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Thread: MCP Rating Question

  1. #1
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    MCP Rating Question

    I have an old table my boss gave me listing various motor horsepowers and a recommended MCP rating for each motor.
    The table recommends (all 460V) a 15A MCP for a 3 and 5hp motor, a 30A MCP for a 7.5 and 10hp motor, a 50A MCP for a 15 and 20hp motor, and so on and so forth.
    My question is, where do these MCP ratings come from?

  2. #2
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    What is MCP?

  3. #3
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    MCP = Motor Circuit Protector

  4. #4
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    The MCP is the short circuit protection (mag-only breaker providing instantaneous trip protection) covered under 430.52.
    Ron

  5. #5
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    As an example an adjustable 15A MCP may provide instantaneous protection settable from 45-165A.
    http://ecatalog.squared.com/pubs/Cir...0580HO9501.pdf
    Ron

  6. #6
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    So basically, the MCP ratings I've listed above for the associated motor sizes are standard MCP sizes that will give me the range of settings I need based on the requirements of table 430.52?

  7. #7
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    Quote Originally Posted by georgeshall
    I have an old table my boss gave me listing various motor horsepowers and a recommended MCP rating for each motor.
    The table recommends (all 460V) a 15A MCP for a 3 and 5hp motor, a 30A MCP for a 7.5 and 10hp motor, a 50A MCP for a 15 and 20hp motor, and so on and so forth.
    My question is, where do these MCP ratings come from?

    Motor Short-Circuit / Ground-Fault protection is sized as per a percentage of Full-Load Amps, not Power rating (HP). There are four types of SC/GF protection for motors. An MCP should be an Instantaneous Breaker. These might be around 800% to 1100% of Motor Full Load Amps (Not nameplate, NEC tabular value). Look at Table 430.52 to get the percentage of full load amps. Look at 430.248 or 430.250 (or ...) to get the full load amps of the motor.
    Eric Stromberg, P.E. Texas

  8. #8
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    Jul 2007
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    Thanks guys.
    Doing a little more digging, I think I found where my boss got his table of recommended settings. I believe he copied all of the info from the 2006 Cutler Hammer Consulting Application Guide. The tables are on Tab 1 page 5-7 (page 1.5-7).

  9. #9
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    Quote Originally Posted by eric stromberg
    ...Motor Short-Circuit / Ground-Fault protection is sized as per a percentage of Full-Load Amps, not Power rating (HP)...
    Where did Spock go?
    Norb
    My Boss is a Tyrant and my Employee is an Idiot.

  10. #10
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    Quote Originally Posted by georgeshall
    MCP = Motor Circuit Protector
    Same thing as us old farts call Heater?
    Greg

    Electrical Inspector in our Nations Capital

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