Circuit Breaker From Grid-Tie Solar Required in Main Electrical Panel?

[Zeigh]

Hello,

Does a grid-tie solar electric installation (for a 3000 square foot home) require connection via double-pole circuit breaker to the household electrical panel? Here is the rest of the story...

I am doing some other work in my electrical panel and wondering if I need to keep open a space on the bus bar for the eventual solar electric installation later this year. I will also be installing a whole house emergency generator further down the line. From some of the schematics of various equipment that I have browsed, it looks as though there is an external main transfer switch that coordinates all of this, but it is unclear if a circuit breaker in the main panel is required. Any help would be appreciated...


More microchips than sense,
Dr. Z.

[Naptown]

if you go the whole house generator the solar will tie into the line side of the transfer switch in what is called a line side tap.
If above is true no space is needed in the panel box.

[inetdog]

Hello,

Does a grid-tie solar electric installation (for a 3000 square foot home) require connection via double-pole circuit breaker to the household electrical panel? Here is the rest of the story...

I am doing some other work in my electrical panel and wondering if I need to keep open a space on the bus bar for the eventual solar electric installation later this year. I will also be installing a whole house emergency generator further down the line. From some of the schematics of various equipment that I have browsed, it looks as though there is an external main transfer switch that coordinates all of this, but it is unclear if a circuit breaker in the main panel is required. Any help would be appreciated...


More microchips than sense,
Dr. Z.

OK.
Here are some points to keep in mind during the design:

1. A grid tied inverter will not work with a generator and to prevent confusion to the GTI and possible damage to the generator should be connected upstream (on the utility side) of any transfer switch you install for use with the generator.
2. Given that, if you are going to put your transfer switch upstream of the main panel so that the generator can power anything in the house you choose to operate, you cannot connect the PV to the main panel. Period.
3. If you install a sub panel off the main panel and put all of your important loads that will run off the generator into that subpanel, you can put the transfer switch between the main panel and the subpanel and run the PV into a breaker in the main panel. There is a constraint on how much PV you can connect this way, called the 120% rule. If you find out the bus amp rating of the main panel (sometimes the same as the main breaker size, but sometimes larger, such as 125A for a nominal 100A main panel) then you multiply that bus rating by 1.2, subtract the amp rating of the main breaker and that will tell you how much PV you can feed back into the panel.
4. As for leaving spaces in the panel, any PV backfeed breakers MUST be at the opposite end of the panel bus from the main breaker. Otherwise the 100% rule applies and you have to subtract the main breaker rating from the bus rating, possibly leaving zero.
5. If you are going to install an automatic transfer switch (ATS) then the generator you use must, under the NEC, be capable of supplying the entire calculated house load or else you have to have automatic load shedding equipment to reduce the load to match the generator output when the ATS switches. If you have a manually operated transfer switch you do not need to follow that design rule, since it is assumed that you will turn off unneeded loads before throwing the switch.
6. If the GTI output is 240 volt, then yes it has to connect via a two-pole 120/240 breaker, but that breaker does not necessarily have to be in the main panel. Going deeper into that is a matter for advice from a local electrician.
7. If the GTI connects to a subpanel rather than to a breaker in the main panel, the 120% rule including the opposite end requirement will also apply to the main panel as well as to the subpanel. You can get into a bind here if you do not plan ahead.

Those are just design suggestions, and any actual installation and wiring needs to be done by a licensed electrician, preferably one with experience in PV installation under your local code variants. Sometimes a local power company (POCO) will not accept wiring which is in complete compliance with the NEC. That is their right in this situation.