Short Circuit Rating

Status
Not open for further replies.
R

raulpaez

New member
As per UL Standard Annex G the minimun amperage rating rating of equipment requiring 65KA is 800 amps I think this is incorrect it should be at least 400 amps as a minimum, but I got no answer from UL. Any comments will be appreciated.
Thnaks
 
I believe the OP is concerning the design SCCR requirements of a switchboard bus structure (without breakers).

It appears they are questioning the UL appendix saying all 800A and larger switchboard bussing must be 'braced' for 65kA as a minimum.
 
Jim
I read it differently. He is saying that the minimum size panel the comes with a 65ka rating is 800 amps.
He thinks a 400 amps panel should be available with a 65 ka rating. I practice I can not see the utility
providing a transformer large enough for 400 amp service that would produce 65 ka.
 
bob said:
He thinks a 400 amps panel should be available with a 65 ka rating. I practice I can not see the utility
providing a transformer large enough for 400 amp service that would produce 65 ka.
Click to expand...

Sorry but I have seen installations where 200A panels required 65kA bus bracing.
Xcel Energies, a POCO in ND, SD, MN, and WI, requires equipment rated for 52kA @ 208V when fed by a 300kVA padmount transformer.
So it is good that almost every panel is available with this rating (for example, a 12 ckt 125A load center, Square D QO112L125G).

I guess we need to know what wording the OP is actually questioning.
 
I design stuff all the time that requires panelboards of many sizes, including 100A that require 65kA all the way up to 200kA. Higher than 65kA, I have to allow sereis rated equipment, so don't think just ebcause it is a small panel it doesn't need a high rating. If we put a 480V panelboard in the service room of a NYC highrise building (fed at 480V), you know it will be rated 200kA in some way.
 
jim dungar said:
I believe the OP is concerning the design SCCR requirements of a switchboard bus structure (without breakers).

It appears they are questioning the UL appendix saying all 800A and larger switchboard bussing must be 'braced' for 65kA as a minimum.
Click to expand...

I agree, I think this is a misinterpretation on the part of the OP. Just because all bus structures 800A and up must have 65kA bracing doesn't mean that all bus structures with 65kA bracing are thereby 800A and up.

Kind of like saying "All squares are rectangles, therefore all rectangles are squares."
 
bob said:
why? are the planning ahead for a larger replacement?
Click to expand...
Yes.

Xcel Energy | Standard for Electric Installation and Use | Section 5: Transformers said:
Tables IA through V in this Section show the available RMS symmetrical fault currents that may be expected at the secondary terminals of distribution transformers. Each fault current value listed in the tables is based on the lowest percent impedance transformer that might be set initially or as a replacement. No primary source or secondary line impedance has been included since it is generally relatively small, may change, and cannot be accurately forecasted.

Note: Because an overloaded transformer is typically replaced by the next larger size transformer, and an under-loaded transformer may be replaced by the next smaller size transformer, the customer is encouraged to use this range of transformers to perform their studies and select equipment such as current limiting fuses, breakers and switchgear bus bar bracing.
Click to expand...

This is typical of the 4 largest POCOs in this area.
 
Status
Not open for further replies.
Top