Hey Wrenches,

There has been a change in the wording of the 99 Code that might make array wiring easier. The use of a combiner box might become a rare thing on domestic PV systems. Let me know if you think this is correct.

In the old code it appeared that every series set of modules needed to have its output run directly to an overcurrent device in a combiner box. Connections could not be made in another module's junction box, if removal of that module would disrupt the "grounded conductor" of another set.

This is how it has often been interpreted in Colorado anyway. With this method, a rack of 8 modules wired for 24 volts requires four flexible conduit "whips" coming down to a combiner box with four fuses. There are often positive and negative busbars to which the system conductors are connected. Below is the old wording.

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(c) Module Connection Arrangement. The connections to a module or panel shall be so arranged that removal of a module or panel from a photovoltaic source circuit does not interrupt a grounded conductor to another photovoltaic source circuit.

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The new code goes into detail about the meaning of "source circuit." It now appears that a source circuit is any configuration of modules rated at 50 volts or less, terminating in a single over current device. Here is the section.

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(c) Module Connection Arrangement. The connections to a module or panel shall be arranged so that removal of a module or panel from a photovoltaic source circuit does not interrupt a grounded conductor to another photovoltaic source circuit. Sets of modules interconnected as systems rated at 50 volts or less, with or without blocking diodes, and having a single overcurrent device shall be considered as a single-source circuit. Supplementary overcurrent devices used for the exclusive protection of the photovoltaic modules are not considered as overcurrent devices for the purpose of this section.

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The first sentence of the new code reads exactly like the old. To simplify the changes, we can eliminate it. The changed part has a lot of modifying wording. Although necessary to consider, we can eliminate the modifiers for present purposes. The reduced statement boils down to:

Sets of modules interconnected as systems rated at 50 volts or less having a single overcurrent device shall be considered as a single-source circuit.

This would seem to imply that a rack of 8 modules wired in a 24 volt series-parallel configuration is a single source circuit. It would therefore need only one whip going to a disconnect, rather than 4 whips connecting to a combiner box.

The series sets apparently may be paralleled in the module junction boxes now. If true, this will not only save hundreds of dollars in materials and labor on large systems, it will also make arrays look a lot cleaner.

Here is an example of how an array might be designed with the new code.

Eight Siemens SP 75s wired in series-parallel for 24 volts, have a short circuit rating of 19.2 amps (4.8 X 4) . Multiply that times 1.25 twice (to comply with code) and you get 30 amps. A pair of number 10 THWN-2 wires should satisfy any inspector. (* Note: The rating of THWN-2 comes on many THHN conductors).

So two #10 AWG wires can be used instead of eight. One conduit is needed instead of four. One overcurrent device is needed instead of four. The wiring can be run directly to a fused disconnect, rather than to a combiner box first. A 16 module, 24 volt array can have two whips run to a 2 pole fused disconnect.

Due to the language, it looks hopeful that this rule will also apply to 48 volt systems. The code now seems to say that photovoltaic source circuits are "Sets of modules interconnected as systems rated at 50 volts or less." The word "rated" seems to imply nominal voltage rather than the actual measured, operating voltage.

Wrenches, let us know if we are on track.

Drake Chamberlin

Don Wallingford