Motor Circuit Protectors & Motor Circuits

Status
Not open for further replies.
T

timosborne

Member
As part of an energy savings initiative, we recently installed VFD's on multiple air-handling units in our facility. The previous motor circuit for each unit consisted of a Square D Mag-Gard motor circuit protector and starter w/ overloads in MCC bucket feeding the respective unit. We removed starter in bucket circuit and wired directly tp MCP. The VFD was installed at the unit. We were told we violated code section 240.4 that states that all current carrying conductors shall be protected against overcurrent in accordance with their ampacities specified in 310.15. The reviewer stated that MCP's do not provide overcurrent protection and we need to replace the MCP with a thermal magnetic breaker.

The way I understand it is that the MCP provides instantaneous-trip magnetic-only and when installed as part of a UL Listed combination controller having motor overload protection it is code compliant. The VFD we installed does have thermal protection capability. From what I've understood, the thermal overload protection can be anywhere in the circuit between the motor & the overcurrent protection device at the source correct - not always at the start of the circuit/source? I believe we have that with the VFD's ability to provide thermal overload protection or do we have a code issue? VFD does not have bypass.

Do we need to replace all MCP's with thermal-mags as stated by the reviewer?

Thanks,
 
I believe your reviewer is correct. Others can explain in much further detail than I can.

My question to you is why adding VFDs to your air handlers will be expected to save money? They should have been sized correctly to begin with. Are you changing speeds often enough to warrant them?
 
ptonsparky said:
I believe your reviewer is correct. Others can explain in much further detail than I can.

My question to you is why adding VFDs to your air handlers will be expected to save money? They should have been sized correctly to begin with. Are you changing speeds often enough to warrant them?
Click to expand...
Maybe the air handling units currently use dampers to control air flow in which case there would be a good case for VFDs.
 
ptonsparky said:
I believe your reviewer is correct. Others can explain in much further detail than I can.

My question to you is why adding VFDs to your air handlers will be expected to save money? They should have been sized correctly to begin with. Are you changing speeds often enough to warrant them?
Click to expand...

System is VAV type. AHU's have vortex dampers controlling air flow. When all boxes downstream are at minimum, static rises and signal is sent to vortex to close yet motor is still running. Removed and/or locked open the respective vortex dampers to each unit and took 4-20mA signal from vortex to new VFD to turn-back motor/reduce air flow and save energy. Tremendous reduction in kWHR.
 
timosborne said:
... MCP provides instantaneous-trip magnetic-only and when installed as part of a UL Listed combination controller having motor overload protection it is code compliant
... Do we need to replace all MCP's with thermal-mags as stated by the reviewer?
Click to expand...

As mentioned, Yes. You already gave the answer:
timosborne said:
... MCP provides instantaneous-trip magnetic-only and when installed as part of a UL Listed combination controller having motor overload protection it is code compliant ...
Click to expand...

430.52.C.3 Instantaneous CB

The instantaneous CB has to be part of a listed with coordinated overload protection.

The starter is removed (and, you did not say, but the overloads as well) The bucket is no longer a "UL listed combination motor controller

Also true if a Motor Short Circuit Protector (430.52.C.7)
 
iceworm said:
As mentioned, Yes. You already gave the answer:


430.52.C.3 Instantaneous CB

The instantaneous CB has to be part of a listed with coordinated overload protection.

The starter is removed (and, you did not say, but the overloads as well) The bucket is no longer a "UL listed combination motor controller

Also true if a Motor Short Circuit Protector (430.52.C.7)
Click to expand...
This.

Even without that, what does the drive require for an overcurrent protective device? Probably not an instantaneous CB.

Might be acceptable to leave the breaker as the disconnecting means and use some supplemental fuses - many drives want expensive semiconductor fuses anyway - though all it really does is lessen how spectacular a front end of drive failure may be should that happen.
 
kwired said:
... Might be acceptable to leave the breaker as the disconnecting means ....
Click to expand...

Nope. The only place one can use an instantaneous is part of a "listed combination ...."

kwired said:
... many drives want expensive semiconductor fuses anyway....
Click to expand...
Yep. Generally the drive will either have them, or the installation guide will specify.
 
kwired said:
I was thinking you could use it basically as a switch but you would need fuses that are listed as branch circuit OCPD to go with it.
Click to expand...

Yes, I figured that was the case.

430.52.C.3 is hard to have an alternate translation:
(3) Instantaneous Trip Circuit Breaker. An instantaneous trip circuit breaker shall be used only if adjustable and if part of a listed combination motor controller having coordinated motor overload and short-circuit and ground-fault protection in each conductor, and the setting is adjusted to no more than the value specified in Table 430.52.​

I am not suggesting that what you are advocating is dangerous. Just consider explaining it to you as the AHJ:

(Following is poking fun - not just poking)

AHJ (you): Lets see this is an industrial grade client. You spent 4 hours, tearing out the starter/overload and installing a $150 block and fuses in an attempt to get around 430.52.

