NEC 70 Article 700

[wbd]

Hello,
I was curious to see if anyone has run into this issue before with a AHJ. The situation is surrounding a installation of a generator and ATS's in an existing health care facility and selective coordination. The facility has mccb's. The comments made by the AHJ concerning selective coordination are:

"Evaluation of selective coordination of overcurrent protection devices down to 0.01 seconds (one cycle) is insufficient per 2008 NEC 700.27 and 701.18. There is no time limitation exceptions included in the code. The full range of overcurrents irrespective of time is required to be evaluated."

Due to the inherent nature of mccbs in the instantaneous region this seems to be an impossible and the AHJ appears to be saying that coordination even to 0.01 s is insufficient.

Thank you in advance for any comments.

 

[davidbeach]

So, you get to use fuses, breakers where you can turn off the instantaneous, or design the system to keep the maximum fault current below the minimum current for an instantaneous trip. That's one that the fuse manufacturer's won.

 

[dpc]

Yes, this is the direct result of the changes to the NEC regarding selective coordination for emergency power systems. Fuse manufacturers have tested their fuses and will state that they will selectively coordinate over the full range of fault current (when the fuses in series are in a 2:1 size ratio or greater). This NEC change was pushed by the fuse manufacturers as David notes.

There are now MCCBs that have been tested by manufacturers for series coordination (not to be confused with "series rating") up to a certain maximum fault current. You can obtain tables of these tested combination from the suppliers, such as GE.

wbd, I think you are an EasyPower user, so I'll also add that in EasyPower you can display this maximum fault current limit for series coordination between breakers provided this has been defined in the library data.

dpc