Water Heaters - tankless water heater

i am installing a tankless water heater and the electrical requirements are 2 40 amp 240V breakers. they suggest 6 awg wire but say 8 gauge is sufficient.
my question is on the wiring of the breakers. any help would be appreciated.

Just my DIY'ing thoughts:

Advantages of #6:
1. The longer the run, the more the benefit of heavier wire to reduce voltage drop.
2. If it's a short run, the extra cost of heavier wire is probably negligable.
3. Some tankless heaters use 2 50A breakers, and would thus require #6 wire...if you ever replace it with one that uses 50A breakers, you'd have the wire.

Advantages of #8:
1. Cheaper.
2. A little easier to work with since it's thinner.


I'd look at the cost savings...unless it is significant, go with the recommendation rather than the minimum.

the run is 40FT. my problem also is with the wiring instructions.
http://www.controlledenergy.com/pdf/ae115_125%20manual_EN.pdf
so in my run will there 4 wires or 5 wires? i also need to know how to wire the breakers, what color of wires to run, etc.

You need a ground wire.

If you use NM type cable, then you will have two ground wires. If you use conduit and individual conductors then you only need one ground wire.

How you connect the wires depends on whether you use individual conductors or two runs of NM.

Please do decide if this is something you should be tackling yourself. Electricity can kill and cause fires if not done properly.

is there a sticky somewhere.... perhaps with diagrams on wiring 240v single phase

I think the diagram would be too trivial to create. Two wires, both connected to the breaker, running to two connections on the tank. Plus the ground wire.

look on th einside of your breaker panel to make sure the ground slots you use are the proper ones...