DIY PV System --
Running the Underground Wiring
Back to the PV
This page covers the layout and digging of
the trench for the underground wiring from the meter/distribution panel
location on the house to PV panel array out in the yard.
The PV system we decided on uses Enphase
micro-inverters mounted at each PV panel to convert the DC generated by the
PV panel into grid compatible 240 volt AC. A set of underground
wires then carries the 240VAC over to the distribution panel and meter
location on the side of the house. This section covers laying out and
digging the trench.
On most grid tie systems, a single larger
inverter is used, and it would typically be mounted on the house wall near
the meter and distribution panel, or inside the house near the circuit
breaker panel. In most cases, a "string" of PV panels would be wired
in series with each other with a resulting voltage of two or three hundred
DC volts for the full string. The output from the string of panels
would then be run back to the house and the big inverter in a trench that is
similar to the one we used. So, with either system you end up needing
an underground wiring trench.
In our case, the panels are a ways from the
house, and by the time you avoid the buried propane tanks, the underground
line ends up being about 90 ft long. The trench needs to be on
the order of 2 ft deep, and our soil is hard digging, so I decided to rent a
fairly big trench digger -- this was a good decision, and saved a
lot of time.
The first thing you want to do after
establishing where the two ends of the wire are going to be is to call the
underground utility locator service in your area. In most places, you
can call them, and they come out and mark where the electric, phone, and gas
lines are (and maybe water?). This is normally a free service, so even
if you think you know where things are, call them anyway. I can't tell
you how dumb you feel if you cut a line that could have been avoided -- well
actually, I could tell you, but I won't :)
The other thing you might encounter is
sprinkler system pipe. Our house seems to have them running all over
the place, and I've given up on trying to avoid it. I just figure on
cutting them in a few places, and having to do some repairs. The
repairs are pretty easy.
After you know where all the utility lines
run, plan a path that stays out of the way of these lines, or if you have to
cross them, figure on digging that part by hand.
If we went with a straight line from the
meter area on the house to the PV array, it would go right over (through)
our underground propane tank, so we ran the wire trench parallel to the
house until clearing the propane tank, and then out to the PV array.
I could not find a consistent number on how
deep the trench needed to be by code -- there were numbers from 18 inches to
30 inches depending on the source, and whether the wire was in conduit or
not. The trencher did not seem to have much trouble with 30 inches, so
I dug the trench to 30 inches to be safe.
I was originally going to use the direct
burial service entry wire with conduit only where the wire comes out on the
two ends, but when I discovered how cheap the PVC conduit is, I just decided
to do it all in conduit to provide some extra protection, and allow for the
potential of pulling new wire through.
Given that I was using conduit, I could
have run individual strands of insulated wire through the conduit, but since
I had already bought the service entry cable, I just used it in the conduit.
I don't think there would have been much difference in price anyway.
the trench digger I rented. It has a chain with teeth mounted on that
dig a trench about 3 to 4 inches wide.
It was heavy and a bit difficult to get set up and going at the start, but
got moving it just chewed its way along with almost no attention. It
was slow, but not tiring.
$50 for overnight.
View from the "drivers" position
with the digging chain up.
View into the trench.
It makes a nice clean cut, and tends to
sort of glaze the sides. It had little
tendency to collapse.
I ran the conduit along side the
trench, and then when all was
ready, just pushed it into the trench.
This approach worked fine.
I ran the wire through the conduit
as I went because there were a several
bends, and I was not sure about
being able to pull it through after.
See the wiring
section for the details on the actual wiring...
Gary November 21, 2009