Do you need to upgrade your electrical panel for a solar installation?
A home solar system can be connected to your electrical panel or bypass it. Connecting to the panel can require an expensive upgrade, but sometimes it's the only option. Here's the lowdown on line-side and load-side taps.
Your home’s electrical panel is often an under appreciated piece of equipment. You probably don’t think about it until something goes wrong.
However, you might need to take a closer look at your electrical panel if you decide to install solar panels on your home. Depending on the capacity of your electrical panel and the size of the solar array you install, the panel might need to upgraded. According to HomeAdvisor, the typical cost of a panel upgrade can be anywhere between $500 to $2,000.
A solar installation is already an expensive project, so finding out that you also need a panel upgrade can be an unpleasant surprise. It may be possible to bypass the need for a panel upgrade with an installation method called a line side tap, but it’s not always an option.
I’ve found that homeowners are sometimes a little skeptical when an installer tells them they need an electrical panel upgrade, so this article will dive into the details of why it’s sometimes necessary. I’ll also describe a line side tap, which is an installation method that can avoid this issue altogether.
What is an electrical panel?
An electrical service panel, also called a service panel or simply a service box, is a metal box into which the main service wires that supply the electricity from your electric utility are fed. A home service panel has two thick metal components called busbars that distribute electricity to the different wires (aka. circuits) of your home.
The electrical panel is where you’ll find safety devices that protect the individual circuits of your home. If appliances in your home try to draw too much current, the devices will automatically shut off the circuit to protect it from overheating. Newer homes use circuit breakers, which are switches that can repeatedly switch off and on. Older homes use fuses, which have a metal strip that melts to disable a circuit, so they can be used only once. If your panel is old enough to have fuses, it will most likely need to be upgraded for a solar installation.
The circuit breakers are attached to the busbars of the panel, and so you’ll see the breakers organized into two rows.
Each breaker has a number that indicates the maximum amperage of the circuit the breaker is protecting. Commonly, these will be 15, 20, 30, or 50 amps.
What size is your electrical panel?
There will be one main breaker that can shut off the entire panel, and it will typically located at the top or bottom of the panel. It will be labelled with a large amperage, such as 150A or 200A.
The size of your main breaker is one number that determines if your panel will need to be upgraded for your solar installation.
Another number to know is the capacity of the panel itself. While the main breaker will often have an amperage that matches the capacity of the panel, that’s not always the case. The capacity of the panel is determined by the busbars. They aren’t visible until you remove the circuit breakers.
An empty panel is pictured below. The busbars are the two thick copper bars in the center, and they determine the carrying capacity of the panel:
To determine the size of the panel, look for a label on the panel itself that lists its maximum amperage. Common sizes for household electrical panels are 100A, 125A, 150A, and 200A.
Connecting solar to your electrical system: load side and line side taps
Solar panels generate direct current (DC) electricity, which is converted into alternating current (AC) electricity. That AC power then needs to be supplied to your home. There are two ways of making this connection.
A load-side tap is when the output from the solar inverter is connected through the electrical panel. This is called a load-side connection because the panel is where the home’s electrical loads are connected.
This type of connection “backfeeds” the electricity into your panel. This means that the solar inverter is pushing electricity into the panel, rather than consuming electricity as the regular circuits do.
In most cases, a load-side tap will be connected to a circuit breaker. This requires an empty space in your panel to add a breaker. If there is no space available, the installer may decide to add a subpanel, which is a smaller electrical panel branched off the main panel.
Where local codes allow, it is also possible for a load-side tap to be connected without a circuit breaker. (This is what I have in my own installation.)
In contrast, a line-side tap is “upstream” of the panel and is connected between the utility meter and your electrical panel. In this configuration, the electricity from your solar panels flows “downstream” into your main breaker and electrical panel. Because a line-side tap is upstream of your electrical panel and the main breaker, the capacity of the panel doesn’t matter.
