Technical Corner
This month’s article will talk about some of the changes in the NEC 2017 codebook sections 440.4(B) and 440.10. These sections are a recent hot topic and deal with labeling multi-motor and Combination Load Equipment with their Short Circuit Current Ratings (SCCR) that we often see in single point of connection roof top mechanical equipment. Going back a few code cycles (2011 and 2014) we can see in the same 440.0(B) section Exception #3 that equipment fed with a 60A branch circuit or less does not need to be labeled with its SCCR.
System designers and installers also are realizing that once the equipment is installed with inadequate SCCR, there are no easy fixes either. The only options are equipment modification and recertification or reduction of fault current (through use of additional conductors, isolation transformers or current limiting fuses). Some other key exceptions apply to dwelling units and cord and plug connected equipment as equipment installed as such are not required to be labeled with their SCCR ratings. Conclusion:
As you can foresee, the 2017 code eliminates the 60A or less branch circuit exception leaving this type of equipment with strict labeling requirements in regards to SCCR. For new design projects this creates a need to specify the ratings of this equipment required based on the Engineers calculations. After speaking with some reputable HVAC mfgs. it was determined that a typical standard rating for this equipment is approximately 5k-10k. The mfgs. indicated they can do any rating that is specified…at a cost. Also, a standard spec option of a 55k rating is typically available for larger units 10 Tons and greater. These ratings could potentially not be large enough depending on where the equipment is fed from in the electrical system. Typically, the higher upstream in a system the larger the SCCR. Is the blanket statement in a Div 23 spec section: “Listed and labeled as defined in NFPA 70” enough..?
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It appears that we need to pay closer attention to the SCCR ratings of this type of equipment and strategize on how to reduce the minimum available SCCR at these points of connection. If not, then it seems like we can have options to get the equipment built with higher SCCR ratings, but I’m sure that comes with a cost. Hopefully this article finds you well and can be used as a reference for your project needs. If anyone would like to contribute to the Rochester Engineer and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.
Brett Eliasz, P.E., LEED AP BD+C RES Director
MARCH 2019 The ROCHESTER ENGINEER | 5