Multiple Generators

[frostyra]

I work for a radio station with a standby 250KVA diesel generator. We are preparing to install ANOTHER similar generator as safety backup. The generators will be in separate, adjacent rooms in the transmitter building. I have had little luck in finding answers to these two questions in the NEC:
1) MUST the disconnect breaker for the newer generator be within sight of the generator, or may it be in the existing panel next to the older generator in the next room?
2) MUST the two breakers be mechanically interconnected so that it is impossible to have both breakers on at the same time? Switching between the two would only be done by trained and authorized engineering personnel.

I'd appreciate any help on this subject; if you can reference code sections as well, I'd appreciate that also.

Thanks.

 

[caj1962]

Re: Multiple Generators

Look in ariticles 700, 701, 702 and 705. While 701 would not specifically affect your instalation the others will.

 

[frostyra]

Re: Multiple Generators

Thanks; will give it a try.

 

[don_resqcapt19]

Re: Multiple Generators

This would be an Article 445 and 702 generator installation. Look at 445.18 for generator disconnect requirements. The location of the disconnect is not specified. I see no requirement to interlock the two generator breakers, but unless you have synchronization equipment to permit the two generators to be operated in parallel, you should interlock these breakers or use a second transfer switch.
Don

 

[brian john]

Re: Multiple Generators

Rather that a mechincal interlock a good safe soloution would be a key interlock system (In our area the generic name is Kirk Key) allowing only one CB to be closed at the same time.

 

[jerryb]

Re: Multiple Generators

I have been told that there is a new UL requirment for having the disconnect at the generator, otherise the generator is not a listed assembly.

Also according to article 445-13 "The ampacity of the conductors from the generator terminals to the first distribution device(s) containing overcurrent protection shall not be less than 115 percent of the nameplate current rating of the generator".

 

[flightline]

Re: Multiple Generators

I guess that this will not answer your questins as to citing code sections, but Don is pretty good, at this. Since paralleling is not desired, my next question would have to inquire if this is to be aan automatic situation. I mean if one generator failed, would an engineer or some other such person have to physically start the second generator, and then opn and close the correct devices in a proper sequence? Then Kirk Key is OK, but will they be doing this in the dark? That installation would seem to be frought with the possibility of problems, injury, or litigation.

I like Don's suggestion of multiple transfer switches. Also would you want both generators to start simuataneously? so if while the "lead" or Primary" generator fails during the utility outage, the secondary generator would be ready to assume load, and when the second transfer swith transfer to it, only a blink would be seen.
If you do go with this, I would also suggest that you apply an alternator that alternates between generators so they wear evenly from this point on. Even though the existing generator will have more hours on it, I believe it's still good practice. Diesel prime movers run with no load, such as trucks ilding or generators spinning with no load on them are prone to what is called "wet stacking"; a procees by which the cylinder walls get polished and fuel passes up the stack, and efficency to some extent has been lost. Pierre may be able to shed some light on this better than I.

 

[sjaniga]

Re: Multiple Generators

Heres a question, If the first generator fails, ok start the second generator, WHAT IF THE SECOND GENERATOR FAILS? I know, you will get a third, right? I dont know, but it sounds wierd that a radio station would need 2 gen sets.

 

[jtb]

Re: Multiple Generators

Starting the generators simultaneously could cause problems if they are not synchronized. Even if isolated, and a transfer from 1 to the other occurs, you could get more than a 'blink'!

I would suggest a wind down time delay before transfer, so any inertial loads (heavy motors) won't be fighting with self induced phase current.

Or you could synchronize all 3 sources for quick transfer if you have the $$$.

[ July 03, 2003, 08:10 PM: Message edited by: jtb ]

 

[gregory]

Re: Multiple Generators

I was in a hotel in Chicago and they had 13 generators that they ran the whole building off of. They would run 11 at a time and use the other two as back ups when they took one of the eleven out of service for maintenance. Why couldn't the two generators that you are talking about be paralled to both feed into the grid? what would be the considerations for doing this? would they have to be the same size or impedance. what would it take to run both at one time?