I have 220 V Electric line 30 amp. Can I convert to two 110 Lines using the same common wire?

Who gave Ron a thumbs down? He has the closest answer.

I assume you have 2 hot wires and a ground for a pump. You cannot call the ground a common wire. I've never heard of a neutral and 2 hots for a motor, but if that is what you have you can have 2 circuits. The bottom line is what is the wires you have now, not what is it being used for. If it is a cable, the 2 wires are white and black and you can use that for one circuit, 20A tops for feeding your lights and receptacles. If it is in conduit (2 black wires for example) you cannot "mark" a wire as hot. That's the part of Ron's answer that is wrong. Remarking conductors hot vs. grounded is restricted, and making a non-white wire a grounded conductor (neutral) is never allowed for smaller conductors.

To have 6 15-20 amp breakers for the new room, you need a breaker panel in the new room, and use the 30A as a feeder. I don't think that is big enough if you really think you need 6 circuits in the new room. Also, what size is the wire? 30A needs 10AWG or larger, but that pump could have used smaller wire. You might not even be able to use 30A for a different purpose.

Since you are calling the ground a common wire, putting in a new panel I would say is beyond what you should be doing. So I recommend getting an electrician.

Yes, depending. If you have three wires coming in, you should have two hots and a ground. If this is the case, you will have to use one of the hot wires as a neutral (mark it clearly as such and change its location in the panel.) If possible, move the old wiring up to approximately eye level and set a single pole 30 amp 6 circuit sub panel. You can feed the rest of the rooms electric from that source.

Even though you have 6-15 amp breakers in this panel, you will generally not draw more than 30 amps unless you are running an AC and many lights and other household applainces.

Each 15 amp breaker will protect each branch from overload, and the 30 amp breaker in the main panel will protect the entire room.

If you have four wires, (two hots, a neutral, and a ground) then you can add a 6 circuit 2 pole subpanel. It will still utilize the same principle, but will not require the moving of one hot to become a neutral.

You should always have a ground. Do not use a ground in place of a neutral.

You can convert to two 110 lines but it would not meet code to do it that way. The best thing would be to replace the dual breaker with a single 20 amp breaker. Then only use one of the lines and the nuetral , leaving one line unused. use 12-2 wire to continue the new room. Or totaly remove the 220 three wire and start fresh.

Whoever gave him thumbs down is the type of person I DO NOT want near my home, after reading the question, I was going to suggest setting a sub panel as he did, 2 thumbs up to him!

I agree with Ron. And since when can't you remark a line? What do you think all those colored tapes are for?

Sounds very reasonable to me.