In determining the largest motor for a service and feeder calculation, is the A/C unit subject to being selected even if the heat is shown to be the larger of the two loads? Even though the heat load may be the larger of the two, you still use the A/C during the summer and if the A/C is the largest motor load, then shouldn't it be considered for the 25% increase?
Selecting Largest Motor
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Re: Selecting Largest Motor
The thermostat that I have in my house, prevents me from running the heat, and AC at the same time.
If the heating load is larger, and I have calculated this into my service/feeder calculation, then why should I need to add any more available capacity to the system?
The thermostat that I have in my house, prevents me from running the heat, and AC at the same time.
If the heating load is larger, and I have calculated this into my service/feeder calculation, then why should I need to add any more available capacity to the system?
Re: Selecting Largest Motor
I understand that the heat and A/C are not going to be on at the same time in a residence but I was just wondering about selecting the largest motor for the 25% increase. Even though the A/C may have removed from the service and feeder calculation because the heating load was larger, isn't it true that the A/C will still be used during the summer? If the A/C is shown to be the largest motor load then shouldn't the 25% increase based on the A/C be added to the service and feeder calculation?
It seems the code stipulates that the largest motor shall be increased 25% and added to the service and feeder calculation. So I guess my question is what do you do when the A/C has been dropped from the service and feeder calculation but the A/C is indeed the largest motor?
I understand that the heat and A/C are not going to be on at the same time in a residence but I was just wondering about selecting the largest motor for the 25% increase. Even though the A/C may have removed from the service and feeder calculation because the heating load was larger, isn't it true that the A/C will still be used during the summer? If the A/C is shown to be the largest motor load then shouldn't the 25% increase based on the A/C be added to the service and feeder calculation?
It seems the code stipulates that the largest motor shall be increased 25% and added to the service and feeder calculation. So I guess my question is what do you do when the A/C has been dropped from the service and feeder calculation but the A/C is indeed the largest motor?
Re: Selecting Largest Motor
What are we including in the heating/ air conditioning calculation.
Depends on the type of system.
If you have a heat pump, or package hvac then its name plate rating is the highest of the two.
If you have electric strip, water, or some other heating system, and a seperate A/C then you choose the larger.
If you are refering to a split system, then the fan coil unit is infact a motor load, in residential these are often 120 volt, and in that case I can see where it could be considered to be the largest motor. in this case it is not part of the heating or ac load and needs to be calculated seperately.
What are we including in the heating/ air conditioning calculation.
Depends on the type of system.
If you have a heat pump, or package hvac then its name plate rating is the highest of the two.
If you have electric strip, water, or some other heating system, and a seperate A/C then you choose the larger.
If you are refering to a split system, then the fan coil unit is infact a motor load, in residential these are often 120 volt, and in that case I can see where it could be considered to be the largest motor. in this case it is not part of the heating or ac load and needs to be calculated seperately.
Re: Selecting Largest Motor
Add 25% to the A/C motor. Is the heat still the larger of the two loads? If so, then you have allowed the needed additional capacity for the motor when it is used, since the heat load won't be used at the same time.
If the 25% makes the A/C unit the larger of the two loads, then use the A/C load and drop the heat.
Hope that helps.
Add 25% to the A/C motor. Is the heat still the larger of the two loads? If so, then you have allowed the needed additional capacity for the motor when it is used, since the heat load won't be used at the same time.
If the 25% makes the A/C unit the larger of the two loads, then use the A/C load and drop the heat.
Hope that helps.
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Re: Selecting Largest Motor
Separate from this, you find the largest motor, and add 25%. If the largest motor is the A/C unit, then that is the motor you use. It's that simple.
You are not looking for the operational scenario that gives you the highest possible combination of loads. You are merely trying to ensure that the house has enough load to meet minimum needs.
I don't agree with this. You look at the A/C and then you look at the heat, and you pick the larger.
Separate from this, you find the largest motor, and add 25%. If the largest motor is the A/C unit, then that is the motor you use. It's that simple.
You are not looking for the operational scenario that gives you the highest possible combination of loads. You are merely trying to ensure that the house has enough load to meet minimum needs.
bphgravity
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Re: Selecting Largest Motor
I can't imagine a single occasion where the 25% would make any difference in the size of the service or feeder. That being said, that same 25% can be all the difference on an exam question.
I can't imagine a single occasion where the 25% would make any difference in the size of the service or feeder. That being said, that same 25% can be all the difference on an exam question.
Re: Selecting Largest Motor
I wish I would have found this thread a little earlier.
If anyone gets a chance, look at my post on Exam Prep...
http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=6;t=000451
I have emailed Mike about this and he has responded. Ryan Jackson is also looking into this problem.
Basically, if you don't want to read the other post...lol... in Mike's 2005 Exam Prep book he states that you never count the A/C motor in the +25% calculation.
There is even an example in the book where service calcs are being done that has an A/C motor and a separate motor. Even though the A/C motor is the larger of the two it is not used in the +25% of the largest motor calc, the other motor is. I don't have the book in front of me or I would be more specific.
Any thoughts?
TXInspect
I wish I would have found this thread a little earlier.
If anyone gets a chance, look at my post on Exam Prep...
http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=6;t=000451
I have emailed Mike about this and he has responded. Ryan Jackson is also looking into this problem.
Basically, if you don't want to read the other post...lol... in Mike's 2005 Exam Prep book he states that you never count the A/C motor in the +25% calculation.
There is even an example in the book where service calcs are being done that has an A/C motor and a separate motor. Even though the A/C motor is the larger of the two it is not used in the +25% of the largest motor calc, the other motor is. I don't have the book in front of me or I would be more specific.
Any thoughts?
TXInspect
Re: Selecting Largest Motor
Alright, I decided not to be lazy and go grab my book.
2005 Electrical NEC Exam Prep
Page 469
11.20 Bank
Without going through all of the calcs we have this for our motors:
10hp, 230V A/C (FLC=50A) 11,500 VA
5hp, 230V Sprinkler Pump (FLC=28A) 8,050VA
If you look at the last calc on this page for the "+25% of the largest motor" you will find the sprinkler pump motor being used -
8,050VA x 25% = 1,610VA.
Once again, I welcome any comments.
Thanks for reading,
TXInspect
Alright, I decided not to be lazy and go grab my book.
2005 Electrical NEC Exam Prep
Page 469
11.20 Bank
Without going through all of the calcs we have this for our motors:
10hp, 230V A/C (FLC=50A) 11,500 VA
5hp, 230V Sprinkler Pump (FLC=28A) 8,050VA
If you look at the last calc on this page for the "+25% of the largest motor" you will find the sprinkler pump motor being used -
8,050VA x 25% = 1,610VA.
Once again, I welcome any comments.
Thanks for reading,
TXInspect
Re: Selecting Largest Motor
Article 220.50 tells us that hermetic compressors are figured differently than motors. (See 440.6) So if this is the case for you, as I imagine it is, the 25% has no bearing in this calculation.
This is a change from the pre 1999 codes. However if you notice, the examples in Annex D still have not caught up with this change. There is a proposed change to correct this for the 2008 Code.
___________________
Wes Gerrans
Instructor
Northwest Kansas technical College
Goodland, KS
Article 220.50 tells us that hermetic compressors are figured differently than motors. (See 440.6) So if this is the case for you, as I imagine it is, the 25% has no bearing in this calculation.
This is a change from the pre 1999 codes. However if you notice, the examples in Annex D still have not caught up with this change. There is a proposed change to correct this for the 2008 Code.
___________________
Wes Gerrans
Instructor
Northwest Kansas technical College
Goodland, KS
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