Standby Generator for Dwelling

[curt swartz]

I am working on a project to install a back up generator for a single family dwelling. Because of my schedule and this Town?s requirements I have asked an electrical engineer to help me with the required drawings. I was planning to use a 30KW generator which is the largest one permitted with out going through an expensive environmental study. My electrical engineer feels that the generator needs to be sized according to the maximum calculated load that could be connected to it. I have always sized these backup generators based on what loads the customer would like to be able to run during an outage. I have installed and seen many generators installed that are substantially smaller than the actual calculated load. Frequently a 200 amp transfer switch gets installed in series with the 200 amp main feeder but a 15 KW back up generator is used. 702.5 requires an optional standby system to have the capacity to supply all required equipment intended to be operated at one time. It also says that the user of the optional required standby system shall be permitted to select the load connected to the system. Since this is not a required system I feel if the home owner decided to turn on every light in the house, do laundry and turn on every other load they think of causing an overload on the generator it would simply cause the overcurrent protection generator to open. I frequently see transfer switches that have the generator side of the switch rated less than the primary supply. If the generator is required to be sized according to the maximum possible load on the switch I don?t see any use for this type of switch. Let me know what you think.

Curt

 

[ron]

Re: Standby Generator for Dwelling

The minimum code. does not require you to size the generator to handle the actual or calculated load.
If the transfer will be automatic, it would make sense for you (or the engineer) as the designer, to account for false tripping of the gen's OCPD when too much load is applied. It is not a code requirement, but it makes sense for design purposes.
If the transfer would be manual, then the homeowner has every opportunity to load shed before transferring to the gen source, and the burden (possible false trip) is off your shoulders, as long as you document the required load shedding procedures to the homeowner.

 

[curt swartz]

Re: Standby Generator for Dwelling

Ron, thanks for the reply. This generator will have an ATS and will actually be supplying a main house and 2 small apts. (in-law or house keeper homes). The heating and cooking is gas. There are 2 small A/C units for the apts that I?m thinking of installing a relay that will prevent them from restarting after a power failure. There will also be 1 standard and 2 small clothes dryers but all they usually don?t automatically restart after a power failure. 30KW is a pretty substantial amount of power for mostly general lighting loads. If the homeowners are aware of what they can and can not do I don?t see a problem here.

I?m interested in comments from others too so don?t be shy.

Curt

 

[msd]

Re: Standby Generator for Dwelling

I was planning to use a 30 KW generator which is the largest one permitted with out going through an expensive environmental study.

Can you put in two generators to get around the "environmental study" (Might Be cheaper)?
Split the load between two smaller subpanels.

 

[petersonra]

Re: Standby Generator for Dwelling

A couple of comments.

1. Why would an engineered set of drawings be required since you almost certainly will be using stuff that already comes with installation drawings. It seems like serious overkill.

2. I can't see any reason why the thing has to be sized for the full load it might see if everything in the house is running at the time of switchover, but I can understand why the engineer might be nervous about such an event, since it seems at least possible that you would be no better off with the standby system in place if it trips due to overcurrent when it tries to come on. You might want to look at only backing up certain loads so this can't happen. The relay interlock to prevent big loads from picking up when running on backup power might be adeqaute insurance.

[ October 12, 2004, 09:07 AM: Message edited by: petersonra ]

 

[charlie b]

Re: Standby Generator for Dwelling

At an assumed power factor of 85%, and the usual household voltage supply of 240 volts, a 30 KW generator can give you almost 150 amps. That should be plenty of power for anything they could wish. But I would suggest starting off by performing a service calculation, as though this were a brand new project that will be constructed on newly cleared land. Find out how much power your would need to supply for the main house and the two smaller facilities. Then, give the owner the information in some clear form of display. Give him a chart of the connected loads and the amount of current each load takes, so that he can make informed choices of what to run.

My opinion of the relay interlock is that it would not serve the owner?s needs. By denying the owner the option of running an item, even if the owner could shed enough other load to leave room, you are taking the ?optional? out of the ?optional standby system.?

Aside to Ron: It?s not a ?false trip.? The OCPD would be reacting to a true overcurrent condition. I suggest the term ?nuisance trip.?

 

[ron]

Re: Standby Generator for Dwelling

Charlie,
Point taken.
Thanks
Ron

 

[curt swartz]

Re: Standby Generator for Dwelling

msd, I don?t know if installing 2 generators world get around the environmental issue but I?m sure the owner does not want to pay for 2. I?m not even sure that using 2 or a larger unit would meet the towns noise requirements.

Peter, the town isn?t requiring engineered drawings but the are requesting load calcs, one line diagrams, etc. Normally I would do this myself but am too busy now so decided to let an engineer take care of it.

Charlie, I have done load calcs for the project. The total calculated load that will be connected to the transfer switch is around 52kva but this includes the 2 A/C units and 3 dryers. Other than the A/C units and dryers most of the load is general lighting or small appliances (disposals, dishwashers, microwaves, etc.). The interlock that I was thinking about possibly using for the A/C units would only prevent the units from restating after a power failure. The owner could have some type of reset switch to turn them back on. The owners have provided a list of what items they would like to be able to operate during a power failure and I could actually use a much smaller unit to meet their needs. My problem now is convincing the engineer that we don?t need to size this unit to the full calculated load.

Thanks for the input. Keep it coming.

Curt

 

[BAHTAH]

Re: Standby Generator for Dwelling

For what its worth, I typically install a separate panel for the stand-by loads with a conduit between it and the Normal-Power panel so you can select the loads the owner needs to run. You do not want the system to open the generator main and have the owner come back from vacation to find some critical load did not operate while they were gone. I have a stand-by system on my home and when I decide I want to change what is on the Em-Panel the connection between the two panels is a great help. I have a 200amp service and a 12kw gen operating at 5000' elevation and have never had a problem.