Sizing OCPD for motors

[Designer69]

I'd like to get a full understanding of this..

It appears that the NEC says size ur OCPD as close to the motof FLA as possible. I used to think it was FLA x 1.25 for some odd reason.


and table 430.52 for polyphace AC mtrs says size Instantaneous Trip Breakers up to 800% of FLA and Inverse time Breaker up to 250% FLA.

 

[raider1]

You have to remember that motors typically have separate overload protection. Overload protecion can be sized up to 115% or 125% of the FLA of the motor from the nameplate in accordance with Part III of Article 430.

Because motors have separate overload protection, the OCPD for the branch circuit/ feeder conductors is used to protect the conductors from short circuits and ground faults only, and therefore you are permitted to use a much larger OCPD.

Table 430.52 is used to size the OCPD to provide ground fault and short circuit protection for the conductors and the motor.

Hope this makes sense.

Chris

 

[Designer69]

yes Chris it does, so basically if I have a 50A FLA motor I can use a 50A or a 150A breaker it doesn't matter?? as long as the overloads are set to no more than 62.5A ??

also Table 430.52 says for polyphase AC mtrs.. inverste time brks can be set up to 250%FLA and instantaneous trip bkrs up to 800% FLA

is a molded case thermal magnetic bkr an inverse time breaker? and it allows an inrush for starting?

is an MCP an instantaneous only breaker? and does it allow any inrush for starting at all?

 

[Jraef]

yes Chris it does, so basically if I have a 50A FLA motor I can use a 50A or a 150A breaker it doesn't matter?? as long as the overloads are set to no more than 62.5A ??

Think that one through a bit. Would you EVER put a 50A breaker on a motor that will draw 62.5FLA? Just because you can, doesn't mean you should.

also Table 430.52 says for polyphase AC mtrs.. inverste time brks can be set up to 250%FLA and instantaneous trip bkrs up to 800% FLA

is a molded case thermal magnetic bkr an inverse time breaker? and it allows an inrush for starting?

Yes, what is commonly referred to as a Molded Case Circuit Breaker is an Inverse Time breaker, if it has a Thermal trip element. The better way to refer to those is as "Thermal Magnetic" (Thermal-Mag, or TM) breakers. They have BOTH the Inverse Time (Thermal) trips and Instantaneous (Magnetic) trips. Some TM breakers have ADJUSTABLE mag trips, some are fixed at some factory value. Those with fixed Mag trips may or may not work on motors very well without serious oversizing, depending on what the manufacturer uses as a fixed value. For example, some are set at 400% of the Thermal rating, some at 1000%. The 400% versions generally nuisance trip on motor inrush. Better to use one's with adjustable Mag trips.

is an MCP an instantaneous only breaker? and does it allow any inrush for starting at all?

Yes, an "MCP" (actually a brand name for the old Westinghouse Magnetic Only breakers) only has the Magnetic, or "Instantaneous Trip" (IT is a better term to use than MCP) settings. There is no Thermal trip element, because they can ONLY be used as part of a motor starter, where the Overload Relay is going to be the Thermal trip device. IT breakers then by nature ALWAYS have adjustable Mag Trips. They are specifically used in order to avoid nuisance tripping on motor inrush.

But technically, both the TM and the IT breakers are both "Molded Case Circuit Breakers", as well as Electronic Trip Unit (ETU) breakers, so that term in and of itself does not differentiate.

 

[glene77is]

From a design standpoint,
I divide the overload protection into three "Time Based" categories.

(1) Short term, dead fault, 1 second, protects the whole system,
handled by Instantaneous Magnetic Breakers, < 800%.

(2) Medium term, 10 seconds, protects the Wire Conductor,
startup inrush protection by Thermal Breakers (Inverse Time), < 250%.

(3) Long term, 10 minutes, protects the Motor itself,
handled by Overloads, < 115%.

Then, I read 430 tables, etc. for the details in my specific application.

Hope that helps.