Because of the unreliable nature of the electrical utilities in my area, I have been working on improving the interface of my backup diesel generator to the electrical system of my house. If you have seen this video:
then you know that I theoretically might have wired it so that I could backfeed my panel (if legal) via the clothes dryer line. Basically I (again, theoretically) made a connection from the dryer outlet (internally, not a plug) to a second breaker box in my workshop that I use to distribute power therein. That is a reletively common practice, by the way: extending the dryer wiring to power some piece of workshop equipment, typically a welder. Although a standard 200-250 amp welder needs more current at full load than the typical 30 or 40 amp dryer circuit can supply, home workshop welding jobs almost never require the full output of the welder. A dryer circuit can supply all the power the average welding installation ever needs. You just have to make sure the welder never gets used while the dryer is running.
I took that basic circuit a step further. I had a load center, complete with a full compliment of breakers, that had been discarded during a home rewire job. I added that to my workshop to distribute power to the various tools. When I installed the generator, I (theoretically) connected it to an unused 30 amp double-pole breaker in that box.
This meant that during a power outage I had to grab my checklist to make sure I did everything correctly and in order, and follow the steps to bring the generator online. It went something like this:
- Turn off the main breaker.
- Turn off the water heater breaker and any other breakers I deem necessary. The dryer breaker stays on. If the dryer was running, it won't restart on its own so it is not a problem.
- Go out to the shop and make sure the generator breaker is off. Turn off all other workshop circuits too, except the house connection. Start the generator and let it warm up.
- Go back in the house and double-check that everything is ready.
- Return to the shop and bring the generator online.
That sounds like a lot of opportunity for a catastrophic failure, but it is really not as bad as it seems. First, having a checklist and following it to the letter every single time you bring the generator online prevents problems. As they say, the best safety is between your ears; or to put it another way, attempting to design a foolproof piece of equipment underestimates the ingenuity of fools.
Second, there are three circuit breakers separating the generator from the house wiring. Two of those are 40 amps and the third is 30 amps. The generator is rated 7,500 watts continuous. 30 amps at 240 volts is 7,200 watts, or 96 percent of the continuous rating of the generator. That is a nice safety margin. On the other side of the main breaker is a whole neighborhood just waiting to be powered up. To a 30 amp breaker, that looks like a dead short. If the generator were brought online with the main breaker in the "on" position, that 30 amp breaker would trip instantly.
Granted, the best solution is a true transfer switch that disconnects the public power before connecting the generator line. But those are not only expensive; they are also not necessarily 100 percent reliable. Sure, if you have one of those big industrial three-pole double-throw knife switches as a transfer switch, that would be as close to 100 percent reliability as you can get. But those are not easy to find, and a new one probably costs a thousand dollars if you do find it. If you find a surplus one cheap, by all means use it. I used to have one myself, but I haven't seen it in over ten years, so I obviously don't have it any more.
The transfer switches that are readily available are nothing more than a pair of breakers placed back to back so the act of making one connection breaks the other. That works, but breakers do fail on occasion, and one of the ways they can fail is sticking contacts so that a breaker that is supposed to be "turned off" is actually still conducting. That is unlikely though, so if you prefer to shell out 300 dollars or more for one of these, have at it. Personally, if I wanted one of these I would make my own from breakers and boxes I already own.
There is another disadvantage to a transfer switch, even the big industrial knife switches: it has to go "upstream" of your main breaker, between the breaker box and the meter. Because you don't control the meter, this means you have to get the power company to come out and disconnect it before you can work on that portion of the circuit. You do not want to work on that wiring while it is live, even if it were legal.
Actually there is a way to install your transfer switch without calling the electric company and telling them about it so they can disconnect the line. Just don't pay your bill. After awhile they will come out and disconnect your power without your asking them to do so. After you install your switch, pay the bill and they will reconnect your power. This is merely a tongue-in-cheek observation, not a recommendation. Some utility companies would probably have you up on charges (no pun intended) if you actually did that.
