OL relay necessary??

lesmar96

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I am still learning UL codes and have looked in the document but wasn't sure the right place to look.

Please see attached picture. The customer sent me this picture. Is it legal to not have any OL relays? From my understanding the MMP would be the branch circuit protection and an OL relay would still be required for each motor? I kinda remembering something about combination of an MMP and a contactor with the correct coupling devices can be ok, but it is vague in my mind.

Anyone with experience help me out??
 
I am not a code expert, but I do believe that is is fine to have the MPCB upstream of the contactor as long as it is sized right and only protecting one motor per MPCB.

I have seen a lot of motor control panels built this way. In your picture there is one to the far right that broke their pattern with an OL relay attached to the contactor.

I'm not a fan of the white wire on boths sides of the the MCP aux. contacts nor of the dangling wirenut, but that is better than hiding the wirenut in the duct.

EDIT: Your jpg is too big (pixel size) for the forum, but allowed full size as a png.
EDIT 2: Thanks Garry, I used the wrong acronym...

IMG_0560.png
 
Last edited:
MCP is not an overload the MCP protects the branch circuit the overload is to protect the motor only
Different rating different uses both are required
A VFD has the overload built in so it's not necessary otherwise it is required
 
If those are actually motor circuit protectors, you are correct. You would need an overload. However, if they are motor protection circuit breakers then that is all you need. An MPCB is designed and rated to protect a SINGLE motor branch circuit and the motor connected to it. You can even purchase them in different overload trip classes.

AB makes both devices in the same body so you really can't tell from the picture if what is shown is OK or not.

Keith
 
If those are actually motor circuit protectors, you are correct. You would need an overload. However, if they are motor protection circuit breakers then that is all you need. An MPCB is designed and rated to protect a SINGLE motor branch circuit and the motor connected to it. You can even purchase them in different overload trip classes.

AB makes both devices in the same body so you really can't tell from the picture if what is shown is OK or not.

Keith

That makes sense. I suppose the different devices have 2 different UL file numbers?
 
Those have an adjustable thermal trip setting, so those are MPCBs and are the OL protection device for those starters. You set the dial for the motor nameplate PLA, the pick-up point is ALREADY designed into it, so don't add 25% like a lot of people do.
 
I hate to be a stickler but UL requires both a branch circuit protector and a motor overload
They are completely different and function differently
Branch circuit protection can be fuses or breakers
When I started out and old mentor electrician I work with told me if want to be sure it will open on a fault use a fuse they never fail breakers can weld closed on high inrush overloads.
As for breakers there are 2 major types thermal and magnetic while both will generally work but each works differently. Pay attention to the current curves of both each in entirely different select carefully.
The branch circuit protector (MCB) is generally select at 125 to 150% of the expected full load of the circuit. If you use this as the overload for the motor you could burn up the motor before it opens the circuit, it offers no protection for the motor. A Motor Overload / Motor Protector on the other hand are selected for the FLA of the motor. That will vary with the motor type ambient air temperature of both the controller and the motor. There are a lot of things to consider when selecting the correct overload most don’t bother they just select the closest FLA. I have been on jobs where the OLs were way oversized and jumped out completely. When I asked why it was because they keep shutting the motor off production was more important than safety. The same is true for fuses if they keep blowing increase the size until they hold. The newer OL’s are electronic and no longer use the thermal or magnetic to trip but they use actual measured motor current to calculate the trip curve.
If you look at the UL manual they still require both on a motor control. You can have one MCP for multiple motors if the motors are small. Remember you are protecting the circuit not the motor
While we all know that some panel shops don’t install the MCB for each motor just because they do it does not make it right or safe.
I for one would not want that liability if something goes wrong.
 
It is a little hard to tell by the picture - but if they have a link module between them and the line spacer on the feed to the MPCB, they are a combination unit (Type F or Type E) and do not require an overload.

I'm not a UL expert, but nearly every panel I build is done this way and I have had several sent to be UL certified.

The biggest concern for me on the panel are the terminal blocks at the bottom. Typically those have to count in the SCCR rating - and usually they are pretty low or unlisted. I always leave the Contactor at the bottom for customers to wire to.
 
MPCBs are UL listed for Branch Short Circuit AND Overload protection. An MPCB + Contactor makes a complete combination motor starter, it needs nothing else. MPCBs have been included in Article 430 of the NEC for providing both functions for a long time now.
 
There is a difference between MPCB's & MCP MPCB's are fine on their own but MCP's must use an O/L as well I believe.
 

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