Motor Control Centers vs. Motor Control Panels

Does anyone have any comments on Motor Control Centers vs. Motor Control Panels? I have been debating this in my head and I am looking for other opinions. Here are a few pros and cons:


An MCC gives you individual lockable switches for every motor or load.

You could door mount individual IEC switches in a MCP but this would get difficult for larger motors.

An MCC allows you to de-energize a bucket for maintenance or troubleshooting and keep all of the other loads on (of course the line side of the switch or circuit breaker is still energized).


For smaller loads an MCC really seems like a waste of money. For example six 1 HP motors might take one whole vertical section of MCC. The same equipment in a panel would take a few feet of DIN rail and some power distribution blocks.

try posting your question on mike holt's electrical website
www.mikeholt.com

Mcc's are big & bulky.
Your idea of placing iec's on the motor panel door needs to be looked into. Your idea is good, but, what's behind the door?
A lot more wires and no true loto.

regards,
james

I quoted a job using a AB Intellecenter with 2 spare blank stacks. I was going to use the spares for the PLC. It worked out cheaper to have the Powerflex VFDs preinstalld by RA. So all I had to do was wire the PLC. I couldn't buy the hardware for what they could deliver it to me for.
I a sure there is a point that the panel and parts are cheaper than the MCC. I haven't done that many large systems so I couldn't tell you where it is.

James Mcquade said:

try posting your question on mike holt's electrical website
www.mikeholt.com

Mcc's are big & bulky.
Your idea of placing iec's on the motor panel door needs to be looked into. Your idea is good, but, what's behind the door?
A lot more wires and no true loto.

regards,
james

Click to expand...

James

Thanks for the link to mikeholt. Let me clarify the IEC's on the door. They would be installed in addition to a main disconnect for the whole panel.

I've been round and round this topic throughout my career of 20 years, which is short to many here on this board. You nailed the pro's in your original post (OP).

The cons of MCC's are space and cost. As previously posted if you have a lot of small HP motors you eat up money and space with MCC's.

Another consideration is automation. With MCC's there are interface issues to deal with. When all the starters are in one panel we wire confirm inputs back to the plc and use a touchscreen to provide xyz functionality. With MCC's that are on site, they might have Hand Off Auto controls and a confirm input and the time it takes to provide a seamless automation solution for the end user is compounded.

In my experience we can provide a control panel with all motor controls and a seamless automation solution for much less money and space than with an MCC. With MCC's the interface to the automation is a concern, not so with a control panel.

When we use MCC's it is at customer request and then we ask for start-stop buttons if the application permits. We wire our plc in parallel with their start, in series with their stop, using interface relays. They end up with local and remote control. If the plc command the motor to run and they stop it locally the same fault occurs as if the motor trips out. But we can add a button to the touchscreen: local control and so bypass this.

The H-O-A switch is problematic because if the motor is running in hand and plant power goes off, when power is restored all motors in Hand will restart. Not good with 200hp fans, etc. So we don't like H-O-A.

With MCC's when a bucket goes bad you can remove it and insert another and be on your way. That benefit cannot be ignored, you'd be changing a motor starter out of a control panel and have to kill the panel (or group if so designed). However if a process is dependent on all the motors to run then this feature is not nearly as important. For a process a control panel is much more appealing for the seamless automation, cost savings, and space savings.

If you have 20 exhaust fans in the roof with no automation then an MCC makes sense. For a process, I would say no.

Hope this helps!

Paul B

Thanks for your comments. About automation with an MCC, I like to install a remote rack right in the MCC. I usually use a 2 space factor empty bucket and mount the Flex I/O in the bucket. The connections from the PLC panel would include a comm cable (Remote I/O or Ethernet) and control power. Wiring the starters is then all internal to the MCC.

With your all in one panels, do you have any rule of thumb limits, like 100 or 200 amps per panel. Or, maybe a HP limit where it makes sense to use an MCC?

Please note most vendor have High-Density Starters which will let you do 6" high Size 1 mags. Now you got decent density with 12 starters per section and you still get all the safety and plug and playability.

I don't think it is cut-and-dried as to when it makes sense to use a motor control center. There are many factors that come into play. It depends on the nature of the project.

MCC Advantages:
Tested and rated for nameplate volts, amps, and interrupting-current rating (custom panels are not).
Factory Warranty.
Factory-wired starters, breakers, panels, and many other items.
Floor-mounted, stand-alone(in some locations, wall space for a motor panel is hard to find).
For large projects, can be quicker than building your own.

For example, if it is a large design-and-build job with several factory buildings and motor locations, then motor control centers become more useful. You can use the Allen-Bradley IntelliCenter software and layout several complete MCCs, call a distributor, and Email the program output, and order 4 or 5 MCCS in less time than it would take to make one layout of the first Motor Control Panel. The software is really great, giving you layout, spreadsheets, elevation drawings, and a complete parts list. You can include in the MCC Devicenet or Ethernet connections to every starter and circuit breaker, where interfacing the MCC to a PLC will require only one cable.

Where the budget is small and the customer is not too picky, I have bought used or reconditioned motor control centers, which are widely available.

If it is a smaller job with one set of motors in a limited area, and you are also doing the PLC and control system, then a custom-designed panel will be much cheaper and probably just as quick.

With your all in one panels, do you have any rule of thumb limits, like 100 or 200 amps per panel. Or, maybe a HP limit where it makes sense to use an MCC?

Click to expand...

200 Amps or less: All-in-one custom panel probably.
200 to 400 Amps: Either way, MCC or custom motor panel, depending on situation.
Above 400 Amps: Probably use MCC, unless other reasons. In this category, you have to start worrying about things like short-circuit currents, excess heat in the panels, grounding difficulties.

The H-O-A switch is problematic because if the motor is running in hand and plant power goes off, when power is restored all motors in Hand will restart. Not good with 200hp fans, etc. So we don't like H-O-A.

Click to expand...

Due to safety concerns, I would never design a motor control circuit with only a H-O-A maintained-contact switch. If you also include Start & Stop pushbuttons along with the HOA, then the safety problem can be eliminated - motors do not start until "Start" is pressed.

Another way I have used is a 3-position selector switch with a spring-return momentary-contact Hand position, so that the switch stays in either Auto or Off, and only goes to Hand momentarily when someone turns and holds it there.