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150Hp 460V 3 ph motor with Rockwell softstart

Merry Christmas
D

Dale001289

Senior Member
Location
Georgia
Not sure why the one line shows a 300fuse in line with 250AF/250AT TM. Cb to a softstart before leaving the MCC to the motor - should I size the motor branch circuit to the 250AT or the 300A fuse?


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petersonra

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Dale001289 said:
Not sure why the one line shows a 300fuse in line with 250AF/250AT TM. Cb to a softstart before leaving the MCC to the motor - should I size the motor branch circuit to the 250AT or the 300A fuse?


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Click to expand...
Could be the fuse is needed to get an appropriate SCCR.

The ampacity of the motor circuit is 125% of the FLA taken from the tables in the code.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The fuse is necessary to attain a higher SCCR, but you don’t really want the fuse to clear for “normal” short circuits. So the fuse is sized higher than the breaker, that way the breaker clears the fault first.
 
I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
Jraef said:
The fuse is necessary to attain a higher SCCR, but you don’t really want the fuse to clear for “normal” short circuits. So the fuse is sized higher than the breaker, that way the breaker clears the fault first.
Click to expand...

Thank you Jraef.
But just to be clear the feeder is sized to 125% of motor FLC per the NEC tables and not to the 250AT of the thermal magnetic CB, right?


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Last edited:
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
The conductor is sized at 125% of motor rated current, per NEC motor current tables for standard sized motors that are in those tables.

The short circuit /ground fault protection can be up to 250% or if that doesn't allow motor to start can be increased even more though that shouldn't be necessary with a soft start controller. The conductor is protected from overloading by the motor overload protection.

Many VFD's and soft starters will require specific fuse types be used if the available fault current exceeds certain levels. These are fast acting fuses during short circuit/ground fault events. Should the fuse blow there is good chance your drive is also toast though. The fuse just lessened whatever fireworks show there possibly could have been in that fault event.
 
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Dale001289

Senior Member
Location
Georgia
kwired said:
The conductor is sized at 125% of motor rated current, per NEC motor current tables for standard sized motors that are in those tables.

The short circuit /ground fault protection can be up to 250% or if that doesn't allow motor to start can be increased even more though that shouldn't be necessary with a soft start controller. The conductor is protected from overloading by the motor overload protection.

Many VFD's and soft starters will require specific fuse types be used if the available fault current exceeds certain levels. These are fast acting fuses during short circuit/ground fault events. Should the fuse blow there is good chance your drive is also toast though. The fuse just lessened whatever fireworks show there possibly could have been in that fault event.
Click to expand...

That pretty much clears it up. Thanks Kwired


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D

Dale001289

Senior Member
Location
Georgia
kwired said:
The conductor is sized at 125% of motor rated current, per NEC motor current tables for standard sized motors that are in those tables.

The short circuit /ground fault protection can be up to 250% or if that doesn't allow motor to start can be increased even more though that shouldn't be necessary with a soft start controller. The conductor is protected from overloading by the motor overload protection.

Many VFD's and soft starters will require specific fuse types be used if the available fault current exceeds certain levels. These are fast acting fuses during short circuit/ground fault events. Should the fuse blow there is good chance your drive is also toast though. The fuse just lessened whatever fireworks show there possibly could have been in that fault event.
Click to expand...

For some reason the E&C firm thinks this cable needs to be shielded and rate 5kV even though the motors are only 460V.
I don’t see any reason for this - seems like huge overkill both in labor and material


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Last edited:
petersonra

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Dale001289 said:
For some reason the E&C firm thinks this cable needs to be shielded and rate 5kV even though the motors are only 460V.
I don’t see any reason for this - seems like huge overkill both in labor and material


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Click to expand...
If it is in your contract you are obligated to do it even if it exceeds nec requirements.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Dale001289 said:
For some reason the E&C firm thinks this cable needs to be shielded and rate 5kV even though the motors are only 460V.
I don’t see any reason for this - seems like huge overkill both in labor and material


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Click to expand...
Shielded cables were typical 'boiler plate' specifications when VFDs were fairly new, some 35 years ago. Many EC used 5kV cable because 600V shielded cable was almost unheard of back then.

However, I have never seen a requirement for shielded cables on electronic soft starts.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Dale001289 said:
I think you meant 310.14(A) right?


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There is no 310.14 in the 2017 Code...

But I did err... 310.104 is the ARTICLE;
Part III. Construction Specifications
310.104 Conductor Constructions and Applications. Insulated
conductors shall comply with the applicable provisions of Table
310.104(A) through Table 310.104(E).
Click to expand...
310.104(A) is the TABLE associated with that article.

My point was that your E&C firm might be trying to make you do something that would be a Code violation.
 
D

Dale001289

Senior Member
Location
Georgia
Jraef said:
There is no 310.14 in the 2017 Code...

But I did err... 310.104 is the ARTICLE;

310.104(A) is the TABLE associated with that article.

My point was that your E&C firm might be trying to make you do something that would be a Code violation.
Click to expand...

I’m using the 2023 Code. Id love to be able turn this into an outright code violation.
Routing 5kV, shielded 133% into a 460V motor peckerhead has many problems - where/how do they terminate the shields?
There’s no room for that. Also the cable OD is so large it won’t even fit into the threaded hub. And lastly all resources I’ve found show no shielding is required on a soft start


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