For instance, could you use normal 14/2 building wire normally used in houses for 115 volt AC lights and outlets and run DC through it provided that the voltage and current does not exceed 120 volts and 15 amps?Could you use normal in-wall light switches in the same way as long as the DC volts and amps does not exceed the AC rating?
Wiring wise there is no problem. If you use a bulb to light up, no problem. Do not use fans, and other inductive loads that require an ac. Even a microwave should not be used in dc.
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No, this is not safe as far as switches are concerned. When you open a switch (turn a light or appliance off), a tiny spark is generated which erodes the contacts slightly. Over a period of many actuations of the switch, it will degrade to the point of becoming dangerous or non-functional. Every 0.02 of a second (50Hz mains) or 0.017 of a second (60Hz mains), the voltage in an AC circuit is instantaneously zero. The tiny spark generated cannot last longer than this at most, and some of the time even less. Therefore the erosion of the switch contact is small and the switch last for many years. In a DC circuit, the spark lasts as long as it naturally would. This may be a lot longer than 0.017 sec and may erode the switch contact a great deal faster than it would for AC. This causes a defective, maybe dangerous, switch much sooner. The longer duration spark itself is a safety hazard from the point of view of fire safety. A DC switch for 115V will be a much more heftily engineered than an AC switch. A standard AC switch must not be used for DC.
They typically need to be derated signficantly for DC. By how much depends on the device - you need to consult the relevant data sheet. To give you some idea, I have some small toggle switches here that are rated for 1A @ 125V AC but only 1A @ 30V DC. That is not at all untypical.