Load-side taps and the 120% and 125% rules
A load-side tap is often the preferred method, and is sometimes the only available method for connecting a solar array. However, it’s limited by the capacity of your panel.
The rules that govern this are set out by the National Electrical Code (NEC). Here are the steps determine whether your panel capacity is adequate under the latest version of the NEC code:
- Find your solar inverter’s output capacity in amps. (In the case of microinverters, add up the output of all the microinverters in the system.) Multiply that value by 125%.
- Add that value to the size of the main breaker.
- The sum of those two values can’t exceed 120% of the panel capacity.
That might be a little confusing, so let’s look at an example.
Let’s say that your solar inverter is the SolarEdge SE6000H-US. It has a maximum output of 6,000 watts or 25 amps.
If you multiply 25 amps by 125%, you get 31.25A. In most cases, this circuit would be backfed through a breaker. You have to round up to the next standard breaker size, which is 35A. (Fortunately, the rules specify that it’s the output current that matters, not the size of the breaker protecting the circuit. This means that we get to use 31.25A instead of 35A for the next calculation.)
Let’s say that you have a 150A panel and a 150A main breaker. Add the size of the main breaker to 31.25A and you get 181.25A.
120% of the panel’s capacity is 180A. Because 181.25A is greater than 180A, we’ve unfortunately exceeded the 120% rule. This means that our panel is slightly undersized for this inverter.
Options for getting around the 120% rule
In our example, we’ve exceeded the capacity of the panel by 1.25A. That means you’ll have to make some changes to do this installation. There are a few options.
- Downgrade your solar array and inverter.
- Downgrade the main breaker.
- Upgrade the panel.
- Do a line-side tap.
Downgrading the solar array and inverter means using few solar panels and a smaller capacity inverter. This is easier with microinverters because you can downgrade in smaller increments than you can with a central inverter. (If you’re lucky, you can remove just a couple panels/microinverters to get under the limit.)
Downgrading the main breaker in our example would mean replacing the 150A main with a smaller size, such as a 125A breaker. This is an easy option, but is only possible if your electricity needs in the home can fit within the smaller breaker size.
Another option is to upgrade your electrical panel either by replacing the main panel with a higher capacity one or adding a subpanel. This is more expensive than downgrading your main breaker, but will give you more capacity for future expansion.
The last option is a line-side tap, which avoids the capacity issue altogether. However, it’s not always an option.
What is a line-side tap?
As mentioned earlier, a line-side connection is upstream of the electrical panel. Because of this, the capacity of the panel doesn’t matter, so you don’t have to worry about upgrading your panel. For the solar installer, it’s often an easier connection and therefore less expensive for you.
This sounds great. So why aren’t all solar installations connected this way?
The reason is that a line-side tap has a few disadvantages, including:
- It requires coordination with the utility company because they must shut off electricity to the home.
- The utility company or local codes may not allow a line-side tap.
- A battery system requires a load-side tap.
In a load-side tap installation, the installer can shut off electricity to the panel by turning off the main breaker. With the breaker off, the installer is free to work safely inside the panel.
However, with a line-side connection, there is no “off” switch that an electrician can use to disable the electricity. Instead, the utility company must do this work. This involves coordination, which adds delays.
Also, while NEC rules specify guidelines for a line-side connection, that doesn’t mean that your local codes or your utility will allow it. They can refuse to allow a line-side tap, leaving you out of luck.
Finally, if you have a battery system, this can only be done with a load-side tap. Batteries are increasingly popular, so this is a constraint that more solar projects will encounter.
If you are able to circumvent all of these challenges, a line-side tap is often the best option for your solar installation.
Preparing for your meeting with the solar installer
If you’re in the early stages of working with a solar installer, it would be a very good idea to familiarize yourself with your electrical panel, taking note of the panel and main breaker sizes. Take a photo and send it to the installer if they haven’t made an onsite visit yet. This will help you ask the right questions and have a knowledgeable discussion if it looks like you might need a panel upgrade.
References
Here are the relevant sections in the NEC code with helpful diagrams:
