I looked through my NEC code book and I cant find all that much specific about low voltage DC system requirements.
If its under 50 volts the requirements are very loosely stated.
Its like the DIY low voltage yard and garden or home lighting. Loosely stated and rather up to the person installing to use common sense about wire size, run locations, and power handling capacity.
Here is the basic condensed version.
If the system is isolated from the primary electrical system and is battery powered the main lead from the battery source must be fused for the size of wire that is being used. All sub feeds of smaller gauge wire coming off of that first feed line should have fuses of relative size for the amp rating of that gauge of wire.
Under 50 volts the electrical wiring is typically considered control wiring and does not necessarily need electrical inspection. But is encouraged if used for larger power applications.