Introduction: Automatic Generator Transfer Switch

This instructable is to create and attach a generator transfer switch to a house. The whole point of this is safety and lazy. I live in a somewhat remote area and we are on a well. With no power it does not take very long before there is no water for flushing toilets. This my friends, will not do. With PG&E deciding that they want to turn off our power whenever the wind blows for multiple days at a time, I needed a solution. If you have priced a prefabricated transfer switch you have had a small heart attack at the price. As I am a control electrician by trade I set out to make one for a whole lot less money using off the shelf parts.

Restrictions: This will not power your whole house. It is not designed to. It will not allow you to use your electric stove, electric dryer or anything that uses a large amount of power. It is designed to give power to selected circuits in your house to make say, the refrigerator, freezer and coffee pot run as well as a few lights maybe the Internet and TV.

The amount of circuits that you can power is determined by the size of the generator that you have. I bought a 9600 watt generator from Costco a few years ago to use when on a camping trip. It is way bigger than I need to run my RV but the price was right and it has electric start! This my friends is a very posh addition to a generator for when it is raining dog hair outside. Yanking on a stubborn generator with water running down your neck is less than fun, but I digress.

So for my application I needed, at the very least, to power my well. (208 VAC (volts AC) @ 15 amp circuit breaker in my panel) 208 VAC x 15 amps is 3120 watts so I was consuming that many watts out of my generators 9600. Due to the technical black magic that is motor engineering, the motor for my well only consumes a portion of that number and only for a half a second when it first starts up. Using a amp draw meter I found that my well actually only drew 11 amps when running.

208 VAC x 11 amps is 2288 watts. Knowing this I have most of my generators capacity left to light the world as it were.

I selected to run in addition to my well, part of the kitchen (the part with the fridge), the garage (the part with the freezers), the bathrooms (showering made somewhat easier with lights) and the bedrooms (thats where the TV and the Internet router are) Since me and the wife can only be in at most two of those rooms at a time, I will not be consuming much power in the other rooms. The major electric draws are the two fridges, the freezer and the well. Taking some numbers off the internet, I discovered the average large fridge draws around 700 watts. Adding the fridges and freezer to the well numbers gives me 4388 watts. (2288+700+700+700=4388) The lights in my house are all LED so their draw all together all on is similar to a fly on a elephants back so I rounded up to 100 watts if I turned them all on at the same time. (This is a great reason to spend the money and make the switch to LED by the way)

So now we are at 4488 watts (2288+700+700+700+100=4488) With 9600 watts to use I think Ill be just fine.

Now on to the build.


This next bit involves you putting your hands inside your main circuit breaker panel.

The chances of getting badly shocked if you don't know what you can touch and what you can't is VERY high.

If you touch something you shouldn't, it very well could kill you and it will hurt the entire time you are dying. If you are comfortable with this adventure then let us continue. If not then feel free to read the rest for educational purposes only. YOU HAVE BEEN WARNED.

Now that we are all on the same page lets move forward shall we?

Step 1: What You Will Need...

I will continue as though you and I have the same situation and the same house. At the end I'll show how you can expand or reduce the system for your situation.

List of materials:

Enclosure - 12 x 12 x 6 - Home Depot - 1 needed - gotta have a box to put it in

Power Relays - 30 amp - 120 VAC coil - Amazon - three needed

Relay, general purpose, SPDT, 120 VAC coil with socket base - Amazon - 1 needed

Circuit breaker, panel, DIN rail mounted, two pole, 15 amp - Amazon - 1 needed for the well pump. This is a two pole version to protect the two phase 208 power.

Circuit breaker, panel, DIN rail mounted, single pole, 15 amp - Amazon - 4 needed for each of the other circuits you will be transferring. These are two single pole versions.

DIN mounting rail - 35mm -McMaster-CarrcMaster-Carr - one section needed to mount circuit breakers and relay to before mounting to back of enclosure

Spade Terminals for 10 gauge wire - Home Depot - box of 50 (not all will be used)

Spade Terminals for 14 gauge wire - Home Depot - box of 50 (not all will be used)

10 gauge stranded wire - Home Depot - at least 50' maybe more depending on how far you have to go

16 gauge stranded wire - Home Depot - 10' or less - you will need black and white - if you feel like recycling then you will be able to cut up an old extension cord and separate the interior wires and they will work just fine for this.

