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I have a wired garage that has 3 wire 10 gauge going in to it.
In the panel this wire is hooked up to 2 individual 15 amp breakers.
I want to know if i can put in a 30 amp breaker in that spot.
The Garage has a 240V heater and 115V plugs and lighting.
I just want to clarify what you're asking.
You have a sub-panel in your garage. How large is the breaker feeding this sub-panel? (located in your house)
This sub-panel in the garage, has two 15a breakers, one of which is connected to the red, one to the black wire in a run of 10-3 wire.
What does this circuit power? I'd assume your 240v heater? Is there anything else connected to this circuit?
Since it's 10 gauge wire (as I understand from your post), it can be used with a 30a breaker, but it will need to be a 240v 30a breaker, there can't be any branches off this circuit with smaller wire, and only if your heater is rated for 30a.
Now, do you need 30a? What is the requirement for your heater? There's nothing wrong with oversized wiring, and if you're not tripping any breakers, it might not need to be changed. YOu WILL need to make sure both 15a breakers are tied together, but that's it.
No, it seems like each phase is wired to 15A putting a single 30A breaker wold pull 30A from one phase, only give you 115V and pull no current from the other phase.
If the two individual breakers are not tied together (e.g. both levers are physically connected), then your current setup is incorrect and unsafe. Both hot wires need to be tied together so they won't be tripped/switched off independently. You should replace the two individual breakers with a 240v breaker.
It is only safe for said 240v breaker to be a 30 amp breaker if all wires and outlets on the entire circuit are 10 gauge. If your outlets are connected with 14 gauge wire (likely), then it is not safe to put in more than 15A breaker. The outlets themselves are likely NEMA 5-15 sockets, which are rated for 20A and thus would not allow more than a 20A breaker. To use a 30 amp breaker, you would need to replace all outlets with NEMA 5-30 sockets and ensure they are connected with 10 guage wire.
