MCC Loading Question 1

[hurlieburlie]

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.

 

[jraef]

Technically, and what you will get from the MCC mfrs, you add up the FLA of all connected loads and subtract it from the bus rating. There is officially no diversification factor for MCC sizing. In reality, everyone does it because we all know you are unlikely to have all devices running at full load at the same time. But if you give a load list to the MCC mfr and the total FLA exceeds the bus rating, their software will flag it as an error and require it being overridden manually.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[hurlieburlie]

So if the MCC already exists, then I would still just add up all the FLAs and subtract it from the bus rating? From what I read, most MCC connected loads are actually larger than the demand loads. So if I turned everything on, then the total FLAs would actually be higher than the bus bar rating.

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!

 

[jraef]

Your question was about finding a spreadsheet or calculator to help you and what I was addressing is that because technically, the MCC mfrs cannot say the MCC can be applied using a diversity factor, a "calculator" is unnecessary from their standpoint because legally THEY don't know how you are going to use it. So from their standpoint the diversity factor is essentially 1 (100%) and you just do the Bus Rating - FLA method. As I said though, people USE a diversity factor on their own all the time, because THEY know how the system is used. You just can't expect support from an official source like the MCC mfrs.

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

 

[waross]

According to the Canadian Electrical Code:
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

 

[VTer]

If this is an existing MCC, why not just meter it for a period of time under regular operations and see the demand vs rated capacity.

"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

 

[hurlieburlie]

Thank you guys so much for that answers.

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!

 

[wroggent]

Just a thought.. If you calculate the connected load (i.e. per Code) and it's in excess of the rating of the MCC (good chance that it is in my experience), you can provide an estimate of what it would have cost to install another MCC to serve the items that are in excess of the rating of the original MCC. You can compare that cost to the cost of fitting the MCC with a meter (and thus being able to confirm spare capacity of original MCC). You may need to state something along the lines that the MCC hasn't burned down despite the excess connected load mostly out of luck, to dismiss statements like: "but John Doe was able to figure out how we could add load to this MCC without needing a meter".

 

[jraef]

... 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. ...

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

 

[waross]

, "if it is working, why spend money on it". I am having a hard time justifying spending the money to install these meters.

If your load is under the rated capacity of the MCC and the new load will not put it over capacity there is no problem.
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:

(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).

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

 

[mls1]

We do a lot of studies for customers with similar needs. Our approach is to set up a data logger, usually for a week, and log the load profile of the MCC under typical operating conditions. One has to be very careful to evaluate if there are times when seasonal variations or operating variations will cause periods of higher load. We then evaluate peak load and continuous load. Continuous load we take as the higher moving average of three hours or more. This is consistent with the NEC definition in article 100. We then apply a conservative factor to the measured continuous load, usually 25% but can be as low as 10% if the load is very consistent, and the difference between that value and the rating is the room to add more load. New load is then evaluated with the typical method of 125% of the largest motor and continuous load plus 100% of everything else. Finally, peak loading is reviewed to ensure no protective devices will operate with the new load transients occurring coincident to the measured peak. Keep in mind, this is thermal consideration so by load I mean current load, not kW. If there's not enough room, then we consider PF correction to reduce current demand for a given real load.

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.

 

[waross]

Actually, that method is sanctioned by the Canadian code;

8-106 Use of demand factors (see Appendix B)
...........................
(10) For loads other than those calculated in accordance with Rules 8-200 and 8-202, feeder and service load
calculations shall be permitted to be based on demonstrated loads, provided that such calculations are
performed by a qualified person, as determined by the regulatory authority having jurisdiction.
Appendix B — Notes on Rules
Rule 8-106(10)
It is intended by this Subrule that demonstrated load data could be used for the purpose of sizing of services or
feeders. It is also intended by this Subrule that the qualified person, as determined by the regulatory authority
having jurisdiction, who is responsible for the design should be able, upon request, to demonstrate to the
regulatory authority having jurisdiction that historical data related to actual demand substantiates the fact that
this historical demand is the maximum possible demand for the specific application.

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

 

[Padlock01]

Hi Guys,
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.

 

[Padlock01]

Hi hurlieburlie
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

 

[GroovyGuy]

I agree with Vter. There is only one way to determine if there is any spare capacity on an existing MCC, and that is to measure it. This assumes that the TCL >> Load carrying capacity of an MCC ( ie if your MCC only has a TCL of 500hp and the MCC is good for 1000hp, then it's a slam dunk).

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)

 

[JBUDA54]

If you are trying to be as accurate as you can on diversity then installing meters (like @VTer suggested) on the incoming section will give you a more accurate representation. The NEC even addresses it in determining existing load. Reference NEC Article 220.87. I was on a reliability project where we rented these meters and logged them over a 30 day period an used that metered data to represent the diversity in the connected load. We were also shifting loads around (moving MCCs to other SWGR, etc.) unlike you just evaluating a single MCC so we had more at stake.

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.