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Thread: FLA > 80% MOCP?

  1. #11
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    Quote Originally Posted by Dale001289 View Post
    As a general rule, the Code requires overcurrent protection for branch circuits and feeders to be sized at 125% of FLA. This compensates for the 80% rating of most molded case CB's (see UL 489, I think?).
    Not exactly.

    For branch circuits

    210.20 Overcurrent Protection.
    (A) Continuous and Noncontinuous Loads. Where a branch
    circuit supplies continuous loads or any combination of continuous
    and noncontinuous loads, the rating of the overcurrent
    device shall not be less than the noncontinuous load plus
    125 percent of the continuous load.

    Exception: Where the assembly, including the overcurrent devices
    protecting the branch circuit(s), is listed for operation at 100 percent of
    its rating, the ampere rating of the overcurrent device shall be permitted
    to be not less than the sum of the continuous load plus the noncontinuous
    load.
    For feeders:

    215.3 Overcurrent Protection. Feeders shall be protected
    against overcurrent in accordance with the provisions of Part I
    of Article 240. Where a feeder supplies continuous loads or any
    combination of continuous and noncontinuous loads, the
    rating of the overcurrent device shall not be less than the
    noncontinuous load plus 125 percent of the continuous load.
    Exception No. 1: Where the assembly, including the overcurrent devices
    protecting the feeder(s), is listed for operation at 100 percent of its
    rating, the ampere rating of the overcurrent device shall be permitted to
    be not less than the sum of the continuous load plus the noncontinuous
    load.
    Note this is the minimum size because of the phrase shall not be less.

    basically if you use a 100% rated breaker you can go to 100% FLC.

    If you do not use a 100% rated breaker it is 100% of the noncontinuous load plus 125% of the continuous load.
    Bob

  2. #12
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    Quote Originally Posted by petersonra View Post
    Not exactly.

    For branch circuits



    For feeders:



    Note this is the minimum size because of the phrase shall not be less.

    basically if you use a 100% rated breaker you can go to 100% FLC.

    If you do not use a 100% rated breaker it is 100% of the noncontinuous load plus 125% of the continuous load.
    That’s why I said “In general”.
    100% rated breaker are a special order.


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  3. #13
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    Quote Originally Posted by HVACEE View Post
    What about the 80% rule for the breaker? 68.4 A is definitely > 80% of 70 A...
    That is general rules in 210 and 215, motors it changes a little. For one thing you already have to multiply motor FLA by 1.25 so that is already factored into minimum conductor ampacity no need to multiply by 1.25 again. Overcurrent device - most common values is 175% for time delay fuses or 250% for inverse time breakers. You can go next standard size higher. If that won't let motor start you have the ability to go even higher.

    Quote Originally Posted by HVACEE View Post
    5 HP @ 480V = 7.6A per NEC

    FLA = 7.6 A x 9 Motors = 68.4 A
    MCA = 7.6 A x 125% + 7.6 A x 8 Motors = 70.3 A
    MOCP = 7.6 A x 225% + 7.6 A x 8 Motors = 77.9 A next lower standard breaker is 70 A
    You must have at least 70 amp conductor, 6 AWG is less then 70, 4 AWG is good for 85 - 4 it is unless other adjustments come into play.

    Feeder overcurrent protection for inverse time breaker can be 250% largest plus all others. gives you 79.8 which can be rounded up to 80, which is less then the ampacity of 4 AWG. I'd likely put it on a 90 just in case it wouldn't hold while starting most/all of them all at once. 4 AWG can be protected by next standard size of 90 pretty much in any case.
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  4. #14
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    Here is an interesting US Chart of average resistivity in ohms/meter. I am surprised that some areas in Texas have a 33 ohms/meter. I assume this means that an 8' ground rod which is almost about 2.5 meters would have a resistivity reading of 13 ohms.

    https://www.rd.usda.gov/files/UTP_Bu..._1751F-802.pdf

    Page 59 is where the chart is located. I have no idea as to how accurate this is...
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  5. #15
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    Quote Originally Posted by kwired View Post
    That is general rules in 210 and 215, motors it changes a little. For one thing you already have to multiply motor FLA by 1.25 so that is already factored into minimum conductor ampacity no need to multiply by 1.25 again. Overcurrent device - most common values is 175% for time delay fuses or 250% for inverse time breakers. You can go next standard size higher. If that won't let motor start you have the ability to go even higher.



