electric ranges

Status
Not open for further replies.
C

charlie06

Member
Location
New York
Hello everyone this is my first time posting. I have 2 questions.
I ran across this today and just wanted an opinion. My friend called and said that his electric range keeps on tripping the c.b. and he said the c.b. is warm. So I took a ride on over, and the wires were loose under the 2 pole 40 amp screws. I removed that c.b. put in a new one and of course tightened the screws. I also put my amprobe on the wires and the amperage was 31amps per leg with the oven on and all 4 burners at once.
I then decided to check the kw rating since I was already there. At 120/240 which is what he has at the feeders the range is rated for 10.7 kw. and 8.1kw at 208/120.
The range is fed with a 6/2.
10.7kw/240v= 44.58amps
I think this should have been a 50amp breaker. When I had the range going at full blast with the 40amp, it held. He says it has been like this for roughly three years. Should I put in a 50amp c.b. or not. The math says that I should.

question 2: He has a second kitchen down in the basement. This electric range is fed with a 6/3. The rating on this one is 120/240 at 12.1kw and 208/120 at 9.1kw. Again the houses feeders read 120/240. With that said this is also fed with a 2 pole 40 amp breaker and he says that he has never had a problem with this range.
12.1kw/240v= 50.4 amps, 50.4*.8=4.03 (table 220.55)
50.4+4.03=54.43amps
Is my math correct and should I change the breakers?

Thanks
Charlie
 
charlie b

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
This is a bit tricky, and I have to think about it. But let me start by asking if you really meant that the first range was fed by 6/2. If it is dual voltage rated (120/240), it needs a third wire (not counting the EGC).

Welcome to the forum.
 
mcclary's electrical

mcclary's electrical

Senior Member
Location
VA
charlie06 said:
Hello everyone this is my first time posting. I have 2 questions.
I ran across this today and just wanted an opinion. My friend called and said that his electric range keeps on tripping the c.b. and he said the c.b. is warm. So I took a ride on over, and the wires were loose under the 2 pole 40 amp screws. I removed that c.b. put in a new one and of course tightened the screws. I also put my amprobe on the wires and the amperage was 31amps per leg with the oven on and all 4 burners at once.
I then decided to check the kw rating since I was already there. At 120/240 which is what he has at the feeders the range is rated for 10.7 kw. and 8.1kw at 208/120.
The range is fed with a 6/2.
10.7kw/240v= 44.58amps
I think this should have been a 50amp breaker. When I had the range going at full blast with the 40amp, it held. He says it has been like this for roughly three years. Should I put in a 50amp c.b. or not. The math says that I should.

question 2: He has a second kitchen down in the basement. This electric range is fed with a 6/3. The rating on this one is 120/240 at 12.1kw and 208/120 at 9.1kw. Again the houses feeders read 120/240. With that said this is also fed with a 2 pole 40 amp breaker and he says that he has never had a problem with this range.
12.1kw/240v= 50.4 amps, 50.4*.8=4.03 (table 220.55)
50.4+4.03=54.43amps
Is my math correct and should I change the breakers?

Thanks
Charlie
Click to expand...


I assume the first stove does not have a 6/3 because of an existing wire? But other than that, Your math is correct, and IMO I would put a new 50 on both circuits.
 
charlie b

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
charlie06 said:
12.1kw/240v= 50.4 amps, 50.4*.8=4.03 (table 220.55)
Click to expand...
charlie06 said:
50.4+4.03=54.43amps
Click to expand...

I don't get this part. First of all, 50 times .8 will not give you around 4. It will give you 40.3. But why are you multiplying those two numbers anyway? And then why are you adding 4 to 50?

I think both ranges should be calculated as 8kW. Both are within the rules for column C. For the one that is 12.1 kW, you take the 8kW and add 5% for each major fraction of a kW above 12. But you only have 0.1 kW above 12, and that is not a major fraction. You can therefore stop at 8.

8000/240 = 33.3. So a 40 amp breaker should be fine.

But there is still a tricky part I am trying to think my way through. More to follow.
 
charlie b

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
OK, I worked out the ?tricky part? to which I refer above. It had to do with 210.19(A)(3). I see now that that article is all about the conductors, not about the breakers. Your conductors are OK. They have to have at least the same ampacity as the branch circuit rating. So whether you use a 40 amp or 50 amp breaker, the #6 conductors will have enough ampacity.

My problem had been with the statement, ?. . . conductors . . . shall have an ampacity . . . not less than the maximum load to be served.? I was not sure if that was referring to the ?maximum calculated load,? which in this case is 8kW, or the maximum ?actual? (i.e., rated) load of 12.1 kW. But it does not matter. The conductors are good for 65 amps, and that is sufficient for a 12.1 kW load.

That brings me back to my previous answer, which I now confirm: the breakers are OK as is. Please refer to Note 4 of Table 220.55. It says you can size the branch circuit for a range per the table above, and the table above sizes both of your ranges at 8kW.

Let me also mention that it would be acceptable to replace the breakers with 50 amp devices. Acceptable, and perhaps a good design choice, but not necessary (per code).
 
C

charlie06

Member
Location
New York
electric ranges

Thanks for the replies.
To charlie B I think I may be misinterpreting the code. But after further reading isn't 220.55 for multiple ranges on the same circuit. These two ranges are on separate circuits.
 
W

W=EI

Member
charlie b said:
This is a bit tricky, and I have to think about it. But let me start by asking if you really meant that the first range was fed by 6/2. If it is dual voltage rated (120/240), it needs a third wire (not counting the EGC).

Welcome to the forum.
Click to expand...

I agree with Mr. Beck as to the 40/2P breakers technically correct. The 6/2 if service cable would be 'old school' correct but not meet new code. Perhaps the range has an element going to fault condition...
 
W

wawireguy

Senior Member
I'd leave the 40 amp breaker. Quit scrutinizing the KW rating on the stove. What does the manufacture require for a branch circuit? Most modern ranges only require a 40 amp circuit. You said it draws 31amps under full load. That's roughly 75% of the the circuit capacity on a 40 amp breaker. That's 5% under what the circuit is rated for on a continuous load. The 4 range burners rarely are used for 3 hours under full load. Oven doesn't stay on continuously either. Even if I call a range a continous load, which I really don't see it being one, your 40 amp circuit should be plenty. If replacing the breaker doesn't resolve it keep looking for the problem.
 
Status
Not open for further replies.
Top