Ampacity, Heater

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Joey94

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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

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Illinois
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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

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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

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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

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So in case of the 3 wire system application there is a need for 40 amp breaker ?


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Joey94

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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

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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

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Thank you so much! This is exactly the answer I was looking for! Now it all makes sense


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robertd

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Maryland
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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

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Illinois
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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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