Clarifying adding %25 for largest motor in single and multifamily units

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Systems

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Hi
Can anyone clarify for me when do I have to add the +%25 for the largest motor in single and multifamily load calculations?
I have read about it and saw many examples online but sometimes I saw that it should be calculated as name plate and sometimes +%25.

Thank you
please see bellow

When calculating dwelling single unit and multifamily units
Single family
Standard:
Optional:

Multifamily
individual unit within building
Standard:
Optional:

For all building
Standard:
Optional:
 
Systems said:
Hi
Can anyone clarify for me when do I have to add the +%25 for the largest motor in single and multifamily load calculations?...
Click to expand...
I don't know of any exception for omitting the extra 25% for the largest motor in any occupancy type.
 
Systems said:
http://www.iaei.org/blogpost/928475...-Calculations-in-the-National-Electrical-Code

Is this wrong ?
thank you
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Which part of the article is wrong? Read the part on the "Largest Motor Load" again on that article that you gave.
Largest motor load

Section 220.14(C) tells us that motor loads shall be calculated in accordance with the requirements in 430.22, 430.24 and 440.6. For the service calculation, this means that we must determine the largest motor load and add 25% of its value to the total calculation. Common motor loads in residential applications include air conditioning, water pumps, disposals, blowers, etc. Often, the largest motor load in a home is the air conditioner. Even if the air conditioning is dropped from the total load calculation in favor of electric heating (see below), you may still be required to use the AC motor load for this calculation. Check with your jurisdiction to see what the policy is locally. Many jurisdictions publish residential load calculation worksheets to help with determining the size of the service.
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I understand and see what your'e reffering to, but just wanted to clarify that

I understand and see what your'e reffering to, but just wanted to clarify that

examples I see online are wrong, such as this that doesn't calculate 25% for largest motor in optional method
13c_chuntertb2.jpg

So to get it clear, in all 6 options in my first post (as well as any other calculation) I have to add the 25%?!
 
Systems said:
examples I see online are wrong, such as this that doesn't calculate 25% for largest motor in optional method
So to get it clear, in all 6 options in my first post (as well as any other calculation) I have to add the 25%?!
Click to expand...

I think I see the source of your confusion here:

The largest motor in all of the calculations is the AC not the hot tub, since the hot tub load is the combination of motor and heater.
So the hot tub will go in as 8000 in both standard and optional.
In the standard, there is an additional amount of 6000 x .25 added in on top of the 6000 base, for the AC motor.

But in the optional calculation the rule is used which says that the AC can be added in at 100% of its nameplate rating rather than breaking it down as the largest motor load. Whether that is in fact correct is open for discussion, but that matches the numbers entered into table 2.
All of the other loads are added into the total which was then reduced using the demand calculation for the general load.
If the calculation had been done using the heating load instead of the AC load, then a separate item of 6000 x .25 would have been necessary. But since the heat was not included (no electric heating?) then the AC can simply be added in at nameplate value.
 
GoldDigger said:
I think I see the source of your confusion here:

The largest motor in all of the calculations is the AC not the hot tub, since the hot tub load is the combination of motor and heater.
So the hot tub will go in as 8000 in both standard and optional.
In the standard, there is an additional amount of 6000 x .25 added in on top of the 6000 base, for the AC motor.

But in the optional calculation the rule is used which says that the AC can be added in at 100% of its nameplate rating rather than breaking it down as the largest motor load. Whether that is in fact correct is open for discussion, but that matches the numbers entered into table 2.
All of the other loads are added into the total which was then reduced using the demand calculation for the general load.
If the calculation had been done using the heating load instead of the AC load, then a separate item of 6000 x .25 would have been necessary. But since the heat was not included (no electric heating?) then the AC can simply be added in at nameplate value.
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Thanks for your reply, so if I understood what you wrote this is how it should be:

Single family
Standard: Largest motor X 1.25
Optional: Nameplate

Multifamily
individual unit within building
Standard: Largest motor X 1.25
Optional: Nameplate

For all building
Standard: Largest motor X 1.25
Optional: Nameplate

is it true?

also, (in red above) if largest motor is heat ill have to calculate Largest motor X 1.25 in all options?

Thank you
 
Systems said:
Thanks for your reply, so if I understood what you wrote this is how it should be:

Single family
Standard: Largest motor X 1.25
Optional: Nameplate

Multifamily
individual unit within building
Standard: Largest motor X 1.25
Optional: Nameplate

For all building
Standard: Largest motor X 1.25
Optional: Nameplate

is it true?

also, (in red above) if largest motor is heat ill have to calculate Largest motor X 1.25 in all options?

Thank you
Click to expand...
As I interpret it, the selection of Nameplate under optional will apply unless both of the following are true:
1. The largest motor is the one in the AC,
AND
2. The heating load was used in the general calculation (one of the heating lines was used instead of one of the A/C lines of the six options shown specifically under Heating and Air Conditioning). [emphasis is mine]
Heating and Air Conditioning

The largest heating and air-conditioning load must be chosen from six options:

100 percent of the nameplate rating of the air conditioning and cooling
100 percent of the nameplate rating of the heat pump when it is used with no supplemental electric heating
100 percent of the nameplate rating of the heat pump compressor and 65 percent of the supplemental electric heating for central electric space-heating systems (If the heat pump compressor is prevented from operating at the same time as the supplementary heat, it does not need to be added to the supplementary heat for the total central space heating load.)
65 percent of the nameplate rating(s) of electric space heating if less than four separately controlled units
40 percent of the nameplate ratings of electric space heating if four or more separately controlled units
100 percent of the nameplate ratings of electric thermal storage and other heating systems where the usual load is expected to be continuous at the full nameplate value.
Click to expand...

You avoid having to add 25% to the A/C motor load because it will be intermittent rather than continuous and yet you are adding it in at full value rather than applying the demand factor to it as part of the General Load total. IMHO, YMMV

PS: If the largest motor load is the heater (heat pump), then you do not have to add an additional 25% to it, based again on the special rule for Heating and Air Conditioning. You have again added the heat pump motor in at full nameplate rating instead of adding it the General Load where it would be reduced by the demand factor.

And there is no provision telling you to add 25% of the second largest motor load instead if the largest one is in Heating and A/C.
 
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Maybe it was said, but if you use the nameplate MCA rating of the air conditioning equipment, the MCA already incorporates 125% of largest motor in the value listed.

IMO if the AC contains the largest motor for the feeder or service being calculated then that 125% is already included in the MCA and you don't have to account for it a second time.
 
Thank you all for trying to help
I appreciate and respect your opinions but I have to know how to calculate it on the exam based on NEC..:)

Smart $ wrote at the top that there is no scenario that I'm allowed to omit the 25%, I not sure its true, because I think that in a new restaurant optional calculations you add all in nameplates without 25%..

on another subject
On the exam, when there is a calculation question and it doesn't specify whether its standard or optional do I have to use standard?

Thank you
 
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