6/4 electrical cord to go from generator to house - by the foot - Home Depot - 5-10 feet depending on how much distance you want between the box and the generator. Keep this distance as short as you can and still be somewhat convenient. Don't go cheap here, this is the main feed cord to the whole system.

Electrical Splice Lugs - Home Depot - 3 needed - come in packs of 2 - to splice several wires together to the large gauge feed wire

Shrink tube - 7/8inch (22.2mm) - Home Depot - 3 needed - come in packs of 1 - to cover and insulate the splice lugs of #6 wire to the #10 wires

High power plug to attach to generator - Home Depot - 1 needed - my generator uses this plug for 50 amps of 208 VAC. Your generator may have a different plug. Smaller generators will have smaller plugs, adjust accordingly.

Super power double stick tape - Home Depot - Get the good stuff. You don't want anything in this box to come loose in the heat of the summer.

Conduit, wire nuts and fittings as needed to connect the panel to your house.

Tools as needed to drill, saw, screw, cut wire, strip wire, measure electricity, and take notes as you go. A label maker is a good idea as to keep track of where you are and what's what.

Now that you have a couple of hundred dollars worth of materials, lets assemble.

Step 2: The Assembly...

I am going to show you how I did mine. Yours will be different. Your results may vary. You will notice the two drawings. The first drawing shows the approximate layout of the parts in the box before the wiring goes in. The second drawings is of how the wiring goes together. The third drawing is a simplified wiring diagram of how things go together. The fourth drawing is the final product of what mine looks like.

I was fortunate to have some space directly below the main power panel at my house. I mounted the panel just below the trim on the panel. After removing the panel, I VERY CAREFULLY cut a piece of the siding that is just below the main panel making sure not to cut so deep as to cut the wires below the breaker panel. (See previous warning about shock and dying.) The piece I cut out was shorter than the trim is tall and narrower than the trim is wide. This allows me to glue the piece of siding back in place and cover the cuts with the trim. The only reason for this hole was to be able to fish the wires from the generator panel to the main breaker panel.

Now we have the breaker panel, a hole just below and the generator panel below that.

I reached inside the hole and pushed the existing house wires inside the wall to one side so that when I drilled the hole in the back of the generator panel through to the wall space behind I would not accidentally drill the wires behind. (Again see warning about shock and dying)

For my hole I used a 2 inch hole saw to drill through the back of the box, through the siding and into the space between the outside paneling and the inside drywall. With that done lets add the parts to the inside of the box and get this thing wired.

NOTE: the next step will have you turning off the power to the whole house and it will stay off for a couple of hours. Send the family to the movies or something so that you can work in peace and not have to deal with a mutiny when there is no TV or Internet. Perhaps leave one person besides yourself at home in case you touch something you shouldn't and need medical assistance. Just throwing that out there.

Step 3: Double Stick Tape the Parts...

Because I had three big relays, 1 little relay, 6 breakers, and a good pile of wire to fit in this box, I had to move things around a couple of times to get the clearance that I needed to get everything in and still wire it. The double stick tape that I used is the REAL double stick tape. Once it is set, it is VERY hard to remove it without breaking the box or the part. Play with your layout BEFORE you peal and stick. Here is what it looks like with all the parts except the breakers installed (they came in the mail later). Yes it is a little tight but a little planning and some pre-assembly before sticking the parts makes it a bit easier. If you are fortunate enough to be able to have an enclosure box with a removable false back that makes things so very much easier. I of course decided to do it the hard way. Why? Well I like a challenge. (as my wife snorts in the background. Women, I swear.) It seem to be easier to route and connect the little wires first and then lay the larger wires over them when routing. It doesn't really matter just something I observed.

You will see in the second picture that I brought in three #6 wires; yellow, red and black. These are the wires that will be coming from the generator. A #6 wire is big enough to handle 50 amps of power which is the size of the plug on my generator as you can see from the third picture.

With the terminal block connectors, attach the #6 from the generator to one side and add three #10 wires to the other side. Do this for the two hots (red and black). Make sure the screws are good and tight before sliding a piece of shrink tube over the terminal blocks and heat with torch or heat gun to make a nice insulated band.

