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    Derating calculation problem

    In the mike holts study master contractor practice exam they derate 10awg @ 70% with 8 current carrying conductors thhn for 21A loads on 30A breaker.

    90° thhn 10awg is 40A x 70% = 28A. they say 10awg can be on 30A breaker derated at 28A since the load is 21A? I thought the derated wire had to be calculated not less than the breaker itself?? Have i been wrong all this time? Or did Mike holt make a mistake?

    Since the wire is derated to 28A doesn't that mean its only good for a 25A breaker?

    #2
    In general you are correct but look at 240.4(B) for your answer

    240.4(B) Overcurrent Devices Rated 800 Amperes or Less. The
    next higher standard overcurrent device rating (above the
    ampacity of the conductors being protected) shall be permitted
    to be used, provided all of the following conditions are met:
    (1) The conductors being protected are not part of a branch
    circuit supplying more than one receptacle for cord-andplug-
    connected portable loads.
    (2) The ampacity of the conductors does not correspond with
    the standard ampere rating of a fuse or a circuit breaker
    without overload trip adjustments above its rating (but
    that shall be permitted to have other trip or rating adjustments).
    (3) The next higher standard rating selected does not exceed
    800 amperes.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky

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      #3
      Let me be clear that the wire is rated 28 amps but you can use a 30 amp overcurrent protective device however, the calculated load must not be greater than 28 amps.
      They say I shot a man named Gray and took his wife to Italy
      She inherited a million bucks and when she died it came to me
      I can't help it if I'm lucky

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        #4
        Originally posted by jonnytsa View Post
        In the mike holts study master contractor practice exam they derate 10awg @ 70% with 8 current carrying conductors thhn for 21A loads on 30A breaker.

        90° thhn 10awg is 40A x 70% = 28A. they say 10awg can be on 30A breaker derated at 28A since the load is 21A? I thought the derated wire had to be calculated not less than the breaker itself?? Have i been wrong all this time? Or did Mike holt make a mistake?

        Since the wire is derated to 28A doesn't that mean its only good for a 25A breaker?
        One other piece of the puzzle is whether or not the load is continuous or not. If it is a continuous 21 amp load then your minimum conductor ampacity is 125% of 21 = 26 amps. But that is still below your adjusted ampacity of 28 amps for the 10 AWG conductor.

        If it is not a continuous load there is some possibility you must use a 25 amp breaker instead of a 30, kind of depends on what the load is and what code sections may apply to it. If it is a motor or refrigeration compressor, it likely still needs 10 AWG but breaker can be even more than 30.
        I live for today, I'm just a day behind.

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          #5
          Originally posted by kwired View Post
          One other piece of the puzzle is whether or not the load is continuous or not. If it is a continuous 21 amp load then your minimum conductor ampacity is 125% of 21 = 26 amps. But that is still below your adjusted ampacity of 28 amps for the 10 AWG conductor.
          The 125% applies before the application of any adjustment or correction factors. After the application of any adjustment or correction factors, the conductor would require an ampacity of at least 21 amps to serve the load, but it would have to be greater than 25A to be protected by the 30A c/b.

          Comment


            #6
            Originally posted by david luchini View Post
            The 125% applies before the application of any adjustment or correction factors. After the application of any adjustment or correction factors, the conductor would require an ampacity of at least 21 amps to serve the load, but it would have to be greater than 25A to be protected by the 30A c/b.

            Yes, had a brain cramp there, 125% applies to selecting conductor based on termination rating.

            Ampacity adjustments and temp correction factors apply for insulation rating of conductor - which only needs to be at 100% of load in all cases.

            OP can still have some situations where the OCPD needs to be 25 amps for his 21 amp load, some where it maybe can be 30 amps, and some where it could be more than 30.
            I live for today, I'm just a day behind.

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              #7
              Could you please provide any code reference..

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                #8
                Originally posted by david luchini View Post
                The 125% applies before the application of any adjustment or correction factors. After the application of any adjustment or correction factors, the conductor would require an ampacity of at least 21 amps to serve the load, but it would have to be greater than 25A to be protected by the 30A c/b.
                Could you please provide any code reference that says 125% applies before the application of any adjustment or correction factor..

