I am installing a tankless hot water heater and I have two options, 2 - 60amp dual pole breakers or 3 - 40 amp dual pole breakers. I have 200 amp service into the house. Is there any advantage for one over the other? Does the 3 - 40 amp system spread the load better than the 2 - 60 amp? I heard sometimes lights may flicker under a heavy draw, does either situation stop that?

Without knowing any more than you have told us, I would prefer the (2) 60 amp branch circuits. My guess is it will be less expensive. Yes, the 40 amp version would spread the load somewhat but I don't see any sizable advantage. Will either version cause flicker?
Hard to say from my desk... many more factors than just one piece of equipment.

Why electric water heater? Why not natural gas?

Natural gas is not available in my area of Florida. Thanks for your help. Any other questions I can answer?

What other loads are in the house?
Did your electrician do a load calculation?

That is a big unit. Typically ones that size cannot be used with a 200A service in an average house.

Large loads in the house are the AC, cooktop and dryer. No load calculation was done, I will have that done. The manufacturer did recommend 200A service in the house for the unit.

Does the length of wire in a house cause any power degradation? For example, I need to run 3 lines of 8 gauge wire, I can run them from the service panel, up to the attic and back down to the heater, about 40 feet total. Or I can run it straight from the panel, through the wall studs, to the heater, about 13 feet. Does that difference in wire length affect the draw going to the heater?
Thanks.

Voltage drop does not begin to be a real factor until you go a distance of 50' or more. Usually the threshold I use is 100 ft.

There will be a bit more voltage drop with the 40' but pretty minimal with #8. Shorter is always better. This is a 240 VAC circuit?

Needshelp,

For the record, the NEC Electrical code does not allow parallel conductors (three #8 AWG 40 amp circuits for the 120 amp load) of less that #1/0 AWG.

If this is the circuit you are setting up for your tankless waterheater.

Needshelp,

For the record, the NEC Electrical code does not allow parallel conductors (three #8 AWG 40 amp circuits for the 120 amp load) of less that #1/0 AWG.

If this is the circuit you are setting up for your tankless waterheater.Don, this is not a case of parallel conductors. These units require one, two or even three separate circuits feeding them.

Needshelp,

For the record, the NEC Electrical code does not allow parallel conductors (three #8 AWG 40 amp circuits for the 120 amp load) of less that #1/0 AWG.

If this is the circuit you are setting up for your tankless waterheater.

Thanks Don, but what does that mean, parallel conductors of less than #1/0 AWG?

Thanks Don, but what does that mean, parallel conductors of less than #1/0 AWG?
This does not apply to your installation.

Stan,

If the heater requires a 120 amp feed and they get this by connecting either two 60 amp branch circuits or three 40 amp circuits, how can this be anything but a parallel connection?

If they used a 120 amp feed then it would not be patrolled connectors.

I'm asking a question of you, not arguing with you.

Needshelp, can you give me the model number and the manufacturer of the water heater. I'd like to look the specs and installation instructions up. Just trying to learn something new.

Stan,

If the heater requires a 120 amp feed and they get this by connecting either two 60 amp branch circuits or three 40 amp circuits, how can this be anything but a parallel connection?

If they used a 120 amp feed then it would not be patrolled connectors.

I'm asking a question of you, not arguing with you.Because it is not a 120A feed. It is two separate 60A feeds, or three separate 40A feeds.

Stan,

I did a search and found a Titian tankless water heater that was 120 amp.

From the installation instructions, I can see why this would not be a "parallel connection" because each branch circuit feeds a separate load within the heater.