Running service from house to garage

I am running service from my house to a detached garage through 90' of buried conduit. I will be operating flourescent lights on one circuit and outlets on another. The outlets will power a fridge, an occasionally a battery charger, small tools, space heater, etc... Initially my plan was not to install a sub panel in the garage, but rather run two separate circuits from the service panel in the house to the garage. In the future I'd like to have the option to expand my voltage in the garage to 240V but at the current time this is not needed. I think I may have made an early mistake that is not to late to fix. So far I have run 2 lines of 14/2 romex through the conduit. I understand that this is only 15 amps on each circuit and I was thinking I should use 20 amp circuits (I know I should have used 12/2 wire). Howver, I purchased the 14/2 and don't have any 12 guage to use. If I leave the 14/2 in will this be enough to power the garage effectively? Also, does this give me the option to create a 240V circuit in the future? If I did decide to switch to 12 guage wire would it make sense to use a single 12/3 wire hooked to 2 separate 20 amp breakers at the main service in the house and split these in the garage to the two 20 amp circuits through 12/2 wire? Both circuits in the garage will be protected with GCFI outlets.
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A buried conduit is considered a wet location, except possibly in dry areas. Romex has type thhn conductors in it, which are NOT for use in wet locations. You don't give the size of the conduit, but without installing panels or grounding rods, you can run two 20 amp circuits, which could be accomplished with a 12/3 UF cable or four individual # 12 THWN or similar conductors. This would give you 20 amps 120/240 volts and would be adequate for what you've described

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The conduit is 3/4"
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If it's a decent, fairly straight and clean run, you could pull 3 #8 and 1#10 for the ground,THWN conductors, use a six circuit panel two ground rods connected with #8 and a 40 amp breaker in the main panel

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To run an electrical service to detached building, you didn't necessarily have to pull a ground conductor with the feeder, you could establish a new ground at the detached building by driving ground rods, which now is a requirement for any service larger than two 20 amp circuits. The down side of establishing an independent grounding system at the detached building is that you can't run anything between the buildings that could become a ground path

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