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In the case of motor loads connected to a long branch circuit, why does NEC Table 430.52 limit selection of an over current protection device (OCPD) to a smaller size than the branch conductor will permit?

My hypothesis is a branch conductor that is made larger to compensate for voltage drop cannot supply a larger motor load where voltage drop was the constraining factor. Therefore, the OCPD is limited in size to add protection against motor damage from an unacceptable voltage drop.

This flies in the face of common reasoning that says branch OCPD are for protecting the branch conductors and are not for motor protection. Separate motor overload protection is installed for that purpose.

What gives? Is the NEC actually providing motor protection against unacceptable branch voltage drop during motor startup by prescribing these size limits on circuit breakers?

I would appreciate others' thoughts on this.

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Here is an excerpt from the code:

430.51 General. Part IV specifies devices intended to protect the motor branch-circuit conductors, the motor control apparatus, and the motors against overcurrent due to short circuits or ground faults. These rules add to or amend the provisions of Article 240. The devices specified in Part IV do not include the types of devices required by 210.8, 230.95, and 590.6.

Referring to that other problem we were nattering over; if you have 1200 feet of #4 at 240 volts during a dead short at the end of the line I calculated 623 amps of current. If it was a ground fault close to the motor and the current flowed through the motor this could be quite damaging. I don't know if a 30 amp breaker can trip much faster than a 70 but I believe the intent of the code, as is stated in 430.51, is to protect the motor and conductors from damage resulting from short circuits and ground faults regardless of what size conductor is used.

So, the code limits the size of the OCPD based on the Table FLA of the motor not the ampacity of the conductors.

IMHO Happy Friday!

  • Code dose actually state that you shall use the fla on the motor nameplate X 125% for the conductor (NEC 430.22) if there is more than 1 motor 125% of the largest motor then 100% of the fla for all other motors. The wire size should be upsized for voltage drop. – Ed Beal Nov 20 '15 at 20:50
  • @ArchonOSX - 430.51 and subsequent passages do make me think that OCPD sizing for motor loads is at least partly about protecting the motor. I think I muddied the question by talking about "FLA" earlier, since service factor can commonly vary from 1.0 - 1.4. To remove this aspect of the question I am going to change it to SFA (service factor amps). – user39367 Nov 21 '15 at 1:30
  • Still doesn't address the original question , we upsize wire for voltage drop, code states voltage drop is not part of the conductor ampacity calculation , and I have provided code reference ,,, cite it to write it is how my state handles this – Ed Beal Nov 21 '15 at 2:02
  • I probably should not say this as it is from memory but code also states the opcd can be upsized beyond the "suggested" value to make sure the motor starts, again there are are 3-4 different areas of code that cover this one area "motors", if you only read 1 or 2 you can quote them or change the subject, I admit when I screw up or missed something , don't think I did here , trying to be civil,,, Ed B – Ed Beal Nov 21 '15 at 2:25
  • @EdBeal actually the code is extremely clear that it is the Table FLA that is used for sizing the conductors and the OCPD and the nameplate FLA that is used for the overloads. The nameplate FLA is only used for the OCPD when it is a torque motor. Not a normal motor. To your second comment, if, as you say, the "code states voltage drop is not part of the conductor ampacity calculation" then that just goes to reinforce the reason for continuing to protect a #4 wire at 30 amps when used as a long distance motor branch circuit. – ArchonOSX Nov 21 '15 at 11:27
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the motor overload is based on the FLA of the motor 115% and this protects the motor, in the US we normaly use class 10 overloads this measn the motor can draw 115% of the full load value on the motor nameplate for 10 seconds before it will trip out. 430.32 the conductor size is 125% these are minimum values the OCPD can be increased if needed for starting as motors generaly draw 200-800% of full load current on startup if the breaker is set two low it will trip as the time of the high current draw is only a few seconds the wiring dosent get hot but if there is a locked rotor/ or shorted phase the motor will draw enough to trip the breaker. so the motor overloads protect the motor, circuit breakers protect the branch conductors, there are many kinds of overloads the 2 major types are electronic , and heaters, the electronic measure the current and many times are adjustable, the heaters have a small coil that is heated when it gets hot the metal in the "operator" melts and a small ratchet is released opening a switch contact that is in line with the "starter" the relay that applys line power to the motor , once the overload is cleared the heater cools and it can be reset, these are adjusted by changing the coil size that heats the "operator"

  • Good explanation of motor overload protection Ed. However Chris was asking about sizing the OCPD for a motor circuit on conductors that have been upsized to compensate for voltage drop. – ArchonOSX Nov 20 '15 at 20:18
  • the conductors are sized at a minimum of 125% other areas of code require less than a 3% voltage drop so if you have a voltage drop above this you have to upsize the wires (there are many opinions about what the drop can be durring startup as it is not specificly addressed with the exception of fire pumps which may not trip the breaker even under locked rotor currents ) – Ed Beal Nov 20 '15 at 20:24
  • Thanks, Ed. I scored AchonOSX as having the best answer because my question related to NEC limiting size of the branch OCPD based on motor size and not motor overload protection. Motor overload is separate and in addition to. As I read it, OCPD is 125%-250% motor rating (inverse time), branch conductor size is at least as big as OCPD and not smaller than that required to limit voltage drop. In case of long branch , OCPD still protects branch but is much smaller than branch ampacity to still provide some degree of motor protection. – user39367 Nov 21 '15 at 2:35
  • Not a problem I am used to giving all the code reff and even getting a negative why do I do this I am studding for the equivalent to professional electrical engineer in most states but here it is called supervising general journeyman a real bugger of a test , I mess up sometimes but admit it and here I have provided code reff that some are ignorant of! – Ed Beal Nov 21 '15 at 2:48

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