piece of equipment.

Please bear with me as this is a first for me

1) If I cannot find the AIC rating, does this mean I have to do a full short circuit study, going all the way upstream and contacting the utility to find out what their transformer's AFC/impedances are?
2) If I did have the AIC rating, would all I need to show to the building department be the available fault current at the secondary of my new transformer and panel, and the appropriate breaker SSCR ratings to show I'm covering the maximum available fault current?

 

[jim dungar]

You need to prove any additions made to the 480V existing equipment have the appropriate AIC, so you need to know the available fault current or a worst case.

If all you need is to determine a minimum AIC/SCCR for devices and equipment after the transformer, you can determine a worst case fault by assuming an infinite source. Simply take the full load amps of the transformer and divide by its per cent impedance (%Z).

 

[dec_primary]

Thanks Jim.

So to check my calcs, I would get 112.5kVA/208*sqrt(3) = 312.27 A (FLA) and then my AFC = 312.27A/.05 (assumption for now) which would land me at 6.25kA.

Making my breaker protecting the transformer a 10KAIC breaker. I'm assuming everything downstream of that is safe to size to the same KAIC value? My panel MCB and branch breakers also at 10KAIC?

When would I need to do a full short circuit study? Is that if this transformer was existing and I was planning on using it to add a panel?

 

[electrofelon]

Thanks Jim.

So to check my calcs, I would get 112.5kVA/208*sqrt(3) = 312.27 A (FLA) and then my AFC = 312.27A/.05 (assumption for now) which would land me at 6.25kA.

Making my breaker protecting the transformer a 10KAIC breaker. I'm assuming everything downstream of that is safe to size to the same KAIC value? My panel MCB and branch breakers also at 10KAIC?

When would I need to do a full short circuit study? Is that if this transformer was existing and I was planning on using it to add a panel?

When you say "breaker protecting the transformer" is that on the primary or secondary side? If on the primary side, the AFC that sees would be based on the upstream transformer and conductor length.

 

[jim dungar]

Thanks Jim.

So to check my calcs, I would get 112.5kVA/208*sqrt(3) = 312.27 A (FLA) and then my AFC = 312.27A/.05 (assumption for now) which would land me at 6.25kA.

Making my breaker protecting the transformer a 10KAIC breaker. I'm assuming everything downstream of that is safe to size to the same KAIC value? My panel MCB and branch breakers also at 10KAIC?

When would I need to do a full short circuit study? Is that if this transformer was existing and I was planning on using it to add a panel?

Yes, your example is correct. Just be aware that transformers less than 500kVA often have impedances <5%.

Short circuit current does not get worse at downstream equipment unless there is another downstream source.

You should do a full short circuit study when the infinite bus method results are not satisfactory or when performing Arc Flash analyses.

 

[dec_primary]

When you say "breaker protecting the transformer" is that on the primary or secondary side? If on the primary side, the AFC that sees would be based on the upstream transformer and conductor length.

Yeah, I meant on the primary side. Okay, so my calculation since my secondary voltage is 208V is for the AFC on the transformer secondary and so my panel main and branch circuits would be rated for 10kAIC. On the primary side, it looks like I'll have some digging to do and contact the utility because there's absolutely no information on the switch gear regarding the kAIC rating. Is that right?

 

[dec_primary]

Yes, your example is correct. Just be aware that transformers less than 500kVA often have impedances <5%.

Short circuit current does not get worse at downstream equipment unless there is another downstream source.

You should do a full short circuit study when the infinite bus method results are not satisfactory or when performing Arc Flash analyses.

Thank you. I just responded to the other comment regarding the primary vs secondary side of the transformer but now come to think of it...knowing what the utility transformer's impedance/voltage is is probably unnecessary because the voltage would be so much higher. So the worst case would indeed be the the secondary of my new transformer? Sorry for all the questions guys, but appreciate all the responses!

 

[electrofelon]

. So the worst case would indeed be the the secondary of my new transformer? Sorry for all the questions guys, but appreciate all the responses!

No the AFC on the primary of your new transformer would be much higher. You will need to either get a figure from the utility or get the utility transformer information and calculate it yourself.

 

[jim dungar]

Thank you. I just responded to the other comment regarding the primary vs secondary side of the transformer but now come to think of it...knowing what the utility transformer's impedance/voltage is is probably unnecessary because the voltage would be so much higher. So the worst case would indeed be the the secondary of my new transformer? Sorry for all the questions guys, but appreciate all the responses!

Protective device AIC selection needs to be done separately for each side of the transformer.

Are you adding a new 480V protective device that will be feeding this new transformer? If you are you need to determine what AIC is needed based on the calculated fault current.

If you are using an existing protective device, how sure are you that its AIC was determined correctly? For example has the utility changed its transformer since the original service installation?

 

[augie47]

Definitely anecdotal but just to give you a ballpark number, the SCA on a couple of 112.5 kva I installed turned out to be in the 15-20k range.

 

[dec_primary]

No the AFC on the primary of your new transformer would be much higher. You will need to either get a figure from the utility or get the utility transformer information and calculate it yourself.

Okay, I think I am getting closer. My new breaker serving my 112.5kVA transformer on the switchgear is going to see a much higher AFC because despite my higher operating voltage, the upstream transformer is going to be significantly larger. If for example's sake the 2000A main on the switchgear was sized exactly for 125% of the kVA rating of the utility transformer, I may have something like a 1.3 MW utility transformer upstream, giving me roughly 1600A FLA and at 5% impedance and roughly 32kA available fault current worst case scenario. I won't actually assume my OCP on the switchgear to obtain my utility transformer's kVA value when actually calculating.

Am I there yet or am I still missing something?

 

[dec_primary]

Definitely anecdotal but just to give you a ballpark number, the SCA on a couple of 112.5 kva I installed turned out to be in the 15-20k range.

Thanks. I'm doing work in a commercial space with relatively low demands but they definitely juiced the power coming into the building. Maybe an uncommon situation, but if the utility transformer is as large as I'm expecting, looks like the SCA might be a good amount higher than that.

 

[augie47]

Thanks. I'm doing work in a commercial space with relatively low demands but they definitely juiced the power coming into the building. Maybe an uncommon situation, but if the utility transformer is as large as I'm expecting, looks like the SCA might be a good amount higher than that.

I was referencing the secondary side of the transformer only not your 480 panel.

 

[jim dungar]

Thanks. I'm doing work in a commercial space with relatively low demands but they definitely juiced the power coming into the building. Maybe an uncommon situation, but if the utility transformer is as large as I'm expecting, looks like the SCA might be a good amount higher than that.

Be careful.
Utilities undersize their transformers as much as 50% versus what an NEC calculated load is. And because they have to pay for transformer losses they often buy units with low %Z.

But, then the utility says it never knows when a transformer might be replaced with a larger unit, so they give you available fault currents based on some possible future condition. For example during a disaster recovery they may install an oversized transformer if it is available and fits on their pad.

 

[topgone]

Definitely anecdotal but just to give you a ballpark number, the SCA on a couple of 112.5 kva I installed turned out to be in the 15-20k range.

True. My notes tell me %Z = 1.7% for a 112.5 kVA transformer (banked 3 X 37.5 kVA, SCA= 18 kA)

 

[kingpb]

Personally, I would never size based on utility fault current, would always use infinite bus for this size of equipment. If you had adjustable setting capability on your device, that's when you use their info for coordination settings. But for sizing, you do device duty calcs and use infinite bus.
BTW: the fault current at the HV side of your transformer would simply be (using topgone Z%)112.5KVA/0.017 = 6.62MVAsc, or 7.96kA assuming infinite bus.
That doesn't solve for what the SC rating of the 480V gear is. If it isn't marked anywhere on the gear or breakers, I would look up the type in a catalog and use the lowest available for it, i.e. if it comes in for example 10kA, 25kA, and 42kA, or 65kA etc, use the 10kA. Although 480V, 2000A, my gut feeling is 42kA is the lowest rating.