Ampacity, Heater

Joey94

Member
Location
Illinois
Occupation
Apprentice
I have a heater that is 6000 watts 240V. So it uses 25Amps. In my 3/4 raceway there are 9 conductors from which 8 is current carrying. I’m wondering if my AWG 10 THHN with a 30 Amp breaker is the right size wire and amperage of breaker for it. Please advise
 

Joey94

Member
Location
Illinois
Occupation
Apprentice
So what would would be the correct wire size and breaker amperage?


This is the heater. Model number: LEH61
The specs say it’s 6000W 25Amp 240V
The wiring diagram says 20Amps per element. Which is kind of confusing.
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infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
It says MAX 20 amps per element not 20 amps per element. The specifications say 6000 watts 25 amp @240 volt which is correct.
 

Joey94

Member
Location
Illinois
Occupation
Apprentice
I’m guessing that’s if you want to have two separate circuits of 240V with two double pole breakers!?


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Joey94

Member
Location
Illinois
Occupation
Apprentice
So in case of the 3 wire system application there is a need for 40 amp breaker ?


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Joey94

Member
Location
Illinois
Occupation
Apprentice
So what do you all think is the correct size wire and breaker ? With multiplying by 125% and derating 70% for 8 ccc


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wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
So what do you all think is the correct size wire and breaker ? With multiplying by 125% and derating 70% for 8 ccc
First, the branch circuit conductors needs to have a table ampacity of 25A * 125% (continuous factor) = 31.25A at 75C, assuming 75C terminations at both ends. So that would be #10 Cu with an ampacity of 35A.

Second, the circuit conductors have to have a table ampacity of 25A / 0.7 (derating factor) = 35.7A at 90C, assuming 90C insulation. So again #10 Cu.

Lastly, the breaker has to be at least 25A * 125% = 31.25A, so a 35A breaker minimum But the small conductor rule limits #10 Cu to 30A breakers, with no exception for this application, so you'll need to use #8 Cu.

If you use #8 Cu with 90 C insulation, that has a table ampacity of 55A. So the derated ampacity is 38.5A. That means you can use a 40A breaker on it, if desired.

But if you used #8 Cu with 75C insulation, that has a table ampacity of 50A, giving a derated ampacity of 35A. For that you'd need to use a 35A breaker.

Cheers, Wayne
 

Joey94

Member
Location
Illinois
Occupation
Apprentice
Thank you so much! This is exactly the answer I was looking for! Now it all makes sense


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robertd

Senior Member
Location
Maryland
Occupation
electrical contractor
I’m guessing that’s if you want to have two separate circuits of 240V with two double pole breakers!?


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If you have two circuits on one switch would need a tie handle on all four breakers, or a 4-pole breaker. That doesn't seem very useful.
 

Joey94

Member
Location
Illinois
Occupation
Apprentice
The switch for the heaters is a duplex. You can turn each element on or off separately. So that’s why the 5 wire application. In case you want them to be on separate circuit with separate double pole brakers. At least that’s how I understand the diagram.


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