DIY PV System -- Running the Underground Wiring


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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.
This shows 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 once it
got moving it just chewed its way along with almost no attention.  It was slow, but not tiring.
It was $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...




Next -- Installing the system components

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Gary November 21, 2009