Range/Oven Calculation, Table 220.55 demand factors?

[switchleg45]

Can I get a ruling on this conclusion I've come to regarding a range/cooktop calculation?

So, I have 8-3 NM cable supplying a J-box with 2 appliances served by each of those circuits, a built-in(wall-mount?) oven and a cooktop. The oven is rated at 8700w and the cooktop at 4200w. I am wanting to verify that this size wire is legal....with reference to Table 220.55 in NEC 2011 "Demand Factors and Loads for.....and other household cooking appliances over 1-3/4 kW rating (Column C to be used in all cases except as otherwise permitted in Note 3).

So, 4200+8700/240 = 53.75a. Now, I figure I use row 2, for 2 appliances, and use Column C. If 65% of 53.75 = 34.9375a, is size 8 AWG sufficient for these loads and satisfy code? Or does the 80% capacity rule apply here?

Thank you in advance for your thoughts.

Kevin

 

[jumper]

I get 35A using notes 1 and 4 of T220.55 and column C

4200W+8700W=12,900 note 4

Demand from C= 8000W+5%=8400, note 1

8400W/240V=35A

 

[infinity]

I agree with Jumper, by using note #4 you can just add the KW rating of the two units together and treat it as one range.

 

[Dennis Alwon]

Just remember if the demand factor is 35 you still have to use 210.19(A)3 for household ranges

(3) Household Ranges and Cooking Appliances. Branchcircuit
conductors supplying household ranges, wall-mounted
ovens, counter-mounted cooking units, and other household
cooking appliances shall have an ampacity not less than the
rating of the branch circuit and not less than the maximum
load to be served. For ranges of 83⁄4 kW or more rating, the
minimum branch-circuit rating shall be 40 amperes.

 

[switchleg45]

Demand factors in general

Demand factors in general

Regarding the "demand factor" I am left thinking about the possibility that exists if all the loads (heating elements) are turned on simultaneously, or at least at the same occasion (think Thanksgiving...or Christmas when all family is there using lots of electricity). If the turkey is 'launched' into the oven and all five cooktop burners are being used for various dishes obviously the possibility exists that the "demand load" will be exceeded and the ocpd will trip. Or is it the intent of the "demand factor" to consider that the heating elements cycle on and off per the stat and are not typically all pulling their maximum amperage at one time? But the possibility does exist...right? And more so at particular events such as holidays like Thanksgiving and Christmas?

Just trying to get a better understanding of this subject. And a huge "Thank you!" to all you seasoned engineers out there trying to pass on your knowledge to us underlings:dunce:

Kevin

 

[jumper]

. Or is it the intent of the "demand factor" to consider that the heating elements cycle on and off per the stat and are not typically all pulling their maximum amperage at one time?

Kevin

Yes.

 

[Sierrasparky]

Regarding the "demand factor" I am left thinking about the possibility that exists if all the loads (heating elements) are turned on simultaneously, or at least at the same occasion (think Thanksgiving...or Christmas when all family is there using lots of electricity). If the turkey is 'launched' into the oven and all five cooktop burners are being used for various dishes obviously the possibility exists that the "demand load" will be exceeded and the ocpd will trip. Or is it the intent of the "demand factor" to consider that the heating elements cycle on and off per the stat and are not typically all pulling their maximum amperage at one time? But the possibility does exist...right? And more so at particular events such as holidays like Thanksgiving and Christmas?

Just trying to get a better understanding of this subject. And a huge "Thank you!" to all you seasoned engineers out there trying to pass on your knowledge to us underlings:dunce:


Kevin

I like your thinking. The demand load is for minimum design. I build like to build for the worst case. Problems always happen during the holidays. I can see a holiday cook feast exceeding 40 amps at warm up.
If this was commercial you would not wire it to the residential minimum. That said some will say you are wasting money doing it any other way.

 

[ActionDave]

Regarding the "demand factor" I am left thinking about the possibility that exists if all the loads (heating elements) are turned on simultaneously, or at least at the same occasion (think Thanksgiving...or Christmas when all family is there using lots of electricity). If the turkey is 'launched' into the oven and all five cooktop burners are being used for various dishes obviously the possibility exists that the "demand load" will be exceeded and the ocpd will trip. Or is it the intent of the "demand factor" to consider that the heating elements cycle on and off per the stat and are not typically all pulling their maximum amperage at one time? But the possibility does exist...right? And more so at particular events such as holidays like Thanksgiving and Christmas?

People have similar thoughts about plumbing in big buildings. The rumour goes that if all the toilets get flushed at once the DWV system will be overwhelmed and the building will flood. The possibility exists, right?

The demand factors have been in the code for a looooonnnnggggg time. The reality is they work fine.

 

[jumper]

The demand factors have been in the code for a looooonnnnggggg time. The reality is they work fine.

Yeppers and if someone wants to oversize and overbid a project that is his/her constitutional right to lose the job.