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The time has come for me to run power to my detached garage. A little bit of background - I am well versed in all aspects of home improvement, am fairly comfortable and have done my fair share of electrical work.

  • The garage is approximately 80ft from the house, and I have already dug and ran 2" conduit from the house to the garage in a straight line. The conduits point of entry into the house is on the corner opposite the houses service main panel (approximately 30ft away in a straight line) so part of the run would be through the drop ceiling of the basement. The total run would be approximately 120ft.

  • The house has a 200A main panel with an existing sub-panel coming off of it on a 50A 2 pole breaker - the sub-panel has a couple unused breaker slots available.

  • The garage is a "1.5" car garage. I would like to have enough amperage in the garage to run inside/outside lights (LED), a 1 head mini split system, garage door opener, standard 120v outlets on all the walls and I would like to have the option of adding a 220v outlet for an electric car charger/welder/some other heavy duty equipment if the need arises at some point in the future.

  • My plan is to run the wires, install the garage sub-panel, lights, outlets, etc and then have a local electrician do the final hook up to the main panel.

So now on to my questions...

  1. Panel sizing: how many amps and spaces would you recommend, based on my needs outline above?

  2. Aluminum vs copper: It seems like the general consensus is that there is significant cost savings from going with aluminum vs copper and is common for this kind of run.

  3. Gauge & individual conductors vs Romex: It seems like most of the time, individual conductors are used in these situations - however I'm having trouble figuring the exact gauge of each wire. I have also seen a few people recommend using 2-2-2-4 Mobile Home Feeder cable. Also, from my understanding the run inside the house (from the conduit entrance point to the panel) would need to be in conduit if using individual conductors or can transition to another type of cable to run without conduit (I would prefer to not have to run conduit inside the house). Specific guidance would be super helpful here!

  4. Grounding rod - it seems that it's recommended that at least 1 grounding rod be installed by the garage panel. I did also read somewhere that 2 rods is recommended. Alternatively, I have seen recommendations to just connect to the houses ground. What is the right course of action here? Is a hammer drill the best way to drive a grounding rod into very rocky clay soil?

Any and all guidance is highly appreciated.

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  • There a whole lot of already asked and answered "garage subpanel" and "garage sub-panel" questions here. If you'd read a few, you should already know that 2 isn't even a question if you don't particularly want to waste money, and 1 is "go big, really big, to get enough spaces." And I'm pretty sure the fact that you can't use Romex (NM/B) outside at all would also be covered several times.
    – Ecnerwal
    Oct 11, 2023 at 18:12

1 Answer 1

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Panel sizing: how many amps and spaces would you recommend, based on my needs outline above?

I'd recommend a minimum of a 100A "main" panel. 200A is fine as well. By using a larger panel than you actually need right now:

  • You have capacity for the future. To upgrade from a 50A feed to a 100A feed (assuming your utility service can support that much), you will only need to replace the feed wires and not the panel.
  • You need a disconnect, and a "main" panel provides that in the form of the main breaker. It doesn't matter that the breaker is larger than the feed as its purpose is to be a disconnect and not a safety breaker.
  • Panels designed to be main panels tend to have more spaces than panels designed to be subpanels. I count around 10 or so spaces easily in your plans, so a little subpanel with 12 spaces will end up nearly full while a 20 space (and 30 space or 40 space is OK!) will have room to spare.
  • Any size panel requires the same open working space in front of it, so a big panel doesn't really take extra space.
  • Large "main" panels are often available in bundles with "bonus breakers" that make the net cost very close to a small subpanel.

As far as spaces, keep in mind that most circuits now require GFCI or AFCI, so half-size breakers are not all that practical. If you see a panel that says 20 spaces/40 circuits or similar, just look at the number of spaces and pretend spaces = circuits.

Aluminum vs copper: It seems like the general consensus is that there is significant cost savings from going with aluminum vs copper and is common for this kind of run.

Correct. While branch circuits are almost always copper for 15A and 20A and usually for 30A, feeders are almost always aluminum. For in-between sizes (e.g., 40A or 50A branch circuits) it depends on local code (NEC allows aluminum but local code sometimes does not) and compatibility (breakers nearly always allow aluminum but many receptacles, EVSE equipment and other devices do not).

Gauge & individual conductors vs romex: It seems like most of the time, individual conductors are used in these situations - however I'm having trouble figuring the exact gauge of each wire. I have also seen a few people recommend using 2-2-2-4 Mobile Home Feeder cable. Also, from my understanding the run inside the house (from the conduit entrance point to the panel) would need to be in conduit if using individual conductors or can transition to another type of cable to run without conduit (I would prefer to not have to run conduit inside the house). Specific guidance would be super helpful here!

You first figure out the size you need based on an ampacity chart. Then you figure out what to buy. One thing to keep in mind is that splicing wires larger than 6 AWG gets expensive. So generally with feeder (whether individual wires or cable) you want to use one "thing" the entire distance from feed breaker to disconnect (which should be "main breaker" of your subpanel).

This is especially the case with your nice 2" conduit. You could start with a 50A feed on 2/2/2/4 wires and then later upgrade to 90A feed (same wires) and then later pull the wires out and replace with larger wires for 150A feed (or whatever). If you instead use cable for part of the run then you have multiple things to deal with when you make a change. Keep in mind also that if you could use multiple types of conduit with one set of wires - you don't need to use PVC in your drop ceiling.

Grounding rod - it seems that it's recommended that at least 1 grounding rod be installed by the garage panel. I did also read somewhere that 2 rods is recommended. Alternatively, I have seen recommendations to just connect to the houses ground. What is the right course of action here? Is a hammer drill the best way to drive a grounding rod into very rocky clay soil?

It is somewhat jurisdiction dependent. But generally "1 rod with a test that nobody does or 2 rods without the test". So most of the time, 2 rods. You also have to run a ground wire (that's the "4" in "2/2/2/4") back to the main panel, but a detached building needs its own set of ground rods. Hammer drill? Maybe. Or a sledge hammer.

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    A post setter will also work for driving the grounding rods.
    – FreeMan
    Oct 11, 2023 at 13:17

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