Power transfer switch for homes.

[tsteahr]

tsteahr:

"If you use a three pole switch you break the neutral along with the two hots and you are totally isolated (because the utility only consists of three conductors). As to the ground, in my area there is no utility ground. The ground is just that, a wire connected to copper rods driven into the ground outside the house. I used three ground rods although only two are required by code here. The genset ground is connected to the same ground as the main panel ground. All grounds in the house must be tied together. It would be against code to have multiple ground paths in my area."

You are correct, I had overlooked that as a possibility as it is very rare, although isolation of the "grounded conductor" or neutral is a requirement in only a few places. So assuming this switch is placed between the transformer and the service entrance appliance and that no grounding path exisits betwwen the cabinent of the transfer switch and the transformer, then it is possible to achieve isolation from the utility. The multiple ground I was referring to in my e-mail is a reference to the fact that in most cases (other than the one I just mentioned) the ground is bonded to the neutral. If the utlility neutral somehow gets energized, the ground is also energized, so isolation is not achieved in that case. By the way, in the case of a commercial emergency power backup, the genset often does carry its own ground, by code. However, you are also correct in that in most situations, not all, multiple grounds are a no-no.

As to the matter of bonding the neutral and the ground mentioned in several posts (not yours tsteahr), the 2008 NEC is crystal clear, the neutral and ground are bonded once within the service entrance applicance, in all subsequent panels they are to be isolated. Theoretically this provides a fully redundant fault path to ground. It is correct that in all subsequent subpanels the ground and the neutral (now called the "grounded conductor" in the NEC) are to be fully isolated from each other.

Few of the ATS devices discussed in this thread are triple pole to allow the neutral to be isolated from the utility. If for some reason the neutral from the utility becomes energized, so does the neutral throughout the circuit system, except that it should be driven to ground through the bond in the service disconnect panel. To be clear here, that event does not cause a potential harm to a lineman from someone running a generator. A fault that is not grounded through a proper neutral/ground bonding system however, could be a hazard to a lineman.

I recently installed a fully interlocked transfer switch in my house. Like many posts here I formerly back fed the panel through a 30 amp breaker and turned off the main breaker. I trust myself to do that correctly and not put anyone at risk. I used a special set of wires and recepticles to avoid any charged male prongs, not good. However, I am now travelling alot. Last two times I have been on the road we lost power. Wife did not know how to set up portable geenrator. So I have since cured that problem with a safe, foolproof, fully interlocked, up to code system so that when the power goes out she can fire up the gen set (I even bought a new one that is electirc start) and have power in the house. I have 11 circuits in total that can be operated from this subpanel either with utility power, or the genset. Works like a charm, as I had to use it three days last week when we lost our power. I have one more enhancement to add and that is an indicator light on the utility line to make it easy to tell if power is back on.

I wish I knew how much it actually costed me to install. Good heavens copper wire is nearly as valuable as gold. Cheaper by the spool, especially if you have future use for the surplus, which I do. But $235 for 500' of AWG 6/19 is just plain crazy!

This has been a really good thread and I hope information here encourages people to install safe gen transfer switches. I do not know about anyone else, but I could never forgive myself if I accidently eloctrocuted a lineman trying to restore power to my neighborhood.

Thanks for clearing that up. What you describe makes perfect sense.

When I installed my transfer switch, I also installed a audible indicator to tell me when the utility power is back on. It is a battery powered gadget that has a sensor wire that is wrapped around one of the hot utility leads inside the transfer switch. When the power goes out I turn this little gadget on. When the utility power comes back it starts beeping. It is loud enough I can hear it in the house. My transfer switch is in my garage and I have this device mounted to the switch cabinet. I shut the device back off when the utility power come back on. It runs on a 9v battery.

Reliance THP108

 

[eepete]

For single phase residential service, the "high voltage" shows up at the transformer on two wires. I have buried service. The 7.5KV has two conductors that come from the "wires on the poles." It's a coaxial sort of cable, with the outside wire being close to ground potential- the insulation to the center conductor is far thicker than the insulation on the outside. Different distribution schemes are in use, but, the high voltage side of the transformer (the primary) is a two wire connection.

After going through the step down transformer that takes the 7500 volts and makes 240 volts (on the secondary coil), there are three terminals. At the transformer pad, the center tap (or center terminal of the three) has a connection to a ground rod at the transformer. Three wires come into the house, you can see the three wires that go from the meter base into the house in the 1st picture.

These three wires make their way into the basement (under the garage, the conduit they are in is in the ground) and go into a 400A disconnect. The three wires come into the top via the 90 degree fitting. The disconnect switch also has a wire that goes to the grounding system for the house- ground rods and copper wires in the earth. See 2nd picture.

Inside the disconnect switch the center tap wire (which I am purposely not calling ground or neutral) turns into two wires- ground and neutral. So at this point and only at this point both wires are connected to the ground that goes into the disconnect switch from the earth ground. From here on out, all the electrical in the house is a four wire system.

The 3rd shot shows the mains disconnect. To the right of that is a breaker panel that is before the generator transfer switch. It has some big loads on it like the resistive electric backup for the heat pumps, and the 100A service to the outbuilding (tractor garage). It also has the block heater for the generator.

