Confused by a wall oven

[George Stolz]

Looking at 220.55, and 210.19(A)(3), I'm left a little confused. Look at this graphic:

If I have a 6 kW wall oven, fed by EMT, with 75?C terminations all around, could I feed this oven with a 25A breaker with 12 AWG THHN in EMT?

 

[augie47]

I would say no.
240.4(G) in referencing Art 422 only lists " motor operated appliance circuits" as "exceptions" to the 240.(4)(D)95) rule that limits #12 to 20 amp OCP.
Your wall mounted oven is not a "motor operated appliance"


(You might get away with it on a convection oven)

 

[George Stolz]

What about 240.4(E)(1)?

 

[Dennis Alwon]

What about 240.4(E)(1)?

How do tap conductors enter into this picture.

 

[augie47]

What about 240.4(E)(1)?

personally, can't see where it applies.

 

[George Stolz]

Yeah, on second thought, 240.4(E)(1) should reference 210.19(A)(3) exception 1, not just 210.19(A)(3).

It's kinda screwy, and I was reading too fast. Thanks guys.

 

[charlie b]

Let me alter the question a tiny bit:

If I have a 6 kW wall oven, fed by EMT, with 75?C terminations all around, could I feed this oven with a 20A breaker with 12 AWG THHN in EMT?

Actually, the first time I read the question, that is what I thought you had said. Then I read it again. My answer to this version of the question is, "Yes it's allowed, but don't do that in my house." :happyno:

 

[George Stolz]

No, you couldn't use a 20, because the nameplate is 25A.

 

[kingpb]

Could someone please explain further?

220.55 forms a part of 220 Part III - Feeder and Service Load Calculations. It would be my understanding 220.55 only pertains to a load diversity/demand factor that can be applied to the KW or KVA rated value based on certain criteria and is related to service/feeder sizing only, i.e. has nothing to do with sizing of the branch circuit itself.

The equipment being 6kW, 25A, I would think the appropriate design would be #8Cu on a 40A breaker. I do not understand the picture that is saying minimum could be #10AWG (assuming Cu).

So, what am I missing?

 

[charlie b]

No, you couldn't use a 20, because the nameplate is 25A.

And what exactly makes us consider 100% of the nameplate? 210.19(A)(1) speaks of the maximum load to be served. So does 210.19(A)(3). How then do we determine the maximum load to be served? Can we not go to 220.55, use column B, take 80% of the nameplate, which calculates at 20 amps, and make the claim that a range is not a continuous load? :happyyes:

By the way, I am aware that I may appear to be stepping across a line that I have, in the past, claimed should not be crossed. That line, and its application to this question, might not be readily apparent. But let's step away from that issue, at least for the moment, and stay on the original topic.

 

[charlie b]

It would be my understanding 220.55 . . . has nothing to do with sizing of the branch circuit itself.

OK, I was typing too slowly. That is the side issue I was alluding to. I see a difference between the notion of, "we must use the rules within 220, as the criteria for sizing branch circuits," and the notion of "we are permitted to use the rules within 220, as the criteria for sizing branch circuits." I have objected to that first notion, in the context of establishing a limit to the number of receptacles on a 20 amp branch circuit.

 

[charlie b]

The equipment being 6kW, 25A, I would think the appropriate design would be #8Cu on a 40A breaker. I do not understand the picture that is saying minimum could be #10AWG (assuming Cu).

Do you agree that the load is not continuous? If so, a #10 is good for as much as 30 amps, so it could be protected by a 25 amp breaker, and serve a 25 amp load.

 

[david luchini]

And what exactly makes us consider 100% of the nameplate? 210.19(A)(1) speaks of the maximum load to be served. So does 210.19(A)(3). How then do we determine the maximum load to be served? Can we not go to 220.55, use column B, take 80% of the nameplate, which calculates at 20 amps, and make the claim that a range is not a continuous load? :happyyes:

I don't believe that we can use 80% of the nameplate for the wall mounted oven. Note 4 of T220.55 says the Branch Circuit Load for one range shall be permissible to be calculated in accordance T220.55. It further says the branch circuit load for one wall mounted oven shall be the nameplate rating of the appliance. I interpret this is requiring the wall mounted oven branch circuit to be based on the full nameplate rating, but I'd be interested in other interpretations.

(Go Irish!)

It would be my understanding 220.55...has nothing to do with sizing of the branch circuit itself.

I don't think that is correct. 220.14(B) says "load calculations shall be permitted as specified...in 220.55 for electric ranges and other cooking appliances." And as noted above, Note 4 in T220.55 is specifically for "Branch Circuit Load." I agree with Charlie that the load for the oven would be non-continuous.

 

[charlie b]

I interpret this (Note 4 of T220.55 ) as requiring the wall mounted oven branch circuit to be based on the full nameplate rating, but I'd be interested in other interpretations.

I misssed that note. I agree, and I withdraw my suggestion of using #12.

(Go Irish!)

I wholeheartedly agree with that!:happyyes:

 

[kingpb]

I wouldn't consider a single oven as non-continuous load, cooking a turkey for thanksgiving could easily exceed three hours, especially if you take it out, then heat the rolls and stuffing, albeit not at full load.

Now I'm getting hungry.

 

[iwire]

I wouldn't consider a single oven as non-continuous load, cooking a turkey for thanksgiving could easily exceed three hours, especially if you take it out, then heat the rolls and stuffing, albeit not at full load.

Then it is not a continuous load to the NEC.

Continuous Load. A load where the maximum current is
expected to continue for 3 hours or more.

 

[charlie b]

I wouldn't consider a single oven as non-continuous load, cooking a turkey for thanksgiving could easily exceed three hours. . . .

To Bob's observation, I will add that the oven's thermostat will be cycling the heating elements on and off the whole time. That also makes it a non-continuous load.

 

[Little Bill]

Then it is not a continuous load to the NEC.

During that 3 hours of cooking the turkey you will notice that the oven will cycle on and off. So....not continuous.

Charlie, that "go irish" comment sped you up and you typed faster than me!