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My garage has two 20 Amp circuits and an unused 10-3 w/ground connected to two 30 amp breakers.

I want to use those unused wires for a subpanel but i'm not sure how big of a panel could I install? I would like to add four 20 amp circuits. Will I also need a grounding rod?

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You can add as big a (sub) panel as you wish so the 4 circuits are not a problem but the circuits are limited as follows:

  1. no more than 60 amps of 120V at the same time

  2. no more than 30 amps on a single leg (there are two legs).

If the garage is attached to the house, the ground on the 10-3 should be sufficient and no grounding rod should be needed. Be sure to keep the ground and neutral separate in the (sub) panel.

For a detached garage, I don't know the details but a lot probably depends on how far away the garage is.

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Great answer - Just to clarify though the OP mentioned 4 20Amp breakers which exceeds your suggested 60Amp@120V . So David. you need to rethink what you would like to do with the new subpael.. –  ppumkin Jul 18 '12 at 10:41
    
i'ts fine to install 4 (or even more) 20A breakers in the proposed subpanel. The 2 30A breakers from the feed will protect the 10-3 feed and make sure that no more than 60A is drawn at any moment. –  mac Jul 18 '12 at 13:46
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The point of the breakers is to protect the downstream (i.e. outlet side wiring). The panel breaker is to protect the upstream wiring. (i.e. service side). The 200A breaker in my main panel isn't to protect ME. It's to protect the wire outside, and the transformer it's attached to. It's fine for me to have 800 A total circuits in my house, as long as each one is individually protected, and I don't try and use my Kiln, Lathe, Tesla Coil and Particle Accelerator all at the same time. –  Chris Cudmore Jul 18 '12 at 15:11
    
"60 amps of 120V at the same time" - this is misleading, as a 120V 40A load on one leg will (correctly) trip the breaker. –  Jay Bazuzi Jul 19 '12 at 5:09
    
Thanks for the clarification, Jay, I have updated my text. –  Philip Ngai Jul 19 '12 at 18:03
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It's true in a sense that you can add any number of loads to a 10/3 feeder as long as all the loads are not on at the same time such that the feeder is overloaded. However, the NEC code (NFPA 70) used by most US jurisdictions requires that in no case should the capacity of the feeder be less than the sum of the factored loads of all branch circuits supplied.

Summing the loads is not done by adding the breaker capacities together, nor just by simply adding the connected appliance loads. There are specific demand factors to apply based on the occupancy, load type, and other conditions. You can only exclude a non-coincident load from the calculation where it's clear that the load would never be applied coincident with another load, such as an A/C load and a heating load. Furthermore, loads must be so arranged to avoid significant unbalanced loading of the supply legs.

That said, you can generally add as many non-dedicated general recep circuits as you want, because they are considered to be part of the lighting load, which is calculated by area, not by number of receps. The number of receps is only a factor for how many are on one branch circuit.

So it is true you can add any number of general recep circuits, but to add anything else, you'll need to consult the NEC to determine if it could overload the feeder.

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Thanks to everyone for info, I am going with a panel that will take eight breakers but since the feeder is only 10-3 I will have to be careful not to overload the system. I am going with the larger panel because I like the idea of the 30 amp plug even though it won't be installed right away. The garage is detached and the box might be outside so I need an exterior box and two grounding rods ( is that mandatory or just good practice?). I have not planned for the number of outlets or their location but it will be done before I drywall the garage. –  David C Jul 20 '12 at 7:39
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You have the ideal situation for adding a subpanel. Woo-hoo!

As @Philip points out, you can use any size subpanel you like. Get one that's bigger than the minimum, so you have plenty of room to work inside.

You can choose either a "main lug" panel, which does not include its own main breaker, or a "main breaker" panel. In the latter case, the breaker can be any size, as the feeder is already protected by the 30A breaker in the main panel.

If the panel is mounted outdoors, it must be an outdoor rated panel.

In my jurisdiction, a pair of grounding rods are required if the garage is detached from the house. If it's very close, you can probably share the same grounding rods as the house uses.

Be sure to keep the ground bus and neutral bus separate in the subpanel . They should only be bonded in the main panel.

Consider adding a 30A 240V recep right off your new subpanel where you can plug in heavy equipment on short notice.

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Curious: What sort of equipment needs that sort of receptacle? I'm rewiring my house with 20 A circuits and not even bothering to use the 20 A T-style receptacles because I have yet to meet anything with that sort of plug, so the cost difference didn't seem worth it. –  Jeremy W. Sherman Jul 19 '12 at 16:03
    
@JeremyW.Sherman: Nothing you can't do on 120V/15A, but bigger versions of many machines. Welder, air compressor, table saw, electric car. –  Jay Bazuzi Jul 19 '12 at 18:06
    
Could the main breaker panel with a 50 amp breaker be misleading to someone who sees this breaker and does not realize that the panel is protected by a 30 amp breaker? Or should it just have a matching 30 amp breaker? Is there an advantage to this type of sub panel other than being able to shutoff the power at the garage rather than the main panel? –  David C Jul 20 '12 at 7:48
    
@DavidC, personally, i'd go with a main lug panel (i.e. no main breaker) for this application. No sense in redundant breakers at both ends of your 10-3 feeder, and if they're mismatched, it could be confusing for the next guy. Unless you have a real need to shut off all the downstream circuits together from that sub-panel (rather than killing them individually, or going to your main panel and shutting off the feeder), I'd keep it simple. just my $0.02 –  mac Jul 20 '12 at 21:44
    
@DavidC, please be more precise in your language instead of phrases like "this type of sub panel" or we'll all be talking past each other. For example, now you bring up a 50 amp breaker. That is news and not in your original question. I don't really follow where the 50 amp breaker is used. If it is in your main panel protecting your 10/3, that would seem at least two sizes too small. –  Philip Ngai Jul 20 '12 at 23:33
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