I am about to start building a cabin and I want to use LED car lights in a string run to my 12-volt fuse box I will use off a conversion box. I anticipate no problems accept for inspections. I wonder is there any codes or rules about running 12 volts in a house? Thoughts or experiences?
I've installed a bunch of 12v in residential applications and, although of course building codes vary by state, county, city, etc., the one thing that seems pretty universal is just protection of the wiring. In most applications I was able to get away with using the plastic cable channel available from Home Depot or wherever; in one particularly ornery city I had to run the 12v wiring inside a metal conduit, but I strongly suspect that was just the inspector being a difficult bastard rather than an actual code issue.
Other than protecting your wiring, the only other thing I'd mention is that, depending on how you're converting, you may need the converter box to be in a proper metal j-box... I usually just used a standalone 120vAC to 12vDC transformer mounted inside a j-box, and that method passed inspection. If you're just using a plug-in transformer, you might run into issues where you tie in your wiring because connections can't be exposed but have to be accessible... usually. Again, it really all depends on your particular city/county/state - there are some places I've been where 12vDC wiring isn't regulated at all.
I did 10 years as a general contractor and honestly the big thing I've found with most inspectors is just: make it right, and make it look right, and they usually won't have an issue unless it's a blatant safety hazard or a blatant code violation. If they can tell you know what you're doing and you made every effort to make it safe and correct, they'll usually be okay with things.