what guage wire do i use between an electrical box of 200 amps and 120 amps?

Question

putting in a "on demand hot water heater" it takes a separate box of 3- 40 amps breakers. I need to go to the main box with "what gauge wire"?
Do I need to install a breaker in the main box as well? If so what amp breaker should it be? The boxes are approximately 15' apart.

Answer 1

From the way you ask this question, I would most strongly
advise you not to attempt to do this on your own.
You need to learn far too much to safely instruct you here.

A 40A. load needs # 8 Awg. copper wire.

Each 'hot` wire is protected by a breaker,

a neutral, (white), wire goes to the Neutral bar in the panel.
a bare, (Ground), wire goes to a different bar.
The two are NOT interchangable.

If I read you right, you need 3 wires for the heater.

Read the documentation that comes with the unit
and the UL nameplate on the unit itself.

Most residential heaters are single phase 240V.
and would require two hots and a ground
and will be marked "240V. or 120/240V. 1 Phase" -
and will need two Breakers

If you do have a three phase heater, it will be
marked "208V. or 120/208V. 3 Phase".
and will need three breakers

The Nameplate on your service box must match
the one on the heater in regard to the above.

Answer 2

You can use #8 AWG copper wire for each of the 40 amp breakers.

Answer 3

I think there is to much about this that you don't understand. Hire an electrician to connect it for you. Be careful taking advice on here from well meaning, but untrained people. There is much more to proper wiring than the wire size. Real electricians are much cheaper than fires.

Answer 4

If we are talking about three phase current you have to have three phase available. I would check with a licensed electrician. My answer was based on, I dont know of many residences( single family homes) wired with a three phase service. Maybe I am not understand your question.

Answer 5

is this on a three phase system?
if it is then you can use #8 wire