multi-dewelling units calculation
- Thread starter alcalde
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mikeackley
Senior Member
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- Washington
Re: multi-dewelling units calculation
alcalde:
Is the context of your question: Calculations for Computing Feeder and Service Loads - Optional Calculation--Dwelling Units?
If so, (2002 NEC) 220.30(C)(1) states the largest of six selections of Heating and Air-Conditioning loads shall be included, with Air-Conditioning at 100%, Heatpumps at 100%, and four other heating unit types are listed at 100% or less depending upon type. I can't find my 1999 NEC book so can't say for certain what rating was applied to A/C back then, 125% or 100%.
Can anyone else add to this?
[ March 25, 2003, 01:58 PM: Message edited by: mikeackley ]
alcalde:
Is the context of your question: Calculations for Computing Feeder and Service Loads - Optional Calculation--Dwelling Units?
If so, (2002 NEC) 220.30(C)(1) states the largest of six selections of Heating and Air-Conditioning loads shall be included, with Air-Conditioning at 100%, Heatpumps at 100%, and four other heating unit types are listed at 100% or less depending upon type. I can't find my 1999 NEC book so can't say for certain what rating was applied to A/C back then, 125% or 100%.
Can anyone else add to this?
[ March 25, 2003, 01:58 PM: Message edited by: mikeackley ]
- Location
- Lockport, IL
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- Retired Electrical Engineer
Re: multi-dewelling units calculation
The 1999 code had 100% for air conditioning in the same paragraph.
Please note, however, that this paragraph is for the ?Optional Calculation ? Dwelling Unit.? I?ve looked through the Section B calculation method. The only place it seems to explicitly mention air conditioning is to say that it can?t be part of the ?75% Demand Factor for 4 or more fixed appliances? given in 220-17.
I think it is possible (whether correctly or incorrectly) to assign a 125% factor for air conditioning via the following path: Start with 220-14, ?Motors.? It says that motor loads shall be computed per 430-24, -25, or -26. Next go to 430-24. It says that the conductor feeding several motors (or one motor with other loads) must have an ampacity based on 125% of the largest motor on the circuit (plus 100% of the other loads). But that relates to the ampacity of the cable, not to the amount of load to be included in the service or feeder calculation. So do you use 100% or 125%? 430-24 does not answer this question.
In the 2003 code, the wording of the 1999 Section 220-14 has been moved to 220.3(B)(3). It points the reader to 440.6 for air conditioning loads. In 440.6, there is no requirement to use a 125% factor.
The 1999 code had 100% for air conditioning in the same paragraph.
Please note, however, that this paragraph is for the ?Optional Calculation ? Dwelling Unit.? I?ve looked through the Section B calculation method. The only place it seems to explicitly mention air conditioning is to say that it can?t be part of the ?75% Demand Factor for 4 or more fixed appliances? given in 220-17.
I think it is possible (whether correctly or incorrectly) to assign a 125% factor for air conditioning via the following path: Start with 220-14, ?Motors.? It says that motor loads shall be computed per 430-24, -25, or -26. Next go to 430-24. It says that the conductor feeding several motors (or one motor with other loads) must have an ampacity based on 125% of the largest motor on the circuit (plus 100% of the other loads). But that relates to the ampacity of the cable, not to the amount of load to be included in the service or feeder calculation. So do you use 100% or 125%? 430-24 does not answer this question.
In the 2003 code, the wording of the 1999 Section 220-14 has been moved to 220.3(B)(3). It points the reader to 440.6 for air conditioning loads. In 440.6, there is no requirement to use a 125% factor.
mikeackley
Senior Member
- Location
- Washington
Re: multi-dewelling units calculation
Bare with me here Charlie but maybe you have found the answer, above 3rd para., without fully realizing it. Maybe 220-14 Motors, is really saying, "Yah, I know 430-24 is dealing with the ampacity of conductors, but I want you to use this 125% figure anyway for Service Feeders and Load Calcs."
Interestingly enough, I just went to Mike Holt's on-line kVar Calculator, here: http://www.mikeholt.com/freestuff.php?id=freegeneral and he is still using the 125% value for A/C. There is a note embeded in the worksheet which reads: "
Motors (Air Conditioning) Motor loads shall be calculated in accordance with Sections 430-24, 430-25, and 430-26. This entry shall be considered the largest A/C motor and calculated at 125%. A/C #2 and A/C #3 shall be calculated at 100%."
Bare with me here Charlie but maybe you have found the answer, above 3rd para., without fully realizing it. Maybe 220-14 Motors, is really saying, "Yah, I know 430-24 is dealing with the ampacity of conductors, but I want you to use this 125% figure anyway for Service Feeders and Load Calcs."
Interestingly enough, I just went to Mike Holt's on-line kVar Calculator, here: http://www.mikeholt.com/freestuff.php?id=freegeneral and he is still using the 125% value for A/C. There is a note embeded in the worksheet which reads: "
Motors (Air Conditioning) Motor loads shall be calculated in accordance with Sections 430-24, 430-25, and 430-26. This entry shall be considered the largest A/C motor and calculated at 125%. A/C #2 and A/C #3 shall be calculated at 100%."
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: multi-dewelling units calculation
My belief is that you always include air conditioning at 100% (assuming it is larger than heating), and never at 125%. HOWEVER, you still need to add 25% of the largest motor, whether it be the A/C unit or some other motor. To be precise, you never include the A/C at 125% and then add 25% of the largest motor on top of that.
To support my position, I cite NEC Example D1(b) (both 1999 and 2002 codes). The A/C unit is included at its nameplate rating (12 amps), and the 25% for the largest motor is added in as a separate line item.
I realized it; I?m just not convinced that it is the right answer. I have two reasons. First, 430-24, -25, and -26 (and while we?re at it, -22) all speak of conductor ampacity. When 220-14 points you in that direction, you should expect to find information about motor loads, not conductor ampacity. So the correlation is not entirely satisfactory (meaning, not completely clear). Secondly, the 2002 code points you in a new direction: to 440-6. That paragraph also speaks of conductor ampacity, but it does not add 25%.
My belief is that you always include air conditioning at 100% (assuming it is larger than heating), and never at 125%. HOWEVER, you still need to add 25% of the largest motor, whether it be the A/C unit or some other motor. To be precise, you never include the A/C at 125% and then add 25% of the largest motor on top of that.
To support my position, I cite NEC Example D1(b) (both 1999 and 2002 codes). The A/C unit is included at its nameplate rating (12 amps), and the 25% for the largest motor is added in as a separate line item.
Thom Peterson
Member
- Location
- New york
Re: multi-dewelling units calculation
FWIW I have seen it done both ways. I have settled on using 100% for A/C. Based on Dwelling Calculator at NFPA site. If that's what they use I assume it's correct.
FWIW I have seen it done both ways. I have settled on using 100% for A/C. Based on Dwelling Calculator at NFPA site. If that's what they use I assume it's correct.
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