MCC Loading Question
MCC Loading Question
(OP)
Hey Everyone:
First time posting so I hope this goes well.
I work at a company that has some old MCCs that are still in use. I am trying to collect all the loading information for said MCCs. I am just wondering if anyone has any examples of how to do a load list.
What I want to know is what is the best way to determine if there is any more room in the MCC for say a 50HP motor or a 100KVA distribution panel? I know that there are other factors to take into account like duty cycle of the load.
Link
Is this the best way to do it? I would love to have a live document (spreadsheet, database, etc.) where I could enter in a load and it will tell me if I am over the capacity of the MCC.
Any help you guys can provide would be awesome.
Thanks.
First time posting so I hope this goes well.
I work at a company that has some old MCCs that are still in use. I am trying to collect all the loading information for said MCCs. I am just wondering if anyone has any examples of how to do a load list.
What I want to know is what is the best way to determine if there is any more room in the MCC for say a 50HP motor or a 100KVA distribution panel? I know that there are other factors to take into account like duty cycle of the load.
Link
Is this the best way to do it? I would love to have a live document (spreadsheet, database, etc.) where I could enter in a load and it will tell me if I am over the capacity of the MCC.
Any help you guys can provide would be awesome.
Thanks.






RE: MCC Loading Question
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: MCC Loading Question
I am just trying to see what I should have in place when a process engineer says that they need a 100HP in this MCC, is it possible (given there is physical space in the MCC)?
Thanks for the reply!
RE: MCC Loading Question
Engineering software packages sometimes offer that, they are a user tool, not a mfr. tool. So for example I've seen one from EasyPower that does allow you to use a diversity factor in your MCC design criteria. Their software is far from free or even cheap, but they do offer a limited time free trial that would include the MCC designer... (hint hint). SKM and ETAP software packages probably offer that too, I just haven't used them in a long time now.
If you are not familiar with what a diversity factor is, it just means that you know that the equipment is only going to be utilized part of the time and only at a partial load. So if for example you run calculations of what runs when and come up with a value of your loads running 75% of the time at 80% of max, you apply a diversity factor of .75 x .8 = .60 or a 60% diversity factor. Therefor if you have an 800A bus and a total FLA of 1200A, but a diversity factor of .6, you should be OK because 1200 x .6 = 720A adjusted load and < the 800A bus.
Just a word of warning though, gathering the data required for a reasonably accurate diversity factor calculation is no small undertaking, especially if your facility users pay little or no attention to what they use when and how hard it is working, which is unfortunately very common. Good luck with that!
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: MCC Loading Question
I would consider the MCC bus bars to be sized as a feeder.
The sum of all loads.
Plus
25% of the largest motor.
Plus
capacitor loads at 135% (Motor power factor correction capacitors directly connected to motors are exempt.)
Plus
25% of the largest transformer.
135% for transformers feeding capacitors.
If this does not show enough capacity for the addition of new equipment you may benefit from this rule;
(9) Where additional loads are to be added to an existing service or feeder, the augmented load shall be
permitted to be calculated by adding the sum of the additional loads, with demand factors as permitted
by this Code to the maximum demand load of the existing installation as measured over the most recent
12-month period, but the new calculated load shall be subject to Rule 8-104(5) and (6).
Check whatever code that you work under.
There are other exceptions in the CEC.
You may have to have a documented history of loading to justify the use of diversity or demand factors.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: MCC Loading Question
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla
RE: MCC Loading Question
jraef: You are correct about facilities not paying attention to what's on and what's off are usually not on their list of priorities. Thank you very much for the helpful information.
waross: Thanks for the reply. I appreciate the information regarding using the code.
VTer: This brings me to another point. I am trying to justify installationg power meters in all of these older MCCs, but I work with a company that has the mentality of, "if it is working, why spend money on it". I am having a hard time justifying spending the money to install these meters. That's one of the reasons I wanted to come ask you people.
Thanks!
RE: MCC Loading Question
RE: MCC Loading Question
One possible option; insist that you need a good portable scope-meter and get one that can record over time. They are too bad now, you can get some decent ones for around $3k. You can at least hook it up temporarily to each MCC main and/or unit and record energy use, then use that data to look for opportunities to save. In many cases if you can DOCUMENT wasted energy and come up with a savings idea, such as adding VFDs to centrifugal pumps or fans, the service utility offers rebate programs to offset the installed cost. I've done that in many cases over the years where I was able to prove a payback of under 18 months WITHOUT the rebate, then when the rebate paid over 50% of the installed cost, it was around 6 months. Even the tightest bean counter can't ignore that.
I helped one guy at a big plant do this at his facility on air quality vent fans that ran full speed all the time and found that most of the time, they needed about half that much air flow, but SOMETIMES then needed it all. So he came up with ways to detect WHEN they needed more and tied that to VFDs on the fans. the entire project paid for itself in 6 months (after the rebates) and was considered so successful, the corporation implemented it nation wide, he got a big "bounty" check based on the cost of 1% of the energy saved, then he retired. But because his retirement was based on his pay in the last year he worked, it bumped his retirement benefit too!
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: MCC Loading Question
However if you must use demand factors or diversity factors, consider the lead time to gather information.
While I am under different rules, this may help:
If you point out a possible 12 month lead time, or whatever monitoring time your AHJ demands, It may be easier to justify the meters.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: MCC Loading Question
The above is the method we use but I would not say it is sanctioned by any standard or code. It is engineering judgment and we do not apply it blindly; we review the distribution system as a whole in the process of calculating available capacity.
RE: MCC Loading Question
If you use one weeks data rather than 52 weeks data, it may be wise to consult with the AHJ and get his written approval for any method used.
That will give you protection in the event that there is a failure and insurance companies become involved.
Another option may be to install load shedding equipment to limit the maximum load to the feeder or MCC capacity.
In any event, the next step may be to consult with the AHJ and determine what data he is willing to accept.
If the AHJ will accept data over a short period it may be best to rent metering equipment.
If the AHJ wants data over a longer period it may be cheaper to buy metering equipment.
Another suggestion that may help;
If the plant is on demand metering, get copies of the power bills for the last 12 or 24 months.
You should be able to show a profile of the plant loading over a year.
This may be helpful in justifying assumptions as to peak demand on an MCC.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: MCC Loading Question
I have uploaded Excel spreadsheet I use for sizing MCC's & transformers etc its mainly for the South African LV voltages.
You may be able to twig it for other voltages.
RE: MCC Loading Question
I have attached a load list we use in South Africa it may be of some use to you. Maybe update LOOKUP with your Motor specification.
Padlock01
RE: MCC Loading Question
Many times I have seen a young engineer try to calculate the load on an MCC, only to be out-to-lunch on diversity and what-runs-when. He then recommends not to add any new load, since he is overly conservative, and then some electrician on the plant says "He's nuts since I went out there and measured only xxx amps).
You should tell your Client to install a recording ammeter on the MCC, for a few days to determine the real loading. These instruments can be rented, or if you have a PLC or DCS, why not buy a single current-transducer, with a 4-20mA output, and log the data in the PLC or DCS.
gg
ps That Young Engineer (mentioned above) was me many years ago in a galaxy far far away.
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: MCC Loading Question
It costs a little bit of money to rent the meters but it gives you piece of mind knowing the true the load diversity. In developing a calculation you are always subject to Garbage In Garbage Out data unless you spend a lot of time gathering data about the various loads and coming up with the diversities.