Two motors 15HP branch circuit breaker sizing

[hhsting]

I have 2 motors 15HP 480V three phase fed from one branch circuit breaker. I am trying to size the branch circuit breaker size for multiple
motors. I looked at 430.53(C) but I am still not sure what the size of the breaker should be. The engineer has put 60A breaker with 50A cable amp city.

I am not sure how 60A breaker size was arrived?

 

[Eddie702]

There is a whole list of things you have to comply with, and more information is needed. See 430.53

It also depends on if the motor controllers are a factory package or field built and a host of other things.

 

[petersonra]

Is this a branch circuit or a feeder?

 

[AC\DC]

Sounds like a feeder

 

[hhsting]

There is a whole list of things you have to comply with, and more information is needed. See 430.53

It also depends on if the motor controllers are a factory package or field built and a host of other things.

The two motors are inside fume hood exhaust fan (FHE-2) they are not individual motor. Drawings show below unit is FHE-2. VFDs have fuses.

 

[infinity]

The circuit should be designed according to the input current of the drives not the motors.

 

[hhsting]

The circuit should be designed according to the input current of the drives not the motors.

Would breaker size in panelboard be feeder?I dont see anything in part X NEC 2017 Article 430 that says sizing breaker for multiple motors on one vfd? I see NEC 2017 Articles 430.130 and 430.131 but that says branch circuits? Nothing about feeder breaker sizing

 

[augie47]

Is the breaker supplying the VFDs ??

 

[hhsting]

Is the breaker supplying the VFDs ??

Yes

 

[augie47]

Yes

You can pretty much ignore the actual motors in that case and use 430 Part X and base the sizes on the manufacturer requirements.
The individual VFDs should have a max overcurrent rating and, in the event they are grouped, Art 409 should have a max overcurrent.

 

[infinity]

It's likely that the two VFD's cannot be on the same circuit unless the motors are very small because the OCPD designed by the manufacturer will be smaller than the one required by having multiple motors.

 

[hhsting]

It's likely that the two VFD's cannot be on the same circuit unless the motors are very small because the OCPD designed by the manufacturer will be smaller than the one required by having multiple motors.

The vfds does supply one motor but their are two vfds and both vfds are fed from one breaker.


Sent from my iPhone using Tapatalk

 

[hhsting]

It's likely that the two VFD's cannot be on the same circuit unless the motors are very small because the OCPD designed by the manufacturer will be smaller than the one required by having multiple motors.

Both motors are 15HP. There are two separate VFDs for each motor of the 2 motors but their is one breaker supplying both vfds.

 

[petersonra]

It's likely that the two VFD's cannot be on the same circuit unless the motors are very small because the OCPD designed by the manufacturer will be smaller than the one required by having multiple motors.

The OP indicated each VFD had fuses. So, the feeder circuit can be pretty much as large as the designer wants as long as it is large "enough".

In any case, most modern VFDs I have worked with allow about a 250% OCPD. Very few would ever trip even a 100% OCPD. There are some older designs that are limited to about 125%.

A 15 HP drive at 480V is probably about 25 Amps of rated input current. Feeder conductors with 50 Amp ampacity and 60 Amp CB seems kind of close.

 

[hhsting]

The OP indicated each VFD had fuses. So, the feeder circuit can be pretty much as large as the designer wants as long as it is large "enough".

In any case, most modern VFDs I have worked with allow about a 250% OCPD. Very few would ever trip even a 100% OCPD. There are some older designs that are limited to about 125%.

A 15 HP drive at 480V is probably about 25 Amps of rated input current. Feeder conductors with 50 Amp ampacity and 60 Amp CB seems kind of close.

How would supply breaker be sized if their is one supply breaker supplying two vfds?

How would the supply conductors be sized for one circuit powering two vfds? Split and spliced.

 

[augie47]

The individual VFD should have a manufacturer design OCP max. Any OCP ahead of it should not exceed that value. Beyond that it can be any size with the conductor sized to match

 

[hhsting]

The individual VFD should have a manufacturer design OCP max. Any OCP ahead of it should not exceed that value. Beyond that it can be any size with the conductor sized to match

Well the vfd has fuse so probably manufacturer gives fuse size. So how can breaker be sized for fuse? I thought if manufacturers gives fuse size they would have to be fuse not breaker?

 

[augie47]

It is all academic without solid data, The OCP device ahead of the drive (breaker or fuse) needs to meet mfg requirements. We would need nameplate/manufacturer data to determine if your particular installation meets NEC. Based on petersonra;'s post it is likely compliant but the devil is in the details.