Actually both are done separate wires fed into the conduit. You can not strip NM cable and use it in conduit. The wires from inside a NM cable are NOT suitable for this use. You must use proper rated THHN/THWN wires.
Nm-b romex run outdoors
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CallMeVilla
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Good point, Joe ... For those who do not know, stand-alone THWN cables are coated in PVC, as opposed to vinyl, to assist in water resistance. THHN wire is typically two-conductor with ground, and found in the familiar ROMEX cabling that wires homes, e.g. 12-2. The coating also explains the slight price difference between the two ... and the noticeable price difference between ROMEX cabling (cheaper) and THWN stranded cabling. In harsh climates these are important differences to know! 
THHN is NOT two conductor with ground. It is single wire, either stranded or solid. The wire inside NM cable is NOT suitable to be used in place of THHN. It is not labelled as THHN and can not be legally used in conduit.
nealtw
Contractor retired
PVC, as opposed to vinyl, as in poly vinyl chloride?
CallMeVilla
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Rather than get caught up in the chemistry of coatings, how about this:
1.The "T" stands for thermoplastic insulated cable.
2.A single "H" means the wire is heat resistant.
3."HH" means that the wire is heat resistant and can withstand a higher temperature. This wire can withstand heat up to 194 degrees Fahrenheit.
4.A "W" means that the wire is approved for damp and wet locations. This wire is also suitable for dry locations.
5.The "X" means the cable is made of a synthetic polymer that is flame-retardant.
6.The "N" is for the nylon coating that covers the wire insulation.
Go here for more: http://electrical.about.com/od/wiringcircuitry/a/wirelettering.htm
1.The "T" stands for thermoplastic insulated cable.
2.A single "H" means the wire is heat resistant.
3."HH" means that the wire is heat resistant and can withstand a higher temperature. This wire can withstand heat up to 194 degrees Fahrenheit.
4.A "W" means that the wire is approved for damp and wet locations. This wire is also suitable for dry locations.
5.The "X" means the cable is made of a synthetic polymer that is flame-retardant.
6.The "N" is for the nylon coating that covers the wire insulation.
Go here for more: http://electrical.about.com/od/wiringcircuitry/a/wirelettering.htm
