Proper way to wire overloads

Maintenance Man · Nov 25, 2009

Hi to all
I have a question about wiring up industrial 3 phase motor starters. At this time I am taking a Mechtronics course at our local tech school in which they had us wire up a motor starter. The plant I work at wires motor over loads with extra protection by wiring the common control wire through the NC contacts on the overload then to A2 on the contactor. A1 of the contactor has the hot wire for 120v ac. The instructor told me that having the common go through the NC contacts on over load is not necessary. What I would like to know, is if wiring a neutral through the NC contacts on the overload to open the neutral, if the overload trips, is used in most industries as an extra protection or is this something that the company I work for is only doing? A maintenance supervisor told me they had some contacts weld together, which did not stop the motor, which damaged it so this was the reason for breaking the neutral going to A2 to stop the motor. Thank you for your help in this question.
Sincerely:
Maintenance Man.

leitmotif · Nov 25, 2009

Neutral should never have a switch in it for reasons you said. Lot of motor control diagrams show the overload contacts in neutral (commonly to right of coil on drawing). They should be on the left side ie in the hot especially so in case of 120 and grounded neutral.

And this is how many decades of NEC not allowing switches in neut (with few exceptions just to keep confusion and arguments going of course) ??

Dan Bentler

Maintenance Man · Nov 25, 2009

To make my post more clear, I was talking about the NC contacts on the overload that will open when it trips and not the NC contacts on the relay which open when it is energized.
Thank You

Maintenance Man · Nov 25, 2009

Thank you very much for your help. So we are out of compliance with the NEC and could be causing a potential problem of a fuse not popping or breaker not tripping? Or someone getting shocked because we have opened a grounding path? I just want some ammo to argue this with upper management to make them do the correct thing or at least let them know the dangers involved. We are using a grounded neutral on our transformers and all our cabinets are earth grounded.
Thank you very much for your help.

Christoff84 · Nov 25, 2009

We also wire our motor starters this way. Wire from A2, to the NC contacts on the overload, then out to Neut or -DC.

As an aside, I was told by one of our electricians, that using the style of overload that hangs from the load side of a contactor, this is the proper way to wire it as it does not have contacts inside itself to interrupt the 3 phase (actually seen where the O/L was tripped, but wasn't wired this way and burned out the motor). Using a completely seperate, self contained overload I don't think this would be necessary as it has self contained contacts, but its a nice backup.

That being said, I agree with Dan that you shouldn't switch the grounded side of a load, always switch the hot side.

dogleg43 · Nov 25, 2009

Any bets that this will ultimately turn into a 500-comment post?

Here is one big problem with leaving them as wired from the factory: If you're not careful and install a "Motor Running" pilot light then the overloads can trip but the light will still be ON. Making a field connection for the pilot light directly to the start coil always looks messy

All NEMA starters are factory wired the way you describe and since it is a factory connection I think (emphasis on think)that the NEC does not apply to that connection. So as long as YOU do not wire anything in that can break the neutral then everything is OK.

Why do they do it that way???? Who knows....tradition probably and that does not necessarily make it right.

jimbo3123 · Nov 25, 2009

This has been discussed before. You might want to search.
Here's one thread: http://www.plctalk.net/qanda/showthread.php?t=18682&highlight=overload

To recap what I posted there, It is not against code to wire an overload on the right or neutral side of the coil.

jimbo3123 said:
NFPA 79 (2002 Edition) deals specifically with this.
Section 8.3 says:
"Control Circuits shall be permitted to be grounded or ungrounded. Where grounding is provided, that side of the circuit common to the coils shall be grounded at the control transformer" ...
"Exception No. 2: Overload relay contacts shall be permitted to be connected between the coil and the grounded conductor where the conductors between such contacts and coils of magnetic devices do not extend beyond the control enclosure"

leitmotif · Nov 25, 2009

jimbo3123 said:
This has been discussed before. You might want to search.
Here's one thread: http://www.plctalk.net/qanda/showthread.php?t=18682&highlight=overload

To recap what I posted there, It is not against code to wire an overload on the right or neutral side of the coil.

Umm boy on the way to 500 posts.
OK so we have NFPA 70 Natl Electric Code
and we have NFPA 79
Which one overrides the other??
OR in other words my code (which one??) is better than yours.

