Upgrading old BX: use new BX or NM near hot pipes?

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Old 02-08-14, 05:48 PM
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Upgrading old BX: use new BX or NM near hot pipes?

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I'm in the slow process of upgrading much of the old BX wiring in my 130 year old home. Four circuits are still running old BX without the bonding ground wire. The newer circuits are all NM of various ages, as upgrades were done over many decades and previous owners.

Many decades ago (40 years maybe?), presumably during a major upgrade, a new load center was installed and set up to feed those four BX circuits by running four NM circuits to a nearby large junction box (looks like a small load center without any breakers, all you see when you open the door is all the NM to BX pigtail wire nut connections) from where the BX cables and branch out through the house. All the newer circuits feed directly back to the load center.

I have two questions:

Question 1: For one of the older BX circuits, I have the opportunity to very easily snake new wiring up to the 2nd floor from the basement by pulling out an old run of BX through two stories of 1-foot wide dead spaced next to a chimney. The BX is only supported by its clamp to a junction box once it reaches the second floor (in a closet) and branches off into two bedrooms. Below the junction box it just hangs free below the junction box all the way to the basement.

Normally I'd run NM but I am concerned that the vertical dead space that the wire will follow, which runs along the chimney, is also shared with a steam radiator supply pipe. It is my understanding that the steam pipes heat to greater than boiling temp. There is no way to guarantee that the new cable won't come into contact with the steam pipe.

For this reason, should I run modern BX (AC) or stick with NM?

Question 2: While the newer circuits in the house have mostly modern NM wiring, for one of them, the NM cable looks old, and there is an outer wrap of some type of shellacked nylon or fabric mesh instead of vinyl. The hot and neutral wires have what seems to be PVC jackets and in good shape, they look like that found with new NM. The outer mesh jacket is labeled "Columbia type NM 12/2 with ground". The ground wire however is much narrower than 12 gauge, it looks like 16 or even 18 gauge. Is this still safe to use with grounded receptacles? Or should it be upgraded. It feeds one 3-gang kitchen outlet and is on a 15A breaker.

Thanks!
 
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  #2  
Old 02-08-14, 07:00 PM
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The outer mesh jacket is labeled "Columbia type NM 12/2 with ground". The ground wire however is much narrower than 12 gauge, it looks like 16 or even 18 gauge. Is this still safe to use with grounded receptacles? Or should it be upgraded. It feeds one 3-gang kitchen outlet and is on a 15A breaker.
Older NM cables with a grounding conductor used to be a size smaller, I'd bet the ground in your is a #14. I have some in my kitchen too. I'd leave it alone especially since you said it's protected by a 15 amp breaker.

Normally I'd run NM but I am concerned that the vertical dead space that the wire will follow, which runs along the chimney, is also shared with a steam radiator supply pipe. It is my understanding that the steam pipes heat to greater than boiling temp. There is no way to guarantee that the new cable won't come into contact with the steam pipe.
I cannot give a definitive answer on this, I never ran into anything quite like what you describe. Keep in mind though, the conductors in new modern NM cables have a 90C insulation. AC cable would have the same temperature rated conductors. I think you'd be OK with using the NM cable, but I cannot say for sure. Another way I look at it is this, if wood is safe to be near those pipes, why wouldn't the NM cables also be safe?
 
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Old 02-08-14, 07:16 PM
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The early NM cables had a #16 ground. I would replace the cable if you have a chance. Kitchens can use the extra capacity.
 
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Old 02-08-14, 07:27 PM
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I cannot give a definitive answer on this, I never ran into anything quite like what you describe. Keep in mind though, the conductors in new modern NM cables have a 90C insulation. AC cable would have the same temperature rated conductors. I think you'd be OK with using the NM cable, but I cannot say for sure. Another way I look at it is this, if wood is safe to be near those pipes, why wouldn't the NM cables also be safe?
Good point, but then the flash point of wood is a lot higher than boiling, or 100C. I'm more concerned with melting, and if the AC sheathing makes contact with the steam pipe it's going to dissipate the heat along a longer stretch, like a heat shield, protecting the conductor insulation from heating too much. But maybe I'm wrong.

Let's put it this way-- is there any reason (other than cost) that I SHOULDN'T use AC cable for this run?
 
