It appears to be outside. You can not run NMB outside. It is not wet rated.

 

You need to use UF-b or THHN/THWN for the portion that is outside even if in conduit. It is okay to put the UF-b in conduit although it is a pain to pull through conduit.

 

Thanks joed and pattenp.
Between ufb and thhn, which is “thinner?” Reason being, the romex soesnt have a whole lot of room in the conduit as is. If either option is much thicker, i will probably need to instll new conduit.
which of course is its own brand of fun

 

THHN is individual conductors without an overall jacket like NM-b so it is easier to pull in conduit.. THHN conductors must be placed in conduit.

 

You can't run Romex outdoor or underground because Romex is not rated for wet locations. It's not a matter of being inside a conduit or not. It just can't be in a wet location. Unlike DWV PVC, electrical conduit PVC connections are not inspected for or assumed to be water-tight. Therefore if conduit is outdoors, it is still considered to be a "wet" location.

UF-B is for direct burial. Like with NM-B, it is not prohibited from being inside a conduit, but it is not designed for that purpose and it is nearly impossible to pull it through a conduit. The fill ratio would also need to be reduced because of temperature derating.

THHN is the ANSI designation for Thermoplastic High Heat Resistant Nylon Coated
THWN is the ANSI designation for Thermoplastic Heat and Water Resistant Nylon Coated.
Most single conductors are rated for both. Take a look at this product image. Notice how the packaging indicates this is THHN conductor, but if you zoom in on the wire itself it says THHN and THWN.


A portion of that run is outdoors and a portion is indoors. The indoor part can be Romex, but I would personally want all outdoor conduit to contain zero NM-B and contain only individual THWN conductors (red, black, white, green). However, since it's pre-existing work, you could make the case to your AHJ that it should remain and see if that's agreeable.

So your conduit leads up the wall and into a LB body and then into your attic. As soon as it enters your attic, it is indoors so you can begin using Romex. You can put a junction box in your attic, directly on the other side of the wall from the LB body. In that jbox you can wire-nut your THWN conductors to your Romex conductors. Not sure what you mean about a home run to the main. Even if you use a couple of junctions, it would still be considered a home run because this isn't a branch circuit.

If you use a metal work box for a junction box, make sure you ground it with a short pigtail and a green grounding screw. AHJ likes that.

THHN/THWN is thinner than UF-B. UF-B has a solid thermoplastic jacket. THHN/THWN are single conductors (no jacket). They are also nylon coated to allow smooth pulling through conduits. UF-B is not designed to be pulled through conduit and will be very hard to pull.

I would definitely recommend leaving the existing conduit in place on the outside of your house. There is nothing wrong with it. Remove the existing Romex from the conduit and pull 4 THWN conductors (black, red, white, green).

Again, that's for the conduit on the outside of your house. The wire inside your house (a.k.a. in your attic) can be Romex. The Romex and THWN can meet within a junction box in your attic.

 

THHN does not qualify. It is not wet rated. You must use a TWWN rated wire. Note that most wires have dual rating but just be sure you have a THWN rating.

 

Electric_dummy, joed, pattenp, thank you very much for your responses. Electric_dummy, thanks especially for your details. This is extremely helpful and I now have a good plan.
Thank you!

 

Glad to help. I wish you luck. Some more pertinant info that I forgot to mention.

You are using 6 AWG which is very dificult to work with. Your junction box should be at least a 6 inch square box.

You can’t bury the box in a wall or ceiling. It must remain accessible in your attic.

Your splices should use either the big blue wirenuts or a split bolt (electrical taped to insulate). More ideal but more expensive is a product called a Polaris that is already insulated.

Split Bolt


Polaris

 

Thanks Electric_dummy!
I was thinking of using a 12x12in box (https://www.homedepot.com/p/12-in-x-...3713/202043349) since the larger guage wires are a pain and I figure there should be a little bit of a service loop in the box. The polaris connectors are nice, if expensive, but perfect for a simple job like this.
Thanks for the additional info!

 

Polaris are nice connectors but the large blue or grey wirenuts are fine for 2 #6s and cost less.

 

If you use THWN, the green ground wire can be #10 gauge on a 50 amp circuit. The ground wire on your 6/3 Romex is probably #10 as well.
Also, it's very likely the neutral can be downsized depending on the actual load, but in no case less than #10. Some local codes may be more strict on neutral downsizing than the NEC.​​​
​​​​​If the load is only a heat pump, a neutral may not even be needed.

Just letting you know in case four #6 wires poses a problem in the existing conduit.