motor circuit protector vs thermal mag and feeder

[mjmike]

This is in reference to a piece of equipment with an integral control panel incorporating a thru-door main disconnect with the main load being a motor. I have been noticing lately manufacturers of packaged equipment utilizing a motor circuit protector type circuit breaker as opposed to a traditional thermal magnetic circuit breaker. Traditionally, with a thermal mag main disconnect, we design the upstream feeder breaker to match the control panel's main breaker with a full size feeder. Or sometimes we go a little heavy; say the panel disconnect is a 175A breaker, we may size a 200A feeder.

However, with the motor circuit protector breakers, you can't simply match the breaker amperage with the upstream feeder. This is because the feeder breaker would be a thermal magnetic and the motor circuit protector breaker will handle the motor inrush and push this to the feeder thermal mag which may trip. So the question is, have anybody else been seeing this and how do you size the feeder and upstream CB if the control panel main disconnect is a motor circuit protector? Lately we have just been specifying the control panel main needs to be a thermal mag, but sometimes we run into pre-purchased equipment.

 

[petersonra]

the feeder ampacity gets sized according to the motor feeder ampacity requirements found in article 430.

the ocpd is also sized according to the requirements found in article 430.

exactly the same way it would be sized if there was a main disconnect with fuses, or a MCCB main.

the rating of the main at the panel is not part of the calculation.

 

[iceworm]

Adding to petersonra's comments (which are excellent):
Most all motor starters I deal with are mag only CBs. Interestingly, the suituation you describe has never come up. But, if it did, I'd be comparing the instantaneous trip on the thermal-magnetic feeder with the instantaneous setting on the control panel MCP - coordination.

Here is my interpretation: A TM CB in an MCC or panelboard is feeding a control panel protected by a Mag only CB. Main load in the control panel is a motor. Mag only CB is sized to protect Branch conductors per 430.52. MCC feeder TM CB is sized to protect Feeder conductors per 430.52

Example:
100hp motor
Branch conductors 124 x 1.25 = 155. use 2/0 = 175A
Ovlds set to 1.4 x FLA = 1.4 x 124 = 170A
Control panel Mag only CB set to 13 x fla = 1600A
Feeder conductors same as branch 2/0 = 175A
Feeder TM CB 2.5 x fla = 310A, next size up 350A
Feeder TM CB instantaneous default 6x rating = 6 x 350 = 2100A

not stuck, just thinking

worm

 

[Jraef]

You are going to need to clarify what you mean by "Motor Circuit Protector" circuit breaker.

Technically, that term "Motor Circuit Protector" is a registered trade name of a Westinghouse (now Eaton) or "MPC". These are Magnetic Only (aka Mag-Only), or in the NEC they are referred to as Instantaneous Trip (IT) breakers, they have no thermal trip elements. Everyone makes them, only Eaton can actually CALL them a "Motor Circuit Protector" (I think, although they may have lost that protection by now). If that is the case, they are ONLY allowed to be used in FACTORY ASSEMBLED and listed combination motor starters. The devices themselves are NOT listed as circuit breakers under UL, they are "Recognized Components", which means they can only be used as part of an assembly that will then be submitted for testing and listing under ANOTHER UL file, such as UL508 for motor starters. However, many many panel builders, especially those who do not get UL listing, use them in their panels, paying no attention to the rules. If they are used in this manner, the ONLY valid use would involve having the entire assembly submitted to UL for short circuit testing and listing, a process that is prohibitively expensive for the average custom panel builder. That doesn't stop many of them from doing it anyway, but then the risk / burden falls on the installer, who may get it red-tagged if the AHJ is aware of this rule.

However, this issue has been muddies recently with the release of new marketing materials that call out "Motor Protection Circuit Breakers". That sounds similar, but they are different. These are, in essence, a "Manual Motor Starter" type of device, but one with the short circuit (mag trip) and an adjustable thermal trip, like an overload relay. Different rules apply to those devices. They CAN be used as part of a SINGLE motor circuit, along with a contactor for example, but CANNOT be used as a feeder or branch OCPD per the NEC, they need to have ANOTHER feeder OCPD ahead of them. That makes them unsuitable as a "main breaker" in a control panel, unless by control panel, you mean just a combination motor starter.

So start by getting clear on your terminology.

 

[iceworm]

You are going to need to clarify what you mean by "Motor Circuit Protector" circuit breaker. ...

