Help with sub-panel

So I would like to set up a new grow in an outbuilding. Unfortunately, its not really going to be possible to run new wire to the site, the neighbors would likely find it suspicious. Currently there is an underground 12/2, 20 amp line going to the site. Is there any reason why I could retro-fit it to carry 240v to a sub-panel at the site? I'm not an electrician so please bear with me. Id like to run 2 1200 watt ballasts (4 600 watt) along with some fans and pumps. The total setup should stay under 40 amps. Would it be safe to rewire the line? Current code requires that a 240v wire have both a ground and a neutral but is this absolutely necessary, I know it was done with only three wires for many years? I intend to run it off of a main panel with a 40 amp breaker and have 4 15 amp breakers at the sub-panel. Assuming that all of the electrical junctions and wiring is done properly, I just want to know if my plan to use the current 12/2 is (relatively) safe?

I've included a little diagram of my plans, sorry I'm no schematician (probably not a word).

bumpitty-bump-bump

12/2 is not going to cut it. This is what I would do. I'd measure the distance from the panel to where your going to put the subpanel. I.E 50 feet, add 10 feet because you will need it. You are going to need a 8/3 wire for an 40 amp subpanel. If not you will pop breakers all day long. Start at the panel and disconnect the wires leading to the outhouse and duct tape the new 8/3 wire to the old wires, now go to the out house and and disonnect the out and get to the wires. Start pulling the wire until you get your new wires. Wire up your subpanel and the go back to your panel and wire that up. This is provided that your wire is ran in conduit under ground like it should be per N.E.C

nope, go get some 6/3 uf direct burial, and run a 60amp 2 pole breaker from main panel out to the sub, , and if you are drawing 40 amps total, your main breaker feeding it must no exceed 80 percent of your FLA

riproor said:

nope, go get some 6/3 uf direct burial, and run a 60amp 2 pole breaker from main panel out to the sub, , and if you are drawing 40 amps total, your main breaker feeding it must no exceed 80 percent of your FLA

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yep VV

Anybody have any diagrams involving subpanel wired to a heavy duty timer with a stove cord?

tmacfl said:

Anybody have any diagrams involving subpanel wired to a heavy duty timer with a stove cord?

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A 'sub panel' doesn't need to be wired to a timer. You would run a circuit for the timer from the sub panel along with any other circuits you want. I'm not sure I understand your question, can you expand a little. VV

That 12/2 is not even close to good enough. I believe 12-2 UF is rated at 25amps, not sure maybe 20A. You really need to up the wire size, 6awg or 8awg may do the trick, How far is your spot from the electrical panel. Would you be able to run a new wire from the panel to your spot. I'd try making friends with an electrician