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: http://youtu.be/SSs5D-Qr0rg 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.
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