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Old 02-08-14, 07:54 PM
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I don't know which would be better. The insulation inside is the same. I am not sure if the metal sheath would act like a heat sink to disapate the heat or not.

Are you sure the pipe temperatures are that high? I have seen boilers that are running about 120 to 140 F.
 
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Old 02-08-14, 08:21 PM
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Are you sure the pipe temperatures are that high? I have seen boilers that are running about 120 to 140 F.
Steam by definition is above boiling. We're not talking hot water heat that's circulated through your house by a pump. We're talking STEAM, so it's traveling vertically by gravity up the pipes as steam pressure is built up at the boiler from boiling water. The radiators themselves reach upwards of 200F which is 93C, so the pipes feeding the radiators are probably getting a bit hotter than that.
 
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Old 02-08-14, 09:01 PM
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Wire in vertical chase

I am concerned that the vertical dead space that the wire will follow, which runs along the chimney, is also shared with a steam radiator supply pipe.
Since you would have to de-rate the conductors going up to the second floor based on ambient temperature, I would suggest hanging a thermometer down from above and getting some idea of how hot the chase gets with the heat on. There are high-temp insulations in the NEC, mainly for fixtures and they are allowed for some other uses but not branch circuits. The NEC allows NMB cable to be fished through concealed access points in a building and unsupported where you can't get in to install supports.
 
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Old 02-09-14, 06:48 AM
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I'm more concerned with melting, and if the AC sheathing makes contact with the steam pipe it's going to dissipate the heat along a longer stretch, like a heat shield, protecting the conductor insulation from heating too much.
It looks to me like the metal sheathing on AC cable would transfer the heat to the conductor insulation very quickly where the PVC jacketed NM cable would act like an insulator. I am not sure if the jacket on NM cable has a temperature rating or not. Might be best to consult your AHJ and have this discussion with them.
 
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Old 02-09-14, 10:37 AM
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Can you run rigid conduit through that space? It might be easier to keep the conduit from coming in contact with the steam pipe than just a regular cable.
 
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Old 02-09-14, 04:35 PM
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Can you run rigid conduit through that space? It might be easier to keep the conduit from coming in contact with the steam pipe than just a regular cable.
Not a bad idea! Since it is a 9-foot run between the basement ceiling and the second floor floor, and my basement has only 6'6" of headroom, I would have to run two 4.5 foot segments, joining the two with a union after the first one was snaked up the void. But I like it!
 
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Old 02-09-14, 04:59 PM
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I do believe you will have to run individual wires through the conduit.
 
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Old 02-09-14, 05:40 PM
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I would have to run two 4.5 foot segments, joining the two with a union after the first one was snaked up the void.
No, not a union. You would either thread both ends of the rigid conduit and join them with a rigid coupling OR join them with a threadless rigid coupling. The threadless rigid couplings come in both setscrew and compression types.
 
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Old 02-09-14, 05:45 PM
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You could use EMT also. Not as expensive and lighter and easier to work with.
 
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Old 02-09-14, 05:59 PM
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You could use EMT also. Not as expensive and lighter and easier to work with.
I agree, much easier to work with.
 
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Old 02-10-14, 08:34 AM
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I have some spare 1/2" EMT conduit that a friend gave me but I have never worked with it. Does code allow running Romex through it or does it have to be individual wires? Are there EMT connectors that can (in a code compliant way) clamp the end of the EMT to the 2nd floor where it pokes through from below? What would those clamps look like? There needs to be something to secure the EMT since there is on access inside the dead space.
 
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Old 02-10-14, 04:26 PM
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One hole EMT straps can be used where the conduit is accessible.

How many circuits and what size do you need to run upstairs?
 
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Old 02-10-14, 07:08 PM
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Does code allow running Romex through it or does it have to be individual wires?
The NEC does not prohibit NM cable from being installed in conduit, but some jurisdictions do. Consult your AHJ on this.
 
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Old 02-10-14, 07:11 PM
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The cable size would influence the size of the EMT sleeve.
 
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Old 02-11-14, 04:35 AM
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One hole EMT straps can be used where the conduit is accessible.

How many circuits and what size do you need to run upstairs?
Just two bedroom receptacles.

The cable size would influence the size of the EMT sleeve.
It would be about 9 feet of NM 12/2. I'll consult the local inspector.
 
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