Hey gents, I've got a question.  My parents have an electric tankless water heater, an Envirotech esi-2000.  The unit is rated at 240V and 120a.  It's got 2 power-in feeders on the unit itself.  Presumably,

these feeders are supposed to be run to 2 separate 240V 60+ amp breakers in the panel.  In their case, I believe they have a single cable run from a 240V 100a breaker in the panel to a j-box in the attic directly above the

tankless heater.  From the J-box I believe there are 2 separate cables run to the power-in feeders on the tankless WH.  They don't seem to be having trouble with tripping breakers or anything, but shouldn't

the breaker in the panel be sized appropriately, i.e. 150a.  Is there any inherent benefit to running each of the line in power-feeders for the tankless unit on separate 240V breakers rather than the way it's run now

with a single 240V breaker split to two individual lines?  If I can, I'll get some pictures up to make this a little more clear.

The tankless may have a single circuit feed accessory that will enable it to be fed from one circuit instead of two.  You will have to check with the manufacturer for this.  If it is available and the wire is correctly sized for 120 amps you could run the wire straight from the main panel to the tankless.  If there is no such accessory available, and the wire is sized correctly, you could install a small 240 volt only sub-panel with two 60 amp breakers next to the tankless.

All this is if the wire is sized for 120 amps.  I'm an HVAC electrician so I don't have much experience sizing wire, especially that big.  But it's going to need to be some really big wire.  

Breakers are designed for an 80% continuous load.  Look for the circuit ampacity on the equipment rating plate.  Each 60 amp circuit is probably ~45 amps.  90 amps on a 100 amp breaker is too much.  You cannot increase the breaker without increasing the wire size.  The breaker is there to protect the wire from overheating, melting the insulation, and causing a fire.

So I had my dad send a few pictures.  I will take some more next time I go down to their house.  The Electrician used 6/3 to the tankless heater, but I'm not sure what he used from the
panel to the J-box I'm assuming is above the WH somewhere. When I go down, I'll figure out more about that piece of it.

On another note, the manufacturer of this WH is no longer in the business of selling tankless heaters.  They sold out several years ago, and the new company does not support or supply
parts for the models sold by the previous manufacturer.  My dad should be bringing the manual by sometime later today.

Here is the rating plate and model information:


Wiring at the water heater:


Close-up of the wiring at the water heater:


And another:


Here's the breaker in the garage panel:

Some panels have a 100 amp per stab limit.

The contacts for the plug in breaker do not have enough area.

Ain't kosher in my book, that 100 amp breaker is too big for #6 wire and I don't believe you can sister them up on a single breaker as you've suggested they've done. Also the ground wire on of the 6/3 runs is not connected - more bad stuff.

I have an Eemax electric water heater that is fed via three double pole 40 amp breakers. I had a single conductor melt it's jacket due to a loose connection on a lug at the contactor yet the breaker never tripped! These type of units pull lots of amperage and the wire needs the correct over current protection & grounding too.

That's what has me asking questions.  This all started when I noticed the lights at their house pulsing when the WH was running.  It's something I've always noticed, but if finally
bothered me enough to do some looking around  I'm thinking there is more to the problem than  what I'm talking here.  

Personally, I think they need their service upgraded and this placed on it's own 200a sub-panel.  Problem is, I'm having a hard time convincing my dad of it.
When I got curious about the pulsing lights and saw how this was all wired, bells, whistles, and red lights starting going off in my head.  Hence the questions.

As for the service upgrade, when their house was built, the electric company laid enough conduit to pull additional wire to feed more, but dad decided to rob a portion of it to run
a line to his shop instead of trenching up his yard.  Guess he didn't think he'd ever need the additional power.  

At this point, I'm thinking of telling him he needs to re-run wire to the unit, sized appropriately, to two separate 240V 60A breakers in the existing panel.  I'm also half interested
in renting a FLIR unit and checking for hot spots.

I don't know if I would call the person that wired that an electrician.  Looks like a real hack job.  Wires not secured where they enter the cabinet and no protective conduit on wires between the heater and ceiling.  You need new wires from the panel to the water heater.

The guy that wired the house initially was a very young electrician.  I cannot even begin to describe how many screw ups there are/were electrically in their house.  To
be honest, this doesn't surprise me in the least.  I had a feeling that everything needed to be re-run back to the panel.  I haven't been back down to look in the attic, but
I'm afraid of what I'll find.  The wire down to the unit is just poked through the ceiling.  They used NMB 6ga from what I believe I will find to be a J-box in the attic.  From
the J-box to the panel, there's not telling what I'll find.

Okay, so I was able to get to the attic yesterday.  The electrician that wired this originally used 2 AWG TC cable to run from the main panel to a
2-gang outlet box in the attic.  In this outlet box, he used split bolts to connect the 2 AWG to 6 AWG which runs to the tankless heater.  The
2-gang box in the attic, along with several others I found did not have covers overt them.  This particular box was full of insulation.  



So the plan moving forward is to wire a 125a sub-panel about 20 feet from the main panel and use two 60a disconnects at the tankless heater.  The breaker
in the main panel will be upgraded to 125a also.  I've read the TC wire that was used is not supposed to be used for indoor feeders.  I'd really rather not have
to pull new wire into the main panel.  Is the 2awg TC cable okay, or should it be upgraded.  I know It's already borderline sized wrong with an ampacity of 130a.

Check the listing on the panel to make sure it is rated to take 125 A breakers.

Many are limited to 100 amps.

I don't understand why we don't treat this WH as any other appliance and feed it via two 60 amp double pole breakers right from the existing main panel, provided it has a 200 main. I don't see the benefit of installing a sub-panel strictly for the WH.

This type of water heater doesn't always pull in both contactors and draw 100 + amps, it may activate only one contactor (circuit) depending on water flow.

Well, from what I've read about tankless water heaters, the general consensus is a subpanel or disconnect(s) should be located within eyesight of the appliance
in the event it should ever need to be worked on.  Something about safety, or something, heck if I know.  In this particular instance, the WH manufacturers UL listing
is predicated, according to the manual, on being fed from what amounts to a sub panel being within eyesight.  

Another reason for not wiring two 60amp breakers from the main panel is that the so called electrician that wired the house left absolutely Zero spaces in this 200 amp
panel.  The panel is a Square D 200amp QO with main breaker.  It is capable of accepting a 125 amp breaker.  The plan now is to run a sub panel which will feed the
WH with two 60amp breakers.  Because the sub cannot be located near the appliance, I will be wiring in two 60amp disconnects near the WH for the purpose of
working on the unit if that ever becomes an issue.  In reality, I may be running three disconnects to prepare for the eventuality of replacing the unit.  Most new
electric tankless heaters utilize 3 40 amp circuits.

Wait till you see the price for a 125 A breaker.

Gotcha, the disco thing makes sense. Mine is right next to the panel so I often forget about the placement factor.

You might consider reviewing the circuit layout, perhaps some circuits be bundled with others verses the sub-panel work & cost. Sounds like you're on the right track - good luck.

I may take another look at what, if any, circuits I can consolidate to save space.  I'd rather not have to try and
Pull more wire into this panel.  Its location is will make it a nightmare to do.  As far as the cost is concerned,
it's not a terrible concern.  I'd rather do it right vs save $30-$40 bucks.