When planning your grid-tie solar installation you have to consider some things about your main service panel because that is where you are going to make your connection to the grid. The larger the solar system, the more likely it is that you will have to make some adjustments or upgrades to your service panel in order for the installation to be code compliant. There are also a few tricks that may get you out of doing that extra work.
First, let’s discuss 2017 NEC 705.12(B)(2)(b) and 705.12(B)(2)(d). These code sections are the reasons why you may have to alter your main service panel in order to install your solar. These code sections describe what is lovingly referred to by solar installers as the 120% rule.
The 120% rule says that for load side interconnections, 125% of the maximum solar output plus the rating of your main service breaker must be less than or equal to 120% of the busbar rating. This is true for all solar installations but we are going to do the math with the popular Enphase microinverters so we can get very specific.
You are installing 16 Enphase IQ7 microinverters which output 1 amp each.
Your total maximum solar output would be 16 amps.
125% of your maximum solar output would be 16 x 1.25 = 20 amps.
Your main service breaker is 100 amps and your main service panel busbar is 100 amps.
120% of the busbar would be 100 x 1.2 = 120 amps.
Your main breaker plus 125% of the solar output would be 100 + 20 = 120 amps.
120 amps is equal to 120 amps so you are just barely okay to install as is.
You are installing 45 Enphase IQ7 microinverters which output 1 amp each.
Your total maximum solar output would be 45 amps.
125% of your maximum solar output would be 45 x 1.25 = 56.25 amps.
Your main service breaker is 200 amps and your main service panel busbar is 200 amps.
120% of the busbar would be 200 x 1.2 = 240 amps.
Your main breaker plus 125% of the solar output would be 200 + 56.25 = 256.25 amps.
256.25 amps is not less than or equal to 240 amps so you can’t do a load side connection like this.
If example 2 is your situation, let’s review your options. You can de-rate your main service breaker to 175 amps. This changes the math to 175 + 56.25 = 231.25 amps which is less than 240 so you are good to go as long as you can do some load calculations to prove that your house will be okay with only 175 amps for the main breaker.
Let’s say you have the service panel in example 2 and you want to install 56 Enphase IQ7 microinverters. Now the math is 56 x 125% = 70 amps. Now the de-rate to 175 won’t work because 175 + 70 = 245 amps which is still too much. De-rating to 150 amps might work, but only if you don’t have a lot of big electrical loads like air conditioners, clothes dryers and the like. So, another option is to upgrade the whole main service panel. You could put one in that has a 225 amp busbar. 225 x 1.2 = 270 amps so now you can leave your main breaker at 200 amps and still be okay to install. Another option would be to upgrade to a 200 amp service panel that has a dedicated space for the solar breaker that is not on the busbar with your other breakers which means that 120% rule no longer applies.
You also may be able to skirt the whole thing by not doing a load side connection. Your options depend on what your local utility and city will allow but in many places, they will allow what is called a “line side” or “supply side” interconnection. This means tapping the wires between your electric meter and your main service breaker so you are connecting on the line side of the main breaker instead of the side with your service panel and loads. If local authorities don’t allow you to tap those wires, the utility company may offer something called a “meter adaptor” where they change your service meter to one that has a place to connect your solar. This is still considered a line side interconnection so the 120% doesn’t apply and you aren’t tapping those conductors on your own so it’s a bit safer.
So that’s the rule and the way around it. The larger the system, the trickier it is to determine the best way to install it. When it doubt, call the experts at GoGreenSolar.com who will not only get you a great deal on your solar kit with Enphase microinverter, they will help you through the process of making sure your interconnection method will be code compliant.