Contractor: Yep that's us.

You: And the local vendor has the correct CB is stock for less than $300
C: yep

You: ??????
 
kwired said:
I was thinking you could use it basically as a switch but you would need fuses that are listed as branch circuit OCPD to go with it.
Click to expand...
The conductors extended from the MCP would have to be feeder tap conductors and comply with those rules.
 
iceworm said:
Nope. The only place one can use an instantaneous is part of a "listed combination ...." ...
Click to expand...

kwired said:
I was thinking you could use it basically as a switch but you would need fuses that are listed as branch circuit OCPD to go with it.
Click to expand...

don_resqcapt19 said:
The conductors extended from the MCP would have to be feeder tap conductors and comply with those rules.
Click to expand...

kwired said:
Correct, and most the time if still supplying same motor as before and they don't leave the MCC bucket, probably are going to be in compliance.
Click to expand...

Note to inspectors listening in: You have my sympathy.
 
iceworm said:
Note to inspectors listening in: You have my sympathy.
Click to expand...
Some inspectors, especially those that don't inspect much industrial, may not be all that sharp on this kind of thing, so the sympathy is that we have now confused them with more information to have to remember.:)
 
Note to inspectors listening in that understand the issue - yes, all three of you:
You have my sympathy
 
Mag only breakers have no legal use outside of tested and listed assemblies. Even as a switch in combination with some other branch or feeder OCPD because nobody will have tested and listed them that way, why would they?

Around 2005, UL even ceased allowing mfrs to list combination controllers using VFDs and Soft Starters with Mag-Only breakers, they all must be Thermal Mag (or fuses) now. Existing listed combos were allowed to continue, but most mfrs have had to redesign drives and soft starters because of RoHS rules so the new versions all have TM breakers or fuses.

In your situation when you removed the contactor and overloads, you voided any listing of that bucket. But even if nobody seems to care about listing after the fact, it’s still a code violation to leave it as is and your insurance company may be a problem with it, as in if anything should go wrong, you may have nullified coverage of any damages.
 
iceworm said:
Yes, I figured that was the case.

430.52.C.3 is hard to have an alternate translation:
(3) Instantaneous Trip Circuit Breaker. An instantaneous trip circuit breaker shall be used only if adjustable and if part of a listed combination motor controller having coordinated motor overload and short-circuit and ground-fault protection in each conductor, and the setting is adjusted to no more than the value specified in Table 430.52.​

I am not suggesting that what you are advocating is dangerous. Just consider explaining it to you as the AHJ:

(Following is poking fun - not just poking)

AHJ (you): Lets see this is an industrial grade client. You spent 4 hours, tearing out the starter/overload and installing a $150 block and fuses in an attempt to get around 430.52.

Contractor: Yep that's us.

You: And the local vendor has the correct CB is stock for less than $300
C: yep

You: ??????
Click to expand...

Thanks for the input ( and the humor)!!!!!
 
Would the modification be code acceptable if the VFD had a main breaker in the drive as its disconnecting means from the source? (By the way, overloads were removed in the starter). Much appreciated!
 
timosborne said:
Would the modification be code acceptable if the VFD had a main breaker in the drive as its disconnecting means from the source? (By the way, overloads were removed in the starter). Much appreciated!
Click to expand...

No, because as I said, the MCP cannot be listed as a stand alone device for any purpose. It’s ONLY valid use is as a COMPONENT in a subsequently listed assembly.

Even if someone were to have listed it as a “Molded Case Switch”, think about it this way:

If it is not the branch, it’s a feeder. So could you legally put a non-fused disconnect in a switchboard and use it as a feeder just because there is a branch OCPD at the other end? No you can’t. So that scenario with trying to use the MCP as a switch is no different.
 
Jraef said:
No, because as I said, the MCP cannot be listed as a stand alone device for any purpose. It’s ONLY valid use is as a COMPONENT in a subsequently listed assembly.

Even if someone were to have listed it as a “Molded Case Switch”, think about it this way:

If it is not the branch, it’s a feeder. So could you legally put a non-fused disconnect in a switchboard and use it as a feeder just because there is a branch OCPD at the other end? No you can’t. So that scenario with trying to use the MCP as a switch is no different.
Click to expand...
I would permit that installation as a feeder tap, even if I had to resort to using 90.4. It is not less safe with the MCP in the circuit as compared to a direct connection to the feeder.
 
Status
Not open for further replies.
Top