Sizing Short current ground fault for 300 hp motor

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mo2004

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I have situation where we are going to install a 300 hp 480 V motor
FLC according to 430.52 361 amp
Wire size is (361)*1.25 = 451.25 500 kmill.
For an instantaneous circuit breaker we have 800 amp can i use it for short circuit and ground fault since the motor have an overload protection.

thank you for all your inputs.:-?
 
mo2004 said:
I have situation where we are going to install a 300 hp 480 V motor
FLC according to 430.52 361 amp
Wire size is (361)*1.25 = 451.25 500 kmill.
For an instantaneous circuit breaker we have 800 amp can i use it for short circuit and ground fault since the motor have an overload protection.

thank you for all your inputs.:-?
Click to expand...

Take another look at 430.52. For an instantaneous you can go 800% of FLA, not 800 amps, HOWEVER, see 430.52(c)(3) for limitations on instantaneous trip breakers.
An 800 amp Inverse Time breaker would be probable, although you could exceed that for an Inverse Time also.
 
Thank you all for your responses

430.52 c 3
Allow the use of the Instantaneous Trip Circuit Breaker

1- Adjustable (yes)
2- Part of listed combination motor controller with coordinated motor overload
And short circuit and ground fault protection.
3- The setting is no more than 800% (800 amps <<< 2888 amps).

With exception of the wiring upgrade to parallel 4/0 this should be good to go.
 
thank you coffee time

I do see your point.
but i just found out that the motor FLC from name plate is 340 AMP.

340 *1.25 = 425 amp
the 500 Kcmill wire which exist from an older installation is rated 430 AMP.

i know i am suppose the NEC FLC for wire sizing but is not close enough that i can use the name plate
 
mo2004 said:
3- The setting is no more than 800% (800 amps <<< 2888 amps).
Click to expand...
mo -
Assumptions: The 300hp motor is starting across the line - no soft start. The motor is a pretty standard Design D.

If your telling us the instaneous setting on the CB is 800A - the motor is not going to start. The CB will trip.

The inrush will likely be in the range of 1400A - 2000A.

If you are telling us the CB is an 800A frame, with an instantaneous only trip, that is a way different story. This will easily adjust up to the range you need.

However, although I'm sure someone makes 800A frame CBs with an instantaneous only trip - I've just never seen one, or an application where I would want to use one.

Just curious: The overloads you mentioned, are they CTs and mechanical thermals? Or an electronic type?

cf
 
mo2004 said:
thank you coffee time

I do see your point.
but i just found out that the motor FLC from name plate is 340 AMP.

340 *1.25 = 425 amp
the 500 Kcmill wire which exist from an older installation is rated 430 AMP.

i know i am suppose the NEC FLC for wire sizing but is not close enough that i can use the name plate
Click to expand...

Where did you find a rating of 430 amps for 500 kcm? You can only use the
75C rating which is 380 amps. If the installation is old, you may have to use the 60C rating.
 
mo2004 said:
thank you coffee time

I do see your point.
but i just found out that the motor FLC from name plate is 340 AMP.

340 *1.25 = 425 amp
the 500 Kcmill wire which exist from an older installation is rated 430 AMP.

i know i am suppose the NEC FLC for wire sizing but is not close enough that i can use the name plate
Click to expand...

For sizing branch circuit conductors you need to use the table values in accordance with 430.6 so for a 300 HP 460 volt motor you would use 361 amps.

Also 500 Kcmil copper wire is rated at 430 amps at 90 degrees, unless the terminals are listed for 90 degrees you can not use that value.

You are most likely limited to the 75 degree column and would have an ampacity of 380 without any adjustment factors.

Chris
 
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