NEC 700

[wbdvt]

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.

 

[dkidd]

NFPA 99-2012 says

6.4.2.1.2* Selective Coordination.
6.4.2.1.2.1 Overcurrent protective devices serving the essential
electrical system shall selectively coordinate for the period
of time that a fault?s duration extends beyond 0.1 second.

 

[don_resqcapt19]

This is part of a fight between the technical committees for NFPA 70 and NFPA 99. If you look at the 2014 code you will find that much of what was part of the "emergency system" for health care in the 2011 code is not part of the emergency system in the 2014 code. In the 2014 code the definition of selective coordination from NFPA 99 applies for Article 517 installations. The new definition in Article 100 of the 2014 code applies to all other cases where selective coordination is required by the NEC.

Coordination (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.

The NEC requires selective coordination from time "zero".

This is also part of the battle between the circuit breaker companies and the fuse companies.

The question here would be what code the installation is under. NFPA 99 is not likely an adopted code that the AHJ would be enforcing. The code he or she would be enforcing would be NFPA 70. Prior to the new definition in Article 2014, there was questions as to what "selective coordination" as covered by the NEC means. There are "experts" (fuse people) that argue that the NEC requirement has always been from time "zero" and other experts (breaker people) that argue it is from time 0.01 seconds.

 

[jim dungar]

The State of Wisconsin does not have any time cut-off. They ask for 'as far as practicable'. The AHJ typically accepts published witness tested breaker combinations.
This has been standard practice since 2008. It is to the point, where my team performs selective coordination for every project with an ATS on it. We still get 'emergency study' requests due to the Certificate of Occupancy being held up because the electrical contractor never wanted to pay for a study.

The time limit in NFPA99 will not help us, as it is not incorporated into our state electrical code.

We have found it is often much easier to use tested breaker combinations, than it is to fit fuses into the system.
Typically one hard part is to coordinate to the generator breaker. Another problem is when there are four to five devices in series (e,g, breaker feeding ATS, main breaker in panel after ATS, feeder breaker to transformer, secondary main breaker, branch breaker).

 

[kingpb]

It sounds to me like the device coordination is taking place at a time interval that is not achievable? MCCB breakers have a minimum tripping time in cycles. I want to say 7 cycles, but I would review the IEEE Blue Book to confirm you have properly coordinated your devices.

 

[BostonEE]

I have had this discussion many times. I would try to argue that the scope of the NEC is to govern installation requirements, not performance requirements. Also, the AHJ has the authority to permit an installation not to meet the letter of the code. So you could try to convince him/her that the design is coordinated to the maximum extent practical.