Example D4(b) Optional Calculation for Multifamily dwelling

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shortcircuit1

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Hello,

I was looking at example D4(b) for load calculations and i ran into a calculation that got me confused.

If you look at example D4(b) in 2014 NEC, minimum size feeder for each dwelling unit consists of general lighting,small appliance only but when you have a water heater dont you include water heater with general lighting,small appliance to apply the demand factor?

In this example it was included after the application of the demand factor and added to the range kVA.

And another question is in the same example while determining the minimum size feeder from service equipment to meter bank for 20 & 40 units total load was calculated and then we simply applied table 220.84 demand factor for multifamily dwelling units..But for the calculation of the neutral example calculated lighting and small appliance and then applied demand factor of table 220.42 and then added 20 ranges @35000VA @70%. In the example they specified that each range is 8kW. so for 20 units it should be 20*8*.28(Table 220.55)=44.4kW. But example has 35kW. Am i missing something?
 
shortcircuit1 said:
Hello,

I was looking at example D4(b) for load calculations and i ran into a calculation that got me confused.

If you look at example D4(b) in 2014 NEC, minimum size feeder for each dwelling unit consists of general lighting,small appliance only but when you have a water heater dont you include water heater with general lighting,small appliance to apply the demand factor?
Click to expand...

No.

In this example it was included after the application of the demand factor and added to the range kVA.
Click to expand...

That is the correct way.

And another question is in the same example while determining the minimum size feeder from service equipment to meter bank for 20 & 40 units total load was calculated and then we simply applied table 220.84 demand factor for multifamily dwelling units..But for the calculation of the neutral example calculated lighting and small appliance and then applied demand factor of table 220.42 and then added 20 ranges @35000VA @70%. In the example they specified that each range is 8kW. so for 20 units it should be 20*8*.28(Table 220.55)=44.4kW. But example has 35kW. Am i missing something?
Click to expand...

Look at the last sentence in the header of the table:

(Column C to be used in all cases except as otherwise
permitted in Note 3.)
Click to expand...

Col.C says 35 kVA for 20 ranges.
 
jumper said:
No.



That is the correct way.



Look at the last sentence in the header of the table:



Col.C says 35 kVA for 20 ranges.
Click to expand...

It says use column C in all cases except as otherwise permitted in Note 3.But in Note 3 it says in lieu of method provided in column C,it shall be permissible to add the nameplate of cooking appliances and multiply the sum by demand factors....Isn't it still okay to do the way i calculated?
 
jumper said:
Most people would want the smaller number.;)
Click to expand...

But lets say if you have 5 8kW ranges...If you do it by column C it would be 20kW for combined load...But if you do it by Column B for 5 8kW ranges it would be 5*8*0.45=18kW. So in this case Column B provides the smaller number..What should i follow then?:?
 
shortcircuit1 said:
But lets say if you have 5 8kW ranges...If you do it by column C it would be 20kW for combined load...But if you do it by Column B for 5 8kW ranges it would be 5*8*0.45=18kW. So in this case Column B provides the smaller number..What should i follow then?:?
Click to expand...


You follow the Table unless you are specifically told not to. 35kw is the answer... In some case it may be lower to use the other method but I am not sure. Use the lowest and for testing purposes, I have never seen anything but column C used in such examples.
 
Dennis Alwon said:
You follow the Table unless you are specifically told not to. 35kw is the answer... In some case it may be lower to use the other method but I am not sure. Use the lowest and for testing purposes, I have never seen anything but column C used in such examples.
Click to expand...

But if you are designing a job anyone will prefer the lower numbers...It will change your wire size depending on how man units you are gonna have...Once we put something on the drawing no one likes it to comeback and say hey i can do it with lower numbers than yours .....:p;)
 
shortcircuit1 said:
But if you are designing a job anyone will prefer the lower numbers...It will change your wire size depending on how man units you are gonna have...Once we put something on the drawing no one likes it to comeback and say hey i can do it with lower numbers than yours .....:p;)
Click to expand...


This is your choice. If you don't feel comfortable then use a larger feeder. These numbers are based on the fact that ranges will not be on at 100% and certainly will not be on at the same time. The code allows a 12kw to be calculated at 35kva and these ranges are only 8 kw.

Suppose the ranges were 5 kw. 20 * 5 * .28 = 28 kw. Here is a case where using Column B works to your advantage
 
Dennis Alwon said:
This is your choice. If you don't feel comfortable then use a larger feeder. These numbers are based on the fact that ranges will not be on at 100% and certainly will not be on at the same time. The code allows a 12kw to be calculated at 35kva and these ranges are only 8 kw.

Suppose the ranges were 5 kw. 20 * 5 * .28 = 28 kw. Here is a case where using Column B works to your advantage
Click to expand...

So basically use whatever you feel comfortable with...Everytime we cant run calcs using two methods and see which one is lower...Right?:cool:
 
shortcircuit1 said:
So basically use whatever you feel comfortable with...Everytime we cant run calcs using two methods and see which one is lower...Right?:cool:
Click to expand...

The op is studying for a test, I assume, so IMO he would use the lowest. Many questions ask for the minimum size anyway so yeah, in that particular case you would need to do both examples.

It is not often that one sees that on an exam. Personally, I would probably have gotten that wrong because I would have used Column B and not even thought about C. The one the columns are set up it makes you think that Col. C is only for Ranges 8 3/4 kw and larger but that is clearly not the case.

I would hope whoever gives the exams makes it clear which column to use.
 
Dennis Alwon said:
The op is studying for a test, I assume, so IMO he would use the lowest. Many questions ask for the minimum size anyway so yeah, in that particular case you would need to do both examples.

It is not often that one sees that on an exam. Personally, I would probably have gotten that wrong because I would have used Column B and not even thought about C. The one the columns are set up it makes you think that Col. C is only for Ranges 8 3/4 kw and larger but that is clearly not the case.

I would hope whoever gives the exams makes it clear which column to use.
Click to expand...

Its not for an exam..I am getting into design slowly and studying these examples as a start..;)
 
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