Overload Relay Wiring

I don't know of any standards off hand but I will offer some general comments.
- All the MCC's I've seen come pre-wired with the OL's in the control ckt to drop out the contactor. This has worked well for years, is simple and reliable, so why go to the expense of runinng wires, reconfiguring the MCC bucket and incurring additional labor costs?
- I know PLC's have gotten more reliable over the years but I wouldn't want them performing protective functions. I still think alot of their strength is in the control area.

WBD

Wbd,
Thanks for your comments, I feel the same way. I thought maybe it was a NEMA standard or maybe the NFPA would have something dealing with the wiring practices for overloads, but I can't seem to find it.

See Article 430 NEC Section C for Motor Overload Protection.
Article 110-3 requires materials to be approved for the purpose used, I do not of any PLC Card or PLC application approved for motor running protection.You can use a NO Overload contact wired to a PLC input to detect OL trip.
As to wiring of Overload relay control elements.... a long long time ago only 2 overload elements were required and these were mounted on both sides of the motor contactor and the NC contacts were connected to each side of the contactor coil. These 2 elements gave running protection and single phasing protection for load side single phasing of supply transformer. To protect against single phasing on the supply side of the transformer the third element was required, starters of new designs were introduced using block style overload relays using only 1 NC contact.
Additionally the the NEC allows only control voltage of less than 150 votls to ground to leave the stater enclosure. Manufactures continued to wire one side of the contactor coil to the NC Overload Relay contact.This wire is generally very short and not subject to physical damage or ground and hence would not present a hazzard. There has been much ringing of hands on this subject, but the devices are UL Listed and hence approved for the purpose. Some manufactures of MCC will have an option as to which side of the contactor the NC contact is to wired. ( Starter = Contactor + OL Relay ) {Combination Starter = Starter + Motor Branch Circuit Protection }

On PLC work I normally hard wire the N/C overload contact to the contactor coil and the N/O contact to the PLC for monitoring purposes.
I have no knowledge of any standard which states a preferred option except where operator safety is concerned. In which case software must never be solely relied upon.

Thanks for pointing out article 110-3 "approved". I have used the NO contact for PLC monitoring in the past on new installations but this is a older MCC with only NC contacts. I could also install a interposing relay to give me a NO and NC.
I'm curious as to the scheme with overload elements on both sides of the contactor. I'm not sure what you mean about the transformer. Also couldn't find the NEC article prohibiting control voltages above 150 to ground from leaving the starter enclosure. I know there are alot of system grandfathered in with 480/277 volt control circuits. If you have more information on these items I would appreciate it. Thanks.

The old style starters had two single overload relays mounted on either side of the contactor coil, one side of the NC contact of the overload relays were wired to the respective coil terminals. Control wiring was connected to the otherside of the NC contacts. You are correct as to NEC motor control wiring, I was refering to the Chicago Electrical Code wiring requirements.

On the purpose of the added 3rd OL relay… Two poles will adequately sense overcurrent in 3ø motors except in the case where there’s a lost phase upstream of a wye/delta or delta/wye transformer. In that case one phase may have up to ~1.73x full-load current, and only ~1x on the other phases. That would be undesirable from a protection standpoint.

Most suppliers will add an additional ol relay to a starter if speclified. You can wire three into the starter circuit in the usual manner and connect three to your PLC. This is usually done with heaters of a "lower" rating than the in circuit heaters that takes the motor off line. This gives you a warning that something is going on.
In OL relays where you can get at each of the three OL elements I have seen one installed with a lower value and that one wired to annunciator.

Suggestion: Visit
and concentrate on overload relays, e.g. SMP-3 connection for starters NEMA size 5 up over CTs. It will provide the clue.