Dan Bentler

jimbo3123 · Nov 25, 2009

leitmotif said:
Umm boy on the way to 500 posts.
OK so we have NFPA 70 Natl Electric Code
and we have NFPA 79
Which one overrides the other??
OR in other words my code (which one??) is better than yours.

Dan Bentler

I'm not sure that either one overrides the other. I believe that NFPA 79, being the standard for industrial machinery, is meant to supplement and clarify NFPA 70 which is the more general code for all electrical installations.

kamenges · Nov 25, 2009

Originally posted by leitmotif:

They should be on the left side ie in the hot especially so in case of 120 and grounded neutral.

So, how do we get the desired protection on a reversing motor starter? These have only one overload but two coils. Generally speaking overloads only have a single NC contact on them. Are we required to run the supply through the overload contacts, to the actuating circuit (which might be a PLC output with 8 or 16 outputs serviced by a common supply) and then back to the motor starter? The dual direction motor starters are why the common switching through the overload is the way to go.

NFPA 79 is the machine specific electrical standard from the Nat'l Fire Protection Association. It holds precidence in machinery and panel construction. NFPA 70 hold precidence in construction and installation.

Keith

jimbo3123 · Nov 25, 2009

OK, hopefully we can put the code bit aside and get back to the basics of your question.

Maintenance Man said:
What I would like to know, is if wiring a neutral through the NC contacts on the overload to open the neutral, if the overload trips, is used in most industries as an extra protection

Let's assume that you have a motor starter circuit that involves a NC contact from an overload relay. What makes you think that wiring the overload contact on the neutral side of the coil would provide extra protection?

Maintenance Man said:
A maintenance supervisor told me they had some contacts weld together, which did not stop the motor, which damaged it so this was the reason for breaking the neutral going to A2 to stop the motor.

I can see someone telling you this, but I suspect that the maintenance supervisor doesn't fully understand why motor starter control circuits are typically wired with the overload on the neutral side of the coil.
Ask yourself (or him) this question: would wiring the overload on the hot side of the starter coil provide any different protection against a welded contact?

leitmotif · Nov 25, 2009

kamenges said:
So, how do we get the desired protection on a reversing motor starter? These have only one overload but two coils. Generally speaking overloads only have a single NC contact on them. Are we required to run the supply through the overload contacts, to the actuating circuit (which might be a PLC output with 8 or 16 outputs serviced by a common supply) and then back to the motor starter? The dual direction motor starters are why the common switching through the overload is the way to go.

NFPA 79 is the machine specific electrical standard from the Nat'l Fire Protection Association. It holds precidence in machinery and panel construction. NFPA 70 hold precidence in construction and installation.

Keith

completely forgot reversing starters - does explain why the overloads are on neut side of coil especially where you have multiple start stop stations. One could put overloads in the "hot side" but oh lawzy it might take some extra wire and we cannot afford that.

I would think 79 in case of machinery would override 70 also.

Dan Bentler

bikerdude · Nov 25, 2009

MaintMan,

What I would like to know, is if wiring a neutral through the NC contacts on the overload to open the neutral, if the overload trips, is used in most industries as an extra protection or is this something that the company I work for is only doing?

That is how its done where I work. I don't see any problems with it after 25 years. Yes.. the "Hot" side of the coil can still be hot when the motor is overloaded. So what! That way is starts back up when the overloads are reset. The aux. contacts that snap onto the side of the contactor can be used as a feedback signal to the controller to operate indicator lights or announce problems. I have seen one cabinet that used the aux signal to tell the controller to de-energize the coil if it faults. Then you have to go to the main panel door & hit the reset button to make it start back up.
MY $0.02
BD

OkiePC · Nov 25, 2009

I have seen some older overload relays that would not open all three phases unless you wire the OL contact to interrupt the coil. They'll single phase the motor unless you drop out the coil.

I always wire the NC contact on the neutral side...never a problem...

Maintenance Man · Nov 26, 2009

Thank you all so very much for your help; this has answered my question about wiring motor overloads. I just wanted to make sure I was doing and being taught the proper way. I can see where having the neutral opening to drop out the contactor coil would not be any help if the contacts become welded together. Opening the neutral would help to prevent single phasing if all 3 legs did not open during an overload. I will continue to wire motor starters, the way our plant wants me to and I will wire motor starters, how the instructor wants me to in class. This has been a very beneficial lesson on motor controls. Again, thank you all so very much.
Sincerely
Maintenance Man

Proper way to wire overloads

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