Main Power Panel Placement for Grid Tie System

[Mike Arant]

I am in the process of designing and building an earth sheltered home and plan to add a pv system later as finances permit. Ultimately I intend to put the solar panels on the roof of an outbuilding which will be 50-75' from the main house, but I will need to put the main electrical panel from the utility company in the main house now since the pv system will be built later.

I understand that it is preferable to have the main panel as close to the pv panels and inverter as possible, but in this case that will not be possible. What problems will it present if I put a subpanel in near the pv system and feed it back to the main panel? Is there a better way to solve the problem? I am interested in the best way to handle the problem now so I can install the optimum wiring and other equipment now to be ready to accept the pv system later. (When your home is made of concrete and covered with 3-5' of earth you really have to plan ahead!)

[Wilco Vercoelen]

1 - Check your local Electrical Code for requirements
2 - Check wire sizes requirements for DC & AC
3 - Put Sub Panel, Main Panel and AC Disconnect in close proximity
4 - Install PV & Grid-Tie inverter with seperate disconnects & fuse in the outbuilding
5 - Install proper lightning arrestors in outbuilding, PV (DC) and Grid Side (AC)
6 - Run a wire from the Outbuilding AC Disconnect to the AC Disconnect at the Sub Panel loc.
7 - Connect AC disconnect to Sub Panel
8 - Connect Sub Panel to Main Panel
9 - Label every disconnect etc.
10 - Have the installation certified by local authorities before use

[Mike Arant]

Wilco,

Thanks for the list, it will come in handy when we get to that point. The real problem, however, is that the pv system and the outbuilding that will support it will probably not be built for a year or more after the main house. Thus, the main panel will need to be on the main house, while the pv system, inverter and subpanel will be on the outbuilding. What problems will it present to have the main panel 50-75' from the pv components?

Mike

[Wilco Vercoelen]

It is not a problem if the outbuilding is to be build later on. We would recommend the sub panel and main panel to be located in close proximity unless the pv system needs to power some things in the outbuilding as well. The sub panel is normally feeding critical loads like a sump pump, fridge, freezer etc. in your main building.
The connection between the AC side of the inverter/disconnect and the sub panel is normally 120V and this does not cause problems for wire loss unless you have a large solar system/inverter/battery bank in conjuction with large loads e.g. 4kW surge power or more. Just make sure you have the right wire in place to carry these heavy (surge) loads and always check your local electrical code.

[Rob Beckers]

Mike, is this going to be a grid-tie system with battery backup, or simply grid-tie without batteries? Reason for asking is that in case of batteries the inverter needs to be close to those (relatively low voltage, large currents, and very thick and thus expensive wiring that you'd want to keep short). Depending on your climate you may not want to put batteries outside in an unheated shed. Also, in this case the critical loads would get powered through the inverter, making it impractical to route first to the shed (to power through the inverter) and back again (to power the pony-panel with critical loads.

For a system without batteries you have more freedom, in fact you probably don't need a pony-panel at all. So then the choice is to put the inverter in the shed, or in the house near the main panel. What's best would depend on what panel configuration you are planning, and what the voltage to the inverter is going to be. Higher voltage means thinner gauge cable (meaning cheaper). For example, the MPPT Sunny Boy inverters handle DC input voltages all the way up to 600 Volt. It would make more sense for such a setup to have the inverter next to the main panel and run longer DC cabling from the solar panels to the inverter, instead of having the inverter in the shed and running 120/240 cabling to the main panel. For a low-voltage string of solar panels (24V, or 48V) it's the reverse, it would make more sense to put the inverter close to the panels.

As Wilco pointed out, there's local code to consider. This could well limit the choices you have.

Hope this helps!

-Rob-

[Mike Arant]

Thanks, guys, that is exactly the type of information I need.

I am so early in the process that I have not decided what panels to use or whether to go high (dc) voltage or low, so your comments will help me decide. One of the reasons I am going to delay (in addition to financial) is the hope that the new roofing panel type solar cells will be perfected and will come down in price per volt, since I will have plenty of roof surface to use.

I plan to use a grid tie system without batteries, and will use a solar contractor for the actual work. The home is in Texas, about 30 miles from Austin, so there will be plenty of solar resource. I expect to have a 2-3 KW system, and rely on the grid for surge loads.


I will be sure the contractor is aware of and follows all codes.

[Rob Beckers]

Hi Mike,

I like the solar roofs, there are some interesting (and aesthetically pleasing) products coming down the line. As you mention though, they're (too) expensive at this time.

If you're thinking of using the roof later, and if you didn't do so already, have them build it so it's facing due south (not magnetic south!), or close to it, and with a pitch equal to your latitude (that'd be around 32 degrees for Austin if my old map here isn't lying, or a 7/12 roof). The latitude-as-pitch is the angle for best year around average production in case of fixed panels.

-Rob-

[Mike Arant]

Rob,

Thanks for the reply and ideas. My lot has a good southern exposure, and it will be easy to specify the roof pitch. I have already decided to use a standing seam metal roof and will specify that it be at least 16" wide, since that seems to be the required width to accept most of the current roofing type panels. I expect I better specify a full subroof, not purlins, so there is plenty of strength for the later walking on and installation of the pv roofing.