Adding Electrical subpanel?

It is cold out in the garage and I am wimping out when I can see my breath out there. In the summer it gets way to hot and I will not go out there either so I need to get some more power to the garage for heat/AC and other things in the future. I have tried the little space heaters but with high ceilings and 3 exposed walls they just don't seem to put out enough heat. I am not a fan of the propane/kerosene fumes and we don't have gas running to the house so electric/heat pump it will end up being.

Currently my power comes into the garage but all the breaker slots are filled (most are even tandem breakers). 14/16 is the hot water heater.
20 is an outlet I added right below the box.
All the even numbered ones on tandem breakers are lighting/outlets/whatever in the house.

The main breaker is a 200A breaker and the panel is a square D QO panel(qo20-40m200). Since in general I hear good things about the QO panels I was planning on sticking with that type unless there is a good reason to go with some other one.

I was thinking about buying something like this. http://www.amazon.com/Square-Schneider-Electric-24-Space-24-Circuit/dp/B00KHVM3E8/

Installing it right next to the current panel and moving the microwave and garage outlet to it and adding additional circuits to that. On the new panel removing the neutral bonding screw, adding ground bar, Hooking up with appropriate size wiring. Is it as easy as that? Can I just knockout one of the side knockouts on the old panel and line up the new one with that and drill thru the 2x4 between them to run the wire?

I would like to add.
1. 240V Heat/AC (Garage)(2 spaces)
2. 240V air compressor (Run to basement)(2 spaces)
3. 240V welder (garage)(2 spaces)
4. More outlets (garage)(2 spaces?)

The garage is 19'x23' and ~11' ceilings and has some insulation but need to be finished insulating. I was looking at one of the 5000W heaters like this . I don't need short sleeves temp in the garage just not freezing my hands and feet.

It looks like yes. If you have a proper 200 amp panel (I worry about the size of the feeder, it looks small in the picture.) Removing one of the 2 pole breakers and replacing it with a 100 amp. Going out the top knock out (looks easier to do) with a short piece of 100 amp rated SER cable and into the next stud cavity to your sub panel. You wouldn't use a main breaker panel but this instead http://www.amazon.com/Square-Schneider-Electric-QO11224L125GC-24-Circuit/dp/B0002YSP1U/ref=sr_1_1?s=hi&ie=UTF8&qid=1453639942&sr=1-1&keywords=24+space+main+lug+qo+panel Just make sure to use anti-oxidant on the aluminum wire connections.

You need to discuss amperage, not voltage.

Yes, the wiring is much like you described, but you have not described the load- neither the existing load, nor the added load.

It appears you have much more than 200A in breakers in that panel (though I can't see). If so, you may need to start putting a load meter on some of the circuits to see if your breakers are oversized (and can be down-sized).

If it was a shop subpanel, I'd be pretty comfortable with overloading it (if it is a 1 man shop, you are not going to use the welder, table saw, and plasma cutter at the same time), but it's not. It's a house panel, therefore you have no control over when the load kicks on or off.

No one can answer your question without knowing the load.

This is all good advice. Study up on the current demands for the equipment you want to add, this will give you a good direction to go in.

While you are at it I would pull the meter and Noalox the feeder terminations in your existing panel. From the picture it doesn't look like this was done. Turn off the main in the panel before you do this!

Current panel looks like this as far as the breakers in it.

The garage includes the two openers. While checking them today I found a few that I could not figure out what they went to so I left them off until I figure them out. They do have wires running to them so something should be on them.

Lighting is all LED/CFL/fluorescent. The house has an electric range, heat pump with supplemental electric heat.

As for size of the wires I can't find anything on the feed wires saying the size but for reference the smaller wire in the back here is 2 AWG.

What I would be adding.
1. Heat/AC - 21 amps for the linked heater any AC should be less than that.
2. Air compressor - 15 amps (most of the ones I am looking at are around 15 amps but a few are a little higher)
3. Welder - ~20 amps? - Don't currently have one but that seems to be a decent range there.

bikerbenz - My understanding is that you can run a main breaker panel as a sub panel?. I was looking at the one I posted because it has more spaces. The one you posted is 24 circuit but only 12 spaces and just what I am looking at adding would fill 8 of the 12 spaces of the panel because the 240V circuits take up 2 spaces each. That is the same problem I have on my current panel (20 spaces 40 circuits rated) so I would rather have a few extra spaces when done in case I need to add in the future. Since most of the stuff I seem to want to add is 240V I would rather have the extra spaces over one rated for tandem breakers.

Wire gauge of the feeder will be printed/embossed on the cable sheath, you can see a little bit of the writing in that picture.

So, you have 510A of breakers currently in a 200A box, and would like to add a 100A subpanel?

Don't go by the rated amperage of all the breakers in the panel and add them up. That's not how it works. Every panel in every house I bet has more than the total rated amperage of the main in its branch circuit breakers. Heck an electric range alone is half of my rated main breaker. It's not what they are rated it is what they are using. There is an alarming amount of tandem breakers in that panel, maybe it was made to do that I don't know, but I have never seen that many in a panel before. I would add a second 100 Amp sub-panel for that reason alone, but moving a few of those circuits could get dicey with the fact that the cabling may be too short to move to the new panel. I don't see a problem with adding the circuits you want to your existing service but you will need more space and therefore a second panel in my opinion. These are not loads that will be drawing very much on a constant basis that you should worry about uprating your service size. Also it appears that there is already Noalox on the main cable terminations if you look at the close up shot of the lugs. Hire somebody to do this work though, please.

Oh by the way what sort of furnace uses both a 60A two-pole breakers and a 25A two pole breaker? I have to see this furnace. Does it heat the house with the flames of hell? I think my boiler is on a single pole 15A circuit.

In reply to Mazdax605:

Since the panel also says "Heat Pump", I'm pretty sure those are strip heaters.

Or, there is more than 1 heating unit (which would be a pretty good reason to reconsider adding a sub-panel to a 200A service).

In reply to Mazdax605:

And yes, I agree. The actual load means more than the rating of the breakers.

Since we have no info on the actual load, I was just pointing out that there is a potential issue here that should be confirmed LONG before suggesting it would be easy to simply tack on another sub-panel.

In reply to Mazdax605:

This sounds like one of those weird all electric houses.

Is the garage insulated? That will make a huge difference with whatever heaters you use. Mine's insulated to better than house code, and it takes very little to keep it at a comfortable working temperature.

In reply to Turboeric:

Good catch, in his climate a well insulated attached garage might not even need heat. Or at least no more than can be provided by leaving the house door open.

Yes all electric house. I did a residential load calc and came in around 150 amps currently.

The garage is partly insulated (doors are, House shared side is, other side is mostly, ceiling is mostly, rear and front are not except the doors). I might need to do some more sealing around the garage doors since this is what it looked like inside during the big snow.

The garage has high ceilings and the floor is 3+ feet down from the house door so with the hot air rising I don't get a lot of heat into the part of the garage I need it in even with the open door.

Since the current box is rated for the tandem breakers in all slots I am not worried about those just that I am out of space in that box.

Just an idea for the future. Add a ceiling fan or two in the garage. That'll help pull the hot air down. You'd probably need less heat than without.