breaker sizing

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On the motor plate, look for the "MAX LOAD AMPS", and multiply that by 125% or 1.25. This would be the 3-pole breaker size needed. Any motor that is UL listed should have that motor plate attached to the casing. Sometimes they are all beat-up and not legible.
 
adrian33773 said:
On the motor plate, look for the "MAX LOAD AMPS", and multiply that by 125% or 1.25. This would be the 3-pole breaker size needed. Any motor that is UL listed should have that motor plate attached to the casing. Sometimes they are all beat-up and not legible.
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For the motor in the OP the NEC requires that you use the ampacities listed in table 430.250 in sizing the ampacity of the conductors, and the branch circuit and ground fault protection device not the nameplate full load current value. This would require that you had the HP value of the motor. You could use 125% but the motor might not start. Look at 430.52(C)(1) and Table 430.52 for the different maximum values that are permitted.
 
One way is:

http://www.squared.com/motodata/calcmotor.html

I have a Square D "slide rule card" that shows this for single phase and three phase motors. In addition, it has wire ampacity and required conduit sizes, and transformer data. I assumed everyone had these slide cards, but I have never heard anyone else mention them.

Steve
 
steve66 said:
One way is:

http://www.squared.com/motodata/calcmotor.html

I have a Square D "slide rule card" that shows this for single phase and three phase motors. In addition, it has wire ampacity and required conduit sizes, and transformer data. I assumed everyone had these slide cards, but I have never heard anyone else mention them.

Steve
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Actually I have one too. A rather valuable piece of cardboard.
 
He only asked about setting the magnetic trip elements for motor starter breakers.

That is covered in 430.52

Synopsis:

If you have a Thermal-Mag breaker, most applications will fall under 452.C.1 exception 2(c) because standard Thermal-Mag breakers without adjustable mag trips will not stay in under typical motor starting current. If the Mag Trips are adjustable, use the table and exception 1

If you have a Mag-Only breaker (a.k.a. MCP), i.e. what is usually provided as part of a listed Combination Starter, use 452.C.3, exception 1
 
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