# 27 kw tankless water heater

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#### JOHNEO99

##### Senior Member
Can anyone tell me if I can put this on a 100 amp panel?

There are plenty of spaces and it takes 3-240v 40-amp breakers. Its a 850 sq. ft. condo that it will be installed in.

If you could point me in the right direction that would be good.

Thanks

#### raider1

##### Senior Member
Staff member
Can anyone tell me if I can put this on a 100 amp panel?

There are plenty of spaces and it takes 3-240v 40-amp breakers. Its a 850 sq. ft. condo that it will be installed in.

If you could point me in the right direction that would be good.

Thanks
27,000 watts divided by 240 volts is 112.5 amps.

Chris

#### jumper

##### Senior Member
27,000 watts divided by 240 volts is 112.5 amps.

Chris

408.30 General. All panelboards shall have a rating not
less than the minimum feeder capacity required for the load
calculated in accordance with Parts II, III, IV, or V of Article
220 as applicable

Right?

#### JOHNEO99

##### Senior Member
Ya I did the ohms law calculation but was wondering if the 3-40 amp breakers made a difference vs. 1-125 amp breaker.

#### Cavie

##### Senior Member
Ya I did the ohms law calculation but was wondering if the 3-40 amp breakers made a difference vs. 1-125 amp breaker.
# of breakers has nothing to do with it. If full load is called for, 112.5 amps, you just exceded your main breaker compacity by 12.5 amps just for hot water. Nothing left for anything else.

#### Npstewart

##### Senior Member
One of two things is going on here, the electrical service is way too small, or the water heater is way too big. Im not sure where this is located, but given a 80 degree temperature rise, thats 139 GPH. That is a lot of water, way too much for a condo. A typical 40 gallon STORAGE water heater has a recovery rate of 25 GPH.

#### JOHNEO99

##### Senior Member
I was afraid of this.

#### raider1

##### Senior Member
Staff member

408.30 General. All panelboards shall have a rating not
less than the minimum feeder capacity required for the load
calculated in accordance with Parts II, III, IV, or V of Article
220 as applicable

Right?
Correct, I was hoping the original poster would see that the demand for this water heater would exceed the rating of the panelboard.

# of breakers has nothing to do with it. If full load is called for, 112.5 amps, you just exceded your main breaker compacity by 12.5 amps just for hot water. Nothing left for anything else.
Agreed, 3 40 amp breakers does not change the demand of the water heater.

Now if there are multiple elements that are designed so that they can't be running at the same time and the maximum demand at any one time is less than 100 amps you may be OK but that would be determined by the manufacture.

Chris

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#### JOHNEO99

##### Senior Member
Thanks Chris

I mean I know how to do simple ohms laws but I was thinking the same thing about how they may not run simultaneously.

They changed this heater from 1-125 to 3-40amp breakers recently and I'm not sure why. I cant find the install pdf for this unit yet.Hopefully all three elements don't run at the same time for better efficiency or more consistent heating or something.

##### Senior Member
Not that You missed anything

Not that You missed anything

It?s also interesting to read all of what is listed in the Index about Water Heaters! : )

#### steve66

##### Senior Member
I've seen tank type heaters that are non-simulatanous. But with an instant heater, I'll bet if the flow rate is high enough, they will all run at the same time.

I think there is a paragraph in the NEC that requires electric heating elements to be divided into groups with not more than 60 amps per group. But I'm not sure that applies to an instant HWH. But if it does, that could expalin why they changed their design.

#### kingpb

##### Senior Member
Based on info I found for a 2 input breaker unit, the power is going to run simultaneously, or at least have the potential too.

The instruction manual makes it very clear that you need to have someone check your electrical supply BEFORE yo attempt to hook-up. I can see why.

If you take the 27kW, divide by three, you get 9kW per heating element. At 240V that works out to 9kW/240V = 37.5A, multiplied by 1.25 and you get 46.87 or 50A breaker.

Exactly where do the 40A breakers work into the equation?

#### bluesmoke...oops

##### Senior Member
why multiply by the 1.25?

#### iwire

##### Moderator
Staff member
If you take the 27kW, divide by three, you get 9kW per heating element. At 240V that works out to 9kW/240V = 37.5A, multiplied by 1.25 and you get 46.87 or 50A breaker.

Exactly where do the 40A breakers work into the equation?
What is wrong with a 40 amp for a 37.5 amp non-continuous load?

As far as I know the NEC does not requires a tankless heater to be figured at 125%.

##### Senior Member
It?s also interesting to read all of what is listed in the Index about Water Heaters! : )
Exactly where do the 40A breakers work into the equation?
Article 422.12(F)(3)

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
I mean I know how to do simple ohms laws but I was thinking the same thing about how they may not run simultaneously.
If that were the case, there'd be no need for multiple supply circuits.

A typical storage water heater is rated "4500w upper element, 4500w lower element, 4500w total." Obviously, non-simultaneous.

They changed this heater from 1-125 to 3-40amp breakers recently and I'm not sure why. I cant find the install pdf for this unit yet.Hopefully all three elements don't run at the same time for better efficiency or more consistent heating or something.
You're talking about heating fast-moving water very rapidly. The elements are definitely running simultaneously.

How the load divides doesn't change the load impressed on the supply. Whether one circuit or three, KW's are KW's.

Staff member

422.11(f)(3)

Staff member

#### kingpb

##### Senior Member
What is wrong with a 40 amp for a 37.5 amp non-continuous load?

As far as I know the NEC does not requires a tankless heater to be figured at 125%.
Apparently you guys don't have two teenage daughters in your house, I could argue that it is a continuous load

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