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crecore 04-11-2006 11:27 AM

Main/Sub/Garage Gen Backup Questions
I built myself a detatched garage (1260 sq ft shop) last Fall (NY) and I am working out my electrical plan.

I buried 4 conductor in conduit to run 100A 220 out.

In my house the panel box is 200a service but it is only a 30 pos panel and it's full. uggh

Here's the plan followed with some questions:

Install new 100a sub panel in house.
Move "emergency breakers to the sub panel."
Feed sub panel with new 2 pole 100a from main panel.
Feed garage from 2nd new 2 pole 100a in sub panel.
Garage will have a new 100a main breaker panel as well.

This will provide me with a mock generator panel. By shutting off the 100A feed from the main panel and shutting off everything in the garage except the welder feed and the main, I can backfeed from the detached garage (Carbon Monoxide Safety) via a welder outlet to the sub panel powering all of the "emergency breakers".

So I'm killing two birds with one stone I hope. Creating the room I need to feed the garage and creating a way to generator power the house. The items I would move btw are double 20 well pump, 20a refridgerator, 20a sub pump (hardly used), 20a furnace, 20a kitchen gfi and one 20a light circuit.

Garage could run a lot but never during a power outage when I was back feeding the house. Garage use at worse might be double 30 compressor, double 40 welder, 20a furnace, 20a lights, 20a receptacles.

Disclaimer... I will be hiring an electrician friend to do the work but I am buying the components and doing a lot of the grunt work.

1. Is there a problem with any theory?
2. Have I sized the sub panel OK?
3. Is a std double 100 ok to feed the sub panel or do I need a main (higher trip? type breaker?... or will those not even plug in to the regular bus?)
4. Does the sub panel need a main breaker at all or just feed direct from the double 100 in the main panel?
5. My welder requires 40a 220. Is there a way to create a way to safely run that AND be able to backfeed from the welder? I'm not worried about flicking breakers, I can handle that... I mean power wise.

btw, I'm using Square-D QO line if anyone is familiar and cares to be specific.

thanks a bunch!

Speedy Petey 04-11-2006 02:42 PM

This is ABSOLUTEY NOT acceptable. What you propose is illegal and very dangerous.
We have all heard this theory and how everyone will be safe and make sure the main breakers are off, etc, etc, etc. It is still illegal, VERY illegal.

The ONLY safe way to back feed a service with a genset is to install a transfer panel with certain critical circuits in it. OR you can get a main tranfer switch to switch over the whole service. OR you can get a main breaker interlock. This is a mechanical device which prevents the main breaker and a back feed breaker from being turned on at the same time.
In ANY case this HAS to be done at the MAIN panel. It is impossible to achieve any type of generator feed from ANY sub-panel.

crecore 04-11-2006 03:15 PM

SPeedy... so backfeeding this way is illegal.. the sub panel is not... right?

During the ice storm up here there was a lot of illegal hook ups believe me.. many worse than this. But after 6 to 9 day w/o power the wives get a little ansy.

The irony is that when I built the house I spec'd a 40 position panel and somehow endup up with a 30. I also had a gen backup panel on the list but it was an easy deletion when the budget started getting close.

I thought I could kill two birds with one stone.... disregarding the generator business how does it look?


Speedy Petey 04-11-2006 03:26 PM

The rest sounds OK.
The sub in the house does not need a main breaker, but the panel in the detached structure does.

I can almost understand the illegal backfeeds during an extreme situation, like a 9 day outage, but this is a rare occurrance.
Even then there are safer ways to go about it since it is only a one time thing.

crecore 04-11-2006 06:53 PM

It appears Square-D literature says I can only run a 70a max out to the shop on any of their 100 main lug sub panels. Guess I'll have to go up to 125a sub.

Thanks for the input,

frenchelectrican 04-11-2006 10:18 PM

i do agree with speedy pete about this set up.

i did see alot of crazy stuff as well in my area too.

the best way to deal with is start at main breaker box as what speedy pete suggest there if need more question we can help you with it

the way you described about using the subfeed for generator set up i dont think it will work what it way it set up now .

the other question is that how big the generator you have ?? it will make the big diffrence by slecting the size of unit.

Merci , Marc

crecore 04-12-2006 07:39 AM

I'm in need of a new generator for constr. purposes. I could size it for the items in the house I would like the ability to back up.

Is there a way I can accomplish making more room in the main panel, doing a gen backup AND getting a dbl 100 to feed the garage w/o adding a sub panel?

My box does accept tandem breakers... I already have two already... my gen rule is never more than 5 but I'd have to look up the specs for my panel.


jwhite 04-15-2006 05:55 AM

crecore, you could run the new subpanel feed first to a manual transfer switch then from the transfer switch to the new sub panel. Then you could run a cable from the transfer switch to a cord cap or reverse rec outside the house where you can put your generator during an emergancy.

Next you can move the circuts that you want to back up to the new sub panel.

crecore 04-17-2006 10:49 AM

Mr White, that makes sense! I've never messed with the back up panels. Do you agree Speedy?

thanks all

Speedy Petey 04-17-2006 06:31 PM

Yes, that would work as well as a pre-made transfer panel.

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