Can I split 30 amp breaker with 10 g wire into a sub panel with 2 15 amp breakers and 12 g wire?

Question

I want to run new electric to a new garage. I was hoping to have 2 lines, one for lights and 1 for outlets. What would be the best way to run this from my main panel in the house about 80ft away?

Answer 1

There are two separate issues here:

Subpanel Feed

A subpanel feed can be pretty much any size you want, as long as it is large enough to do what you want it to do. The feed is normally 240V from a double breaker. A 30A feed means 30A @ 240V, which is actually 60A @ 120V if balanced. So yes, 30A is enough to feed two basic garage circuits. The wire must be sized appropriately, so that means 10 AWG or larger. But consider a little more - explanation below.

Subpanel Size and Type

A subpanel in the same building does not need a disconnect. However, a subpanel in a separate building, like this detached garage, needs a disconnect. That can be a separate box, but the simplest thing is to just use a "main panel" as a subpanel. There are four differences between typical main panels and typical subpanels:

• Neutral/ground bond - add for main panel, remove for subpanel - easy.
• Ground bar - must have a separate ground bar in a subpanel, optional in a main panel. If a panel doesn't have one, it is easy (and inexpensive) to add one.
• Main breaker - a main panel has one, a subpanel generally doesn't. But if you have a subpanel in another building it needs a disconnect and a main breaker is one of the cheapest (main panels bundle everything together) and easiest (it comes pre-installed) ways to get a disconnect. It can be a 200A main breaker with a 60A feed - doesn't matter as long as the "main breaker" is >= the size of the feed breaker in the actual main panel.
• Size - subpanels are typically 4 - 12 spaces. Main panels are typically 20 - 40 spaces. But they all need the same empty space in front of the panel (30" x 36" space kept clear) so the extra breaker spaces don't affect the physical space you need (except a few inches in the wall, but that doesn't matter).

A subpanel can have anywhere from a few circuits on up to dozens. Generally we recommend more spaces for breakers so that you don't have to replace the subpanel in the future. You only need 2 breaker spaces right now (each 120V circuit takes 1 space, each 240V circuit takes 2 spaces), but anything less than a 12 space panel just doesn't make much sense as the cost is minimal. But a 20, 30 or even 40 space panel can make sense - perhaps $50 to $200 extra but then you have lots of room for expansion.

The two circuits you need right now are actually:

• 20A GFCI-protected receptacle circuit. This is required. An old garage might not have this, but you are effectively building (or at least electrifying) a new garage, so you need to meet current code. The receptacles can be 15A or 20A receptacles.
• 15A or 20A for lighting. It can power additional receptacles, but it must be a separate circuit from the first 20A circuit.

A 15A circuit can use 14 AWG or larger (e.g., 12 AWG) wire. A 20A circuit can use 12 AWG or larger wire.

Why more?

A 30A subpanel feed can easily power 2 x 20A 120V circuits, and in fact depending on usage could generally handle 4 x 20A 120V circuits. However, I would not stop there. I would seriously consider a 50A or 60A feed, provided your main panel/service has the capacity for it. Why? Electric vehicle charging. EV charging does not require 50A or 60A by itself (despite what some people seem to think). But 20A @ 240V or 30A @ 240V is typical for EV charging. If you allow for 30A @ 240V (for EV charging) plus 20A @ 120V x 2 (your required circuits) you essentially have a 50A feed filled up. Bump it to 60A and then you have some extra room.

You may even want to consider using aluminum 2 AWG wire (4 wires actually, details get complicated, but any subpanel feed is 4 wires - hot, hot, neutral, ground). That will allow for up to 90A. You don't need to put in 90A now - and in fact you may not be able to do so if you don't have service and main panel capacity for it. But running that cable or wires now gives you room for the future in case you want to put in a workshop or HVAC or other things.

But minimum, if possible, 50A. That gives you room for EV charging.

What kind of receptacle for the EV charging? Actually, none right now, and most likely when you get an EV you can put in a hardwired EVSE (a.k.a., charger). That relates to GFCI requirements. Any receptacles in a garage need GFCI. EVSE does not need GFCI if it is hardwires. So get a big panel and a big feed and then you can EVSE when you get an EV.

Answer 2

Before COVID shortages, I would have told you to run 2-2-2-4 feeder (90A) since it was about the same price as 10/3 UF cable (30A). Now, things have gotten weird, so it's not quite the same price anymore, but the logic still holds. The hard/valuable part is the wire route - once you have that, run the fattest wire that is cheap. Large aluminum feeder is proven safe at these large sizes - they never suffered the incompetent-installation scandal the small wires did.

Now why on earth would someone want more than 30A in a garage? No reason at all, REALLY... but all the EV newborns think they need a big honkin' 50A circuit to drive electric, and nobody ever lost money catering to -- well, for the sake of our Be Nice Policy, let's just say you'll get higher offers on your house if it has a whole bunch of power in the garage. To be ethical, it'd be real nice if that were in conduit, so they could add the necessary communication wires for an EVEMS if it is needed to shoehorn EV charging into their Load Calculation. (no one ever needs a service upgrade to charge EVs.)

Also, a lot of interesting things are happening with home solar-battery and Vehicle To Home (accelerated by a California mandate for that)... and having a nice conduit between home and garage would make that stuff easy/cheap to install. Even if it's just a PowerWall type thing, nicer to put it in the garage, yes?