NEC code - Genset Breaker Feeding MTS

[dnormanGT_ATL]

On a multi-family highrise renovation job in Atlanta, the engineer of record issued a late in the schedule change as they realized they undersized the natural gas generator for the building. We are now being told to provide a 300kW nat gas genset with (2) separate breakers (400A/3P for emergnecy panels, 300A/3P for fire pump) feeding (1) new 800A manual transfer switch. The manufactuer of MTS (Trystar) has stated that this design does not meet NEC code and that we will have to go with just (1) breaker (800A/3P) in the genset and then have a 400A/3P and 300A/3P breaker built into the MTS. Trystar has not said what code they are referencing, however, it would make sense that if we fed the MTS with (2) separate breakers then when the owner has scheduled maintenance, they would have to rent (2) generators to keep the entire MTS backed up....but other than that I've been trying to figure out any other practical reason and what NEC code the manufactuer may be using to determine the current design does not meet NEC? Any thoughts? Thanks

 

[zbang]

Assuming the MTS has a single set of backup power terminals, then you'd be putting the 400a and 300a breakers in parallel, which I'm sure is not to code and is IMHO a d&mn fool idea.

Does the fire pump controller have an integrated transfer switch? What the engineer may be thinking of is a 400a generator breaker feeding a 800a MTS and a separate 300a breaker directly feeding the fire pump controller. That aside, I'm not sure I've ever seen multiple output breakers in generators but only a single main; could happen with enough $$ applied.

A one-line would be helpful to understand what the engineer wants.

 

[hillbilly1]

The fire pump should have its own transferswitch, not through the 800. That is one problem. Though not required, a lot of engineers will tap ahead of the main in the generator to feed the fire pump transferswitch. If the generator is not large enough to start and run the fire pump (locked rotor not required) all non essential loads must be shed. That’s probably why they upsized the generator.

 

[Birken Vogt]

The manufactuer of MTS (Trystar) has stated that this design does not meet NEC code and that we will have to go with just (1) breaker (800A/3P) in the genset and then have a 400A/3P and 300A/3P breaker built into the MTS.

As stated above, the fire pump will have its own ATS, and breaker at the generator.

The way you described it sounds perfectly normal, and I don't know why Trystar has anything to say about the generator config, that is not a part of their scope.