Standby Generator Supplying Multiple Circuits 3

[CuriousElectron]

Greetings, All-
I wanted to get feedback if it is kosher to have two circuits supplied from the same standby generator. I believe it is permitted to have a single breaker supply two different circuits. The tap rule would not apply here, since both circuit conductors ampacity would be sized to be higher than the GEN breaker. MY concern is if the feeder circuits would be properly protected by GEN breaker. Also, I haven't seen too many installations where two circuits are fed from the same genset; usually it's one circuit that goes to the transfer switch.
Any thoughts?
Thanks for your feedback!
EE

 

[waross]

I have had occasion to feed two transfer switches from one generator.
You haven't given enough information for a better answer.
Good practice is for each circuit to have suitable over-current protection.
I am sure that from your perspective it makes perfect sense, but I have worked on generators from 1.2 KW to 1500 KW and a plant with a total installed capacity of 2.2 MW.
It depends.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[CuriousElectron]

Waross,
Thanks for the response. Is the distance of the circuit for each run determine if dedicated over-current protection should be provided. If the circuits from the genset to the switches have a short run, why would GEN breaker not be considered adequate protection for the feeders? I get that the function of the gen breaker is primarily to protect the genset, but it can also provide over-current protection to the downstream feeders.
Thanks,
EE

 

[waross]

This depends on specific conditions. It may be a judgment based on experience and good practice.
You seem to have more faith in generator circuit breakers than do I.
I normally consider the generator breaker to to be a convenient disconnect and,if the generator is equipped with PMG, protection in the event of a short circuit.
Normally I expect circuits to be provided with code compliant overload protection and over-current protection, and leave that protection in place when installing a standby generator.
There may be exceptions but you have not given me any information that would warrant an exception.
You leave me wondering about two things.
Without a transfer switch, how will you connect the circuits to the generator?
If the generator breaker has a low enough rating to protect either of the conductors, will it be able to supply both loads simultaneously?
Is this a 1500 Watt generator supplying two lightly loaded 15 Amp circuits?
If the conductors are oversized to overcome voltage drop, you must consider any devices installed at the load end.
For instance a 30 Amp receptacle in most instances must be protected by a 30 Amp breaker, regardless of the ampacity of the conductors.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[CuriousElectron]

This is a supervised industrial installation. I don't see how you could protect the feeders at their origin point other than by the GEN breaker in this configuration. Besides, both runs are short from the genset to their respective switches.

Regards,
EE

 

[LionelHutz]

Doesn't the tap rule still apply, as in it didn't get violated?

If everything leaving the generator is "bigger" than the generator breaker then I don't see a problem.

 

[waross]

I am more concerned with usability, Lionel.
As an example, two circuits have 40 Amp loads and are protected with 50 Amp breakers and 50 Amp ampacity conductors.
This presents three issues:
How will the two circuits be combined into one circuit when on the generator.
If the generator breaker is less than 50 Amps will the generator be able to support both loads.
There is a possibility that some normal operating conditions may overload the generator and may cause the generator breaker to trip.
Yes, the tap rule is not violated, but we have both seen too many times when a dodgy installation was hidden behind one specific question.
My hesitancy is based on information not given, and that is based on sad experience.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

One possible arrangement that I can visualize, within the parameters of the given information, may be solved with the use of a load mizer and a multi-pole transfer switch.
But, It Depends.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[CuriousElectron]

Gentlemen,
Thank you for your replies. I wanted to provide additional information on this setup. The generator feeder will terminate at a power terminal blocks panel and the two circuits will then branch out from this termination point(one can be considered a feeder and the other a tap). I believe this is permitted by NEC 445.13(B), if the genset is equipped with listed overcurrent protection. The tap rules can then be applied here. Also, the feeder(conductors from generator terminals) are permitted to be protected against overload by the generator overload protective device per NEC 240.21(G). To Waross's point, I'm not concerned with overload conditions here, the genset and the feeder/tap are oversized and load shedding will be employed.

Regards,
EE