Line Side Tap vs. Load Side Tap: Everything You Need To Know
Sharon Lee • Published January 31, 2023 • Updated on February 20, 2023 • [rt_reading_time postfix=”minute”] read
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If your solar photovoltaic (PV) system produces more electricity than you can use, the excess is sent to the grid where it flows to your neighbor and their neighbor and so on. The process of connecting a solar PV system to the larger electric grid is called interconnection and it’s often the final step in the solar panel installation process.
The physical connection between your solar system and the grid can be made either with a line side tap or a load side tap. If you have no idea what that means, read on. This article is designed to explain the basics in a way that doesn’t require you to be a licensed electrician or have a degree in electrical engineering.
Table of Contents
Interconnection Specialists Required
There are very specific codes and regulations that need to be followed in order for your grid-connected solar PV system to pass inspection and receive the all-important permission to operate from the utility.
That means you need both interconnection specialists designing your system and electricians well-versed in line side tap and load side tap interconnection techniques doing the actual installation. Reputable solar partners like Velo Solar will have both on their team, and they’ll recommend which interconnection technique is appropriate for your situation.
Electrical Engineering 101
We’re assuming that if you’ve read this far, you’re not an electrical engineer or an electrician who already understands this. That said, what follows will make a lot more sense if we establish some basic definitions. First, let’s talk about some of the components of your existing electrical system and your solar system.
Note, the discussion that follows assumes a single-phase system.
Of note, there are a few variations of the ‘single phase’ configuration, based on the number of wires. Three-phase systems, often found in factories or large office buildings have slightly different considerations, though the concept is the same.
The Components
Main Panel
You can think of the main electrical panel, otherwise known as the service panel, as the switchboard for the electricity that flows into your business. There are two main service wires that come into the panel’s main breaker from the utility connection after first passing through the meter. Each carries 120-volts of power. Flipping the main breaker off stops the flow of electricity to your entire facility.
Busbar
After flowing through the main breaker, the electricity next comes to the two busbars in the panel. Busbars (also commonly spelled bus bars or bussbars) distribute the electricity to the two columns of circuit breakers in the panel.
Circuit Breakers
The panel will likely have 120-volt circuit breakers, also known as overcurrent protection devices (OCPD), that are connected to one busbar or the other. Depending on what you’re powering, you may also have 240-volt breakers that are connected to both bus bars. Each should be labeled allowing you to shut off the power to a specific part of your facility if necessary (showroom lights, workshop, exterior lights, etc.).
Subpanels
Because there are a finite number of spaces in each panel for circuit breakers, your business may have one or more subpanels that are fed by the main panel. Subpanels can be the same size and work the same way as your main panel. They likely provide power to a certain area of your facility, such as a workshop.
Solar Inverter
Every solar PV system includes an inverter that converts the direct current (DC) electricity generated by your solar system to the alternating current (AC) electricity used to power your facility and its equipment.
Understanding Line Side and Load Side
Throughout this article, we’ll be discussing the concepts of line side and load side. Here’s how to differentiate between the two: the line side supplies power while the load side uses the power; your utility power meter or main breaker is typically the line of demarcation between the two.
In short, the current could be on the line side or the load side of your electrical system, depending on where it is at the moment. For example, if it’s in between the main meter and the main electrical panel, it’s on the line side. The instant it comes out of the main panel and into your building it’s considered load side.
So, with that basic information in mind, let’s talk about the two ways you can connect your solar system to the grid.
Load Side Taps
With a load side tap, your solar inverter is wired directly to your electrical panel through a circuit breaker. When you have more power than you need, it flows from that breaker through the bus bars, the main breaker, the meter, and then ultimately out to the grid.
Any electrician will tell you that working on a live (or electrified) system is dangerous. When connecting the solar inverter to the panel, the electricity must be shut off.
This is easily done with a load side tap because all you have to do is flip the main breaker in the main panel to the off position; once you’ve done that, then everything from the panel on in is de-energized. As we’ll discuss in a moment, a line side tap is a more complicated process.
Load side taps are common for residential systems and smaller commercial systems.
