malachi constant
Senior Member
- Location
- Minneapolis
I am the engineer on a project currently under construction. We have a chiller being installed in the vicinity of a 208V utility transformer. The chiller is fed directly from the transformer, with a fused switch at the chiller. The electrical inspector noted that the nameplate data shows the chiller (which is on-site) has a nameplate SCCR of 5kA, and he stated that he expects the available short circuit current to be less than this at the enclosure.
The available current at the utility transformer is on the order of 80kA, by the time it reaches the disconnect it is on the order of 25kA. Using the over-up-down method on the 500A fuses yields a let-through current on the order of 10kA. As near I can tell he is correct that this does not meet code, is therefore unsafe and will not pass inspection.
I have done some preliminary research and believe my best options are to install an impedance transformer or see if modifications can be made to the control panel to increase the SCCR. Both of these would be new territory for me. Does anyone hear have any insight as to which is more feasible / cost effective? Or have other viable solutions?
On a related note, how does one go about preventing this? I typically either perform the short circuit study prior to issuing the drawings, but in this case the Owner wanted a third party to perform them. In this case I still ball-parked the panel ratings. Do I now need to be coordinating known or estimated SCCR ratings with the mechanical engineer prior to him writing his spec/schedule? Should mechanical start specifying minimum SCCRs at chillers and other HVAC equipment? (Or whatever the code says needs rated - I don't have it in front of me.) Previously just getting panels and switchboards coordinated was sufficient, but it looks like I need to step up to a different level. Makes sense, and is probably not that big of a deal...but I doubt it is standard practice for 90% of the engineering world, and the coordination involved is a little daunting.
Anyway. The question at hand is what to do now, with this situation. Thoughts on those options? Thanks!
The available current at the utility transformer is on the order of 80kA, by the time it reaches the disconnect it is on the order of 25kA. Using the over-up-down method on the 500A fuses yields a let-through current on the order of 10kA. As near I can tell he is correct that this does not meet code, is therefore unsafe and will not pass inspection.
I have done some preliminary research and believe my best options are to install an impedance transformer or see if modifications can be made to the control panel to increase the SCCR. Both of these would be new territory for me. Does anyone hear have any insight as to which is more feasible / cost effective? Or have other viable solutions?
On a related note, how does one go about preventing this? I typically either perform the short circuit study prior to issuing the drawings, but in this case the Owner wanted a third party to perform them. In this case I still ball-parked the panel ratings. Do I now need to be coordinating known or estimated SCCR ratings with the mechanical engineer prior to him writing his spec/schedule? Should mechanical start specifying minimum SCCRs at chillers and other HVAC equipment? (Or whatever the code says needs rated - I don't have it in front of me.) Previously just getting panels and switchboards coordinated was sufficient, but it looks like I need to step up to a different level. Makes sense, and is probably not that big of a deal...but I doubt it is standard practice for 90% of the engineering world, and the coordination involved is a little daunting.
Anyway. The question at hand is what to do now, with this situation. Thoughts on those options? Thanks!