Fastened in place appliances

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bradlittle

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OK, I have read the archive, and i see that there is ongoing confusion on 'what if one of my fastened in place appliances is also my largest rated motor?' My question is this: with the largest rated motor as one of the 4 fastened in place appliance, do you ignore the fact it is a motor and only figure at 75%; take the largest rated motor X 125% then apply the demand factor (for a total of 93.75% FLC) ; apply the demand factor and then add 25% (for a total of 100%FLC); or (my vote) take the demand (75%) then add an attitional 50% to meet the 125% total from 430?
 
Re: Fastened in place appliances

The highest rated demand factor take precident here,and use 125% to calculate your ocpd
 
Re: Fastened in place appliances

I find the best way to handle calculation questions is to perform one for an example. Lets say I have my 4 appliances that qualify for 200.17:

4500 va Water Heater
1500 va Dishwasher
1000 va Disposal
800 va Compactor

7,800 va Total X 75% Demand = 5850 va

Now, largest motor = 1000 va X 25% = 250 va added

5850 va + 250 va = 6100 va applied to the service.

So, as you can see, I take the nameplate totals, apply the demand, consider 220.14 and 430.24. 430.24 requires me to use the full load current of the largest motor not the demand factor.
Also, I only consider appliances where the primary function is motor drive, and not those that have motors as apart of a process like dishwashers. :)
 
Re: Fastened in place appliances

Bryan, I agree that the most common way to interpret this it that you apply the demand and then add 25%. But that still only gets us to 100%

Figure it this way and see what i mean...

4500 va Water Heater X 75% = 3375
1500 va Dishwasher X 75% = 1125
1000 va Disposal X 75% = 750 - Largest Rated Motor
800 va Compactor X 75% = 600
TOTAL VA demand = 5850 VA
1000 va largest rated motor X 25% = 250 va

if you add the disposal demand to the extra 25% we added for 430.24, we only equal 1000 va. :confused: That is why in my original post i suggested adding 50% to achieve a full 125%

if i were doing an actual calculation for an actual installation, err on the high side. we are only talking about a few amps, which most likely will not affect wire and service size, (200 amps is minimum in our jurisdiction anyway). Then it is only a very isolated incident, as the AC is generally going to be the largest rated motor load, making the discussion moot. however, when teaching apprentices how to do the calc, and preparing them for the journeyman exam, how is it going to be interpreted on the test? is the most widely accepted method going to fly? to me, the code does not address the specifics of 'what if' in this situation, and someone ages ago, said that by applying a demand factor of 75% IF there are 4 or more fastened in place, and IF one of them is also your highest rated motor, that you add 25% of the FLC of that motor, thinking that 75% demand is the same as 100% continuous duty, and adding 25% is equal to 125% of the motor under a demand factor.

[ April 19, 2004, 09:40 AM: Message edited by: bradlittle ]
 
Re: Fastened in place appliances

I completely understand the point your are trying to make and agree the issue is not clear. When performing service calculations, you are required to mix logic. That is, the demand allowance is for an opposite reason than the 125% requirement.

The code knows that the probability of 4 or more appliances putting a demand on a service all at the same time is small, so allows for a decreasing factor. On the other hand, the code rule for motors wants an increase of 25% for the probablity that two or more motors on the same service might start and/or run together.

When all is said and done, I follow each rule and requirement for each said item and not necessarily look at the complete logic of it. :)
 
Re: Fastened in place appliances

We are on the same sheet of music, and i will, as we need to with several other issues on the code, explain logic doesn't always win on code issues. i will tell them to contest it if they get a question that has this specific issue, and if they are right, great, and if they are wrong, maybe the testing authority can 'splain why! :roll:

thanks!

[ April 19, 2004, 12:05 PM: Message edited by: bradlittle ]
 
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