Wire calculations for class

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Crabcakes&football

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I'm in tech school and would appreciate if someone checked my work. Thank you
We have a 480/277 panel with a 3-pole 45amp breaker feeding a vfd. The vfd main breaker is 50amps 460 volts. I figured #8 cu thhn with a #10 cu for the ground. We don't know what the load is yet until we get this answer correct. Appreciate it.
 
You have not supplied enough information to size the conductor between the breaker and the VFD. The wire is required to be 125% of the rated input amps. 430.122(A).
 
don_resqcapt19 said:
You have not supplied enough information to size the conductor between the breaker and the VFD. The wire is required to be 125% of the rated input amps. 430.122(A).
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Here is the nameplate of the vfd in class. So 21amps x 125% = 26.25 amps. #10 cu thhn & #10 ground on a 45amp 3 pole breaker.
 

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Crabcakes&football said:
Thank you sir! What has me confused, the breaker feeding the vfd is 45amps, #10 is rated for 40amps. Isn't that a violation?
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Take a look at 430.130(A). So, you now have to look to the motor current from the Article 430 Tables to apply the rule in 430.130(A).
It is like the OCPD for a motor starter...the branch circuit device is providing only short circuit and ground fault protection and the VFD is providing the over load protection. However there may be cases where the maximum permitted OCPD as found in the VFD instructions is less than what is permitted by the use of the rule in 430.130(A).
 
don_resqcapt19 said:
Take a look at 430.130(A). So, you now have to look to the motor current from the Article 430 Tables to apply the rule in 430.130(A).
It is like the OCPD for a motor starter...the branch circuit device is providing only short circuit and ground fault protection and the VFD is providing the over load protection. However there may be cases where the maximum permitted OCPD as found in the VFD instructions is less than what is permitted by the use of the rule in 430.130(A).
Click to expand...
Thank you for your help. I'm probably over thinking this question. Like our instructor is giving us a trick question.
 
So we can make a guess about the motor load since you are not given info about the motor.

430.52 tables says we can use 250% of FLC.

45a OCP / 250% = 18 amps

We can guess the motor might have an FLC of 18a. The VFD has a working current of 21a so that makes sense.
 
mikeames said:
So we can make a guess about the motor load since you are not given info about the motor.

430.52 tables says we can use 250% of FLC.

45a OCP / 250% = 18 amps

We can guess the motor might have an FLC of 18a. The VFD has a working current of 21a so that makes sense.
Click to expand...
If my answer is correct with the #10 cu and #10 ground then I'll get the motor information today. That makes sense mike,thank you.
 
Since the motor has overload protection you can use an overcurrent protective device that is larger than the conductor rating. In this case the overcurrent protective device just deals with short circuit and ground fault. The overload takes care of protecting the conductor from over current.

btw, #10 is not good for 40 amps because that is the 90c column. You cannot size a conductor based on 90C since the equipment etc is at best 75C. It is the weakest link theory.
 
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Dennis Alwon said:
Since the motor has overload protection you can use an overcurrent protective device that is larger than the conductor rating. In this case the overcurrent protective device just deals with short circuit and ground fault. The overload takes care of protecting the conductor from over current.

btw, #10 is not good for 40 amps because that is the 90c column. You cannot size a conductor based on 90C since the equipment etc is at best 75C. It is the weakest link theory.
Click to expand...
#10's should still be good for this scenario? Correct.
 
mikeames said:
So we can make a guess about the motor load since you are not given info about the motor.

430.52 tables says we can use 250% of FLC.

45a OCP / 250% = 18 amps

We can guess the motor might have an FLC of 18a. The VFD has a working current of 21a so that makes sense.
Click to expand...
Pretty sure you guessed right on the motor amps. We got the nameplate today.
460volts at 16.6 amps.
 
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