ECN Forum Forums Code Related Discussion NEC & other Code issues Microwave

Steve:
UL says '2-Hr wall' and others are 2-hr wall, 1-hr ceiling on rated assemblies.

BTW, the Arlington plastic extenders (1-gang & some others) are also UL rated for FR assemblies.

Hope this helps

I'm curious... has anyone ever tried to burn one? I find it hard to believe that those boxes would last 2 hours in a fire. I imagine the wall rating has more to do with the size of the hole being too small for significant flame penetration, kind of like how smoke rolls in the corners of rooms and won't set off fire alarms too close to the wall or ceiling.

I keep talking about it, but one of these days, I'm actually going to build a 2x4 wall with fiberglass insulation and boxes and all, and light that sucker up to see what happens. Might even plug #14 romex into a 60A outlet to test that, too

Perhaps a more complete explanation is needed here ...

Fire testing of wall assemblies focuses on the wall keeping the fire on one side - that is, the unexposed side of the wall may not exceed a certain temperature during the test. The side of the wall exposed to the fire is typically completely destroyed within the first half hour of the test.

There are additional standards for things like the plastic used in electrical boxes. One of the tests is pretty straightforward; a sample has one edge briefly exposed to a flame, to see if it ignites ... and, if so, what happens when the flame is removed, and how fast the flame progresses up the sample. Pass this test, and the material is considered 'non combustible,' at least as far as the NEC is concerned.

Thanks, Reno

Do you know what the procedure for the flame test is? I'm curious if cabinet wood (solid or MDF) or even sheathing plywood passes. When I think of "combustible" material in relation to storage, etc, I think of old paint cans and oily rags, not wood.

I don't recall the exact details, but here is what I do remember ...

The sample - think a piece maybe 1" wide and 6" long - is held at an angle just shy of vertical. A large Bunsen burner is then placed under it, with the flame in contact with the lower edge for a brief period. Then the results are observed.

NOW ... for the bad news ..... this simple test has been shown to be essentially worthless in evaluating the actual fire behavior of materials. Only in the crudest manner does it provide any real means of comparing the hazards posed by the materials.

For example, nearly any wood product will ignite, and the flame will march up the face of the material. Eventually all the wood will be consumed. Simple, clearly flammable foam insulation will pass, though, as the material melts and falls to the bench as soon as it gets warm; the remaining sample is too far from the flame to ignite.

I do not recall the particular manner in which test results are 'scored.'

Eventhough some argue that cord is exposed, but in realty all the items that usually go in the cabinet above the built in microwave are things that will not in any way damage the cord. besides most microwaves get installed flush on the top and at the back and the cord comes off of the top of the unit. So the only way it can be installed is through the hole that is made on the bottom shelf of the cabinet.

I guess this can also fall under the gray area where the AHJ must make the final decision.

Edward