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RMH: safe clearance to electrical wiring?  RSS feed

 
Philip Rothwell
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I am preparing to build a RMH in my home, which is constructed with 2' thick stone walls and 4-6" thick mud plaster. Using interior walls themselves as part of the thermal mass is my goal, but these walls contain electrical wires embedded in the mud plaster. I understand that common household wire is rated for 194 degrees F; a temperature I am sure the stove's thermal mass can exceed. What do you folks consider to be the minimum safe distance between (un-additionally-insulated) electrical wiring and exhaust ductwork, assuming both are encased in the thermal mass? What (additional) wire insulation do you recommend when this minimum distance cannot be achieved? The area that concerns me most is the final vertical run of duct going to the roof, where electrical wire must pass within 6". This topic does not seem to be covered online, so I appreciate you all weighing in.
 
Erik Weaver
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Location: S.W. Missouri, Zone 6B
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I think I would look up building standards for masonry stoves. I don't recall the ASTM-E1602 specifically mentioning this. However, if you build according to that standard, I should expect you'll be fine. The long and short of that, in terms of what may be applicable, I would say is to make sure your stove thermal mass is a minimum of 8-inches from your other walls. Personally, I would also isolate your stove mass with insulation from the rest of your house, unless that is already isolated with insulation from the earth.
 
Brett Hammond
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Location: Maryland, USA
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If you have a stone wall being heated, and your electrical cable is one foot away from your heat source, both in the wall, you have essentially R1 insulation between them, which is almost nil. If you additionally insulate the cable, but leave it there, you only slow down how long it takes to heat up, but it will still heat up if you never let your wall cool down, unless you find some way of cooling the cable.

For example, you could run the cable in insulated conduit that is much larger than the cable, and is open to the room air or outside air on both ends, and it will stay cooler as convection will move the warmer air out the top and suçk cooler air into the conduit. not very practical.

Or you could insulated around 3 sides of the cable in the wall and leave the 4th side of the cable very near the surface of the wall so the room air cooled that part of your wall, but very different temps in a plaster wall may crack it due to different rates of expansion.

If you were building from scratch, I think I would run the cables very near the exterior of the wall, perhaps on the outside of the insulation, if there is any, so the cold outside air cooled that side of the wall and cables. Or run all your cables entirely inside your living space behind baseboards and other trim, or in conduit inside the living space. Even if they are touching the wall, they might be ok since the cooler room air is on one side.

I can't think of any simple answer to your problem.

Maybe just use the interior walls between rooms as your heat source, and run all your wiring in your exterior walls. And insulate the junctions where the interior walls attach to the exterior walls so the exterior walls don't get that hot.


 
Glenn Herbert
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Location: Upstate NY, zone 5
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If building new, I would agree that the best course is to simply keep all electrical components out of any thermal mass that could get above say 150F or so. That said, if you have a mass bench integrated with a cob wall, the wall a foot from the bench is highly unlikely to ever get significantly hot. Similarly, if you have electric wiring in an existing cob wall and want to add a thermal mass bench, I would put enough new cob against the portion of the wall that contains the wiring that the thermal gradient would keep the wiring safe from getting too hot. Depending on the heat source temperature and the air temperature on the far side of the wall, that could be a little or a lot.
 
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