adding 220 line

Mikejr05 said:

Can I add a 220 line to an existing 30 amp dbl pole breaker with an existing 220 line. panel is 100 amp with 4 20 amp breakers and 1 30 amp dbl pole and 1 40 amp dble pole.

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You could replace the existing 240V line with the new one on that 30A breaker, but you cannot attach both to the breaker at the same time, no. (Called double-tapping; generally not allowed unless the model of breaker is listed for it. 30A and up are [AFAIS] never listed for it).

If you have the space in the panel, you'd have to add a new breaker for the new line.

well do I need to have 30 amp dbl pole or will 15 amp dbl pole work don't want to overload panel

Mikejr05 said:

well do I need to have 30 amp dbl pole or will 15 amp dbl pole work don't want to overload panel

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That depends entirely on what is on the circuit.

What are the other circuits?

using line for ac window unit

well only using line for ac window unit 220 plug , what do you recommend

Mikejr05 said:

well only using line for ac window unit 220 plug , what do you recommend

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What is the load of the A/C unit? (In Watts or Amps).

1 30 amp dbl pole and 1 40 amp dble pole

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What is on those 2 circuits?

And are the 20A circuits all for lights and receptacles, or do you have any appliances permanently connected to those?

Mikejr05 said:

well only using line for ac window unit 220 plug , what do you recommend

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OK Mike. Let's try this from the top.

You have a 240v-30a circuits and receptacle being used for an A/C unit. Correct?
You want to add another 240v receptacle to this existing line. Correct?
For what? What is the draw or circuit requirement?