Article 702 NEC

[mikelp]

I have a question relating to properly sizing an optional standby generator for a commercial installation.

I have read rule 702.5 of the 2005 NEC, and interperet this as the size of the generator should not be an issue so long as the conductors that lead from the generator are rated for the full current output of the generator and the output CB is sized accordingly. I am having a problem convincing the AHJ otherwise; however.

I have a commercial installation with 120/208 3 phase, parallel x 2 feeders of 300MCM. There is no main disconnect as there was the six disconnect rule employed at the time of installation. Although I have never seen the load calculation for the building, I surmise based on the wire size that it was intended as a 600A service. The service is fed overhead, via a CT can on the exterior of the building, then enters the building underground and terminates into a service gutter where the feeders are then tapped to the various disconnecting means. I have logged the load data with a data logger(maximum amps, average amps and maximum KW)from the service consumption over a seven day period and come up with an average load of only 100A per phase and the maximum load on any phase over those seven days was 321 Amps. This is a 24/7 operation.


What I submitted to the AHJ was that we interrupt the service feeders at the CT can and reroute into a new 600A service entrance rated automatic transfer switch (with 600A MCB for the utility feed only). We would then re-feed the existing electrical gutter with the same 300MCM parallel feeders from the load side of the ATS. Based on the load readings that I obtained over the seven days of normal business operation, I have chosen to install a 100KW 3 phase generator with a maximum output of 364A per phase using a 400 A MCB and 2 x parallel 4/0 feeders to the generator input side of the ATS. I know that this generator will support the loads including the starting currents of the motor and compressor loads as the data I collected over seven days of normal operations at the facility falls well below the rated output of my generator.

The AHJ tells me that I must size my generator to 600A; however, as that must have been the original calculation for the building. I do not see the issue as he does. If my wire size from the utility remains unchanged and the wire size I have installed from my generator is protected by the generators MCB, I do not see any code violation. As far as the customer is concerned I have diligently ascertained his load requirements and I believe I have done him a favor by not over sizing a generator that would run with little load and cost him more money. It is my understanding that an under loaded generator is subject to maintenance issues such as "Wet Stacking" and "glazing".

Any comments on interperetation of rule 702.5 of the NEC and the application of the optional standby design described above greatly appreciated.

 

[dpc]

I agree with your interpretation and have used a similar approach many times. For an optional standby system, you can use whatever size generator is large enough to run the loads the owner wants to run, even if this requires manual control of the loads.

Unfortunately, the only opinion that really matters is the AHJ. Can you escalate this up to the State Electrical Inspector?

 

[alehman]

I agree with dpc. That is ridiculous.

 

[EEJaime]

This is probably a done deal at this point, but my colleagues are both correct. There is no code requirement for you to serve the entire service with emergency power. This is not even a legally required standby system. The owner can choose to shed any load he selects and size the system for his most important loads. I would definitely take this up the ladder a step and speak with the head of the AHJ's department. One method I've used, but which involves added cost which the owner should not have to foot is to serve the ATS with a 400A sub-breaker from the main service to limit the current to the ATS to the ATS's maximum rating. Good luck.

 

[alehman]

I've found some AHJ's do not understand the difference between optional standby systems and legally required standby systems. I've had experience with serveral over the years that require optional systems be tested and witnessed as though they were legally required. Attempting to explain the difference seems to fall on deaf ears in those cases.