Hello, my current hot water setup is through a summer winter hookup in the boiler. The boiler is leaking. I need to save the money to replace the boiler, so I was going to install an electric water heater in the meantime. I have a propane stove in place of an electric range that we replaced. My question is can I use the circuit and wiring for the range for the water heater if I change the breakers in the panel from 40 amp to 30 amp? I would also have to add a workbox in place of the old stove outlet and run 10-2 from the box to the water heater. The wiring from the panel to the box is 6 gauge, so I am also not sure if it is OK to connect it to a different gauge.
Thanks in advance for any advice!

If I understand correctly you have 6-2 from the circuit panel to a box with a 40 amp breaker. You want to run 10-2 from the box to the water heater and change the breaker to 30 amps.

Yes, you can do that, it is the way I would do it.

That is OK. You can connect different gauges as long as the lower amperage is the protected source. Was the range, 120/240 or straight 240? The WH is 240. You can use the range wiring but if it is a 4 wire 120/240 setup, you would cap off the neutral.

Thank you very much for the quick responses! Yes, the range was 120/240. I was thinking eventually the range plug can be put back in the future if need be and what I wanted to do will avoid tearing out the entire basement ceiling to run a new wire to the panel. Just wanted to make sure I was not making a problem for myself. This will save me a lot of aggravation until I have the boiler replaced. Thanks again!

The only thing you might need to be careful with is wire nutting the #6 strand to the #10 solid. Make sure you get good contact with a big wire nut. You may not need a new workbox if you pull the old receptacle, add an extension ring to the wall box and then add a cover. I believe the extension ring has a knockout for the Romex.

Just out of curiosity, I happen to have some 8-2 that would reach from the connection to where the water heater would be. If I were to use that, could I just leave the 40 amp breakers as is? Or is that a potential problem? I realize it is overkill for what the heater needs.

I don't know if the HW heater wiring would handle 40 Amps, most are 4500 watts. If a short were to occur, it might fry the heater before the breaker tripped. You can run 8, but I would change the breaker to 30. Breakers are cheap and that is your protection, not the wire size. You could run a 15 Amp breaker on a #6 but couldn't run a 30 Amp on a #12, the breaker might not trip until the HOUSE FIRE consumes everything!!

I am not going to condone or recommend this... BUT...
I have seen dryer / range pigtails wired to an electric water heater to utilize the existing plug. The breaker was changed to suit the 30amp requirements.
Now this is the Ozarks and I have seen many other unusual methods of wiring, plumbing and construction. I'm just sayin'...

I happen to have some 8-2
I wouldn't mess with the 8-2. 10-2 is not that expensive, Electrically you can use it but its not worth the savings for extra difficulty pulling it and making your wiring connections.