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I bought a house recently and I have some experience with electrical but I am not fully sure what would be the best course of action since I've never had a detached garage where the cable would be buried between the house and the garage.

I already have a 20 amp breaker in the main panel and a subpanel with 2 breakers at 15 amps in the garage. They are currently wired with a 12/3 gauge cable.

I would like to upgrade to a 50A breaker on the main panel so that I could have in the garage sub-panel:

  • 20A 120v breaker
  • 20A 240v breaker
  • 15A 120v breaker

Would you all be able to help me try to figure out what I should do?

My current plan was to buy about 100 feet of 6/3 Romex and use the currently buried conduit to pass the new wire and install on the current main panel a 50A breaker.

My main panel is 200A, currently in NJ in case anyone knows any code problems that I should be aware of.

Where the conduit leaves the house:

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    Are you sure that the existing cable is in a continuous conduit run? What size and type is that conduit, for that matter? – ThreePhaseEel Dec 11 '20 at 0:21
  • One continuous conduit from the house to the garage, but from main panel to the external wall of the house where the conduit starts it’s on the ceiling of a finished basement. I am not sure of the conduit size but I think it’s 1 1/2 in or 1in. – Bordoni Dec 11 '20 at 0:34
  • Can you measure it and get back to us? Also, is there room for a junction box where the conduit enters the house at? – ThreePhaseEel Dec 11 '20 at 0:38
  • What type of conduit is it? i.e. is it plastic or metal? – Ecnerwal Dec 11 '20 at 0:54
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    1/2 inch plastic? that's not a typo? I'm amazed anybody got 12/3 UF into that, and you'll need something bigger to get 6 AWG wies run. – Ecnerwal Dec 11 '20 at 1:37
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"Romex" (brand name) is usually used to refer to NM/B cable, which cannot be run in conduit that runs outdoors. It its not rated for wet locations, and all exterior conduits are wet by definition and in reality.

It's also a huge pain and requires huge conduit to run the UF cable (similar to NM/B, but waterproof, with a gray jacket, typically) in conduit.

Use THWN wires in conduit. If you don't want to run conduit the whole way, use a junction box at the end of the conduit to transition from wires in conduit to cable not in conduit.

Edit: if you want 50A service to the garage, you'll need to dig a new trench for new, larger conduit to the garage, since your conduit is 1/2" PVC. Be aware that your conduit MAY not actually extend the whole way - you may just have two shrt sections where the present cable goes into and comes up from the ground, with the UF cable "direct buried" with no conduit in between. That is a legitimate install method with UF cable (the conduit is "protection from physical damage" at the ends where it comes up) but less desirable (no ability to remove and replace the wire without digging it up - also less protection in the ground than wire in conduit.)

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    Thank you so much! Yes, it's a UF cable, I was wondering why that cable was diff then the other ones. All of my EXP comes from Brazil where some of these standards are diff, completely diff names. – Bordoni Dec 11 '20 at 0:59
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    @Bordoni since you're from Brazil, please take your time and be sure to ask plenty of questions to ensure that you're doing things to American (I presume that's where you are now) standards! (Not that there's anything wrong with Brazilian standards, just that they're different.) – FreeMan Dec 11 '20 at 12:35
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    Ah, yes, after a more careful reading, I see that you are in NJ, @Bordoni. Do take care, take your time, read lots of Q&A here and make sure you do things safely. Sounds like you've got the background, you just need to learn some new rules. – FreeMan Dec 11 '20 at 14:40
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Buried conduit is considered wet location. Romex typically refers to NM cable, which isn't approved for wet location. Depending on conduit size you might be able to run 6/3 UF if you can find in quad (not flat) form, but it could be very hard to pull.

Also 6/3 costs about $2.50/ft, you would be better running 3@ #6 THWN at 0.54/ft and #10 ground @ 0.40/ft. (Actual price at my local Lowes). It would be cheaper and much easier to pull. You would need at least 3/4" conduit.

Protect the feeder with a 60A breaker in your main panel, and use a panel in the garage with a 60A or greater main disconnect, and ground rods. Buy a panel with many more spaces than you think you will ever need.

250 feet does have some voltage drop considerations, but the 40A load you describe will not create a problem even if full load at once.

Edit: I see you added it is 1/2" pvc, you'll never get #6 to fit, the largest wire you could legally fit is #10 THWN protected at 30A. I also see you added it's only 60 feet, voltage drop should not be a problem.

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You don't 'technically' need to have a 50 amp drop from the main panel to support what you want in the garage:

20A 120v breaker
20A 240v breaker
15A 120v breaker

You can do that with the existing wiring. You only need 50 (55) amps from the main if you are going to be drawing from all those circuits at their maximum, at the same time.

It is a misnomer to think you need to 'add em all up' and the supply to the sub panel must be that much or greater.

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