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toofast 08-27-2013 05:54 AM

Generator install - unique home wiring or not
I was not sure what to call this here we go.

I bought a Generator that would power 10 - 20 amp circuits in my house. We live in the country and often lose power, especially in the dead of night in the I travel. So wife does not love getting up in the middle of the night in a snow storm to stay in a hotel with our 3 year old. the problem.

The Generator Installer got everything set, then went to wire the 10 circuits and said OPPPS, you are in trouble. I can only use "5" of the circuits you wanted, because the electricians when they built the house "cheated" and you don't really have dedicated circuits. For hot water tank is gas, but on its OWN 20 AMP circuit, however coming into the main panel is a 12 - 3 line, one of the hots feeding the hot water tank, and another hot feeding the foyer lights, sharing a neutral.

So the way the generator is setup, he says I have to move BOTH circuits because the generator circuit takes control and I can't "split" the neutral.

Hopefully that made sense ?

With that in mind, what options do I have. My neighbor who is a EE told me that I could CHEAT YET again, and it might not be CODE, but the way your house is wired you would never have an issue. What he means is that I have 200 amp service with "dedicated" circuits in every room. In the example above he said I could combine the hot water line and the foyer line (runs a single light with 4 40 watt bulbs) together and I would NEVER come close to overloading that circuit.

He said I could do this with MOST of the circuits that are setup with this 12-3 shared neutral and I should be ok.

That does not feel right, but it might be my only option. I can't afford to re-wire my entire house. Is there any other way to perhaps put in a junction box and somehow split that back out...meaning move the 12-3 into a box, then take two 12-2 and run one to the main (aka foyer) and then run the other 12-2 into the genny panel for hot water. Or will this cause an issue with the neutral and when the genny kicks in all heck will break loose.

Ok...hopefully this is enough to get some guidance and of course if you need more details just yell.

toofast 08-27-2013 06:19 AM

Ok, I did a bit more research and I THINK this is called multi-wire branch circuit (MWBC).

Hope this helps.

JoeD 08-27-2013 07:24 AM

You are correct. They are called multi wire branch circuits. They are legal and not cheating. It saves cable and expense when wiring a house.
He is correct that both of them must be moved.
I would not be following your neighbour's directions.

toofast 08-28-2013 02:28 PM


Originally Posted by JoeD (Post 90882)
You are correct. They are called multi wire branch circuits. They are legal and not cheating. It saves cable and expense when wiring a house.
He is correct that both of them must be moved.
I would not be following your neighbour's directions.

Yeah I agree with not listening to the neighbor.

As a side there ANY WAY to split out these circuits...without rewiring the whole house ?

kok328 08-28-2013 03:42 PM

It is my understanding that in order to utilize a MWBC you have to share the neutral between the two phases of hot.

JoeD 08-28-2013 05:48 PM

The only way to separate them is replace the 12/3 line from the panel to the point where it separates into two circuits With two separate 12/2 cables. The new cables do not need to follow the same route as the 12/3, as long as you get them from the panel the separation point. The old 12/3 can be abandoned in the wall.

Actually you only need one 12/2 cable. The 12/3 can remain as one of the circuits with one of the hots simply being left unused.

bud16415 08-29-2013 05:52 AM

I don’t know much about backup generators but have a few questions based around what the OP said he had.

If he has a generator capable of (10) 20 amp circuits the unit then should be large enough to power his whole house assuming he has a 200 amp service. Why would they isolate the power of individual circuits in this manner rather than one large switch that isolates the whole house?

I understand the rational of doing that with a small unit sized to supply only emergency needs heat, water pump, fridge, and a few lights etc. Around here many people have a small gasoline unit sitting in the garage and will hook up some extension cords even if needed in the winter to keep the necessities going, and others have whole house natural gas rig outside with automatic transfer. I assumed the big units took the house off line and then fired up replacing the power.

JoeD 08-29-2013 06:55 AM

20 circuits does not mean it is large enough to support the whole house.
Also a 200 amp whole house transfer switch is much more expensive.

toofast 08-29-2013 10:11 AM main panel has 42 20 amp breakers. My Genny (if I ever get it going) will power most of the critical circuits but not all.

Keep in mind (although I am no expert) the total amps of the breakers installed nearly always exceed the service as these are not directly related. Although it seems like it should be. Perhaps an expert will explain this better.

As for my problem...I think I will try to FIX the circuits that I can get to - without tearing into the walls. Hopefully it will give me enough to power the critical areas.

bud16415 08-29-2013 11:09 AM

How big is your gen, KVA rating?

A auto disconnect for a whole 200 amp house is about $700 and a manual one is about $350 so ya pretty pricy.

The reason the sum of the parts is greater than the whole I believe is that’s a max rating. No one would ever have every circuit maxed out at the same time. That’s kind of what your neighbor was suggesting. You said one of the items was your gas water heater? How much current could that draw?

Do you have a well and pump? That would and should be a dedicated line now what other combinations and items do you want to break out?

If you do like JoeD said and use the 12 / 3 as a 12 / 2 for the lighting then it might not be as hard to run a new wire to your water heater. Other things might just work out the same.

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