There are a number of issues to using AC and DC together in the same electrical system. Briefly, they are: circuit boxes and hardware, outlets, wiring schemes and sizes, and switches.
The Electrical Code prohibits AC and DC in the same box. You’ll need two distribution boxes – one for AC and one for DC.
Circuit breakers rated for AC won’t work for DC. Expect to pay more for DC breakers. On the other hand, fuses are mostly indifferent to AC or DC, or even differences in voltage. Older-style circuit boxes employing fuses that no longer meet Code for AC wiring will work fine for DC circuits. Up to ratings of 30 Amps, the new style of automotive fuses are also great for DC systems.
Standard receptacles will work for DC or AC. You must exercise caution in using both in the same household. Plugging a 12V DC load into a 120V AC socket may fry the load if the circuit breaker/fuse doesn’t pop first.
Plugging a 120V AC load into a 12V DC circuit may hurt the load or blow a fuse or simply do nothing. Still, who needs this worry. Amateur electricians have many
ways to handle this situation. One scheme uses the same type of receptacle for AC and DC circuits but colorcodes or labels the receptacle plate itself. This works okay for hermits but it’s lousy for guests, children, and the uninformed.
A second scheme is to wire AC and DC into the same receptacle, with a shared common (bad idea). Another scheme is to wire the 12V appliances to a unique auto cigarette
lighter plug/receptacle (light loads only, please). Or a plug/receptacle of the style found in older RVs (recreational vehicles) for 12V circuits (mostly inadequate).
Polarity is another issue with DC. Incandescent lights and simple heating circuits don’t really care about polarity, but you must observe proper polarity (pos. or neg.) for LEDs, high-frequency fluorescent lamps, stereos, and many other DC loads. This is easily handled by the newer style of plugs and receptacles that permit insertion in only one way. These will ensure correct polarity in wiring plug and receptacle, as will the use of 3-prong plugs.
Overall, 12V DC wiring will require a larger gauge of wire for even modest loads. Wire size increases rapidly with any length. Here, preparation and creativity go a long way
toward minimizing the expense and labor while retaining full capability.
What do you want to do and where? Special low-voltage wiring tables will assist you in sizing wire for specific loads at varying distances.
There is also merit in the idea of running a branch line of large wire to the far side of the house where it can be distributed from a second, smaller fuse box to loads in that area. Large-gauge wire is stiff and awkward to route; plan accordingly. Use 12-gauge wire “fingers” from a bigger gauge wire to ease connections to receptacles and switches.
Use junction boxes for wire gauges of #8 and larger. Relatively short lengths of #12 wire leading from these to loads and receptacles will incur only small losses.
Switches designed to handle 120V AC may fail in use with 12V DC. The arc produced when a standard AC lightswitch opens (turns off) a DC circuit will be hotter and last longer.
Absolutely avoid “silent” switch types; they open way too slow. Either way, the DC arc will eventually (if not immediately) burn a switch’s contacts. It is possible to add a capacitor across the switch to suppress this arc (Figure 1).
Or to wire a switch with multiple poles in series (not parallel; see Figure 2) to help it survive this arc. Of course, you may also find and install switches rated to switch DC current.
Resource: January/February 2000 Backwoods Home Magazine