Generator size vs. Service size question???

[jango]

Can someone refer me to the NEC and also shed some info on the requirements of sizing a whole-house back up generator in relation to the actual service size? i.e. If I have a residence with a 400A service but the average annual draw is less than 150A on each phase, can the generator be sized smaller than 400A and still be code compliant? I do plan on using a 400A rated ATS but I'm hoping to be able to use a much smaller generator. I would appreciate as much input as possible. Thanks.

 

[iwire]

Check out 702.5 Capacity and Rating.

 

[charlie b]

The generator can be non-existent, and be code compliant. That is the "optional" part of an "optional standby generator."

 

[jango]

The generator can be non-existent, and be code compliant. That is the "optional" part of an "optional standby generator."

Does this mean that the generator can be of a smaller (or even much smaller) amperage size rating as compared to the main service size? In other words, should I have a problem (code wise) with having say a 150A or 200A rated generator as a back system to my 400A service? Again the typical draw on the system is calculated at 125A-150A max for a given year. Any more input on this would be greatly appreciated.

 

[charlie b]

Yes, you can have a 400 amp normal service and a 100 amp backup generator. The constraint is that you must find a way to make sure you don't overload the generator with more than the 100 amps it is rated to handle. If you try to place the whole building on the generator, then hope somehow that the owner will not try to run too many loads, you might come into a code compliance problem. One common way to solve the problem would be to have a separate panel to supply a limited number of loads when the generator is on line. The transfer switch supplies only this panel, and the remaining 300 amps worth of normal service loads do not have access to the generator.

 

[augie47]

Honestly, I don't fully understand charlie b's post so I'm not sure I am at odds with him or not.
Art 702 covers optional standby systems. 702.1 states, in part:
The systems covered by this article consist of those that are permanently installed in their entirety, including prime movers, and those that are arranged for a connection to a premises wiring system from a portable alternate power supply.
To me the "optional" is the option of having a system or not as opposed to one being required.
If you elect to install an optional standby system whether the optional supply be permanent or portable, and you employ automatic transfer equipment, then you must comply with 702.5(B)(2):
(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.
(b) Load Management. Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system.

In my opinion, this would mean your system would need to be designed
to supply the calculated load unless you employee some load management system.

 

[jwelectric]

Does this mean that the generator can be of a smaller (or even much smaller) amperage size rating as compared to the main service size? In other words, should I have a problem (code wise) with having say a 150A or 200A rated generator as a back system to my 400A service? Again the typical draw on the system is calculated at 125A-150A max for a given year. Any more input on this would be greatly appreciated.

702.5 Capacity and Rating.
(B) System Capacity. The calculations of load on the standby source shall be made in accordance with Article 220 or by another approved method.
(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.
(b) Load Management. Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system.

What is the calculated load? Then size the generator to this calculation or install a load management system

Edited to add:

Sorry guys seems we were typing at the same time

 

[tryinghard]

Does this mean that the generator can be of a smaller (or even much smaller) amperage size rating as compared to the main service size? In other words, should I have a problem (code wise) with having say a 150A or 200A rated generator as a back system to my 400A service? Again the typical draw on the system is calculated at 125A-150A max for a given year. Any more input on this would be greatly appreciated.

Possibly but either case must be sized per Article 220; compare 230.79 to 702.5(B). It's not uncommon the service disconnects oversized but it may not be calc the load for the gen set or see other method in 220.87

 

[iwire]

Honestly, I don't fully understand charlie b's post so I'm not sure I am at odds with him or not.

That was the spot I was in a well.

 

[charlie b]

Honestly, I don't fully understand charlie b's post . . . .

Me too. I seldom understand what he says.

. . . so I'm not sure I am at odds with him or not.

As it turns out, you are not. I will explain.

To me the "optional" is the option of having a system or not as opposed to one being required.

To me, in addition to agreeing with that statement, the ?optional? is also the option of choosing which loads will be supplied by a panel downstream of the transfer switch, and which loads will be supplied from the normal source only (i.e., not via the transfer switch, and thus not accessible to the generator).

(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.

That does not require that all loads in the building be supplied via the transfer switch. It merely says that you must determine how much load is to be transferred by the ATS, and verify that the generator is rated to handle that amount of load.

In my opinion, this would mean your system would need to be designed to supply the calculated load unless you employee some load management system.

To which I will add that it need only supply the calculated load that is served by the transfer switch.

 

[tryinghard]

If you do size it for the whole house I suggest using one of the Annex D examples like D2 pages 763-764 2008. The thing about matching the existing service disconnect is not entirely ridicules because someone reasoned it would be needed, but when assessing the costs of a generator system the picture can quickly change and you may portion it out.