Because NM-B (Romex) and THHN are only suitable for use in dry conditions, there are a few provisions that must be met before NM-B or THHN can be used in your design. The main thing to remember is whatever component the AC branches connect to must be indoor. This means that, depending on the configuration of your design, you may need to set the following:

  • The MSP (if combining on roof or if a single branch) set to indoor.

  • If using a wall-mounted AC combiner, it set to indoor.

  • Choose an interior wiring strategy.

Set Equipment to Indoor

The first step is to make sure that your equipment is specified as Indoor when placing the equipment locations for your design.


Specify the MSP as being indoors after placing the pin by using the Properties Panel on the right.

Choose an Interior Wiring Strategy

When we get to the Electrical BOS step of the System Design, make sure to indicate that the Preferred Array Wiring Strategy is to run wire on the interior of the residence.

Wall-Mounted AC Combiners

If using a wall-mounted AC combiner (such as the Enphase IQ Combiners), specify the wall-mounted AC combiner as being indoor during the Generate Locations step of the Electrical BOS page in the System Design.

Continuing on to review the Circuit Table , we'll be able to specify that we're using NM-B or THHN for interior AC wiring in our design. This will then be referenced on our Single-Line or Three-Line Diagram, Bill of Materials, etc.

Did this answer your question?