NEC 430.24

[deee]

What is the maximum size dual-element (time delay) fuse permitted to protect a feeder sized by section 230.24, which supplies the following group of no code letter, 230 volt single phase, squirrel cage induction motors? One 10 HP at 50 amperes and three 5 HP at 28 amperes.

 

[deee]

Is the 125% in 430.24 used to calculate this load along with the Table 430.52.

 

[david luchini]

Is the 125% in 430.24 used to calculate this load along with the Table 430.52.

No, the 125% from 430.24 doesn't factor into this. See 430.62 for determining the correct size.

 

[deee]

No, the 125% from 430.24 doesn't factor into this. See 430.62 for determining the correct size.

So 430.24 only applies to the feeder calculations. Correct???

 

[deee]

What is the maximum size dual-element (time delay) fuse permitted to protect a feeder sized by section 230.24, which supplies the following group of no code letter, 230 volt single phase, squirrel cage induction motors?One 10 HP at 50 amperes and three 5 HP at 28 amperes.

Check my work. 240.62 says the protective device has to be sized to the largest motor in the group in accordance with 430.52. Article 430.52 says to use, in this case 430.52(C)(1) and the exception. Article 240.6 gives the standard amp ratings.

50 amps x 1.75 = 87.5amps. The overcurrent device to use would be 90 amps
90 amps + 28amps + 28amps +28amps = 171.5amps. The overcurrent device to use for all the motors would be 175 amps.

 

[david luchini]

50 amps x 1.75 = 87.5amps. The overcurrent device to use would be 90 amps
90 amps + 28amps + 28amps +28amps = 171.5amps. The overcurrent device to use for all the motors would be 175 amps.

430.62 says that the feeder OCPD shall be not greater than 171.5 amps. So 150A would be the correct answer.

 

[deee]

430.52 Exception 1

430.52 Exception 1

430.62 says that the feeder OCPD shall be not greater than 171.5 amps. So 150A would be the correct answer.

430.62 refers to 430.52 to calculate the higher load, which refers you to Table 430.52. Per the conditions in the question 175% is used to calculate the largest motor. Look at the Exception in 43.52, and tell me what you think. From my understanding, it says you can use a higher size, rating or setting that does not exceed the next higher standard rating. So 50amps at 175% is 87.5 amps So I interpret 430.52 (Exception) as using a 90 amp rating and adding the other motors to this amount which is 174 amps. Since this is not a standard size per 430.52(Exception) then I can increase the size to the next standard size that does not exceed the next higher size, which is 175 amp OCPD. 1

 

[david luchini]

430.62 refers to 430.52 to calculate the higher load, which refers you to Table 430.52. Per the conditions in the question 175% is used to calculate the largest motor. Look at the Exception in 43.52, and tell me what you think. From my understanding, it says you can use a higher size, rating or setting that does not exceed the next higher standard rating. So 50amps at 175% is 87.5 amps So I interpret 430.52 (Exception) as using a 90 amp rating and adding the other motors to this amount which is 174 amps. Since this is not a standard size per 430.52(Exception) then I can increase the size to the next standard size that does not exceed the next higher size, which is 175 amp OCPD. 1

430.52 refers you to 430.52 to calculate the largest rating or setting of the OCPD for your largest motor. You've done that...

50A*1.75= 87.5A...430.52 allows you to use the next size up, so 90A.

430.62 then tells you to add the FLAs of the remaining motor and select a Feeder OCPD that is NOT LARGER THAN that sum.

90A+28A+28A+28A = 174A... 175A is LARGER than 174A, so it does not comply with 430.62. 150A would be correct.

 

[deee]

430.52 Exception

430.52 Exception

430.52 refers you to 430.52 to calculate the largest rating or setting of the OCPD for your largest motor. You've done that...

50A*1.75= 87.5A...430.52 allows you to use the next size up, so 90A.

430.62 then tells you to add the FLAs of the remaining motor and select a Feeder OCPD that is NOT LARGER THAN that sum.

90A+28A+28A+28A = 174A... 175A is LARGER than 174A, so it does not comply with 430.62. 150A would be correct.

I was taught to do it this way by a PE, and now I am completely baffled. I appreciate your response. I had my doubts about the interpretations in the exception.