Transitioning from NM to Conduit

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Old 09-25-09, 03:09 PM
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Transitioning from NM to Conduit

Hi,

I am replacing gas+A/C system with a heat pump. The emergency heat kit is requiring a new circuit to be run to the main panel. The equipment is also requiring a line set upgrade so the plan is to run the new circuit in conjunction with the line set run. Given the location of the equipment (central closet) and multiple finished walls/ceilings, the circuit would run to an exterior wall and then use conduit to reach the exterior mounted main panel (common in Texes).

I fully understand the NM and Conduit rules and had planned to transition from 6/2 NM-WG to 3x#6 THHN (conduit) at the exterior wall in a junction box. Then I saw NM wire advertised as PVC clad THHN wires (see link below).

Is this a "red herring" or is there a viable wire/cable solution that allows the jacket to remain while inside (between floor joists) and then allows the jacket to be removed when transitioning to conduit? This wire is not ideal (solid vs strand, cost) but I am wondering if I have missed an alternative solution (eg. no need to splice, etc).

6/3 WOG x 125' Building Wire # 6/3WOGX125 by Essex Electric Inc
 
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Old 09-25-09, 03:18 PM
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Stripping the sheath off of NM cable creates a violation of the listing requireemnts of the Code. Also THHN, if that is the only rating, cannot be used outdoors.

You would need THWN rated insulation outdoors.
 
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Old 09-25-09, 03:28 PM
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thanks!

Seemed to good to be true. As for the #6 wires in the conduit.. I got confused. Are you saying THHN is only viable for conduit runs inside the building? So a transition from interior to exterior conduit (above ground) would require a transition from THHN to THWN? or using THWN for the entire run?
 
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Old 09-25-09, 04:33 PM
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Most single conductor wire sold is dual rated THHN/THWN but you need to check the markings on the wire to be sure.. Do not rely on guys in orange or blue aprons or red vests, check yourself.
 
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Old 09-25-09, 08:34 PM
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Most of the time THHN and THWN are identical; however the conductor must have the THWN marking on the conductor insulation or it cannot be used outside. Most of the manufacturers state that their NM cables contain THHN conductors, but the conductors themselves are not marked as such so they cannot be used without the outer jacket.

If the rating of the HVAC equipment is 50A or less, you can use #8 in the conduit and #6 in the cable due to the different temperature ratings.
 
 

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