That is one of the reasons I prefer to keep all my electrical work downstream of the main breaker. In some jurisdictions (mine, for example), the wiring downstream of the main breaker is the responsibility of the owner. The power company doesn't worry too much about it, as long as the main breaker is in place. So the first time I saw a generator circuit interlock, I was excited about it. Basically it is just a sliding bar that blocks one breaker while leaving another free. In one position it allows the main breaker to be on while blocking another breaker (for the generator input) in the off position. In the other position it allows you to switch the generator circuit on, but you must first switch the main breaker off. Electrically it is nothing more than an additional breaker that connects the generator just as if it were a water heater or other 240 volt appliance. The sliding bar is what makes it different.
It is not only simpler, but also cheaper than a transfer switch. The companies that produce them make different versions to fit specific breaker panels. Also, some load center manufacturers sell a kit to fit their own panel. Most of these interlock kits sell for around 100 dollars, which seems like a lot for a metal bar and some screws (and is), but it is far cheaper than a transfer switch, especially when you factor in utility company service charges and labor charges for the licensed electrician that the utility will likely demand that you hire to install the switch.
Depending on your specific panel, you may be able to make your own interlock. I was lucky enough to have a Square D QO panel. In this panel the main breaker is centered at the top and switches horizontally, while the individual circuit breakers are in two vertical rows and also switch horizontally. This makes for the simplest possible interlock. The generator circuit must go in the top right position. This allows a simple sliding steel bar about 3/8-inch wide and 1 1/2-inch long to block either the main breaker or the generator breaker, making it impossible for both to be energized at the same time.
I have decided to implement such a device, and have taken the first steps to do so. The dryer circuit is less than ideal anyway because I can hear a slight hesitation in the air conditioning compressor when it cycles on during generator operation. I have listened to the generator when the compressor kicks in and it doesn't strain at all, so the culprit has to be voltage drop across the wiring and all its breakers.
Fortuitously, the load center in my workshop is also a Square D QO, and it contained an unused 30 amp double-pole breaker. So I absconded with that breaker and used it for my new dedicated generator circuit in the main panel. To do so and put it in the correct location for an interlock, I had to move one single-pole breaker from the right to the left side, open another slot at the bottom of the right bank, and move all of the remaining breakers in the right bank down by two slots to vacate the top two slots for the generator breaker. This went off without a hitch.
Also fortuitously, about a year ago I came into possession of a section of 6/3 (with ground) UL direct-burial wire long enough to reach from the generator to the main breaker. There won't be much voltage drop on that with 7.2 kilowatts maximum.
The last thing I did was install a real fused disconnect at the generator end of the line, and put 30 amp fuses in it. I will leave that connected even when I am not using the generator. I'm using that because fuses are more reliable safety devices than mechanical breakers, and because (you guessed it) I salvaged that from a rewire job, too.
Now the generator changeover procedure goes like this:
- Switch off the main breaker.
- Switch off the circuit(s) I do not wish to power right now, such as the water heater.
- Go out to the generator shed and start the genny.
- Return to the breaker box and bring the generator circuit online.
As you can see, that is easier and less prone to accidents than the way it was before.
By the way, if you have never had a diesel generator and have been thinking about buying one for either backup or offgrid prime power, allow me to offer a bit of advice: you do not need or want a big enough generator to power everything in your home at the same time. Not only will a generator that big cost far more money than you need to spend, it will also use more fuel than necessary and furthermore, will give you more problems than a properly-sized generator. The reason this is true is because diesels like to work. They are happy running in the range of 50 to 90 percent of their rated capacity. If they are running at 20 percent of capacity for extended periods, not only will they use more fuel than a smaller generator powering the same load, but that excess fuel will produce more soot and other deposits, which will build up in the engine (especially around the exhaust valves and in the exhaust system) and gradually reduce its performance, making it use even more fuel and become harder and harder to start. The cure for that condition is to hook it up to a near-maximum load and let it run that way for an extended time, but it is better to prevent the condition by sizing the generator such that you can run it in the upper 50 percent of its capacity most of the time.
There is an old adage among (mostly retired now) truckers who are familiar with the old 2-cycle Detroit Diesel engine that the way to ensure good service from a truck powered by one of those is to slam your hand in the door first thing in the morning so you will be mad at it, and drive it like you are trying to kill it! That is somewhat of an exaggeration of course, but it does contain a kernel of truth. The same thing applies to the more common 4-cycle diesels, too.