Adding a garage subpanel

Question

Currently I have a 200-amp main panel located in the basement. I would like to install a subpanel in the garage. I will use to power my electrical tools, heater, and PC. The garage is part of the house. Most likely I will need a 100-amp 2-2-2-4 wire to run from the main panel to the new sub-panel. Couple double pole breaker of 100-amp. I would like to have at least 4 breakers for the tools, one breaker for the heater, one breaker for the PC to start. Does the 100-AMP sub-panel will be enough? Do I need to install an additional ground rod? Any suggestion and advice will be appreciated. Respectfully, Bernard

Answer 1

Go big, Big, BIG on the subpanel

Especially if your main panel is getting full, and having double-stuff breakers in it definitely counts as "getting full".

We have so many questions on here of "My panel is full. What can I do?" And our answers are expensive and regretful. But that we could go back in time, to when that panel was being bought, and say "Spend a couple lattes on a bigger panel". So, we say it now. Go big. Really big.

You're at 7 breakers so far (2 for the heater) + a foreseeable dust collector and EVSE, so 10. I like to finish with all full-size breakers and >50% of spaces unused, so a 20-24 space panel. Disregard "x circuits" as that calls for double-stuff breakers which are no longer viable due to Code changes. (they don't mention that in the marketing materials :b)

Don't let panel bus rating hold you back: An "R" rated tire is only rated for 85 mph, would you put that on your car? Not willingly! So it's not a win to get a panel with 100A busing. So don't think twice about a main-lug breaker that's 225A/24 space if you can find a happy price on it.

The main breaker doesn't buy you anything (sometimes main-breaker panels come bundled with some bonus breakers, so that can be a savings), but the main breaker per se is irrelevant. You can't make the more convenient breaker trip first, because of main vs branch trip curves and Murphy's Law. Even if you needed a main disconnect switch, the trip value is irrelevant; feel free to feed a 225A subpanel from a 30A feed breaker. Totally legit.

Your feeder wire

2/2/2/4 is an unnecessary size, but I understand why it seems correct.

First, if it's convenient to run conduit from the main to subpanel, and you want to do that, use THHN/THWN-2 individual wires inside the conduit.

Don't bother with copper unless it's like a 10 foot run or something. The lugs the subpanel will be aluminum anyway because aluminum lugs play nicely with both. So why create a dissimilar metal "problem"? (isn't actually much of a problem on these fat feeders). But if you do go copper, #3 will suffice and #8 Cu ground is all you need.

If you go aluminum (most of us would), you need #1 AWG.

Loads

You're in very good shape. Your PC and tools will chomp only 15A of your 100A. Electric garage heaters range from 15A to 50A, but you're definitely covered, even with a 60A sub. But stay at 100A.

Here's why I don't think 100A in a garage is overkill. The automotive world is changing so very fast. "Range anxiety" which used to stop the electric car industry dead in its tracks, is being alleviated from both ends: First, consumers are becoming much more aware of their actual driving patterns. And second, Tesla and others are making is making enroute recharging look stupid-easy. I do lots of transcon trips, and Tesla is at the point where I could recharge in the time I already stop for gas, bathroom, and meals. So EVs are happening, so having 50-80A of headroom in a garage panel is great, and can only help resale value.

Answer 2

Without knowing your existing loads on the house , let’s look at your new sub panel.

First you will need a double pole 100 amp breaker in your main , your wire size is based on the 60 degree table per 110.14.C.1.A (100 amp or less / 14-1awg require the 60 degree table. So you are at #1 copper or 1/0 aluminum (aluminum is much cheaper and aluminum feeders pose no risk like branch circuit wiring did in the past). You want 4 circuits not a problem We would need the size of the heater electric heat will gobble up a lot of power but a 30a 240v heater will warm a well insulated garage. A branch circuit for lighting 120v 3 ea branch circuits for receptacles 120v If you install 20 amp in each of these with the 30 amp heater you are not even using 70 amps so you still have enough room for a 30 amp 240 v welder or air compressor . And a few 15 or 20 amp circuits more with a 100 amp panel and wiring. I would get at least a 125a panel with a main breaker.

You do not need a ground rod since the garage is attached but you will need a 4 wire feed as you had identified. With the 2017 code all receptacles will require GFCI Protection (even ceiling mounted lights or garage door opener) . Hard wired lights do not require GFCI protection. Get a larger panel than you think you will ever need , you said 4 and the heater , I would not get less than a 12 full sized space panel and 20 space will probably be the same price or have more choices thus cheaper for larger.

Hope this helps , note look at the table again and if you go to a 125 panel the #1 wire will you to use all of it because now you can use the 75 degree table. So there are many advantages to going larger (you could put a 150 amp panel or larger but at this point #1 wire will only support a load of 130a but finding a breaker bigger than 125 for your main panel may be difficult depending on the brand.