MCC Bus Rating Sizing 6

[ThePunisher]

When sizing MCC busbars based on load lists, are we sizing the busbars based on 100% load factor for the motors?

Right now, load factors have been considered in sizing the upstream main transformers connected to the switchgear that distributes power to these MCCs.

A load factor of 80% was considered for the motors.

 

[rcw retired EE]

It depends. How much allowance for future growth do you want? Are the motors all running at the same time or are there redundant motors where only one of them will operate? Does the MCC feed hundreds of small motors or just four large ones? Many motors implies more diversity.

Sizing the MCC bus larger than the transformer feeding it isn't economical either.

In my experience, motors operate 75% of the time at 80% of nameplate amps for a total divsersity/demand factor of .60. I size the MCC for 75%-100% of the total load depending on how comfortable I am with the load list, client's plans for future growth, and the type of service. If it is a material handling system where everything could run for 4 hours at close to 100% load I upsize. If it is a building service with a lot of diversity I use smaller demand factors.

For many years we sized transformers and MCC's based on 75% of the preliminary load list. Actual load readings during plant performance tests showed the transformers and MCC's were operating at 75% load. 75% x 75% = 56% But we did find a couple that were operating at 125% of rating and had to be replaced.

Bottom line, use engineering judgement and be prepared to justify your sizing. An inspector could hold you to sizing it for 100% of all loads plus 25% of the largest per the NEC.

 

[dpc]

For sizing overall system power demand, we've used 70% to 80% of the MCC connected load. But for sizing the horizontal bus, I would be a little more conservative, and for the vertical bus, even more conservative. The closer you get to the individual loads, the less you can count on diversity and demand factor.

Again, this will depend on the type of system and how much future growth you want/need to plan on.

 

[waross]

I was always under the impression that the CEC or NEC applied to any motor feeds and that MCC bus bars must be sized at 100% + 25%.
Is there a rule that allows other sizing or exempts MCCs from the 100% + 25% rule??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Bill,

Yes, that is the basic NEC requirement. The load calculations in the NEC do not address industrial facilities very well. Generally, AHJ will accept load calculations done by an engineer that take diversity and demand into account, especially for overall load calculations - especially for industrial plants "under engineering supervision".

 

[rcw retired EE]

Bill- You are correct, codes require the sizing based on nameplate amps with no diversity or demand factors. A strict code interpretation requires that all motor loads be included at 100%. Even duplicate loads have to be included at 100% unless they are interlocked to prevent simultaneous operation. One inspector did not approve software interlocks in the PLC, they had to be hard wired.

NEC does allow some adjustments:

Section 220.60 "Noncoincident Loads. Where it is unlikely that two or more nonconincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service."

Section 430.24 has an exception "where circuitry is interlocked" allowing motor feeder sizing based on the maximum load operating simultaneously.

My 2006 edition of the CEC has a similar exception for interlocked loads, [Paragraph 8-106 (3)], and a more liberal criteria for noncoincident loads in 8-106 (5) “Where a feeder supplies loads of a cyclic or similar nature such that the maximum connected load will not be supplied at the same time, the ampacity of the feeder conductors shall be permitted to be based on the maximum load that may be connected at any one time.”

In the CEC Section 28 for Motors, 28-108 (3) addresses demand factors “Demand factors shall be permitted to be applied where the character of the motor loading justifies reduction of the ampacity of the conductor…..”

For most industrial installations, the AHJ's allow engineering judgement, but not always. That is why I say be be prepared to justify your assumptions and calculations.

 

[jraef]

Yes, as rcwilson and dpc have said, there is no official exemption for MCC (or switchgear) bus sizing, there can't be because if there were, every Tom, Dick and Harry equipment salesman would immediately say they qualified in order to keep their cost down, whether or not they did. But as noted, most AHJs will make load diversity allowances for a system that has been configured by an engineer. It's unofficial but a common practice.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Thank you gentlemen. A round of stars.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ThePunisher]

Thank you for all the replies. In our case, all our motors are conveyor motors...hence it will be quite hard to get proper loading factors from conveyor manufacturers at the time of design phase. Initially, we have considered 80% load factor for purpose of sizing our transformer. The switchgear bus bars and cables connected to the transformer secondary are rated at 100% tranformer kVA rating. However, the MCCs busbars connected to the switchgear were sized according to motor load factor.

If majority of the loads are conveyors, is it going to be safe due to the nature of the motor load to consider 100% in the absence of valid load factor information?

 

[jraef]

100% load factor would likely be safe, yes. 125% would be LEGAL and safe assuming you are in North America).

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

A relevant anecdote:
I did a design for a small shingle mill. Under the Canadian Electrical code, the motor load plus lighting and then raising to the next available transformer size, we were looking at a sizing of one KVA per Horse Power. This would give us lower primary metering rates.
If we went for secondary metering the utility would supply a transformer sized at one KVA per two HP.
The project was cancelled before the decision was made, but I was paid for my work.
In this instance the code required a transformer sized at 200% of the capacity that the utility had found to be adequate for this type of mill.
Remember that when you design an installation, it is expected to function with no transformer upgrades required.
The utility is in a position to exchange a transformer for a larger transformer if it develops that the chosen demand factor is not adequate. Also, the code frowns on overloading transformers whereas the utility may decide to allow a small over load or larger overloads for short durations.
However there is another aspect to code sized transformers. The Canadian Electrical Code allows the addition of loads if it can be shown that the service has sufficient unused capacity for the proposed additional load.
We had a customer who wanted to add equipment to his plant. By code the service had no extra capacity. However the demand charges from the previous twelve months power bills showed that there was unused capacity and the AHJ allowed the addition of the extra equipment on the existing service.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Huh, I've never heard of using Demand charges as a way of determining that there is unused capacity. Makes sense from a logical standpoint, but I don't know if I would trust the utility's engineering to be in synch with their revenue department. I have seen some very questionable practices on their side and here in the US they are "above the code" for the most part.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[jkristinn]

A note on the original topic:

Busbar sizing can be optimized by the physical location of incoming feeders with respect to load feeders. E.g. if the incomers are at the center of a busbar with 50% of the load to the right hand side and 50% of the load on the left hand side, you'd only need to size the busbar for 50% of the total load.

 

[dpc]

Regarding actual load data: The NEC (220.87) allows existing load data to be used when sizing capacity for additions. You are supposed to have 12 months of maximum demand info, and then use 125% of the max existing demand plus 100% of the new connected load.

 

[waross]

At the time, it was common practice in our area to connect recording demand meters to a service for a number of months to demonstrate that there was unused capacity. The Canadian code at that time did not specify the time period. When we had a customer who wanted to add equipment without waiting, we included copies of the last twelve months power bills with the permit application. Twelve months of revenue quality monthly demand KVA values exceeded the minimum requirements of the AHJ at that time. A phone call to the clients accounting department to request copies of the power bills was a lot cheaper than the prevailing practice of installing extra metering for a few months.

Bill
--------------------
"Why not the best?"
Jimmy Carter