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| Subject - Dwelling calculations | |
| dwion |
I'm trying to understand these calculation better. My problem is with Fastened in place appliances (220.17) and with the largest motor load (220.14). My question is if you have 4 fastened in place appliances and one of those appliances is your largest motor. Do you apply 220.17 and take your 75% demand factor and then add 25% of the largest motor F.L.A. to meet the requirements of 430.24. To me that does not meet 430.24 because you do not take 125% of the largest motor F.L.A. you are only allowing total of 100% of that motor. In my opinion you should consider you have 4 fastened in place appliances and apply the 75% demand factor to only three of them and take 125% of the largest motor load. This way it appears to meet all of the code section requirements. I would like to get more opions on this subject. Thank You Dwion |
| Scott Vickrey | Ok, here's my two cents. Figure it the way you first mentioned.quote: Remember your figuring the total minimum load for the service or feeder not the branch circuit. Right? |
| David Hyatt | Think of it as two different steps in your calculation. First apply 75% to 4 or more appliances. Then find your largest motor in FLC or HP and add 25% and add that seperate to your total. Only multiply times 25%. Note: Dishwashers will not be largest motor. It has to be an all motor load. Like a disposal, AC or pump motors. Then the demand factor of 75% does not apply to ranges, dryers, heating, AC. Note if you omit ac in the calculation you can't use it as largest motor because it is omitted in the whole calculation. 1/2hp disposal: 9.8FLC X 115V= 1127va X 25% =282va. This was probably more than you wanted but hope I helped. |
| David Hyatt | Very good question. I just knew to do it like I stated . So I checked why. It appears that the 125% section 430.24 is for a branch circuit for more than one motor, not a service calculation. In your calculation you already did the 100% derated at 75% so stands to reason all you would have to do is add the extra 25%. Atleast this is the way see it. |
| dwion | Thank You for your response. I'm a new instructor and when the apprentices ask questions when we are doing calculations I like to be able to answer them correctly. This question came up and I didn't know why they calculated it the way they did. I teach them about the NEC and what certain words mean when used in the code. 90.5 Mandatory Rules:Specifically required are characterized by shall or shall not . 430.24 Shall have an ampacity not less than 125% of the F.L.C.. 220.17 It shall be permissible to apply a demand factor [/b[b]]of 75%. I know you understand the calculations and I think I understand the calculations but explaining this is not as easy as it might seem. Take your 1/2hp disposal: 9.8FLC X 115V= 1127va my students see this as 9.8FLC x 125% x 115V =1408.75 or 1409va. and this is what should be added according to 430.24. When I was trying to explain to them the calculation thats when all of these questions came up. The calculation 9.8FLC x 115v =1127 1127 X 75% = 845VA 1127 X 25% = 282VA For a total of 1127va where is the 1409 va? we never added the all of the 125% of the motor shall means it is required permissible means you can but not required where did the 25% come from the code says 125% So you see why a person might question these calculations and i have not found anyone who can explain all of this to me. The most I get is basicly this is the way it is done. I would like to here more on this subject if you have more. I hope you can see my problem. Thank You Dwion |
| dwion | Thanks Dave You have given me a great deal of help with this subject. I have enough information to explain this subject. I just can't accept “that’s the way it's done" without a little explanation. I want to thank you again for the help and I will be asking more questions in the future. |