    You must have at least 70 amp conductor, 6 AWG is less then 70, 4 AWG is good for 85 - 4 it is unless other adjustments come into play.

    Feeder overcurrent protection for inverse time breaker can be 250% largest plus all others. gives you 79.8 which can be rounded up to 80, which is less then the ampacity of 4 AWG. I'd likely put it on a 90 just in case it wouldn't hold while starting most/all of them all at once. 4 AWG can be protected by next standard size of 90 pretty much in any case.
    The 125% of FLA for motor branch circuits is based on a typical TEFC motor Service Factor of 1.15 - which drives the thermal (or electronic) overload device set typically at 125% of Motor FLA. Explosion proof motors typically have a SF of 1.0 and a tighter tolerance for OL settings, nonetheless the circuit is basically sized at 125% to match the overloads.
    The CB is normally ‘high magnetic’ only for short circuit protection.
    Cutler Hammer for example standardizes its HMCP trip settings at 1300% of FLA for todays high efficiency motors.


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  6. #16
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    Quote Originally Posted by Dennis Alwon View Post
    Here is an interesting US Chart of average resistivity in ohms/meter. I am surprised that some areas in Texas have a 33 ohms/meter. I assume this means that an 8' ground rod which is almost about 2.5 meters would have a resistivity reading of 13 ohms.

    https://www.rd.usda.gov/files/UTP_Bu..._1751F-802.pdf

    Page 59 is where the chart is located. I have no idea as to how accurate this is...
    I have seen this chart before but not all the calculations and studies - very impressive stuff — thanks for sharing.
    Those who think Grounding is simple...simply don’t know Grounding.


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  7. #17
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    Quote Originally Posted by Dale001289 View Post
    The 125% of FLA for motor branch circuits is based on a typical TEFC motor Service Factor of 1.15 - which drives the thermal (or electronic) overload device set typically at 125% of Motor FLA. Explosion proof motors typically have a SF of 1.0 and a tighter tolerance for OL settings, nonetheless the circuit is basically sized at 125% to match the overloads.
    The CB is normally ‘high magnetic’ only for short circuit protection.
    Cutler Hammer for example standardizes its HMCP trip settings at 1300% of FLA for todays high efficiency motors.


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    To add to that, for other then 100% rated devices - the reason we size conductors at 125% of continuous load is because of termination temp rating. Typical ocpd sinks some heat into the conductor meaning terminations will be the hot spots and need more material there to sink that heat into. Otherwise we typically have conductors with 90C insulation connected to 75C rated terminals - so the conductor can take it, the terminal is the reason for the additional 25%.

    intermittent duty applications is a different game - some you situations you can have conductor ampacity of only 85% motor rated current others you need 200%.
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  8. #18
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    Quote Originally Posted by kwired View Post
    To add to that, for other then 100% rated devices - the reason we size conductors at 125% of continuous load is because of termination temp rating. Typical ocpd sinks some heat into the conductor meaning terminations will be the hot spots and need more material there to sink that heat into. Otherwise we typically have conductors with 90C insulation connected to 75C rated terminals - so the conductor can take it, the terminal is the reason for the additional 25%.

    intermittent duty applications is a different game - some you situations you can have conductor ampacity of only 85% motor rated current others you need 200%.
    True - but ‘heat sink at terminals’ applications is more for thermal-magnetic CB applications vs magnetic only. Intermittent operations many times get lost in the shuffle. It pays (literally) to investigate all motor loads since not always continuous duty.


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  9. #19
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    Quote Originally Posted by Dale001289 View Post
    True - but ‘heat sink at terminals’ applications is more for thermal-magnetic CB applications vs magnetic only. Intermittent operations many times get lost in the shuffle. It pays (literally) to investigate all motor loads since not always continuous duty.


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    But at same time motors is a different ball game then most other applications and we are sent to art 430 instead of following the general rules for motor circuit conductors and overcurrent protection on those circuits. The conductor still gets sized at 125% for majority of continuous duty applications even if it will be on an instantaneous trip only breaker. I suppose the fact that overload protection can be 125% is a contributing factor to that, but at same time the rule for conductor ampacity doesn't change when you have a 1.0 SF motor.
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