The Evolution of DC Combiners

As safety requirements for PV systems change, balance of system manufacturers are playing an increasingly important role in the solar industry ecosystem.

As recently as 8 years ago, it was not uncommon for system integrators to build combiner boxes in the field. This practice went away under NEC 2008, as the Code-making panel revised 690.4(D) to require that source-circuit combiners intended for use in PV power systems be “identified and listed for the application.” To comply, system integrators had to use source-circuit combiners listed to UL 1741. Today, these listing requirements apply to all dc combiners.

More recent Code changes have further influenced the design and construction of listed dc combiners. For example, the Code-making panel added general requirements for fuse-servicing disconnecting means in 690.16(B) of NEC 2011 and more specific requirements for roof-mounted dc combiner disconnecting means in 690.15(C) of NEC 2014. It also added dc arc-fault circuit protection requirements in 690.11 of NEC 2011 and rapid-shutdown requirements in 690.12 of NEC 2014. The revised ground-fault detection and interruption requirements in 690.5(A) of NEC 2014 also have implications for dc combiners used in PV systems deployed with central inverters.

To get a feel for how dc combiners and PV system design practices are evolving in light of these new Code requirements, I reached out to equipment vendors and system engineers. The following responses address changes in the dc combiner market, popular product features, challenging Code requirements, cost concerns and new products or features on the horizon.

On Balance

> We published our last combiner box article in 2011. How has the market for dc combiners changed in the last 3 years? What dynamics are driving those changes?

“We have seen a significant change in design and certification requirements for dc combiners. Balance of system (BOS) products are no longer left to the installer’s discretion on-site. Engineers, developers and contractors are looking at BOS products as an integral part of the overall system and are requiring products that carry third-party certification. Further, as PV projects have increased in capacity, the throughput of dc combiners has increased accordingly. Code requirements are also driving demand for circuit-breaker PV output-circuit combiners. Circuit breakers are a reliable and cost-effective means of providing overcurrent protection for high-value equipment, as well as a disconnecting means at the inverter input.”

—Patrick Kane, product manager, Eaton

“Large system integrators used to be hesitant to use combiners with disconnects. Now, the largest integrators all use disconnect combiners. The dynamic driving that decision is system operation and maintenance (O&M). While O&M was rarely discussed 3 years ago, there are entire conferences dedicated to the topic today. The market for PV output-circuit combiners is also greatly changed. Prior to NEC 2011, it was rare to come across PV output-circuit combiners with disconnects or circuit breakers. Now most combiner manufacturers offer both fused-disconnect and circuit breaker options for these products.”

—Bill Brooks, PE, principal, Brooks Engineering

“One of the biggest changes we have implemented is the use of dc circuit breakers in lieu of fuses in our recombiner [PV output-circuit combiner] boxes and cabinets. Circuit breakers allow end users to reset tripped circuits without replacing expensive fuses, which reduces spare parts inventory costs. Some inverter manufacturers are also moving away from using fuses at the inverter input bus. The market has also transitioned to using 1,000 V–rated PV source-circuit combiners with an integral 250 A or 400 A load-break–rated disconnect switch. This change was Code driven and makes it safer for technicians installing or servicing the system.”

—Tom Willis, director of sales, AMtec Solar


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