... .So start by getting clear on your terminology

uh ... J -
Don't be having an apolexy fit right here visible to the rest of us. Yuck - we would have to bleach out our eyeballs. (shudder)

The OP is asking about sizing a TM CB to feed a control panel that is using a mag-only Cb as the disconnect. And yes, the OP was clear the element is a Mag-only CB.

I highly suspect he was not looking for a disertation on if/when the "FACTORY ASSEMBLED" police were planning a raid because he knowingly and with malice received a control panel supplied as part of an equipment package.

ice

 

[don_resqcapt19]

To add to the confusion we also have a "motor short circuit protector" in 430.52(C)(7) which is a fuse type device.

 

[iceworm]

To add to the confusion we also have a "motor short circuit protector" in 430.52(C)(7) which is a fuse type device.

I'm going to question your authority on that

 

[Jraef]

uh ... J -
Don't be having an apolexy fit right here visible to the rest of us. Yuck - we would have to bleach out our eyeballs. (shudder)

The OP is asking about sizing a TM CB to feed a control panel that is using a mag-only Cb as the disconnect. And yes, the OP was clear the element is a Mag-only CB.

I highly suspect he was not looking for a disertation on if/when the "FACTORY ASSEMBLED" police were planning a raid because he knowingly and with malice received a control panel supplied as part of an equipment package.

ice

He never actually says it is a mag-only breaker, he says " motor circuit protector". I'm pointing out that people get a little loose with that terminology, when in fact they are looking at (or reading) something else.

So is it a "control panel" with a mag-only breaker being used as a main breaker? If so, it's an inappropriate use of the device and he should make the OEM replace it, or risk getting red tagged by the AHJ.

Or, is it a combo-motor starter for one motor, built by a factory, with a few extra control components used inside? If so, then he need not worry about the size of the breaker used IN the starter, he has no choice in the matter anyway. So all he needs to do is size his feeder circuit conductors per the appropriate code rules and size the feeder CB ahead of it to protect them.

Was I verbose? Well, yes... one of my faults. :weeping:

 

[don_resqcapt19]

I'm going to question your authority on that

Take a look at the informational note that follows 430.52(C)(7).

Informational Note: A motor short-circuit protector, as used in this section, is a fused device and is not an instantaneous trip circuit breaker.

The following note follows 430.52(C)(3).

Informational Note No. 1: Instantaneous trip circuit breakers are also known as motor-circuit protectors (MCPs).

 

[Jraef]

By the way, I just found out that "MCP" and "Motor Circuit Protector" are no longer trademarked by Eaton, it apparently expired not too long ago.

 

[mjmike]

Thanks for the info. I am still absorbing it. I was about to say MCP is not a trademark, but didn't know it once was. Thanks for that info. In fact the grounding is different for a motor circuit protector and in fact the NEC specifically references motor circuit protector as an option when sizing a motor EGC.

 

[Jraef]

Thanks for the info. I am still absorbing it. I was about to say MCP is not a trademark, but didn't know it once was. Thanks for that info. In fact the grounding is different for a motor circuit protector and in fact the NEC specifically references motor circuit protector as an option when sizing a motor EGC.

???

Something's not right there. What are you reading that you are interpreting this way?

 

[don_resqcapt19]

???

Something's not right there. What are you reading that you are interpreting this way?

There is a special rule for the motor EGC where an instantaneous trip breaker is used.

250.122(D)(2) Instantaneous-Trip Circuit Breaker and Motor Short-Circuit Protector. Where the overcurrent device is an instantaneous-
trip circuit breaker or a motor short-circuit protector, the equipment grounding conductor shall be sized not smaller than that given by 250.122(A) using the maximum permitted rating of a dual element time-delay fuse selected for branch circuit short-circuit and ground-fault protection in accordance with 430.52(C)(1), Exception No. 1.

Prior to the 2008 code, the motor circuit EGC for a starter that had an instantaneous trip OCPD, was based on the rating of the motor overload.

250.122(D). Motor Circuits. Where the overcurrent device consists of an instantaneous trip circuit breaker or an motor short-circuit protector, as allowed in 430.52, the equipment grounding conductor size shall be permitted to be based on the rating of the motor overload protective device, but shall not be less than the size shown in Table 250.122

 

[petersonra]

There is a special rule for the motor EGC where an instantaneous trip breaker is used.

Prior to the 2008 code, the motor circuit EGC for a starter that had an instantaneous trip OCPD, was based on the rating of the motor overload.

i am thinking that it still would not ever have to be larger than the circuit conductors.

 

[don_resqcapt19]

i am thinking that it still would not ever have to be larger than the circuit conductors.

Correct.