# Motor Control Center



## Speedy_az (Feb 10, 2008)

I was asked at a job interview, by a Master electrician " What is the lowest legal Control voltage allowed in a MCC?" I didn't know the answer. I looked in the Code book can't seem to find the answer.....


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## 480sparky (Sep 20, 2007)

Where in the Code are you looking?


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## Speedy_az (Feb 10, 2008)

*nec 08*

Article 430 VIII


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## nap (Dec 26, 2007)

So, have you found anything, especially in that section, that would limit the voltage allowed in an MCC?


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## micromind (Aug 11, 2007)

I've never heard of a 'lowest control voltage' in an MCC. I guess it would need to be higher than 0. lol. 

I've done plenty of MCCs that had PLCs built into them. The communication voltage to a PLC is pretty low. 

The lowest voltage I've ever connected to a starter coil is 24AC. 

Rob


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## Speedy_az (Feb 10, 2008)

I was thinking the same thing... I have took whatever control voltage the mcc was using an installed interposing relays to start/stop the motor.... Maybe a trick question???? Thanks for your help.


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## nap (Dec 26, 2007)

Speedy_az said:


> I was thinking the same thing... I have took whatever control voltage the mcc was using an installed interposing relays to start/stop the motor.... Maybe a trick question???? Thanks for your help.


since this was a question at an interview, i suspect it was a trick question merely testing your knowledge.

So, what did you give them for an answer? Did you get the job?


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## Speedy_az (Feb 10, 2008)

Don't know if I got the job yet. I'm sure they had more interviews. I said 24v but I wasn't sure.


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## 480sparky (Sep 20, 2007)

Speedy_az said:


> .... Maybe a trick question???? ....


 
That's my thinking.

I wonder if it's possible the answer is zero. Where is it written in stone it has to be _electrical_ energy? Why can't a controller be pneumatic, or hydaulic, or some other form of energy?


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## g17guy (Oct 2, 2008)

I have pulled a lot of polyflow into plc cabinets, but have yet into a MCC. I don't see why you would not be able to. 

I was also thinking about 4-20 mA; however, I can not think of any reason where you would need to pull that to or from an MCC as it is usually just from the PLC to the device.


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## MDShunk (Jan 7, 2007)

4-20ma is 10 volts. The lowest I've ever seen to a coil is 24 volts. I think he was just screwing with your head.


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## sparky970 (Mar 19, 2008)

MDShunk said:


> 4-20ma is 10 volts. The lowest I've ever seen to a coil is 24 volts. I think he was just screwing with your head.


 
How is 4-20mA equal to 10 volts?


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## MDShunk (Jan 7, 2007)

sparky970 said:


> How is 4-20mA equal to 10 volts?


Typically that's close to the voltage on the loop.


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## sparky970 (Mar 19, 2008)

MDShunk said:


> Typically that's close to the voltage on the loop.


All of the loops at our plant require 24Vdc and have a lower threshold voltage for them to operate.


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## nap (Dec 26, 2007)

sparky970 said:


> All of the loops at our plant require 24Vdc and have a lower threshold voltage for them to operate.


it's not what your plant requires but what the equipment requires. The mA inputs utilize an ~10 volts source. You cannot arbitrarily change that. They are self supplying.




> g17guy
> I was also thinking about 4-20 mA; however, I can not think of any reason where you would need to pull that to or from an MCC as it is usually just from the PLC to the device.


but there are MCC's with PLC's built into them so getting from the PLC to the device and the inputs and outputs of the PLC require the conductors going through the MCC.


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## randomkiller (Sep 28, 2007)

Never figured there would be a minimum voltage because as other stated most VFD's either run from a 0-10vdc or 4-20ma signal.


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## bjfred (Oct 31, 2008)

More often than not,the lowest control voltage used in an MCC is 24Vac, this may be used to energise some lowly rated contactor coil or some relays present in the circuit.


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## John Valdes (May 17, 2007)

I have to agree with Speedy. The answer has to be (0)? The master that asked the question probably does not even know himself, or thinks he knows. He probably heard a number and just believed it. There is really no correct answer to this question. Hopefully if you do get this job, he will not be your boss. I would probably last less than hour with this guy.

Ps.....If you get the job, ask for the correct answer and lets us know what he says.


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## Kletis (Jan 18, 2008)

Well...if you had a 480VAC coil on a contactor, technically you would have negative 480V 60 times per second...in fact you would have negative 680V at your peak negative voltage. I would say that the lowest control voltage would depend on the actual voltage rating on the MCC...I don't think this is what he was looking for...but I have never read a code that tells me that there is a minimum control voltage!! When in doubt, guess!!


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## Nodoggie (Oct 17, 2008)

look, the fact of the matter is...this is a trick question, and actually rather nonsensical. There _is_ no minimum control voltage mandated by the NEC. Point blank. So any response is meaningless. You obviously can have no current flow if there is a zero potential, so anything over 0 volts (or under 0 volts in a negative-potential system) would be the minimum that could make a circuit work. 

The minimum voltage you could _make _would be a single electron or negatively charged particle flowing in the circuit. However, the NEC does not define a minimum. It's a ridiculous question. Your so-called "master" is either pulling your leg, or trying to sound intelligent, or just a plain idiot.


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