Transfer switch / sub-panel some distance from main panel

macgyvers2000December 29, 2010

Looking for feedback on soundness of this...

The main panel is currently in one corner of the basement, obviously where the line comes in, in what is currently my workshop. This corner has become the unofficial wiring closet for everything from TV cabling distribution to Cat6 Ether. Combine the fact that this room may turn into a "den" at some point, as well as my desire to add a backup generator, something has to change.

I'd like to add a transfer switch / subpanel to a more centrally-located room that already contains the HVAC and water heater. This room, about 30' from the main panel, is also very close to the pellet stove, and the kitchen fridge is practically right above. By putting the transfer switch / subpanel in there, I could hook up all of the things I would want to run from the generator during a loss of grid power. I'm considering a generator in the 10-14kW range.

So, I'm looking at two thick lines from that corner of the house to the subpanel, one from the main panel and one from the backup generator that will be located just outside that corner. I'm thinking 6/3 (copper) and a 60A subpanel as a bare minimum, but depending upon the actual needs of the combined items (the HVAC's two-stage emergency heater is a beefy unit, around 3kW itself... this was installed before the pellet stove and is probably no longer necessary) may go as high as a 2/3 and a 100A subpanel. This is all off of a 200A main panel.

So, am I missing something important, like some code that says the transfer switch needs to be within so many feet of the main panel? Am I silly in thinking two runs of thick copper from that corner of the house is a good idea, even though it probably kills a number of future upgrade birds with one stone? I'm estimating about $2/ft for the copper, so $100-150 for that piece of the puzzle isn't bad compared to the multi-$k price of the generator.

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Since no one else has, I have a few generic observations. Obviously, I can't be aware of the physical aspects of the install so some may not apply.

(1) if your HVAC has a 3kw emergency heat, it is the smallest I have heard of- 8kw+ is typical and 15 to 20kw not uncommon. Your "10-14kw" set will probably not be happy running that HVAC along with more than a light bulb or two.

(2) If your generator is going outside near the existing service entry, use a 3R rated switch and put the transfer switch outside, short feed from main panel to the transfer, and a longer one to the subpanel. Less wire and maybe less labor. Plus the switch isn't making your "den" ugly.

(3) Read the post linked below. A good set isn't inexpensive.

(4) Call a real generator vendor, not some guy from the big box store, and ask for a load survey to find out what size set you really need. Be aware that late revisions of the NEC tend to make residential genset installs considerably more complicated and expensive than they once were.

Here is a link that might be useful: generator discussion

    Bookmark   December 30, 2010 at 8:49AM
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Thanks for replying.

1) I was adding another circuit to the main panel later in the day of my original posting, so I checked the heater circuit. It was 5kW, not the 3kW I originally posted, but it's in addition to the heating element installed as part of the HVAC (probably wasn't clear in my original post)... the cost was $100 if I had the extra circuit installed by the time they put in the main HVAC unit, so I said "why not". This is the circuit I think I can now do away with as it's not needed with the new pellet stove keeping us warm. The "standard" heating element in the HVAC is now the "emergency" element, if you will.

2) So you're suggesting bringing a short main panel circuit outside to the transfer switch, thereby allowing for a single long run from the switch to the subpanel. Hmmm, that idea has a lot of merit. I would definitely want to go with an ATS, then, so I don't have to walk outside to swap things out (and shoveling snow to get to that side of the house... like the 4' we had in 3 days last year :( ).

3) I'll read it after posting this...

4) I was thinking of a Kohler or a Cummins-Onan, though I hear Kohler is often more expensive in the long run because they do their best to force you into using their own guys for general service. In that case, Cummins-Onan has a serious edge.

    Bookmark   January 3, 2011 at 1:33PM
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