The part of the installation often overlooked by system designers is the method in which the panels on the roof are to be wired. If the wires become loose, it is possible that they will fray if wind underneath the array happens to cause the insulation to rub against the asphalt shingles, which is basically as rough as sandpaper.
Aside from that, it's also important to accomplish the following, in your wiring methods:
- Use the least amount of wire (cuts down on expenses and voltage drop)
- Create the least amount of problems (cuts down on lost time wondering what to plug in).
Think about it. If you have 24 panels, and each one has two 2' leads, that's already 96' of wire, not in conduit, underneath your array. Not to mention that there need to be wires that connect to those leads (the proverbial 'homeruns' mentioned elsewhere in this blog).
These examples show you how to minimize the trouble involved in wiring panels. What I learned as an installer is that there is an art to this procedure. Being an appreciator of the arts, I thought this was noteworthy. So I came up with a neat way of describing how to wire panels on the roof.
Throughout this part, you'll see a yellow arrow, which essentially means "this panel is wired to that one." If you wonder which lead is connected, the answer is at the end of the string.
You'll find a square with a number at the beginning of a string. This number represents the 'homerun' which is a lead that goes to the combiner box. The grey colored square represents the grounded conductor of that string (see grounded conductor article for more info).
At the end of a string, you will see a red box with a corresponding number. That's the homerun for the ungrounded conductor. Each set of arrows leads from a grey square to a red square, indicating which panels are wired to which other panels. So here we go.
Here is a simple roof. The panels are wired in series of 8, as you can see, which is fortunate because they're also in rows of 8. How convenient! These are the types of situations that an installer generally likes to see, because it means that the process of preparing the roof for wiring is quite simple, compared to the next example. As you can see, all of the grounded conductor homeruns are on the left, and all the ungrounded conductor homeruns are on the right, at the end of the rail.
Let's take a look at a more complicated example.
Here's a weird roof. This one has all kinds of odd, peculiar stuff going on. The two rows at the bottom need to be connected to rows above. The method described in this illustration is far preferred, compared to other possible methods, for a few reasons.
- It doesn't involve lots of 'up & down' - - connecting panels to the ones above or below can make the wiring much more difficult and prone to problems later (and by later, I mean 25 years +). Nothing makes an installation last longer than tight wiring from one panel that is directly next to another panel.
- It doesn't involve any connecting parts or diagonal wiring. Panels generally can't connect diagonally without some stress put on the wire.
This is an example of four panels that need to be wired together. Despite how similar each example may appear, the one on the right is twice as easy for an installer to wire, and half as likely to have problems, because of fewer vertical crossings. All that's involved is a different placement of the grounded conductor homerun in each example.