220.18(A) and Load Calculations

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k-rag

k-rag

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I have heard it two ways when it comes to considering the largest motor in a load calculation. Can someone help clarify this for me?

I am teaching a class on load calculations right now and I need to makes sure I have this right. The curriculum doesn't seem to spell it out clearly, (though, I admit, I'm probably missing something obvious)

The question (as an example):

Using the standard method, calculate the service load for a 2,200 square foot single family dwelling containing the following loads: range: 14kw at 240v, dryer 4.20kw at 240v, dishwasher 1.20kVA at 120v, waste disposer 1/4HP at 120v, water heater 4.2kw, a/c: 2 units 12A at 120V, electric heat 6kw at 240v, and attic fan 1.5HP at 120v.

the answer:

STEP 1: GENERAL LIGHTING
2200SQFT x 3 = 6600VA
+small appliance minimum: 3000VA
+laundry minimum: 1500VA
=11,100VA
FIRST 3000VA AT 100% = 3000VA
8100VA AT 35% = 2835VA
TOTAL GL: 5835VA

STEP 2: FASTENED IN PLACE APPLIANCES:
DISHWASHER: 1200VA
DISPOSAL: 696VA (PER 430.248)
ATTIC FAN: 2400VA (PER 430.248)
WATER HEATER: 4200VA
TOTAL: 8496 X 75% = 6372VA
TOTAL APPLIANCE LOAD: 6372VA

STEP 3: DRYER LOAD MINIMUM = 5000VA

STEP 4: COOKING EQUIPMENT: 14KW PER TABLE 220.55
COLUMN C = 8KW
+ 5% PER KW OVER 12 (2 X 5% = 10%)
8000 X 110% = 8800VA

STEP 5: NON-COINCIDENTAL LOADS: A/C AND HEAT
WE CAN OMIT A/C AND USE 6000VA AT 100%


STEP 6: LARGEST MOTOR. This is where the curriculum deviates from my interpretation. I would think you would need to take the 1.5 HP motor and consider the additional 25% for the calculation. OR if the A/C were the largest motor I would add 25% of that VA rating to the calculation.
I would think:
Largest motor: Attic Fan 2400VA
100% has alread been considered in the appliance load, but we need to add the 25%
2400VA x 25% = 600VA
 
Good question In (2020 NEC) 220.18(A) seems to be dealing with branch circuit calcs.
220 part III -> 220.50 addresses motors in service calculations and says to use 430.24 , 430.26 and or 440.6.
430.24(1) seems to require the 125%
 
Agreed 220.18 is BC calcs and doe snot apply to OP question.

I do not think the AC is calculated at 125% for the feeder and service load calcs if its hermetic.
220.50 specifically speaks to hermetic and says those are calculated by 440.6. That speaks to nameplate rating. I do not see anything in 440.6

430.24 is is Part 2 of Article 430 which is only speaking to Motor circuit conductors or motor feeders, and not Service load or feeder calcs.

I may be wrong if I am I would love to learn.
 
Thank you for the responses! That's very helpful. Reading 220.18 again, it's a little embarrassing that I never made that connection. :)

As far as 220.50...

For A/C, that seems clear now. Since there is not a requirement to increase by 25%, I see no inconsistency.

But that still leaves me wondering...
Why would 220 part3 (Feeder and Service Load Calculations) refer to 430.24 if it didn't mean for it to be considered in the service load calculation?

If I remember correctly (from my apprentice training school), the idea of adding the 25% to the top (though not expressly detailed in the NEC) dealt with the apparent contradiction:
A demand factor of 75% (4 or more appliances), but also an increase of 25% for the largest motor....

This may be a negligible point in the practical world, but it's important to me that I'm not perpetuating misinformation.

Thanks again for the help.
 
Article 220.50 mentions 430.24 for "Motor Loads" then it specifically calls out 440.6 for "hermetic refrigerant motor-compressors.

Residential all in one hermetic compressors are different than a larger commercial size unit with separate motors, as a result we can use nameplate.

I can follow the 125% for other calculations but where does the code refer to 125% of largest hermetic AC motor a for Service?
 
I'm convinced about air conditioning at this point. I see where I was mistaken there.

But what still remains unclear is other motor loads such as attic fans, irrigation pumps, etc. Since 430.24 refers specifically to multiple motors, it may be arguable that if there is only one it the whole house, it need not apply. But usually that's not the case since there are other motors integral with appliances and otherwise. In the example I shared above, the attic fan is the largest motor, and there are other motors in the load such as a garbage disposal, a dishwasher, and even the dryer.

Then I thought, "maybe it doesn't apply to the loads dealt with in 220.52-220.55.". But if that's the case, I can't think of anything that is a motor load that isn't also either a small appliance, appliance fastened in place, a dryer, or an air conditioner. Nor do I find any code to support this. So what does that leave us with?