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                  #9
                  Originally posted by mohsan514 View Post
                  Could you please provide any code reference..

                  Sent from my SM-J700F using Tapatalk
                  There are several that can apply, any particular part of the process you have a question about?

                  First you must determine if you are applying to a branch circuit, feeder or a service conductor, to know whether to start in 210, 215 or 230 though they all have mostly same general rules. Then you go to 310.15 and select a conductor based on minimum required ampacity, but may need to make adjustments/corrections to ampacity in accordance with number of conductors in a raceway or cable or because of ambient temperature.

                  Then it is on to art 240 to determine what overcurrent protection is permitted, but conditions of the application may send you to different parts of 240 or even to other parts of code for some specific applications.
                  I live for today, I'm just a day behind.

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                    #10
                    Originally posted by kwired View Post
                    There are several that can apply, any particular part of the process you have a question about?

                    First you must determine if you are applying to a branch circuit, feeder or a service conductor, to know whether to start in 210, 215 or 230 though they all have mostly same general rules. Then you go to 310.15 and select a conductor based on minimum required ampacity, but may need to make adjustments/corrections to ampacity in accordance with number of conductors in a raceway or cable or because of ambient temperature.

                    Then it is on to art 240 to determine what overcurrent protection is permitted, but conditions of the application may send you to different parts of 240 or even to other parts of code for some specific applications.
                    I have chiller units and each unit is 600 amps and required OCPD is 800amp. I used the 2 runs of cable and total ampacity is 820 amp and these are laid in perforated cable tray . After applying grouping factor / correction factor , the cable ampacity is reduced to 720 amps. But i am worried that how 800 amp CB will protect the conductor in case of any abnormal condition.
                    Since nec 240.4 allows to use the higher rated OCPD. Even the conductor is less rated..
                    Please advise

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                      #11
                      Originally posted by mohsan514 View Post
                      I have chiller units and each unit is 600 amps and required OCPD is 800amp. I used the 2 runs of cable and total ampacity is 820 amp and these are laid in perforated cable tray . After applying grouping factor / correction factor , the cable ampacity is reduced to 720 amps. But i am worried that how 800 amp CB will protect the conductor in case of any abnormal condition.
                      Since nec 240.4 allows to use the higher rated OCPD. Even the conductor is less rated..
                      Please advise

                      Sent from my SM-J700F using Tapatalk
                      Motors (and refrigeration units) have higher branch circuit devices than conductor ampacity quite often, this to allow for overcurrent device to hold during starting. There is still motor overload protection that will limit current to conductor ampacity or only allow limited overloading before it interrupts the circuit.

                      Take a simple 10 Hp 480 volt motor - FLA may be about 12 amps, so overload protection is set according to 12 amps (probably going to be 115% to 125% of that most cases but based on that 12 amps) A branch circuit breaker could possibly be as high as 35 amps. You are still allowed 14 AWG conductor because the motor overload protection will never allow 14 AWG to be overloaded for long enough to cause any damage to the conductor.
                      I live for today, I'm just a day behind.

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                        #12
                        I am looking for correction factor or de-rating factor of touching single core cables installed in trefoil arrangement at cable tray . 3 ckt at each cable tray. Please advise with code reference.
                        Thanks

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                          #13
                          Originally posted by mohsan514 View Post
                          I am looking for correction factor or de-rating factor of touching single core cables installed in trefoil arrangement at cable tray . 3 ckt at each cable tray. Please advise with code reference.
                          Thanks

                          Sent from my SM-J700F using Tapatalk

                          Is this what you are looking for [MENTION=160389]mohsan514[/MENTION]

                          Table 392.22(A)(5) Allowable Cable Fill Area for
                          Multiconductor Cables in Ventilated Channel Cable Trays for
                          Cables Rated 2000 Volts or Less
                          They say I shot a man named Gray and took his wife to Italy
                          She inherited a million bucks and when she died it came to me
                          I can't help it if I'm lucky

                          Comment


                            #14
                            Thank you.. same i was looking for..

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