How to select the right combiner box for your next solar project

After selecting all of the panels, wires, inverters and any analytic software or batteries or storage, you wouldn’t want to select the wrong combiner box and accidentally undermine the entire setup. Like with any product selection, the project’s type, size and scope mean everything when selecting a combiner box, and what works best for a residential install doesn’t translate to commercial, and so on.

Choosing the right combiner box for the job isn’t difficult, but you have to understand the site, the other components and their relation to the combiner. Keep these questions in mind when specing your next job.

How easy is it to install?

Often, the right combiner comes down to its simplicity and the headaches it removes from the project — its ease of deployment and installation. A box with pre-wired fuse holders with pigtails coming out can be a plug-and-play solution that wouldn’t require a licensed electrician to install.

For example, OutBack Power released its Integrated Combiner Solution (ICS) in May, which is an all-in-one solution that includes prewiring, strain-relief cable glands, touch-safe distribution blocks and bidirectional fuse holders.

“If we make it time-saving, economical and as easy as possible with a turnkey solution, installers will include them in every project,” says Andrea Hixson, product specialist with OutBack Power.

What functions do you need?

Your combiner box selection might just come down to a price point and availability. For a residential installation, there are off-the-shelf solutions that pack in a variety of potential configurations, saving the time and extra expense involved with a custom solution.

OutBack Power ICS

But there are so many new possible configurations of panels, and depending on the other components in the system, the combiner may need to perform more than the basic functions of combining circuits and fuses. Not all manufacturers have the perfect off-the-shelf combiner box that’s ready for every individual scenario. Do you need the flexibility, or do you just need simplicity? Say you have two separate, completely different systems of solar that both run into the same box and will then shoot out to independent controllers. Some boxes can handle that no problem, and others may need to be custom-built.

It used to be the case that all inverters were just grounded and installers would parallel strings into a combiner and then into the inverter. Now there are transformerless inverters that are ungrounded, which means installers have to fuse the negatives. The arrangement is more sophisticated and calls for a combiner box that makes it all come together.

“Basically, before selecting a combiner today, you first have to define the inverter. What inverter is being used?” says Kim Silva, sales with MidNite Solar. “With so many inverter options out there, from traditional string inverters, transformerless and transformerless with dual channel MPPT, we had to narrow down our code-compliant disconnecting combiners to several solutions that would cover all configurations.”

If it is grounded, that’s old school straight paralleling. If it is transformerless, you’ll have to fuse the negatives and be able to disconnect both negative and positive. And then there is the inverter size. With so many inverters now hitting 1,000 volts, you’ll need a combiner to match.

And again, some combiners can handle multiple tasks. MidNite’s MNPV8HV for example, in one configuration, can do three things at once: A straight parallel and then shoot out to two separate inverters. Or, that same box, could handle a transformerless job and fuse up to four negatives and four positives.

Have you considered monitoring?

Some manufacturers can bundle in wireless monitoring technology into the combiner box that allows for panel-level, string-level monitoring across current, voltage and temperature. In addition to the inherent benefits over the life of the install, monitoring provides real-time feedback when commissioning the field. This way, any mistakes or issues will be discovered at the outset, versus much later, preventing bigger problems down the road.

“It’s a way of checking yourself,” says Peter Maros, executive director at Shoals. “There’s an element of human error in any construction project, and it’s far cheaper to check early and often.”

Does is it protect the system?

First and foremost, be sure whatever you choose meets the code requirements for your particular jurisdiction. The 2011 code made everyone place a disconnecting combiner within 6 ft of the array for a roof mount. The 690.12 codes require rapid system shutdown.

An arc fault is a high power discharge of electricity between conductors that creates an abundance of heat. This breaks down wire insulation and will possibly lead to an electrical fire.

It is important to pair the arc fault solution to the inverter. For central inverters, detecting an arc fault at the inverter is very difficult, says Eric Every, senior applications engineer at Solectria – a Yaskawa company.

“If the inverter has 100 to 200 strings, a fault on one string does not generate enough signals indicative of an arc compared to the natural electrical noises in the system,” Every says. “Above 50 kW, detecting an arc becomes even more difficult. Instead of installing arc fault circuitry at the inverter, implementing arc fault detection at the combiner level provides more accurate detection.”

Solectria’s arc fault-enabled combiner box, the ARCCOM, for example, includes string-level arc fault detection where each string input is monitored for arc faults. If an arc is detected, a DC contactor in the combiner box opens, isolating that section of the array. The inverter is able to continue producing energy with the remaining combiners.

This saves customers a ton of time when looking for arc fault damage.

“When the combiner detects an arc, the combiner turns off and requires a person to perform a manual reset to resume production from that combiner,” Every says. “When the technician goes to investigate the issue, there are LEDs that indicate which sensor tripped. Instead of looking for arc damage on all 24 strings that feed the combiner, the sensor will indicate the group of four strings that triggered the combiner box to trip off.”

That would mean 83 percent of the strings would no longer need to be inspected.

Arc fault detection works because DC arcs generate a lot of electrical noise. The arc fault sensors look for noise signatures that indicate there is an arc fault in the system. One of the challenges with implementing arc fault detection is that the frequency range for noise indicative of arcs is similar to the natural switching noises from inverters.

OutBack previewed its FLEXware ICS Plus line last fall that includes arc-fault circuit interruption (AFCI), rapid shutdown and DC disconnecting means. With other grid-tied systems, AFCI may be provided by the inverter, but for battery-based systems the inverter is isolated from the PV array. Hixson says placing the AFCI in the combiner box, as close to the main source of arcing events as possible, not only helps visibility, but also reduces the likelihood of “nuisance tripping.”

“Nuisance tripping is one of the biggest concerns with AFCI solutions, as all arc fault events have to be manually reset per the 2014 NEC,” Hixson said. “This can be a big problem when it means a truck roll each time an AFCI event occurs, whether it is an actual arcing event or just a nuisance trip. AFCI nuisance tripping can cost an installer both time and money.”

OutBack also recommends ground-level reset and default safe contactors when possible to keep the technician from needing to head up to the roof.

Also, be sure that all of the components you’re installing to ensure rapid system shutdown are compatible — specifically the inverter, conductor and the combiner box.

“There is a requirement to bleed down residual voltage, even though a third-party rapid shutdown system might be used,” says Sagar Khare, ABB’s residential inverters product manager for NA. “So even if the inverter is shut down, there can be voltage on the line. We’ve had some customers just install what was available at the time, and an inspector checked it out and it did not always comply.”

Do you even need a combiner?

Some sites, depending on the other materials selected, may be able to connect everything without a combiner. A new solution from Shoals made its debut at Intersolar NA, the Big Lead Assembly (BLA) harness that was essentially just a thicker gauge of wire that is capable of handling current voltage of the strings without using a combiner.

Such a solution for the right project (one that isn’t extremely complex) could significantly decrease install time.
“For our core group of customers, we only save them about 20 percent with the BLA, but with customers who don’t look at wiring solar fields the same way we do, we’re at about 60 percent cost savings with the BLA versus if you were to home run from each string,” Maros says.

But when removing the combiner box from the equation, be sure you have done your homework and aren’t just wiring up a work-around that reduces costs without fully protecting the install.

“We have seen problems on installations without combiner boxes, most often in emerging markets where installers/customers combine the PV source circuits in some other way, typically to bring costs down,” Hixson says.

“This can result in a hazardous condition because, should a fault occur in one of the PV source circuits, not only could the conductor potentially see backfeed current from parallel PV strings, but it could also see backfeed current coming from the battery bank.”