Wiring a Home Generator Transfer Switch

[RonMar]

Rob,
You could just put an on-off-on switch between your meter and main panel with the first ON being your electric company power, the OFF to isolate the two services, and the second ON being your generator. Then you manage the load with your breakers. This method takes the least amount of wiring, is most likely code legal as the two power sources cannot be mixed, and probably the cheapest in the long run, although you will be shocked at the price of an on-off-on switch at first. But the ease of installation and no need to pull circuits from existing panels may make it more attractive.

Mossroad
A manual transfer switch placed between meter and panel is probably the most expensive option of any discussed here... To install it, you have to have the meter pulled to access and replace the conduit and wiring between meter and main panel and locate the switch in a reachable location. A liscensed electrician will most likley have to do the final wiring, or at the very least you will be paying for him to come and sign off on it, so it can pass an inspection before the local utility will re-install the meter. This is the same as if you had a new main panel installed. The prices of a 2 pole switch capable of switching several hundred amps current capacity(has to handle your normal commercial capacity, so 200+ amps)are also WAY up there. If I was going to do a whole house backup generator, then sure why not. But that is a huge investment for a 5KW emergency generator.

In all of my research(over the past several years), the main panel interlock kits, at around $150, are the least expensive UL approved method of adding hard wired manual control backup power that I have come across. The backup-ready main panels with the interlock plate already installed are starting to be quite common on the shelf in the bigbox and electrical supply stores. The kits are also no more difficult to install than installing a new circuit breaker into the panel. You shutoff the main, remove the cover plate and remove the top right 240 breaker. You relocate it and it's wireing to an open slot down lower in the panel. You install a new 240 breaker sized for your generators capability into that top right slot and wire the generator hots to that. the generator neutral and ground go to the main panel busses just like the commercial powerlines do. You then drill the panel cover plate with a template and rivet the interlock plate in place. Once the panel coverplate is re-installed, the main breaker and top right generator breaker can't physically be on at the same time...

 

[RobS]

Thanks for all the inputs. So far, I'm leaning towards the transfer switch. It sounds like it can tie into both my boxes, is safe and easy enough for anyone to operate in case I'm out of town. Not the cheapest option, but not too bad. A buddy at work put his own in and is bringing the instructions tomorrow for me to have a look.

The conventional backfeed options either don't work, or become more costly for me as my circuits are in the two boxes. The backfeed only feeds one and with the mains disconnected there is no link between the two. Also not the safest option.

I did see the panel interlock and it is intrigueing but I think suffers the same multi-panel problem.

And Dave, our service up here in Michigan has been much improved over what we had in Indiana. I think we've only had a couple of outages that were more than a minute or so. I had always wanted a generator but it was low on the priority list. This one just came up as a deal I couldn't pass. Peace of mind, and I get some tinkering time!

 

[ronjhall]

There are some good manuals on the B&S site for installing a transfer switch.
30 amp transfer switch
Here is a link to all their Manuals
I have 200amp transfer switch on our generator. Works great. I did not install it. Had an electrician do that and some other work. There is enough info in the manual that I could have done the work myself.

 

[kennyd]

I also had 2 panels/400amp service, now I have 3.

When I put the Generac 15kw unit in, it came with it's own 16 space panel/transfer switch, so all the backed-up circuits are wired to this new panel, then the new panel is fed from one of the existing panels. It is wired the same way a sub-panel is.

IMHO, this is the simplest way to do it if you are only backing up SOME circuits with a smaller genset. You could pay the $$$ for a 200 or 400 amp transfer switch, but when your on the genset you will have to turn breakers off to keep from overloading the generator.

 

[MossRoad]

Mossroad
A manual transfer switch placed between meter and panel is probably the most expensive option of any discussed here... To install it, you have to have the meter pulled to access and replace the conduit and wiring between meter and main panel and locate the switch in a reachable location. A liscensed electrician will most likley have to do the final wiring, or at the very least you will be paying for him to come and sign off on it, so it can pass an inspection before the local utility will re-install the meter. This is the same as if you had a new main panel installed. The prices of a 2 pole switch capable of switching several hundred amps current capacity(has to handle your normal commercial capacity, so 200+ amps)are also WAY up there. If I was going to do a whole house backup generator, then sure why not. But that is a huge investment for a 5KW emergency generator.

In all of my research(over the past several years), the main panel interlock kits, at around $150, are the least expensive UL approved method of adding hard wired manual control backup power that I have come across. The backup-ready main panels with the interlock plate already installed are starting to be quite common on the shelf in the bigbox and electrical supply stores. The kits are also no more difficult to install than installing a new circuit breaker into the panel. You shutoff the main, remove the cover plate and remove the top right 240 breaker. You relocate it and it's wireing to an open slot down lower in the panel. You install a new 240 breaker sized for your generators capability into that top right slot and wire the generator hots to that. the generator neutral and ground go to the main panel busses just like the commercial powerlines do. You then drill the panel cover plate with a template and rivet the interlock plate in place. Once the panel coverplate is re-installed, the main breaker and top right generator breaker can't physically be on at the same time...

Yes, I have seen those interlocks just recently and had forgotten about them. They look like a very cost effective solution, but will not work if you have two boxes wired to a common meter and want to run some circuits from each box. It would work very well if the second panel was wired off the first panel. Definately something to look at to see if it can work because the intelock method is so inexpensive.

Our electric service comes out of the meter, into the main disconnect box, then through the exterior wall and into the house and the main sub-panel. There is no conduit. All that is required is to pull the main disconnect, pull the feed out of the main sub-panel, slip it into the on-off-on switch and run a short new wire from the switch to the main sub-panel. No electrician required. Although, around here, you probably have to get a permit to do it because you are changing things, not replacing existing, and you will definately have to have it inspected by the county guy/gal. I also only have 100 amp service. The cost difference between a 100 amp switch and a 200 amp switch is crazy, that's why I put in the disclaimer in my previous post.

