1.5MV Generator breaker size/location

[mull982]

We are looking to install a new 480V 1.5MW generator which will feed a step-up transformer to serve some 4.16kV auxilar loads (matching existing 4.16kV backup source for redundancy).

The generator will feed directly into a quick-connection switchboard which will then be close-coupled to the primary of the transformer. This generator will first be operated as a temporary generator with flexible cables for quick connection (due to permitting reasons) however will later be a permanent installation with cables between generator and transformer run in conduit.

My question is where the generator breaker should be located in this situation. Currently the design shown no breaker on the generator itself (I usually see on-board generator) with the breaker being located at the quick connect SWBD (aprox 50ft away). From a code standpoint I think this would be ok per NEC 445.13 as long as conductors are rated at 115%? My concern however is the protection of the cables between genset and breaker especially if there is a fault on the temporary cables.

Is it good practice to always have the generator breaker located at the genset itself or are there certain situations where its better to have downstream? How are generator breakers usually sized in relation to gen-set size (code reference?)

 

[ron]

We locate the OCPD within 10' of the alternator output unless the generator controller has a listed protective relay for overcurrent and short circuit protection..

As an application note, if the the transformer is outside, it will be de-energized most of its life and will end up with a shortened life because of moisture intrusion and will eventually blow when energized when you need it

 

[mull982]

We locate the OCPD within 10' of the alternator output unless the generator controller has a listed protective relay for overcurrent and short circuit protection..

As an application note, if the the transformer is outside, it will be de-energized most of its life and will end up with a shortened life because of moisture intrusion and will eventually blow when energized when you need it

Any particular reason for the 10' guidelines?

How do you typically size OCPD in relation to genset size (kVA rating)?

 

[ron]

We use 445.13 for ampacity of the conductors and the 10' rule reference.
445.13(A) has us 115% of the nameplate current rating of the generator.
445.13(B) is much more of an intent interpretation than literal. We go to the feeder tap rules as referred in that section and consider the protection provided in the UL2200 generator controller allowing us to consider the alternator terminals to be the "protected terminals" and we stick to the 10' tap rule.