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I am advising some homeowners who need to rewire their first floor on Long Beach Island, NJ after hurricane Sandy flooded the place. The house has a few complications. Wires need to be run along brick walls with no wire cavities inside framing to get to their destinations. In the past wires were routed through the crawl space which is a very damp space with an almost always wet rock floor and very little room to work. Electrical work was done poorly with unnecessary junction boxes. Everything metal had rust. That can all be fixed more easily because the subfloor is removed and the floor joists are exposed. Wires can be run along joist and over running boards with no junctions in the crawl space. There is no consensus in my research about using THWN wire for damp locations or just the regular Romex in a damp crawl space. My feeling was to put in the THWN for wet locations in all runs under the floor through the crawl space and up to the outlet boxes they feed since no one can complain about THWN being in parts of the run. There is a question in my mind whether the Romex feeding the crawl space THWN wire should not also be THWN since its paper wrapped grounding wire would be vulnerable to any water that climbed up the THWN wire running in the crawl space into the junction where the Romex feed the THWN.
The homeowners are worried about running any wires in the crawl space and wanted a way to keep all runs in the upper living area. The two complications are the brick walls running the length of the house and the fact that a very nice wood ceiling not so easily messed with is the only thing not ruined, plus it is a good idea not to drill holes through the main support beams in the ceiling/upper floor of this 100+ year old house. I suggested building a little soffit along the brick wall where they have a piece of wood trim anyway. The soffit will run the wires and be the trim. I cannot speak with confidence about the aesthetic result, but it should be fine if its done well. My electrical concern is the fire code parameters of the soffit for the wiring. The soffit will run three 12/2 wires and maybe one 14/2. The soffit will have an exterior measurement of 4 by 4" and the interior about 3 by 3". It will be made of wood that can either be painted of maybe left to stain. So it won't necessarily have drywall around it. They wanted the front to be removable--to unscrew so that if another flood damages the outlets the wires running to them can be removed and rerun more easily.
Does a soffit with three 12/2 wires and a 14/2 wire need to be of a certain cubic size that would be larger than the size I am suggesting (3 by 3 inches interior space)?
Does the soffit need to be dry-walled with the edges dry-walled to meed the fire blocking standards or can a wood soffit with seams where the edges meet satisfy the fire code burn rate for where wires are run?
Is there a preference to run wires in a soffit above verse a problematically wet crawl space difficult to get to and work in? And what sort of wire should be run? THWN?