I wonder what the total cost difference is between installing one switch like I suggested VS installing an interlock in one of the panels and rewiring all the circuits to one of Rob's two panels would be VS installing a transfer switch with dedicated circuits to the genny, etc... ?

Rob,
You need to do a cost analysis of the several choices you have. Get out that pencil.

Just curious though.... you mentioned that you have two 200 amp panels. What is your amp service rated at at your house?
How does the power come into your house?

Meter, disconnect, branches to the two sub-panels
or
Meter, disconnect, first sub-panel, second sub-panel fed from the first sub-panel?

Where are the panels located?

Pretty nosey, aren't I?

 

[RobS]

Just curious though.... you mentioned that you have two 200 amp panels. What is your amp service rated at at your house?
How does the power come into your house?

Meter, disconnect, branches to the two sub-panels
or
Meter, disconnect, first sub-panel, second sub-panel fed from the first sub-panel?

Where are the panels located?

Pretty nosey, aren't I?

Hey, aren't we all pretty nosey when it comes right down to it? I mean, how many have NOT peeked into the medicine cabinet at someone elses house???

Our power starts at the street (well, it may start at Cook Nuclear Plant for all I know). The house sits too far from the street for a pole mount transformer so we have a high voltage underground feed to our own ground level transformer about 50 feet from the house (it's lovely, shrubs not growing fast enough there). From there the 240V comes to the house and into the meter base. From the meter base there are two sets of aluminum feeds, one to each panel. They are basically mirrors of each other. I'm not sure the rating of all this incoming cable but the aluminum feeds are plenty beefy. If there is a seperate disconnect between the meter and the panels, it must be inside the meter box on the outside of the house. I don't recall any other boxes out there. And I believe the electrician called our service 400 amp, simply adding the two boxes together.

The electricians pretty much split the house between the two panels which are in my utility room in the basement. We probably could have done just one, but that would have left no room for expansion so they talked us into the two and I'm glad they did. I've added at least a half dozen circuits with my basement finish and still have plenty of open spots in the boxes. As someone else mentioned, the total draw of all the circuits in a box can easily exceed it's rated capacity but the chances of maxing all that out are pretty slim. I do remember popping the main in our old house with AC, well and probably my table saw all starting/running in addition to the normal loads. That was a single 200A box.

 

[RobS]

My buddy did bring in his instructions from his transfer switch. He has a Briggs & Stratton/Cutler-Hammer 30A 10 circuit, pretty much what I'm looking at. I won't say it's simple, but it looks straightforward enough. For each generator circuit there are two wires that come into the existing panel. One replaces the load line on the breaker and the other is wire-nutted to that load line. Looks to be a single neutral and a single ground as well. This is where I need some consultation as I do not know for sure if I can tie my two panels neutrals together. They are tied together for grounds.

The whole transfer switch is much more clear to me now. Basically, it goes between the breaker and the load so the load can only be fed from EITHER the breaker (line) or the genny. Now I understand how you can feed the circuits off the genny without disconnecting the main. And it solves my sons marvelous question of "how do you know when the power comes back on?" Answer is that the remaining circuits in the house will simply come back on.

As we all know, copper and such has gotten a lot more expensive. What my buddy paid about $200 for is now over $400!!!

 

[kennyd]

Rob,
Your setup sound the same as mine.

You can find the Generac transfer switches for about $200 if you look around.

 

[RonMar]

Mossroad, In my experience, most domestic services do not have a dissconnect between meter and main breaker in the panel, just like Rob describes with his split panels. If you have to replace a main breaker or do work between meter and panel, the only way to cut power is to pull the meter. I didn't think that the interlock would work well for him having split panels, and the sub-panel transfer switchbox method is probably his most cost effective approach.

Robs
Your main panels should share a common neutral already. It is comming in with the two hot leads from the utility. It is basically at ground potential but the utilities must guarantee it's presence as it is the center tap to the utilities 240V transformer that allows you to derive two 120V legs from a single phase of 240VAC. Most code requires that it be grounded at the main service panel. I think this is a backup incase just the incomming neutral leg from the utility should fail.

 

[MossRoad]

Mossroad, In my experience, most domestic services do not have a dissconnect between meter and main breaker in the panel, just like Rob describes with his split panels. If you have to replace a main breaker or do work between meter and panel, the only way to cut power is to pull the meter. I didn't think that the interlock would work well for him having split panels, and the sub-panel transfer switchbox method is probably his most cost effective approach.

Robs
Your main panels should share a common neutral already. It is comming in with the two hot leads from the utility. It is basically at ground potential but the utilities must guarantee it's presence as it is the center tap to the utilities 240V transformer that allows you to derive two 120V legs from a single phase of 240VAC. Most code requires that it be grounded at the main service panel. I think this is a backup incase just the incomming neutral leg from the utility should fail.

In our area, if the main panel is more than 12 feet from the meter, there has to be a service disconnect near the meter. If it is 12 feet or less from the meter, the main breakers in the panel can serve ase the service disonnect. Several years back, I replaced our main panel, so I had an electrician add a service disconnect under the meter as described before. That makes it so much safer for me to do my own work.

In Rob's case, which panel is considered the main panel? Is there a service disconnect anywhere? It would seem that you would still have power in half the house if you turned off one of the main breakers in one of the panels.

We cannot get 400 amp residential service here. We can get 100, 200, and something in the 300s out in some parts of the county.