Re-Thinking Wire Sizing Rules of Thumb for Solar PV

For many years, the solar industry has followed rules of thumb about oversizing wires to reduce energy losses in solar PV systems.

The rules of thumb call for the PV industry to oversize wires to ensure no more than a 2% voltage drop when transmitting power across conductors. As the energy is transmitted across conductors, power is lost as it goes from the solar array to the inverter to the utility connection, says Ryan Mayfield, instructor, along with Randy Batchelor, of HeatSpring’s Megawatt Design course, which focuses on large-scale solar PV design.

“But in today’s world, this is not the best way to go financially,”  he says.

Rules of Thumb Put in Place When PV was More Expensive

“The idea is PV power is expensive, when you’re putting in a PV array, you don’t want to lose power,” he says. About 20 years ago, the industry adopted the 2% voltage drop rule of thumb, when the cost of PV modules was much higher than it is today. 

In the early 2000s, modules cost $4 to $5 per watt at the wholesale level, but now they cost less than $1 per watt, he says.

The bigger the wires, the lower the losses. However, at some point the law of diminishing returns kicks in, and this guideline doesn’t make sense. The cost of using bigger wires may be higher than the amount that system owners can recoup over the life of the system.

How Site-Specific Analyses Can Help Identify Power Losses

It makes more sense to conduct a site-specific analysis to estimate the power losses, Mayfield says. This involves comparing a code-minimum wire size to upsizing the wires to meet the 2% voltage drop rule of thumb.

“You can look at that over the life of the system and come up with a prediction of where it makes sense to upsize the wires,” Mayfield says.

Using Modeling to Better Understand Losses

Modeling is becoming more sophisticated, allowing for this type of analysis. Mayfield can model systems and create an accurate representation of line losses.

“I can change the wire size in the model and it will show me the ramifications,” he says.

Software companies, including HelioScope and Aurora Solar,  provide modeling software that allows industry members to look at PV production for every hour in a year. This allows users to evaluate wire sizing in a more granular way. 

“You can specify wire size and the software will take the properties of the different wires, the resistance those wires are producing and tell you the losses they will have at that level,” he explains.

The Importance of Educating Clients

Designers can use these tools to help educate clients about the importance of evaluating the voltage drop rule of thumb. It’s common for clients and financiers to include the 2% voltage drop language in contracts.

“One thing you can do is to push back on that, and question what that means,” says Mayfield.  “People get hung up on the  2% number.”  Designers should ask them to look at how the 2% voltage drop affects costs for the entire year, not just at one point in time. 

It’s important to educate clients, given that the 2% voltage drop guideline can increase costs, rather than decrease them.

“We have this hang up on the 2% voltage drop. People who’ve been in the industry for a long time as a whole haven’t questioned or tried to change it,” Mayfield says.

Megawatt Design Updated and Beginning Soon

Come join Ryan and Randy in a new and fully updated version of their Megawatt Design course which begins next week!