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.
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 Locations Step of the BOS menu.
Choose an Interior Wiring Strategy
When we get to the Electrical Components Step of the BOS menu, make sure to indicate that the Preferred Array Wiring Strategy is to run wire on the interior of the residence.
Continuing on to the "Wire and Conduit Options" on the last page of the BOS menu, 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 Diagram, Bill of Materials, etc.