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 Post subject: Lump Motor Contribution and SKM
PostPosted: Mon Dec 09, 2019 5:21 am 

Joined: Thu Jun 09, 2016 6:17 pm
Posts: 6
Good Morning All,

I have recently been in contact with SKM regarding lump loads of motors less than 50hp. When conducting an arc flash evaluation within SKM, there is an option to exclude motors < than a defined HP (50hp in my case). With that said, this made me question how a lump motor load was handled in this situation. For example, in one instance I have (15) 20hp motors. Within SKM, the total HP size is calculated as 300HP (15x20hp). However, I have been informed that the exclude motors < defined HP option applies to the rated HP and not total HP. Therefore, the above example would be ignored because the individual rated HP is defined as 20HP.

I have seen several articles advocating for including lump loads less than 50hp because the contribution could potentially be significant. I understand that the operating sequence/qty of motors running at the same time must be considered as well.

With all that said, I am curious if anyone is utilizing SKM and how they handle a situation like this? Are you just not utilizing the exclude motors < defined HP option and allowing every motor to be included?

Thanks for the input.


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 Post subject: Re: Lump Motor Contribution and SKM
PostPosted: Mon Dec 09, 2019 11:35 am 
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Joined: Mon May 25, 2015 1:17 pm
Posts: 13
Location: Northern California
That is a great question. I just took a look and did not find where the option was to exclude motors less than 50hp? But I use SKM and I include all motors except smaller ones for example on 120V panels. On a manufacturing site the 2009 SKM study I inherited had no motors modeled and the typical substation fault current at 480V was around 30kA. By the time I added motors it was 42kA. I don't know why we would want to exclude small motors, there must be a good reason for this to be an option? But smaller motors are the most likely to be across-the-line started and therefore provide the highest contribution.

On a separate but related note, in my area Sacramento I have seen a surge in the need for power studies. Partly because in the past year the County for the first time to my knowledge requested AF labels in the field. Previously plan check required a note on drawings about compliance with NEC 110.16 which many contractors did with generic non-specific AF labels, like the one that continues to be depicted in the 2017 NEC Exhibit 110.8, that as far as I know some manufacturers continue to supply with their equipment. But another reason for the surge is that new language in 110.24 now has me tell clients that if they want me to do electrical engineering then they must either hire me or hire someone else to do the study and labeling, but it needs to be done the same time as the engineering.

So yes I am utilizing SKM more that ever, right now it is probably 20 hours a week or half time.


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 Post subject: Re: Lump Motor Contribution and SKM
PostPosted: Tue Dec 10, 2019 12:19 pm 

Joined: Mon Dec 07, 2015 9:45 am
Posts: 33
Location: Massachusetts
We do not use the Exclude <50HP motor options when we run our studies for reports and labeling for customers. Especially with 1584-2018 I have not run into a reason to exclude them from final report and labeling purposes, however I can see the potential use for engineering analysis purposes.

When we account for individual motor loads <50HP we will lump them together as you appear to be doing, and we will base the total on the number of motors that can be expected to operate simultaneously. If motors are identified as redundant(ie n+1), then those motor's HP will not be included in the lump sum so as not to exaggerate that total HP calculation.


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 Post subject: Re: Lump Motor Contribution and SKM
PostPosted: Mon Jan 06, 2020 2:05 pm 
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Joined: Tue Oct 26, 2010 5:00 pm
Posts: 188
Location: Maple Valley, WA.
I agree with SteveA. We do the same thing, lump the small motors together and included them in the studies.

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Robert Fuhr, P.E.; P.Eng.
PowerStudies


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 Post subject: Re: Lump Motor Contribution and SKM
PostPosted: Fri Mar 13, 2020 12:12 pm 

Joined: Tue May 29, 2018 8:19 am
Posts: 29
I have generally used a similar approach (lumping small motors) for low-voltage system calculations.

Another engineer has suggested that the standards say it is acceptable to neglect motors <50 hp. IEEE 1584 is silent on how to calculate bolted fault current for arc flash evaluation.

IEEE 141 "Red Book" suggests it is acceptable to neglect induction motors <50 hp for calculating interrupting duty on high voltage circuit breakers, which makes sense. But I'm wondering if this is being misinterpreted as applying to low-voltage systems. IEEE 399 recommends including all motors <50 hp at 1.67X"d for calculating first cycle interrupting duty on low-voltage systems.

Has anyone seen anything else in the published standards that provides clarification on motor contributions to the calculated "bolted fault" current?

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