PV Combiner Box Buyers Guide

by Lena Wilensky

Combiner boxes are an integral part of many PV installations, serving as the "meeting place" where the wiring from array series strings come together in parallel connections.

In all but the smallest PV systems, modules are wired together in series strings, where the positive leads of one module are connected to the negative leads of the next module. This results in cumulative voltage output, with current (amps) staying the same. The box where the output wires from multiple series strings are joined is the combiner box. Rated for outdoor use, it contains overcurrent protection devices (OCPDs) and the necessary bus bars and terminals for combining the inputs.

Many installers used to build their own combiner boxes for lack of availability, but now there are plenty of commercial products to choose from. Some manufacturers will custom-build combiner boxes to meet specific system requirements. Article 690.4(D) of the 2008 National Electrical Code includes PV combiner boxes as equipment that must be identified for appropriate use and listed by an approved testing laboratory, such as Underwriters Laboratories (UL) or ETL labs. The days of "homemade" combiner boxes are quickly disappearing.

If you live in a locale where the 2008 NEC is in effect, your electrical inspector will likely require one that is listed by an approved testing laboratory, as mandated by NEC Article 690.4(D).

Grid-Direct Systems

Every inverter and charge controller, whether in a utility-interactive or stand-alone system, has a DC input voltage window that must be adhered to. In batteryless grid-tied

Combiner boxes, like these models from OutBack, take incoming wires from multiple PV series strings, add overcurrent protection, then parallel the strings for output to the inverter or charge controller.

systems, inverters require relatively high DC input voltages, from 150 to 600 V (typically 7 to 13 modules wired in series).

Small grid-tied PV systems (less than 5 kW) often have only one or two series strings of modules. With few wires and no need for series fusing, a PV combiner box isn't necessary: Strings can be terminated directly in the inverter (see the "Why Series Fusing?" sidebar). However, larger residential systems (between 5 and 10 kW) often have three or more series strings, and will likely need fusing. Systems above 5 kW also typically have more wires to deal with, which can mean more installation time, larger conduit, and increased wire costs if all the wires need to be run to the inverter. A combiner box offers a place to house series fuses and parallel the series string inputs, reducing the number of array output wires needed to run to the inverter location.

Some grid-direct inverter manufacturers include series fusing within the inverter or its attached disconnect box, which

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Responses

  • rudolph goodbody
    How many wires are considered parallel from combiner box to inverter?
    7 years ago

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