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I'm planning on wiring my house with CAT6 cable. I'm finding a large jump in price between CMR and CMP cabling, and I'm confused by what counts as a 'plenum' and what does not.

As I'm wiring my house, I'm planning on going through interior (hollow) walls, and through the attic and crawlspace to get to the points I wish to wire. I will be going between floors at one point, which leads me to believe I need at least CMR, and obviously CMP wouldn't hurt either.

I don't mind spending the extra money if I need to, but is it overkill going for CMP if the bulk of the wires are either going vertical, or through a crawlspace or attic?

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If you will never need to run it through air-handling ducts/spaces, just get Riser rated. But, for retrofits, that's often the easiest route from basement to the upper floors... and sometimes a few basement ceiling joist spaces are covered over with tin (or foil-covered cardboard) to use as inexpensive cold air return ducts - if you have to go through those, Plenum rating is required, too. If there's ever a hot enough fire to melt CMR's covering, any toxic fumes it might emit will be among the least of your worries... but the Riser rating means its fire-resistance will keep the flames from following it between floors. DO use firestop caulk to seal the holes in the sill plates or flooring wherever they're penetrated with wiring/cables or pipes. Even if you didn't make the holes, taking care of that detail can mean having many extra minutes to find/reach someone trapped on upper floors while they're still alive.

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In most cases, Plenum is a space that acts as part of the HVAC system. A typical suspended ceiling that allows air back into the HVAC system would be a plenum. The Plenum rated cable will produce lees noxious fumes if it burns. This would be critical if a fire in one part of a building pumped deadly fumes into occupied spaces through the HVAC system.

It does not sound like you need plenum rated cable.

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Check with your local electrical building code. Some cities require plenum cable to be used regardless if you need it or not.

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