For the yellow neutral wire it needs to to end up in the main power panel. Cut a second piece of the #6 yellow long enough to span between the switch panel and the main power panel. Connect one end to the common/ground terminal in the main power panel and the other end to one side of a terminal block connector. Slide a piece of shrink tube over the terminal connector up on to the wire. On the other side of the terminal block connector connect the yellow #6 wire from the generator and a two foot piece of #16 white wire that you will linked all the relays together with. After all screws are good and tight slide the shrink tube down over the connector and shrink it.

I shouldn't have to say turn the power off before doing this but alas, I will....please turn off the power first at the main breaker. Now that you have all the big wire connections done lets get to the box wiring.

Step 4: The First Wires...

Start with the relay coil wires. Each relay uses 120 VAC so each will need a neutral wire. As you can see in the drawings all of the relays have one terminal connected to all the others. The #14 wire that you connected to the neutral will be used for this. Using the crimp wire connectors, attach one coil terminal from each relay including the small one to the #14 neutral wire. I found that daisy chaining them together one to another works the cleanest. All three of the big relays and the one little relay all get connected.

The generator outputs 2 "hot" lines. For a US wired generator, if you were to measure from either hot line (red or black) to the neutral line (yellow) you would get 120 VAC +/-. If you were to measure from hot to hot you would get 208-230 VAC +/-. My well uses 230 so I will need both poles of one of the relay to control both hots from the generator to feed the well. The other two relays are to switch one hot for common wall power for things like televisions, Internet, lights, fans, phone chargers, this computer and the like.

Lets start with the hard one, the well. Because I am using both of the hots I need to make sure that both of them are powered before closing the relay and allowing power to goto the well pump. This can happen if a breaker is tripped on the generator or the breakers inside the switch panel are tripped. Some pump motors do not react well to only having half of the power present and they search to find a path to ground overheating the motor. So the whole point of the small relay is to make sure both hots are present before the big relay closes. One of the hots goes to through the coil of the small relay and to neutral. When this hot is present the relay closes. The other hot goes to a set of contacts on the small relay, then through the coil of the big well relay. When this hot is present and the contacts are closed on the small relay, power flows through to the well relay. In this manner we must have both hots to get power to the well relay. Slick and easy.

So what is going on here is the wire that is currently connected to the breaker in your power panel will be re-routed to the common terminal of the relay. A new wire from the recently vacated breaker is routed to the normally closed terminal of the relay. This way where there is no power to the relay power flows from the breaker through the relay and back to the circuit in your house. The normally open contact is connected to the feed from the generator. When power is supplied from the generator the relay switches the contacts from the normally closed set to the normally open set and power is shifted from the street to the generator. Easy squeezy.

So starting with the well I pulled the two wires that were on the well circuit breaker and connected one each to the common terminal of the bottom most relay. This is the relay that is controlled by the smaller relay that we spoke about earlier. Two new wires from the breaker in the main panel were routed to the normally closed terminals on the relay.

I then used the remaining contact sets to feed 4 of the normal 120 volt circuits that are in the house in the same manner. Wire removed from breaker and re-routed to the relay and new wire from breaker to normally closed terminal on relay.

Now that all of that is done this is a good stopping point if you are not able to complete the entire project in one shot. This way you can turn the power back on to the house and power will from from the main power breakers through the relays and back up to the circuits and the family will not know the difference. Since there is no power from the generator the relays will just stay as they are. If you have more time then let us continue.

Step 5: More Wires...

From this point we will be routing all the generator power. You will not have to completely shut off all the power in the house, only the breakers that feed power to the switch panel. MAKE SURE YOU KNOW WHICH ONES FEED THE PANEL. Use a volt meter to test from the feeds from the breaker that land on the normally closed terminal to the small white wire that is connected the side of all the relays. If there is more than say 5 volts then you have a breaker still turned on.

Now that you are sure that there is no power inside the panel lets begin.

Start by taking all 6 of the feed wires and connect each one to the input side of the internal breakers inside the switch panel. Make sure that the double breaker gets one lead from each of the hots. This protects each of the circuits with a second breaker that is smaller that the 50 amp breaker that is built into the generator. (see first picture)

Now connect the output side of the double breaker to the normally open side of the well relay. One wire for each normally open terminal.

Take two feed wires from the bottom of the breakers that originate from the same hot and route them to one of the remaining relays and connect them to the normally open terminals. From one of the normally open terminals you just connected wires to make a small jumper wire from it to the unconnected coil wire on that relay. When power is present at that hot it will power the coil to that relay and it will shift.

