Upgrading wiring to detached garage subpanel

Question

My garage is a 2 car detached garage is approximately 33ft from the house.

My current setup (previous owners doing) is

• a 15a breaker (main panel) to 12/2 UF in a metal (aluminum?) piping that runs along the outside of the house, then underground, and comes up in the Garage.
• Garage has a 4 space subpanel with 3 20a breakers, one of which controls all power to the garage.
• Up until recently, the garage had
  • 14/2 wiring
  • 4 outlets,
  • 2 external lights and
  • one light inside,
• at which point I upgraded the wiring to 12/2.

I want to upgrade the 120a circuits (to better handle running a small heater and power tools) as well as add 240V so I can run a welder.

• Would a 60a double pole breaker (main panel) and 8/3 UF suffice?
• What size breaker would you recommend for the subpanel main?

I would keep the 12/2 to the sockets/lights I have currently and upgrade to GFCI sockets.

Pics of conduit:

^{Click images to embiggen}

Answer 1

On another forum, after a long diatribe about choosing correct wire sizes, I said "... OR, just use 2-2-2-4 aluminum for everything".

It covers every base and it's very favorably priced. There's nothing wrong with aluminum wire at such a large feeder size. (some novices have undue fear of aluminum, but they are not versed in the full details/lessons learned.) You didn't aim to be at such a large size, but #2 is a pricing sweet spot, due to popularity and the still-negligible mineral value in the wire. It's also cheaper than ANY copper option, even 30A #10.

#2 aluminum gets you 90A legally, so it serves almost all needs.

You want 90A to a garage.

The reason is resale value. Many home buyers foresee electric vehicles, and will pay extra for a house already wired for big power in the garage. How big is 'big'? The new, emerging "gold standard" is 80 amps for multiple EVs to share using "Share2" technology. (the enabling tech was baked into the original J1772 EV spec, which Tesla also follows. It's really quite ingenious.)

Provisioning that, leaves you 10 amps "left over" on each phase. One phase you can assign to lighting. The other phase can power the mandatory "20A" circuit with up to six 15A receptacles (provisioning 180 VA per receptacle as a rule of thumb borrowed from the commercial Codes).

"That was easy!"

See, this is why I opened with a discussion of wire economics. I wouldn't want you to think I was proposing #3 Cu, which would cost a fortune.

Cable types

Your familiar friend UF is basically NM cable improved for use outdoors. Both are limited to 60C thermal. That means #8 UF is limited to 40A, and #6 UF is limited to 55A and that's the largest UF made. It is not possible to get a 60A feeder with UF. NM goes bigger, but it's not allowed outdoors.

For an outdoor run, you will either do direct burial at 24" of cover, or conduit all the way with 18" of cover (PVC) or 6" of cover (pricey RMC). Note that RMC needs 12" under vehicle pathways.

A popular direct-burial cable is "MH Feeder" (Mobile Home Feeder) which is allowed underground but not indoors (unless it's in conduit indoors). 2-2-2-4 is the most popular size of that, since most mobile home installations use it.

For all-conduit runs, you use individual wires of either THWN or XHHW type. You can get any size of any metal - again #2AL is a popular size. The ground only needs to be #6AL.

Your distance is not far enough to worry about up-sizing the wires for voltage drop.

Note that the lugs on the subpanel are themselves aluminum, because (as the lessons taught) it's a 'universal donor', playing well with both copper and aluminum wire. Cheaper panels also use aluminum buses.

Panel size

Running out of panel spaces, and being stuck on a project, is the #1 topic on here relating to panels. Spaces are laughably cheap when buying a panel. As such, we really really recommend spending the price of a pizza on way more breaker spaces than you ever imagine you'll use. If you ever build a shop, you go through spaces like water - 240V tools use them 2 at a time, and 24 spaces is hardly excessive.

Since it's an outbuilding, your garage needs a "main disconnect" at the outbuilding. Due to a loophole, this "main disconnect" can be up to six hand movements, i.e. 6 circuit breaker throws. (2-3 120V breakers can be tied with handle-ties, making them 1 throw.)

If you exceed the 6 throw rule, you'll need to retrofit a "main" disconnect switch. The cheapest disconnect switches made are circuit breakers (just due to economies of scale). Many 14+ space panels allow retrofitting a main breaker, which is the cheapest path to doing that.

Answer 2

Looking at the picture of the conduit (when there was only one image) it looks like maybe 3/4" Rigid Metallic Conduit (RMC), it is possible the 12/2 in the pipe exceeds NEC the maximum fill rule that you have to calculate flat cable as single round conductor using the widest dimension of the cable. It's not even worth the time to do the measurements and calculations, your improvements using UF in conduit will be insubstantial.

Your highest capacity option for 240v in 3/4" RMC (assuming the conduit is complete) is 75°C rated THWN #6 copper for the 3 circuit conductors and a #8 ground. The circuit conductors are rated for 65A, but 65A is not a standard breaker size so you are allowed to round up to 70A. #8 ground is good for 100A. If the conduit is not complete then this is not an option since THWN conductors must be installed in a raceway.

Functionally the breaker in the source panel can be rated for a maximum of 70A. This protects everything downstream at the 70A level. The sub-panel doesn't technically require a single main breaker, but the Code does require a maximum of 6 disconnecting switches. The most practical at this amperage is a main breaker panel. Usually it is more economical to buy a 100A rated main breaker panel with a 100A main breaker than it is to provision a panel for 70A. Trying to go with a 6 disconnect maximum saves a little, but is often confusing and savings are defeated when you need to expand.

Higher amperage options would require replacing the conduit. If going the 2-2-2-4 AL method as described by Harper the wire above ground will need to be a minimum 1.25" (and I would use PVC). That is a valid option. It might be easier to complete the 3/4" than to replace the above ground with the bigger pipe.

If you decide to pass or delay doing improvements you still need to get that taped splice fixed, the splice needs to be in a junction box.

Please also note you are improving beyond a 2 pole 20A feed you will need grounding electrodes (ground rods) at the garage.

Answer 3

Breaker in sub panel isn't important just so it's rated at least the capacity of the feed. What's important is the rating of the breaker in your main panel FEEDING the sub-panel and that it's not oversized given the wire size. Attached is a chart showing wire ampacities in various conditions.

Short answer is NO, you can't run 60 amps on 8 ga. Also, please double check the wire sizes in your question, they don't make sense. 10 ga is larger than 12 ga so swapping is not an upgrade.