Demand Factor Tables used for Ampacity/OC Protection?

[hitehm]

Hey All, I'm about to take the Journeyman exam. I'm using several study books that seem to be contradicting whether or not the demand factors in Art 220 for "load" calculations should be applied when determining a branch circuit ampacity, OC protection and the circuit conductor size.
Here is a question from Dewalt: " Given: A 240V single phase 17kW rated electric range is to be installed in the kitchen of a 1 family dwelling unit. Determine the minimum size copper NM cable the NEC requires to supply the electric range."

To me, the question is asking about conductor size so I immediately think to find the needed ampacity and whether or not the load is considered continuous and simply to ohm's law. In this case, not continuous and therefore, 17000w/240v = 70.83 amps which would give me a size 3 copper conductor under the 60deg C column for NM cable. Since no derating was needed I couldn't use the 90deg C column - 334.80. The author on the other hand applied the demand factor table 220.55 to the range wattage using column C row 1 for the single range. He added the 25% increase for the 5 kilowatts over 12kW and correctly came up with 10kW total. The problem is, I DON'T SEE WHY HE USED THIS AT ALL! This is NOT a load calculation, it's a cable size calc via the "ampacity rating" of the circuit. That range is capable of pulling 70.83 amps at full load. He sited note 4 from table 220.55 for reason for doing this. Is he correct? What am I missing? Please Help!!

 

[david luchini]

The branch circuit conductors need an ampacity sufficient for the load to be supplied. The load is calculated per Art. 220.

If the calculated load for the range is 10kW, 240V, then you need a conductor with an ampacity of at least 42A.

 

[hitehm]

The branch circuit conductors need an ampacity sufficient for the load to be supplied. The load is calculated per Art. 220.

210.3 Branch Circuit Rating: Branch circuits recognized by this article shall be RATED in accordance with the maximum permitted rating or setting of the OC device.
210.19A.3 Branch circuit conductors supplying household ranges.....shall have an ampacity not less than the RATING OF THE BRANCH CIRCUIT AND NOT LESS THAN THE MAXIMUM OF THE LOAD TO BE SERVED.

Isn't the rating of the branch circuit based on the minimum ampacity and size of the conductors determined by Art 310.15 and Art 240 OC Protection. and Isn't the "maximum load to be served" (not the derated load for the feeders and service) in this case 17kW?

Is there anywhere in the code under Art 210 Branch Circuits that specifically points to Art 220 to calculate the ampacity?

 

[Dennis Alwon]

220.55 is used for branch circuit calculations as well as service demands, therefore, a 17kw range is rated 10kw after demand. A range is never on with all burners and oven at the same time. The ovens are cyclical so the demands are low.

Look at this example-- 2- 16kw ranges are calculated at 13.2kw

 

[Dennis Alwon]

Article 220 is labeled branch circuits, feeder and service calculation

 

[hitehm]

220.55 is used for branch circuit calculations as well as service demands, therefore, a 17kw range is rated 10kw after demand. A range is never on with all burners and oven at the same time. The ovens are cyclical so the demands are low.

Look at this example-- 2- 16kw ranges are calculated at 13.2kw

Hey Dennis, Thanks for reply. What you guys are saying makes sense it's just that the code doesn't seem to specifically point to the demand factors from branch circuit ratings in Art 210. Art 210 specifically points to Art 240 OC protection for the "rating" of the circuit. Art 240 specifically points to Art 310.15 for "Ampacity" - 240.4: Protection of Conductors. Conductors shall be protected from overcurrent in accordance with there ampacity specified in 310.15. The bus seems to stop there with nothing specifically pointing to demand factor when calculating circuit "rating" or "ampacity", etc. Sorry, don't mean to beat this to death but other books did not derate the circuit for this same type of question. Maybe I need a very clear definition of "circuit rating"!

 

[hitehm]

I totally agree with your example and that's what I thought the demand factors were for, to properly size the service and feeder equipment based on the sum of all the loads for entire buildings. The entire building won't be completely "on" all at the same time so demand factor seems to make sense. I didn't realize it was done to size the conductors for a branch ckt. I thought the branch ckt load calculations were used as the addends that we needed to sum together to get our total feeder or service load and THEN apply the demand factor the building. I'll have to rethink this. Thanks for all the help.

 

[david luchini]

Hey Dennis, Thanks for reply. What you guys are saying makes sense it's just that the code doesn't seem to specifically point to the demand factors from branch circuit ratings in Art 210. Art 210 specifically points to Art 240 OC protection for the "rating" of the circuit. Art 240 specifically points to Art 310.15 for "Ampacity" - 240.4: Protection of Conductors. Conductors shall be protected from overcurrent in accordance with there ampacity specified in 310.15. The bus seems to stop there with nothing specifically pointing to demand factor when calculating circuit "rating" or "ampacity", etc. Sorry, don't mean to beat this to death but other books did not derate the circuit for this same type of question. Maybe I need a very clear definition of "circuit rating"!

I think what you are missing is 220.18 Maximum Loads. Specifically, 220.18(C) Range Loads.

 

[infinity]

Take a look at note #4 at the bottom of Table 220.55.