Emergency Circuit

[eee]

I have a diesel genset connected to a transfer switch connected to a main distribution panel. I want only a partial amount of circuits powered when power is converted to the diesel genset. Presently, the whole building is emergency powered by the diesel genset.

It is anticipated that the emergency circuit should be re-wired to the main distribution panel so that only partial loads will be supplied by the diesel genset. It was suggested that the same main distribution panel used for partial loading by the emergency diesel genset could be re-configured such that the whole building is powered from normal source power when power is automatically transfrerred back to normal source power.

A separate MDP for normal source power may be the solution leaving the existing MDP just for the emergency circuit.

 

[templdl]

Personally I would move all of the circuits that you would want to isolate to be supplied by emergency power to a sub panel. Then feed the normal side of your xfrsw with suitably sized breaker in the main distribution panel and naturally the emergency side from the generator.
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[eee]

emergency circuit

emergency circuit

Understand,

You are suggesting that the load side of the transfer switch than feed the sub-panel designated for the isolated circuits feeding from the diesel genset.

During normal source power operation than for the building, the isolated circuits will be fed from this subpanel also and the rest of the building will be fed from the main distribution panel.

 

[templdl]

Yes. To confirm what I had described install a subpanel of a suitable ampacity and spaces to serve you emergency load. I always suggest that you use the same manufactures panels so that you need only relocate the breakers.
Install a properly sized feed breaker in the main panel which will feed the normal source size of the automatic transfer switch (ATS). Feed the emergency side of the ATS with the generator and camplete the connection from the load side of the ATS to the new subpanel.
When the ATS senses a lose of power it with start the genset. When the Genset is up to voltage it will transfer from normal to emergency and feed the subpanel with the genset.
When the normal source is restored it ill transfer back to the normal source.
I alway recommend that (2) modifications be specified.
1) a time delay from emergency to normal (TDEN). The feature will keep the ATS in the emergency source mode for a specified length of time after the normal source has been restored. This feature often prevents the ATS from switching back before the normal souse has stabilized.
2) depending upon the type of ATS that you have, breaker or contactor type, a time delay neutral (TDN) is important to have. Often time an ATS transfers from on available source to the other that load will be out of phase with the normal supply and the counter EMF of the load will create a fault condition which will trip a breaker. A short TDN will allow the load's EMF to collapse a bit before be re-energized.
Also, pay close attention to your EGC and how you wire the grounded neutral conductor. This is extremely important. You will most likely be specifying a 3P ATS. With the neutral of you main panel already grounded the neutral and ground must not be bonded together again at the generator.
The last item not to forget is the holes that must be filled in the main panel. So make sure that you get some fillers for the open spaces. There is always that last minute panic to find fillers to pass inspection.
Dave