I have a detached garage about 55’ from the back of my house and my main panel is in my basement. So Im planning to run about 160’ #3 THHN wire in conduit (About 90’ inside the basement and 70’ outside through a trench to the garage). I want to put a sub panel in the garage and also install a Tesla Wall connector from that sub panel. The Tesla wall connector needs a 60A circuit. Can I put a 100A sub panel in the garage with this configuration? From what I have read, code says #3 THHN can carry 110A so it should be safe to use a 100A breaker in the main panel and the sub panel? Also, what size ground wire can I use for this? Is #6 THHN enough? Im doing this using a licensed electrician but he didnt seem really confident about his answers so wanted a second opinion.
First, I would like you to consider using aluminum wiring instead of copper. The cost difference is staggering and there's no measurable disadvantage to using aluminum if it's installed properly.
If you're dead set on copper, #3 THHN is fine. You'll need 1-1/4 SCH 80 PVC for areas exposed to damage. In particular, this is generally where the conduit exits the building (usually via an LB conduit body) and to the point where it is below the required burial depth, which is 18" in this case. So your preformed elbow will need to be SCH 80 as well. The lateral run to the garage can be SCH 40, which will save you some money. But you can certainly run the whole thing in SCH 80.
As for your equipment ground conductor (EGC), you'll need a #8 copper wire or #6 aluminum wire for a 100 amp feeder.
If you decide the savings is worth it, and you decide to go aluminum, you'll need #1 AWG XHHW wire for the hots and neutral. You can by code stuff 3 x #1 and 1 x #6 XHHW wires into 1-1/4 SCH 80 PVC, but you'll be at about 37% wire fill, which is really close to the NEC maximum of 40%. To aid in making the pull easier, it may be advantageous to up your conduit size to 1-1/2. You'll still be saving a ton of money.
Another thing to consider is that #1 XHHW may not be readily available at your local big box store, so you may have to resort to your local electrical supply house, most of whom have counter sales departments.
Now, as for your EV charger, some manufacturers recommend copper wiring, and you can certainly use copper wiring from your sub panel to the EV plug. The reality is, we electricians use aluminum wire for feeders and services pretty much exclusively now days because copper is just too prohibitively expensive. Chances are the service wires coming from your meter to your main panel are aluminum and the wires that the power company ran from the transformer to your meter are definitely aluminum.
Lastly, I'd like to add a few more pointers to assure you make a safe, code compliant installation.
The grounds and neutrals in your sub panel need to remain separate. You may have noticed that in your main panel, both the white neutral wires and bare copper wires (assuming your house is wired with Romex) are terminated on the neutral bar. They may not be, but usually are. So you'll need to purchase a separate ground bar to install in your sub panel as most do not come with one. I actually like to put one on each side for convenience, just like the neutral bars are.
Every panel comes with a main bonding jumper, in the form of either a green screw or copper strap. You will not install this and simply discard it. The neutral to ground bond should only occur in the main panel or disconnect.
You're going to need an additional ground rod for your sub panel because it's a detached garage. So, you'll need to get a length of #6 bare copper wire to run from the ground bar in your sub panel to the ground rod outside. This is the one place where you have to use copper wire, as you cannot direct bury aluminum wire. Technically you only need #8 but if you use #8 you have to protect it in conduit. If you use #6 you can just run it bare. Although some people like to use an LB and short length of PVC on the outside for aesthetic purposes.
Think again about wire
First, seriously consider aluminum feeder. Some people are afraid of aluminum because of stuff they heard on the Internet: that was about small branch circuits (lighting, receptacles) and really, it was the copper connectors on receptacles and sockets not playing well with aluminum wire. As such, the lugs on subpanels are aluminum, since it plays well with both.
You're talking about $1500 in copper wire, what we're advising would be more like $400. If you wanted to spend that $1100 improving safety, we have a laundry list of good choices that will make a real difference.
100A requires 1 AWG aluminum.
Second, rethink 100A. If 90A will do, you may find #2 aluminum is more readily available and better priced. It's a commodity because it's used on 100A services, which have a favorable derate due to powering an entire residence. This is a feeder.
Breaker spaces are more important
Since it is an outbuilding, you will need a disconnect switch of some kind and it can be inside the garage. Far and away the cheapest way to get a disconnect switch is simply select a panel that has a main breaker. You're not even using the breaker as a breaker, it's only there to be a switch. But that's fine.
At that point, any subpanel size will do. 225A subpanel off a 100A feed breaker, that is fine.
So since that doesn't matter, what rises to the top is number of breaker spaces. Last thing you want to do is chintz out on the number of breaker spaces, only a few years later to want to add another circuit, only you can't because you're out of spaces. Then you have an awkward and expensive refitting to do as you must replace this sub with a bigger one. Much easier to decisively solve that problem today with a wildly oversized panel. Spaces are cheap. 24 space is not excessive.
The tactic of using tandem/twin breakers to get 2 circuits in 1 space is fading fast, since most breakers these days need to be GFCI or AFCI.
Can I put a 100A sub panel in the garage with this configuration? From what I have read, code says #3 THHN can carry 110A so it should be safe ..
Not sure where you got that, Table 310.15(B)(16) gives 100A @ 75C thermal, or 115A @ 90C thermal.
If you want to use the 90C thermal column, you need to pigtail both ends of the wire to a larger wire that is rated for your ampacity at 75C. Using a Polaris or other connector that is rated at 90C.