NFPA 70 Question...

North of 60,

I assume you are referring to section 517.17(C). Note that the "Life Safety Branch" definition in 517.2 has a reference to NFPA 99-2005, section 3.3.96. I don't see any markings to indicate this is a change from previous code editions; the same wording is present in the 2002 edition's section 517.17(B). Am I in the wrong section? No personal experience; just some anecdotal. Although it is a burden to do correctly, I feel that this is needed. I remember a contractor friend caused much trouble in one hospital with a portable tool's ground fault that tripped a 20 amp branch breaker, the 200 amp panel main, and the 2000 amp feeder!

--JMM

Yes, that is what I am referring to. Sorry for the delay in responding, been busy.

I had thought the requirement was new in the last code cycle, thanks for correcting that assumption on my part.

We have just added the requirement in the 2009 CEC and it is raising some questions about what some US states have done with respect to that particular article of the NEC. Apparently a few of them are not enforcing the requirement.

The major issue appears to be due to the increase in frame size of OCPDs and switchgear that is required to handle the increased fault current that must be provided for to meet the time requirements of the selective coordination plan.

North of 60,

The topic and your experience both are "over my head". I can understand the concerns about frame sizes and increased fault energy (I²t) involved in providing the coordination. Perhaps the manufacturers will show some ingenuity in developing methods for this. I suspect that another aspect will be the increasing development of electronic/digital trip units that would provide a closer tolerance in time/current curves (making the 100% coordination requirement more reliable). The ultimate approach would be a method for the breakers to communicate so the decision of the downstream one to trip will automatically increase the time delay for the upstream ones. Another approach would be the introduction of a steeper time/current curve on the smaller downstream breakers.

--JMM