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I have a detached garage that has a single 20A, 120V circuit running to it. My breaker is on the east side of my house, while the garage is on the west side, meaning my 12-2 NM-B cable spans my entire attic.

The problem is that the NM-B currently splices into some UF and comes out the north side of my house, goes around a corner to get on the west side, then goes underground to get to the east side of my garage. I think it would be better if the UF were in PVC (LB coming out of my house, PVC buried ~18", then LB into garage) and if it came out of the west side of my house, rather than coming out the north side and then wrapping around. To do this, I may need a longer run of NM-B cable in my attic so it can reach the west wall, and if so, it'd be easiest to splice it in a junction box in the attic and run it to the west wall.

The current run looks like this:

20A, 120V breaker -> ~50ft 12-2 NM-B -> ~25ft 12-2 UF (exits north wall outside, wraps around to west wall, buried until garage)

I'm proposing:

20A, 120V breaker -> ~50ft 12-2 NM-B -> ~10ft 12-2 NM-B -> ~25ft 12-2 UF (exits west wall into LB outside, buried in PVC, LB into garage)

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From what you are proposing, you are only lengthening the run by 10 feet. Your circuit is still only 85 feet long.

Short answer: Unless you are really loading this circuit heavily (~16 or more amps) you will be fine. There is no limit to the number of boxes or splices, only a length limit.

In the specifications of most of the commercial and industrial jobs we wire they require an increase in branch circuit wire size if we exceed 100 feet. This is a "rule of thumb" without doing a voltage drop calculation.

The definitive answer comes with that voltage drop calculation. There are many web sites online where you can enter your particular information and get a voltage drop estimate.

Also, If you are running PVC you don't need the UF cable you can use THWN rated wire. If you just want to protect the UF it only needs the PVC where it is exposed as long as you bury the UF to the depth required by the Code.

Good luck and stay safe!

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