THWN vs UF-B in my unusual conduit installation

rmully · Mar 31, 2024

Hello, I have 50 ft of corrugated pipe buried between my house and a pool shed. I placed it where I knew future landscaping would make it very difficult to do any trenching. It's four inches in diameter and about two feet down. So far I have pulled two direct burial cables through it, one for a 120V circuit and one for a 240V 30A circuit. There are short distances from either end of the buried pipe to the terminations of the run at either building. Since I used direct burial cable, I just run both ends out of the pipe through the ground until they enter the structures. After each pull I plug up both ends of the pipe to keep mud out. Now I must run yet another cable, for a dedicated 240V 6/2 50A circuit. I need to decide between UF-B WG and THWN. I have had no problem pulling (used lube), but despite this being the third pull, I am fairly confident I can pull the new UF-B WG. Now, some people are suggesting I pull THWN. My concern is this: with THWN I will have a length at both ends of the run where the THWN will exit the main conduit and need to go into another form of conduit. I would have to McGiver two types of conduit together and I am sure there will be no seal. Rather than trying to seal, I could simply push the new conduit, say 3/4" size, about two to four feet into the existing pipe at either end. I know the THWN job would be easier, but don't know if its a good idea. Thanks!

davesparks · Mar 31, 2024

Carry on doing with what you know is ok and up to code.

I am not familiar with much of the US cable types or codes but I do know that it is never best practice to McGiver things together.

Unless the people suggesting THWN are going to accept responsibility and pay for it to be changed if it turns out to be wrong then I'd say stick to what you know is good.

Megawatt · Mar 31, 2024

rmully said:
Hello, I have 50 ft of corrugated pipe buried between my house and a pool shed. I placed it where I knew future landscaping would make it very difficult to do any trenching. It's four inches in diameter and about two feet down. So far I have pulled two direct burial cables through it, one for a 120V circuit and one for a 240V 30A circuit. There are short distances from either end of the buried pipe to the terminations of the run at either building. Since I used direct burial cable, I just run both ends out of the pipe through the ground until they enter the structures. After each pull I plug up both ends of the pipe to keep mud out. Now I must run yet another cable, for a dedicated 240V 6/2 50A circuit. I need to decide between UF-B WG and THWN. I have had no problem pulling (used lube), but despite this being the third pull, I am fairly confident I can pull the new UF-B WG. Now, some people are suggesting I pull THWN. My concern is this: with THWN I will have a length at both ends of the run where the THWN will exit the main conduit and need to go into another form of conduit. I would have to McGiver two types of conduit together and I am sure there will be no seal. Rather than trying to seal, I could simply push the new conduit, say 3/4" size, about two to four feet into the existing pipe at either end. I know the THWN job would be easier, but don't know if its a good idea. Thanks!

No it’s not a good idea and why don’t you just add a sub panel in your pool shed using UF cable. THWN wire is required to be installed by listed conduit from start to finish.