It’s not so easy to size wires in solar arrays, says Sean White, instructor of HeatSpring’s NABCEP PV Installation Professional Certification Exam Prep and CEUs course.

The problem: the National Electrical Code (NEC) can be befuddling. His courses that focus on the NEC aim to take the confusion out of the code and help installers choose efficiency and cost-effectiveness, says White, the 2014 Interstate Renewable Energy Council Trainer of the Year and author of several solar books.

“Even some people who write books on solar get wire sizing wrong because there is so much to it,” says White.

Here are common no-nos for sizing the wires that go between the PV modules and inverters on the grid.

Don’t Mix Multiple Wire-Sizing Steps

“There are many checks and steps to sizing wires,” says White. You can choose among numerous strategies to size wires. “Sometimes you’ll do it with “method one” or “method two,” but can’t do it both ways at once.” But it’s common for installers to make the mistake of using both “step one” and “step two” at the same time.

You Don’t Have to Size Wires Larger than Necessary

Many people size wires bigger than required in an attempt to be extra safe and compliant with the NEC. The downside, however, is installers can spend more by oversizing wires. For example, installers and designers historically used 10-gauge wire for PV source circuits, but they don’t realize they can use the smaller, 14-gauge wire for houses. It’s okay to oversize the wire, but if you’re in a bidding competition for a big job, it might affect whether you get the job–or how much profit you’ll realize.

Consider Using More PV and Smaller Wires

Sometimes it makes economic sense to use a smaller wire and more PV, says White.

It all depends on the price of PV and the price of copper wire.

Which HeatSpring Course is Best for You?

In your efforts to understand the intricacies of sizing wires for PV systems, consider two courses that White offers.

The first, NABCEP PV Installation Professional Certification Exam Prep and CEUs includes less detail and 18 hours of continuing education units (CEUs). It’s less extensive, and designed for engineers or those who feel very confident about how much they’ve studied for the NABCEP exam.

If you already have the Certification, until the end of 2019, you can take this course to meet NABCEP CEU requirements. However, after that, you’ll need 30 hours of CEUs, White notes.

NABCEP certifications run for three years; that means that taking this course now would keep you certified until 2022, and you could avoid the extra hours of training until it expires, says White.

The other, more popular, path is to take the 40-Hour Solar Advanced PV Installer Training course. It includes all the material that’s in the other course–and more.

“It has more than double the material,” says White. He recommends that most people interested in getting NABCEP-certified take this course.

“Wiring for PV is very complex,” he says. “It’s best to take a course to learn about all the details before taking the NABCEP exam.”