Do the same with the remaining two feeds from the switch box breakers and route to the other relay and make the same jumper with it.

Refer to the wiring diagram if this too confusing.

Step 6: Assembly Accomplished!

As you can see from the picture this is what is looks like after the assembly is complete. I took my label maker and labeled everything as in a couple of years of not touching this project I will have no idea what I did here.

Testing. OK this is the moment when you figure out if you wired it correctly. Don't worry to much if you did mess something up the breakers within the switch panel will protect you and the panel. If you really messes things up the breaker that is built into the generator itself will protect you although you may sustain some damage to the panel.

First things first. Turn the power off to the main breaker panel. This does two things; it simulates a power outage and removes the street power from the switch box just in case a major short happens you don't incinerate your generator by directly powering it with street power.

Second thing; turn off all the breakers inside the switch box. This will allow you to energize the panel a bit at a time to make sure that everything is working OK. When you energize the switch panel do wear gloves, long sleeves, eye protection (face shield like for weed eating) and a hat. If a short does happen any molten metal or large arc that may be thrown will be a least partially blocked from doing you bodily damage. I know this sounds either scary or entirely unnecessary depending on your comfort level with electricity. I just throw this out there because I have a pretty scar on my hand from just such an arc. It only takes a couple more minutes to gather this stuff anyway so just do it.

Don't plug your generator in before starting it. Get it running and then plug it in to the panel. Once you have the generator running and plugged in go to the switch panel with your volt meter and start testing. Start with the input side of the double breaker. You should have 230 VAC +/- at the input side. Then place one probe on one terminal of the input side of the double breaker and test to the bottom of the other 4 breakers. Two of them will show 230 VAC +/- and the other two show nothing. Switch your first probe to the second terminal on the input side of the double breaker. Test against the two other breakers that showed 0 on the last test. You should have 230 VAC +/-here as well.

OK if all of those tests went well gear up and then flip the switch to the double breaker. If the breaker does not trip then so far everything is going well. Two things should have happened. 1) the small relay should have clicked shut and the little light on it will be illuminated and 2) the big well relay should have also closed and if your well needs to fill then the pumps should be running. All is well? then let move to the other ones.

The single breakers if you remember are in pairs from the the same hot. There are two breakers that are connected to the coils on the relays and two that just provide power through the relay to the circuit beyond. So start with the first single relay. If the big relay doesn't close turn it back off and move to the next one. If the relay closes and all is well then move to the third or four breaker until the third relay closes. If all is still good then flip the other two breakers one at a time. No sparks or drama? Well done the wiring was done right the first time. Feel free to do a happy dance in full gear in front of the neighbors and make sure to yell "Hurray I didn't burn down my house!!" Go inside and turn on the lights and make sure that everything that is supposed to have power does and everything else doesn't. If everything checks out then go out to the main power panel and flip the main breaker. Still here? Then complete the second of the happy dance because everything works as designed.

To turn off the generator this is very important. DO NOT JUST SHUT OFF THE GENERATOR. If you do as the generator slows down the amount of power that it creates goes from a nice smooth output to a pulsed output that gets more severe as the rotor slows to a stop. This WILL damage you very expensive TV, computer, cable box and anything else of the sort. Before shutting off the generator unplug the generator and that will make a clean cut to the power. The relays will shift back and power will be restored back to street power. Do this every time you start and stop the generator. This also means do not allow your generator to run out of fuel while it is connected. Keep and eye on it. Check the fuel regularly until you know how long your generator will run on a tank of fuel keeping in mind that the more power you need the faster you will consume fuel.

Step 7: Modifications and Adaptations...

Now that you saw how I did mine the system can be up or down sized to more or less relays if you desire. Take into account the amount of wattage that your generator puts out and always give your generator a margin of working room of at least 15% more capacity than all power possible that you could consume to keep your generator from overheating. A note for those who are using this setup in a hot or very humid area make sure that your generator gets good cooling and further reduce the amount of power that you draw from a gen set. If you overtax a generator for long periods the insulation that covers the windings inside the generator will deteriorate more rapidly and will eventually short out destroying the windings. (much smoke and sparks)

Good luck with your version and please send pictures of your and include any improvements that you make.


A Machine Tech.