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Is there any standard out there in terms of ranking residential electrical cable options for fire safety, specifically in enclosed and/or insulated spaces?

I've read that old knob & tube is actually very safe, if undamaged, due to the distance between the wires. What about as compared to NM, or armored cable (MC), or NM-in-conduit?

I'm new to this stuff, so I apologize if my terminology/usage is incorrect...

(We have knob & tube in our walls and attic, and I'm wondering if having an electrician replace some of it, in areas where we want various electrical and insulation changes/upgrades, is actually going to reduce fire safety.)

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Metal conduit is considered "noncombustible", which means if there's a fire, it shouldn't burn. However, if the wires/cables inside the conduit produce heat, the conduit does nothing to prevent the transfer of that heat (aside from possibly spreading it out). Therefore, it does nothing to prevent fires. Though it may reduce toxic fumes, and will not become a fuel in the event of a fire.

If you're looking to reduce the risk of a fire being started by the electrical system. You should be looking at circuit breakers and other similar devices protecting the wires, not so much the wires themselves.

If you can accurately detect potential fire hazards as they happen, and take steps to reduce and/or eliminate them. The wires themselves don't matter much. Installing and maintaining high quality short-circuit, overload, overcurrent, arc-fault, and ground-fault devices will reduce fire hazards.

If you're really concerned about electrical fires, installing larger wires than are required can help. Larger wires will be able to dissipate more heat, and therefore will be less likely to start a fire (when properly protected). For example, using 12 AWG conductors on 15 ampere circuits, will provide the protection device additional time to respond to a hazard.

Increasing the number of circuits, can reduce the tendency to overload the circuits, thus reducing one fire hazard. If you have a room where a lot of equipment will be plugged in, consider installing two or three general receptacle circuits instead of one.

  • "spreading the heat out" is not an insignificant difference, especially if the circuit breaker gets around to tripping - the ignition-worthy sparks and arcs may be entirely contained in the conduit until it does. Of course, not being nibbled on by rodents to make shorts in the first place is a larger difference, but AC should be adequate for that. – Ecnerwal Oct 26 '15 at 15:47
  • When talking about temperatures high enough to ignite nearby combustibles, conduit isn't going to offer much help defusing the heat. Especially conduit surrounded by insulation. Conduit will protect the wires from rodents though, that is true. In my area we do our best to keep rodents out of homes, and don't lube our cables with peanut butter. So rodent damage isn't a large concern around here. – Tester101 Oct 26 '15 at 15:56
  • Thanks for the response! It sounds like a combination of AFCI breaker + AC (or conduit) should be at least as fire-safe as the current non-AFCI/non-GFCI breaker + old k&t. Given the benefits of the upgrade from other perspectives (usability! we really need more receptacles, better switches, better-located switches, etc), it sounds like replacing the k&t and the old breaker will be a good choice. And yes, as long as we're making the changes, then we can certainly add more circuits, and larger wires, to increase fire safety. – PhilPDX Oct 26 '15 at 18:12
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NM-in-conduit is a bad idea on several fronts. Hard to pull, abysmal fill, etc.

THN, THW etc. are appropriate in conduit. XHHW-2 seems like the highest temp in common insulation (teflon, and fiberglass sleeves go higher, but I've never seen those outside of laboratory equipment.)

When looking for "fire paranoia" and electrical code, Chicago is the default, and they appear (on a quick web search) to favor conduit, only.

Elsewise, consider that the US national electrical code is published by the national fire prevention association. But when feeling paranoid about it, follow Chicago.

Not upgrading your ungrounded, elderly knob and tube does not seem like the best approach, to me.

  • NM in conduit may be bad for a number of practical reasons, but if only looking at fire safety it doesn't seem like it would be any worse then THN/THW in conduit? – Johnny Oct 26 '15 at 0:33
  • You have to install it, which means you face the practical problems, which means you experience just how bad an idea it is. Or you do the right thing and don't put it in conduit. Learn by whatever means makes you happy. – Ecnerwal Oct 26 '15 at 0:37
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I replace old electric especially knob-and-tube whenever I get the chance.

It's easy to romanticize about "old and solid" but in reality, while the knobs and tubes may be reliable, the wire insulation is far from it.

The last time I remodeled and found knob-and-tube I also found burnt spots in the blown-in insulation where one conductor's insulation had cracked and it arced between the conductor and the exterior stucco.

If you're at all concerned about safety, don't tolerate outdated wiring.

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