What is Voltage Drop?
When current moves through an electrical circuit, a small amount of voltage is lost due to resistance in the wires. This concept, known as voltage drop, leads to a slight production loss from your solar array.
Voltage drop is more pronounced over longer distances. A longer wiring run introduces more resistance to the circuit, which leads to greater voltage drop.
When you go solar, one of the goals is to design a system with minimal voltage drop so that your array can perform close to its peak rated output.
It is generally considered best practice to keep voltage drop at 3% or less, though many systems come in well under that mark. These recommendations are outlined in the National Electric Code (2017 NEC 210.19).
Why Voltage Drop Matters
It’s pretty simple. Voltage drop has a direct impact on system production. If your wiring run is too long, your panels may not provide enough voltage to the inverter. The efficiency of the entire system will suffer and your real-world production won’t live up to the nameplate rating of the components.
With that in mind, let’s look at ways you can reduce voltage drop as you design your system.
How to Prevent Voltage Drop
There are four main approaches to counteracting voltage drop:
- Minimize the length of the wiring run.
- Consider your inverter placement carefully.
- Use a bigger wire size. Larger wire = less resistance.
- Design your system with higher voltage to overcome resistance.
This is designed to be a beginner-friendly article, so we’ll just run through a quick overview of the concept of voltage drop. If you work with a solar designer, they should take this into account as part of the design process. For example, our tech team checks every system for voltage drop concerns when we provide our electrical wiring diagrams as part of the permitting process.
How to Reduce Voltage Drop
1. Minimize the length of the wiring run.
Since longer wiring runs lead to more voltage drop, the simplest solution is to make the wiring run as short as possible.
As you design your system, plan for a layout that keeps system components close to each other.
If your wiring run is less than 100’, your system may already have less than the allowable voltage drop of 3% without any further design changes.
2. Consider your inverter placement carefully.
AC wiring (from your inverter to your service panel) can be more prone to voltage drop than high voltage DC wiring (the wires running from the panels to the inverter or controller), though sometimes the reverse is true. It all depends on the voltage of the circuit: different equipment operates at different voltage ratings.
The side of the circuit that is operating at a higher voltage essentially has more “push” behind it, which reduces the impact of voltage drop.
As a result, the inverter should be placed close to the lower-voltage end of the circuit, to minimize the effects of voltage drop in that wiring run.
If the DC voltage from the solar array is higher than the utility service panel, install the inverter closer to your utility service panel.
If the DC voltage from the solar array is lower than the utility service panel, install the inverter closer to your solar array.
Please note that this is just a general rule of thumb, and that the guidelines change depending on what products you use. For example, off-grid systems typically have a lower DC voltage, but there are high-voltage charge controllers to overcome that.
To evaluate your own project, use our voltage drop calculator to input the specs for the products you are considering and calculate voltage drop over the length of your run. You can tweak the wire length, size and other variables to find the sweet spot for your system.
(Of course, if you work with us to design your system, we take care of these calculations for you.)
3. Use a bigger wire size.
Some people need to go with a longer wiring run purely for logistical reasons. For example, you might need to run wires from your home to a barn, which may be several hundred feet apart.
In these cases, upgrade to a larger size wire. This is just like using a bigger hose. The wires have greater capacity, which means less resistance, ultimately making the system more efficient.
Large wires cost more, but they make your system more efficient. The extra output retained over the life of the system more than makes up for slightly higher wiring cost up front.
4. Design a system with higher voltage to overcome resistance.
Instead of (or in addition to) using a larger wire to reduce resistance, you can overcome that resistance by using higher-voltage products.
In some cases, you might prefer specific brands and products that are designed to operate at higher voltages.
For example, SolarEdge-based systems operate at 380V / 400V depending on the inverter model. The power optimizers regulate the panel strings to a fixed voltage, which allows you to design a system that consistently pushes the maximum power voltage through the circuit.
In contrast, string inverters like the SMA Sunny Boy don’t have power optimizers, so the voltage changes based on the number of panels in the string, as shown below.
The Sunny Boy’s ideal operating range is 195V-480V, so you can end up on either side of the 240V service panel depending on how many solar panels are in a single string. In these situations, favoring larger strings can help overcome voltage drop.
Learn more about why string sizing matters in our string sizing guide.
And off-grid systems have entirely different considerations. If you are off the grid, it’s mandatory to install the inverter inside so it’s protected from the elements. That means you rarely have the luxury of placing the inverter next to the solar array.
To counteract this limitation, off-grid systems use high-voltage charge controllers (up to 600V) to minimize voltage drop over long wiring runs. Of course, these changes need to be accounted for during the design process.
Voltage Rise: The Opposite of Voltage Drop
For grid-tied systems, voltage rise matters as well. Voltage rise is an equal-but-opposite effect that happens at the start of the circuit (the inverter). The calculations are the same, but the effects happen on opposite ends of the circuit.
Voltage drop is a loss of voltage (and subsequent loss of production) as the current is pushed from the inverter to the service panel. Voltage drop is measured at the end of the circuit, where voltage rise is measured at the start. If a grid-tied inverter is sending power into the grid, you would see voltage rise at the inverter terminals and voltage drop at the end of the wiring run, at the service panel.
Because the voltage is lower at the end of the circuit (the service panel), it follows that voltage should be higher at the start of the circuit (the inverter). That’s voltage rise – an increase in voltage at the start of the circuit.
To continue the hose analogy, picture what happens when you turn a hose on. Right at the spout, the pressure is highest because all of the current is being forced through a small tube. By the time it comes out, the pressure is lower because it had to push its way through the hose.
So a hose that operates at a pressure of 50 PSI might be 55 PSI at the spout and 45 PSI when it comes out of the end of the hose.
The same goes for solar design. Due to voltage rise, voltage is at its highest where the current originates from the inverter. If that voltage exceeds the upper limit of the inverter’s AC voltage limit, it will cause a high voltage fault, causing your system to shut down.
As a result, systems need to be designed to account for voltage rise as well to ensure the extra voltage doesn’t push the inverter past its max AC voltage. Some grid-tie inverter manufacturers, like SolarEdge, recommend maintaining <1% voltage drop/rise to prevent issues.
This is a lot to take in, so let’s boil it down to what really matters.
Voltage drop matters because it causes you to lose wattage from your panels. More voltage drop = less production = less value from your investment into solar.
When designing a system, it helps to take a holistic approach. You should figure out where you plan to place your components, then pick equipment with those considerations in mind.
If you have a long wiring run that can’t be avoided, it may be smarter to invest an extra $500 in high-voltage equipment to save yourself $2000 on a larger wire. Take a high-level view of the project and consider the most efficient design options given the constraints.
If you’d like guidance on the design process, reach out to us to request a free design consultation. We’re happy to help you design a system that is tailored to your needs.