I have 2 6-3 for a 60ft run and 1 6-3 for another run. Question, what size conduit do I use. This circuit is for a tankless water heater. All will in my garage which is drywalled.

While there is a calculation to determine the size conduit for one cable, I would not recommend pulling any large cable through conduit for any great distance.

Since your using conduit, you should be sure the conduit system is complete with all necessary fittings from point to point, and pull in individual wires using THHN or XHHW insulation.

If you insist pulling #6-3 cable through conduit, give me the outside dimension of the cable, or better, the type and brand cable, I can find the manufacturer's spec sheet, from there I can do the conduit fill calcs.

Also, are you sure your existing service can handle the high amp load of these water heaters?

While there is a calculation to determine the size conduit for one cable, I would not recommend pulling any large cable thru conduit for any great distance.

Since your using conduit, you should be sure the conduit system is complete with all necessary fittings from point to point, and pull in individual wires using THHN or XHHW insulation.

If you insist pulling #6-3 cable thru conduit, give me the outside dimension of the cable, or better, the type and brand cable, I can find the manufacturer's spec sheet, from there I can do the conduit fill calcs.

Also, are you sure your existing service can handle the high amp load of these water heaters?
I am Installing 2 Heaters in Parallel, this is supposed to be required for each Heater. Have a 200 amp service.
Wire is marked as follows:
E18679 (UL) Romex (R) SIMPULL (TM) awg 6 cu 2 cdr w/awg 10 grnd TYPE NM-B 600 volt 9/07/07

You should have a flat cable, the larger dimension is .693", and one cable can fill up to 53% of a conduit, so you need 1-1/2" conduit.

You can staple or strap the cable to the upper parts, out of harms way, in the garage, suing short pieces of straight conduit as protection. Even fairly longs runs, as long as it is straight. Believe me, if you try to pull this through a conduit bend, you will have a great deal of problems.

You should have a flat cable, the larger dimension is .693", and one cable can fill up to 53% of a conduit, so you need 1-1/2" conduit.

You can staple or strap the cable to the upper parts, out of harms way, in the garage, suing short pieces of straight conduit as protection. Even fairly longs runs, as long as it is straight. Believe me, if you try to pull this thru a conduit bend, you will have a great deal of problems.

Thank you very much for the input, I truly appreciate it.

Gene