120% Rule
When designing your solar system, the provider must consider the 120% rule. This rule was established by the National Electrical Code (NEC) to identify how much power can be safely back-fed through the load side of the existing electrical panel. Of note, the 120% rule covers 2 out of about 6 ways that a load-side interconnection can be sized. While it is one of the more popular methods, there are situations where another of the 6 rules could be used, allowing you to exceed what would have been possible using the 120% rule.
Every service panel has a capacity limit, which is determined by the busbar rating and is measured in amperes (amps). If the current flowing through the panel exceeds the capacity rating, there is a chance that the busbars can melt, creating a fire hazard.
The NEC’s 120% rule says that the sum of the solar breaker and the main service panel breaker can be no more than 120% of the busbar rating. In other words, given that the main breaker matches the rating of the busbar (which is often the case), the solar breaker can be no more than 20% of the main electrical panel breaker rating.
For example, if your panel has a 200 amp busbar rating and a 200 amp main breaker, the 120% rule is calculated as follows:
200*120% = 240 amps
240-200 = 40 amps
In this scenario, the solar breaker can only be 40 amps, which is fine for most residential systems, but insufficient for most commercial solar arrays.
Exceeding the 120% rule is dangerous and neither the inspector nor the utility will allow your system to be connected to the grid in that way.
Different Rules May Apply
It’s important to note that there are still some commercial and industrial applications that adhere to the 100% rule established by the NEC in 2005.
Additionally, more recent editions of the NEC say that the solar breaker must be at least 125% of the system output. For example, if your solar inverter output is 32 amps:
32*125% = 40 amps
In the scenario above, a 32 amp inverter would not cause the system to exceed the 120% rule. However, if the output of the inverter is 34 amps, you would need a 42.5 amp solar breaker (34*1.25), which would exceed the 120% rule.
Your solar provider will help you understand how the NEC rules apply to your situation.
Downsizing the Main Breaker
If your system will exceed the 120% rule, one solution is to downsize the main breaker. For example, if your main breaker is 175 amps versus 200, the equation looks like this:
200*120% = 240 amps
240-175 = 65 amps
The smaller main breaker means your connected solar system can safely output 65 amps of power. This could be a viable approach if your load, or the amount of power you need at once, doesn’t exceed 175 amps. If it does, you’ll be tripping circuit breakers which will be a nuisance.
Alternatively, your solar provider could replace your existing panel with a model that has a higher rating, though this can cost between $20,000 to $50,000 in a typical commercial building. Velo Solar can provide this upgrade service if needed.
Because of the current cost and availability of electrical equipment for commercial solar systems, line side taps are becoming increasingly common with large solar systems as a cheaper alternative.
Line Side Taps
With a line side tap, also called a supply side connection, the solar inverter is connected to a PV service fused disconnect and/or a solar only circuit breaker panel, which in turn is connected to a junction box. The junction box sits in between the main meter and the main service panel and houses the connections between the main breaker, the utility meter, and the solar system.
The PV service fused disconnect, sometimes called the fusible AC disconnect, is most commonly used when your solar array has a single large inverter. In the case of a multiple inverters, a PV panel and a fused disconnect are both used, as required by code and the utility.
The advantage to this approach is that it allows you to install a supply side connection that is limited only by the amperage of the existing panel. That means that if you have a panel rated for 200 amps, your line side tap connection can also be as much as 200 amps.
With this type of installation, the inverter is essentially wired into the system before the main panel. That means you can’t just flip a circuit breaker to interrupt the power coming into the building, making the installation more complicated.
As previously discussed, with a load side tap you can flip the main breaker, shutting off the power to the system and rendering it safe for the electricians as they make their connections.
The line side method requires your solar provider to work with the utility to disconnect your facility from the transformer (the typical connection point between your business and the grid) before the interconnection process can start.
This process, often called pulling the meter, prevents electricity from flowing to the electrical system, so that it’s safe to work on.
Is a Supply Side Connection Right for Your Solar Array?
It’s critical that you work with a reputable and experienced commercial solar provider to ensure that your renewable energy system is safely connected to the grid.
Velo’s interconnection experts will recommend the appropriate connection method during the design phase of your solar project. Whether it be a line side tap or a load side tap, Velo will help you navigate the interconnection process to make sure you’re generating your own renewable energy as quickly as possible.
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