Validating the kAIC rating of the breakers used in project

[FaradayFF]

Hi Guys,

Can someone validate my thinking process regarding my approach of specifying circuit breaker kAIC rating in lieu of doing a proper short circuit study for a small project I've been working on. I have a service coming in into a shed on industrial site, from there the feeder is going to a distribution panel which feeds several loads.
I've estimated the maximum utility fault contribution using the infinite bus and the utility overhead transformer data. This value would give me the maximum fault current value and my breakers downstream of the utility transformer would be sized to have interrupting rating higher than the maximum fault current at the secorndaries of the transformer. I'm on the conservative side here, since I'm not taking into account the impedance of the cables.

Thanks,
EE

 

[electrofelon]

That is fine. Many times, a series rating system is used so the available fault current and AIC past the service doesn't matter. If you aren't using a series rated system or there isn't a series rating available for what you need, it may behoove you to take into account feeder impedance so you are not spending money on KAIC capacity that you don't need.

 

[paulengr]

Any inductive loads that are large enough that they would contribute to fault current?

SCCR does not have to be complicated for AIC purposes. This is back to old school when we did “conservative” (upper bound) estimates unlike arc flash where you need to be as accurate as possible due to time being a critical factor.

 

[electrofelon]

SCCR does not have to be complicated for AIC purposes. This is back to old school when we did “conservative” (upper bound) estimates.....

I think it's worth also noting that, at least in my experience, calculating using transformer data and infinite primary will usually be a lot lower than what POCO will tell you, so I don't really consider infinite primary and transformer data to be worst case.

 

[FaradayFF]

I think it's worth also noting that, at least in my experience, calculating using transformer data and infinite primary will usually be a lot lower than what POCO will tell you, so I don't really consider infinite primary and transformer data to be worst case.

That doesn't makes sense to me. The fault at the secondary of the transformer is a function of transformer impedance.

 

[FaradayFF]

Any inductive loads that are large enough that they would contribute to fault current?

SCCR does not have to be complicated for AIC purposes. This is back to old school when we did “conservative” (upper bound) estimates unlike arc flash where you need to be as accurate as possible due to time being a critical factor.

No large motors in this system.

 

[petersonra]

You really should not rely on anything other than the utility published values for available short circuit current. They are oriented around the worst case you might see no matter what transformer they install or change to down the road.

Most cases even a relatively short run of wire gets the scc down to manageable numbers so why not use the conductors to help you.

 

[electrofelon]

You really should not rely on anything other than the utility published values for available short circuit current.

I dont necessarily agree. Often these utility numbers are just plain out of the ballpark, even taking into account a potentially bigger and lower impedance transformer. There are certainly some situations where you should listen to the utility and not go off of what is there, but in my experience, more often than not you will get a completely ridiculous number from the utility.

 

[electrofelon]

That doesn't makes sense to me. The fault at the secondary of the transformer is a function of transformer impedance.

I pretty much addressed this in my previous post, but consider that utilities have ZERO incentive to give you an actual/realistic fault current value.

 

[petersonra]

I dont necessarily agree. Often these utility numbers are just plain out of the ballpark, even taking into account a potentially bigger and lower impedance transformer. There are certainly some situations where you should listen to the utility and not go off of what is there, but in my experience, more often than not you will get a completely ridiculous number from the utility.

What happens down the road when they install a different xfmr? Or change the network upstream? The number they are giving out is effectively a promise it won't get any worse. If you make up your own number you are on the hook.

 

[wbdvt]

In my experience, utilities will provide when asked for the short circuit current the infinite bus short circuit current based on the size of the transformer installed at the location. The impedance used is based on the lowest in their purchasing spec or transformer spec. This is fine for specifying the AIC and SCCR for equipment.
If a study is being done of the system, then I request from the utility the available short circuit current at the transformer primary fuses, riser cable data, transformer data (actual nameplate). Then when I model the system, especially for an older facility, not all the equipment is overdutied as may be the case for using infinite bus values.

 

[electrofelon]

What happens down the road when they install a different xfmr? Or change the network upstream? The number they are giving out is effectively a promise it won't get any worse. If you make up your own number you are on the hook.

I just think it depends on the situation. The world is full of what ifs. If the transformer is serving multiple customers then that is certainly a situation where one should take into account the possibility of a larger transformer. Same for a customer with a dedicated unit likely to add significant load.

But on the other side of the coin, take this building I am Working on now: Dedicated vault, %Z as low as it gets, nameplate is 33% larger than service size. POCO still gives me an AFC 30% higher, even with me using infinite primary

Network changes/upgrades would not be an issue if infinite primary was used.

The potential $$$ savings of course comes into play to. If you are looking at having to use several hundred "H" branch breakers at $20 a piece, or adding a $5k main breaker then I will look closer at realistic AFC numbers. To save $100, probably not.

 

[paulengr]

You really should not rely on anything other than the utility published values for available short circuit current. They are oriented around the worst case you might see no matter what transformer they install or change to down the road.

Most cases even a relatively short run of wire gets the scc down to manageable numbers so why not use the conductors to help you.

Agree partly here. Cable impedance is easily had by the point to point method in the Cooper book, same method by the way that the IEEE 1584-2002 spreadsheet uses. That spreadsheet also has the utility fault current and transformer model data too so it’s done for you. You can just ignore the arc flash result since that’s not being used. So essentially almost SKM grade results for the excellent price of $0 since the spreadsheet is widely available even without paying $699 for the standard.