tankless water heater question

KWH

Senior Member
The unit is 24kw 3phase 208, nameplate listed at 67amps with wire size #6

How could a #6 at 75degree be used.
 

mike7330

Senior Member
Location
North America
I believe water heaters terminals are rated for 90c but Infinity is correct on " they think they can use 90c" but I think they forgot about the breaker
 

Canton

Senior Member
Location
Virginia
Occupation
Electrical Inspector
Are you sure it doesn't call for more than one circuit? I have wired one that took 3 single phase 40 amp circuits
Ya....I had a service call that felt with a whole house instant hot..it was being fed with (3) 240 volt 40 amp breakers....(3) 8/2 cables went to it :huh: could not believe it until I saw the paperwork with it.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Ya....I had a service call that felt with a whole house instant hot..it was being fed with (3) 240 volt 40 amp breakers....(3) 8/2 cables went to it :huh: could not believe it until I saw the paperwork with it.
This is why electric is not a good choice for tankless water, all other things being equal. And if you're in CA, prepare for some awesome resi peak usage charges, 'cause that's the way they're going with the smart meters.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I also thought we were limited to 60 amps thru 422.11(F)

(F) Electric Heating Appliances Employing Resistance-
Type Heating Elements Rated More Than 48 Amperes.
(1) Electric Heating Appliances. Electric heating appliances
employing resistance-type heating elements rated
more than 48 amperes, other than household appliances
with surface heating elements covered by 422.11(B), and
commercial-type heating appliances covered by 422.11(D),
shall have the heating elements subdivided. Each subdivided
load shall not exceed 48 amperes and shall be protected
at not more than 60 amperes.
These supplementary overcurrent protective devices
shall be (1) factory-installed within or on the heater enclosure
or provided as a separate assembly by the heater
manufacturer; (2) accessible; and (3) suitable for branchcircuit
protection.
The main conductors supplying these overcurrent protective
devices shall be considered branch-circuit conductors.
 

Canton

Senior Member
Location
Virginia
Occupation
Electrical Inspector
And there was no suplimentary ocpd on the unit.....? (3) 8/2 went straight to the heater, I had to go down stairs to turn it off.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
You are limited to max 60A per circuit but get to run more than one circuit to the water heater.

I know that but I thought the OP was saying one feed. I have done one that was 90 amps and we had to use 1 feed but there were internal fuses in the unit.
 

KWH

Senior Member
I was talking to a friend over the phone who was asking about the wire size. I asked a few times to confirm there was only one circuit and he states there is only one, this is a commercial application.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I was talking to a friend over the phone who was asking about the wire size. I asked a few times to confirm there was only one circuit and he states there is only one, this is a commercial application.

Are there internal fuses in the unit???? I realize this doesn't answer your question about wire size--- obviously 65 amps is not 67 amps so imo you need a number 4 awg. I am assuming there are fuses as is required
 

KWH

Senior Member
Dennis, I will follow up on this and find out, it's not my job but is still good info.
 

kwired

Electron manager
Location
NE Nebraska
Ya....I had a service call that felt with a whole house instant hot..it was being fed with (3) 240 volt 40 amp breakers....(3) 8/2 cables went to it :huh: could not believe it until I saw the paperwork with it.
Why is it hard to believe? No way a 4500 watt element that is typical in storage type water heater can raise water temp instantaneously as it is used, unless it is fairly warm to begin with. The size of these units will vary some depending on incoming water temp, some places you may have to raise it 75 to 80 deg F, that either takes time in a tank type heater or a lot of energy nearly instantaneously in a tankless heater.
 

Canton

Senior Member
Location
Virginia
Occupation
Electrical Inspector
Why is it hard to believe? No way a 4500 watt element that is typical in storage type water heater can raise water temp instantaneously as it is used, unless it is fairly warm to begin with. The size of these units will vary some depending on incoming water temp, some places you may have to raise it 75 to 80 deg F, that either takes time in a tank type heater or a lot of energy nearly instantaneously in a tankless heater.
I was referring to the (3) 8/2 cables going to it with no local disconnecting means, not the 3 feeds themselves
 
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