On the right, you can see a 200A disconnect for the generator. In the same way that you need disconnects on outside heat pump compressors so that you have a disconnect right there at the work area, I have to have a disconnect for the power from the generator. At the far right is the 400A transfer switch. It is fed from the commercial mains disconnect and the generator disconnect, and it's output runs off the the main breaker panels for the house.

Throughout all the wiring after the big disconnect switch, ground and neutral are kept separate and are never switched. All manners of switches, disconnects, and breakers just work on the one (120V) or both (240v) of the two hot wires that came from the secondary of the transformer pad.

There is a shot of the generator (45KW - I don't suffer lack of power lightly). There is code for how far away it has to be from a window or opening in a house, and this is far enough way. It is also on the opposite side of the house from the master bedroom for noise reasons.

So there's a setup of a fully automatic system with whole house backup. Yeah, it's some bucks but the goal is that everything happens automatically and does not have to be done by someone who knows what breakers to throw and how to plug in a suicide cord into a dryer vent etc, etc. Note that a more common configuration for residential is a 200 amp service, disconnect, and transfer switch. All this is very expensive to retrofit, the ability to do it during construction is a big win.

When the transfer switch is thrown, or the disconnect is cut off, the two hot wires are not connected to the transformer at the pad. The center tap for the secondary winding goes to ground and neutral in the house. And it has the connection to ground at the pad. So everything and everyone is save because you can't have a 1 wire circuit, it takes two to tango. And if the one wire neutral circuit opens, well, it really wasn't a circuit that could pass power in the first place. The windings on each side of the transformer are isolated- the goal of a transformer is step-down and isolation. So having one wire (the center tap) connected to a winding means you can't pass any energy. Don't flame on exotic "What ifs" like someone wired the hot terminal of the generator to the neutral during a power fail by mistake and the ground failed because a chipmonk at the wire... I'm dealing with systems that were properly installed and normal faults like opens and shorts.

Open neutrals into a house when the two other lines are connect is a big problem, that's more or less been discussed. I have MOVs (kinda like whole house protectors) in the breaker panels. Inside the house any 120V device that's not a lightbulb is on a protected strip with a fuse or breaker in it. That's my open neutral plan.

Pete

 

[hr3]

The utility actually consist from four wires. The fourth is the ground. If the lineman is staying on the ground he could get electric shock from the neutral.
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Ladia

Sorry but no,, not from the house with a generator and a proper transfer switch,, and I think that transfer switches are what we are talking about,,

 

[MichKubota]

I purchase this unit from Home Depot 10 years ago for about $160. It came with video on how to install. I added a permanent extension cable through the outside wall of house. This eliminated the extension cord to generator having to be run through door when needed.

The generator (8000watts), switching box and extension cord came to less then $1100 and it was a DIY project.

It doesn't power whole house and it is not something the wife feels comfortable hooking up. We use it about once every 2 years. It would be nice to have whole house unit with automatic switching but for $1100 it works.

 

[Redneck in training]

Sorry but no,, not from the house with a generator and a proper transfer switch,, and I think that transfer switches are what we are talking about,,

The question was if it is enough to break only the hot wires or also the neutral. I looked inside of the tombstone with manual transfer switch installed by my transformer. It breaks all three wires. Therefore I am guessing that the automatic transfer switched do the same. Some people just trip the main breaker and back feed power to some outlet. So there is a potential to have voltage between neutral and ground.

 

[hr3]

The question was if it is enough to break only the hot wires or also the neutral. I looked inside of the tombstone with manual transfer switch installed by my transformer. It breaks all three wires. Therefore I am guessing that the automatic transfer switched do the same. Some people just trip the main breaker and back feed power to some outlet. So there is a potential to have voltage between neutral and ground.
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Ladia

Then I guess the answer is,, yes because all automatic transfer switches only switch the two hots on single phase...

 

[eepete]

If you look at most 100 to 400 amp transfer switches, you'll see that the plastic molding are made to have three contracts. This is so that the various molded pieces can be used in either a single phase (like 240 in a house, two lugs) or three phase (and hence three lugs) uses.

My switch just has a set of contacts and moving parts missing for the middle group of contacts since it's a single phase switch. I wonder if the people who are switching their neutral with the transfer switch happen to get a 3 phase switch, either by accident, because it was surplus/a good deal, or a supply house just sells 3 phase switches so there is only one type of transfer switch to stock The contacts were they and they felt they should use them?

Pete

 

[Obed]

Look at this Ive put a couple of them in.

Generator InterLock Kit

if you have the space it saves a lot of wiring.
tom

That thing is a great idea. I haven't been thrilled about the thoughts of buying an expensive generator transfer switch. Thanks for the idea!

Obed

 

[whistlepig]

That thing is a great idea. I haven't been thrilled about the thoughts of buying an expensive generator transfer switch. Thanks for the idea!

Obed

This is "back feeding" a house. Please make sure you read about the implications of " back feeding" a house before doing so.

 

[tommu56]

This is "back feeding" a house. Please make sure you read about the implications of " back feeding" a house before doing so.

As long as the panel in in place you can't turn on the main and the gen breakers it will not back feed to the utility.

The load control is the only issue with these and if you can only run 1 or 2 things at a time hopefully the owner can keep track of it.
Then the only problem would be the breaker tripping on gen or in the panel for the gen.

tom