Perhaps more to the point are these questions:

1. If it's not meant to apply to service load calculations, what is the point of 220.50 (other than the reference to air conditioning)?
2. If it is, and 220.50 does not apply to fastened appliances, or dryers, or anything else classified, by 220.52,53, or 54, than what does it apply to?
3. If it does apply to those loads, what are we supposed to do with it?

I get the sense, that I'm still missing something obvious, but I'm not seeing it yet.

thanks again.
 
k-rag said:
Then I thought, "maybe it doesn't apply to the loads dealt with in 220.52-220.55.". But if that's the case, I can't think of anything that is a motor load that isn't also either a small appliance, appliance fastened in place, a dryer, or an air conditioner. Nor do I find any code to support this. So what does that leave us with?
Click to expand...
Pool pumps, shop equipment like a 5HP planer, well pump, sewage pump, water jet pump, sump pump, hydronic water circulation pump etc...

The point of 220.50 is motors in general.
If you had a well pump and a grinder pump in the basement. You would include those loads and add an extra 25% of the larger of the two. If you don't have motors that fit that then you don't need to add the calculation.
 
mikeames said:
Pool pumps, shop equipment like a 5HP planer, well pump, sewage pump, water jet pump, sump pump, hydronic water circulation pump etc...

The point of 220.50 is motors in general.
If you had a well pump and a grinder pump in the basement. You would include those loads and add an extra 25% of the larger of the two. If you don't have motors that fit that then you don't need to add the calculation.
Click to expand...

Article 100 says:
"Appliance. Utilization equipment, generally other than industrial, that is normally built in standardized sizes or types and is installed or connected as a unit to perform one or more functions such as clothes washing, air-conditioning, food mixing,deep frying, and so forth."

Do you consider those examples to be "appliances fastened in place"? (5HP planer, well pump, sewage pump, water jet pump, sump pump, hydronic water circulation pump etc...)
Is a 1-1/2HP attic ventilator an "appliance"? Are you saying the example doesn't consider the 25% because there is only one large motor? How large is large enough to consider?
What motors do and do not qualify to be considered?

In your example of the well pump and grinder pump, if they are among 4 or more fastened appliances, do you agree that they get the 75% demand factor plus the 25%?
 
I guess I don't know. I consider things like a well pump a motor. That's all they are. They are not appliances as we may think of them. Appliance stores don't sell pumps. Their examples in the definition are what we would call appliance.

That said, my opinion obviously means squat. The code says what it says, and so I see your point. How would the NEC define a motor? The way they define an appliance would also qualify a motor as an appliance. IF you put those into the "appliance" category then I would not add the 25% because they are appliances. IF your classify them as motors then they don't get the 75% demand factor for 4 or more. Somebody may chime in and school me, that's fine.

I don't see an attic fan as being an appliance. In my opinion you define appliances, and then you identify motors. I have provided examples of what I consider each to be. Once you know what your motor loads are then you calculate them accordingly.

The one flaw I see in my logic is garbage disposals are considered appliances even though they are just a motor. So I guess I don't know. Perhaps someone else??
 
mikeames said:
I guess I don't know. I consider things like a well pump a motor. That's all they are. They are not appliances as we may think of them. Appliance stores don't sell pumps. Their examples in the definition are what we would call appliance.

That said, my opinion obviously means squat. The code says what it says, and so I see your point. How would the NEC define a motor? The way they define an appliance would also qualify a motor as an appliance. IF you put those into the "appliance" category then I would not add the 25% because they are appliances. IF your classify them as motors then they don't get the 75% demand factor for 4 or more. Somebody may chime in and school me, that's fine.

I don't see an attic fan as being an appliance. In my opinion you define appliances, and then you identify motors. I have provided examples of what I consider each to be. Once you know what your motor loads are then you calculate them accordingly.

The one flaw I see in my logic is garbage disposals are considered appliances even though they are just a motor. So I guess I don't know. Perhaps someone else??
Click to expand...

Thanks again for the dialogue.

In the practical world, (in my experience) this only becomes relevant in those rare moments when a few hundred watts push the final number just over a standard breaker size. Usually though, I'll calculate a load, then oversize it for expansion, thus making a few hundred watts irrelevant.

Like I said, this is more to make sure I'm teaching others correctly. And if questions like the one above show up on a Journeyman's exam for one of my apprentices, I'd hate to be the reason they got it wrong.
 
Yea I get frustrated as well with how unclear it all is. A service load calc should not be a scavenger hunt through the code. I don't think Appliances is well defined. I think the code could make it more clear on appliances just like they did for dryers, cooktops, AC and Heat. I agree with your earlier comment if fans, and pumps etc are all considered appliances then what the heck isn't? If those are are not considered appliances then why is a garbage disposal that is simply a motor under the sink considered an appliance? Inconsistencies.
 
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