HUH? If you can isolate entire circuits to move to the CRITICAL side, (as opposed to having to move stuff off an existing circuit), the effort to move those circuits to a transfer panel is minimal. Even with an automatic transfer switch this is frequently how it is done, you put the transfer swtich's sub panel next to the main panel and then move the critical cricuits over to that (often the manfuacture gives you a premade whip with wires you you connect to the main panel and then wirenut them to the conductors on the switched circuits as well as a feeder for that panel (you disconnect the circuits from the main panel breakers htey were connected to).

An automatic switched generator needs to handle the full load, but as you realize, a manual transfer can rely on you turning off loads prior to switching so as to not overload the generator.

You'll have to do your own load analysis to find out if 15KVA is sufficient. Make sure to account for startup loads on things with motors such as HVAC, water pumps, etc...

Frankly, I'd go with the largest generator within reason. In fact, my 80KVA generator tends to only be asked to deliver about 6KVA in most circumstances.

This post was edited by ronnatalie on Fri, Oct 4, 13 at 13:12

In order to isolate my critical circuits most of my critical circuits are in the basement sub panel the farthestt subpanel from the main panel. In order to get these distant critical circuits isolated and to the transfer switch sub, i would need to run wire back to the distant transfer switch sub. The cost and disruption to do that I was told is the same or more than just a larger whole house generator connected to the main panel. Did I read your reply correctly?

You don't necessarily need to run each circuit back to the main panel, you can put a second subpanel in if you want and then run a feeder. Still that may indeed be expensive depending on the distances (and construction) involved.

If 15kVA will run what you can reasonably shed your load down to before kicking in the manual transfer, then go for it. That's about the equivalent of an 80A service.

What are you calling "critical"?

fridge, freezer, sump, sewage pump, oil furnace, no well pump, or heat pump AC

Everything but the 'fridge is in an unfinished basement? if that is the case, it should be easy to run new cables to the critical loads. The refrig might be harder, but how hard can it be? I guess the basement is finished or the cost difference between 8 and 15 kW is trivial.

Thanks for all your input and if you have the time I have a few questions. Looking at propane as a fuel. I have 2, 120 gal tanks and only a water heater in use now so extra capacity.
1. Because of my main panel and sub panel configuration one contractor recommends a 15kW . Generac to the main panel to power the whole house. Another contractor says an 8kW would be minimum if I was using a portable wired to the main panel to run the house. Why the difference?
2. Can I dedicate a portable generator in an outdoor location within an enclosure and run it only when needed and to exercise periodically? My propane supplier will only hookup a dedicated unit. Does a dedicated portable have the same setback code requirements?
3. Rather than #2 can I get an 8kW automatic dedicated Generac and only run it manually when needed? I don't need auto, whole house convenience and if 8K is too small for a whole house but an 8KW manually operated after unneeded circuits are closed will work it seems like a less expensive option to a 15Kw and a more reliable version of a dedicated portable unit. One contractor was unsure of the manual mode because of communication between the auto generator and transfer switch and signals to auto exercise.

I know this may sound confusing but any input would be appreciated.

Let me make sure that understand that a generator that will handle the whole house is mandated in the case that an automatic transfer switch is used and no sub panel is used to pare off the overload. That means that you can have as small a generator as you want if you have a manual transfer switch.

Ionized, i understand that if all/most all the loads in my house were active at once and power went out, I would need a large generator (reportedly 15kw) to auto kick in and continue to power the whole house. Alternative 1 is for a smaller generator (reportedly 8kw) and an auto transfer switch and sub panel that will only power critical circuits. In my case that is not a cost effective option because of my main panel and 2 distant subpanel setup with critical circuits in all 3. Alternative 2 is a portable generator (reportedly 8kw) with a manual transfer switch. Power goes out and I manually shed non critical loads in my 3 panels (including AC) fireup the portable and power just the active loads (fridge, freezer, sump, etc.) Even with this setup one contractor says 8 kw is minimum, yet an electrical engineer colleague does just that (alt 2 whole house critical circuits only) with a 3.5kw. The only reason auto backup is being considered is because I have ample propane supply and my propane supplier will only plumb a dedicated generator. I like the propane idea for ease of fuel use, no gas storage or carb clogging with age plus less wear and tear on the engine. I could go gasoline portable for Alt 2 and not get the benefits of propane or convert the gasoline to propane. But the supplier will not plumb it unless its dedicated, hence the question of dedicating a portable to outside use under a cover/enclosure and thus what are the required setbacks. Lastly Alternative 3 is a modified Alt 2. Can I install a dedicated 8kw outside fed by propane but not run it with an auto transfer? Since its not a portable it makes my gas guy happy and he can connect. And when power is out, I manually shed loads in all 3 panels and powerup the dedicated, propane fed 8KW for critical circuits. I would likley need to exercise it manually if I overrode the auto setting but are there any other draw backs to manual operation (battery trickle charge, battery heater, signals between generator and manual switch I am not aware of , warranty, hour meter, low oil indicator, etc.? Thanks for reading through this mess.

I just glanced over the posts, not reading carefully. Looks as if an interlock on the main panel might be the best choice-- all operations are manual. Then wire for 15KW generator input. That input can be connected to any lesser supply with the proper plug on the cord. And that enables using a larger generator if desired.

Your EE colleague can calculate your need better than anyone else since you are his vested interest rather than making money. 8 kW may be what you need if you want to make sure all of your motor loads are starting up at the same time. If you are up to repeatedly selecting loads so they are not running simultaneously, you can get away with smaller.

Propane would be my choice in your circumstances. Why does the supplier dictate what you do with the propane. Just hire a plumber to add pipe for the genset. I suppose you are in a bind if the regulator is not a high enough capacity and they own the regulator/tank.

"Can I install a dedicated 8kw outside fed by propane but not run it with an auto transfer?"

I don't see why you can not do precisely that. The manual transfer will cost less. It has been explained to me that one reason that smaller gensets are less expensive to install is that a small number of strong fellows can move them into place rather than renting equipment for a larger one. There is the smaller transfer switch and conductors on top of that.