6-pole disconnect current rating with poles paralleled

[Steve-R]

We have an existing 6-pole, 60A disconnect that was used with a two-speed 60 horsepower 480v motor, which is 77A per NEC. I'm assuming that the 60A disconnect is acceptable because the load is effectively split between two sides of the disconnect, meaning the full load will be closer to 39 amps, is that correct?

We are now changing speed control to a VFD. Is the 60A disconnect sufficient if both "sides" of the disconnect are connected in parallel to split the load? I can't find anything in the NEC that would not allow this.

Would this be considered poor practice or a bad idea for any other reason?

Thanks!

 

[augie47]

The only 6 pole disconnect I have seen was rated 60 amp on all poles.

 

[justdavemamm]

We have an existing 6-pole, 60A disconnect that was used with a two-speed 60 horsepower 480v motor, which is 77A per NEC. I'm assuming that the 60A disconnect is acceptable because the load is effectively split between two sides of the disconnect, meaning the full load will be closer to 39 amps, is that correct?

I'm presuming that you are referencing Table 430.250.

We are now changing speed control to a VFD. Is the 60A disconnect sufficient if both "sides" of the disconnect are connected in parallel to split the load? I can't find anything in the NEC that would not allow this.

Would this be considered poor practice or a bad idea for any other reason?

Thanks!

Where is the VFD being inserted ? Before or after the disconnect ?

I kinda doubt that the manufacture of the disconnect would agree that each "side" of the disconnect will open / close at the same time and therefore when paralleled up would increase the allowed amperage. Best to call them and ask.

For instance, a typical relay with bifurcated contacts, when compared to another in the same family & model, has only 1/2 the amp rating per contact.

Let's also look at the needs of a VFD since you are wanting to put one in. A typical one (Automation Direct DURAPulse for instance) for 60hp has an input amp rating of 90 amps. Your wire and upstream components must be able to meet the VFD supply specs + NEC rules. If you put in a larger VFD, no matter what the motor HP size is, your wiring and source needs to satisfy that demand.

Let me also throw this in: Many VFD's do not like to have their motor disconnected while under load. I have always provided an early-break Aux. contact on the disconnect wired to the VFD to insure it knows the motor circuit is being interrupted.

While 240.8 permits fuses and CB's to be connected in parallel (when factory assembled and listed as a unit), no mention is made of disconnects since they are not overcurrent protection devices.

As an engineer, I would not accept a design of what the OP describes since I consider the load on each pole to be that of the motor FLA when closing/opening, unless the mfg. allowed it.

 

[benaround]

Steve,

I,ll bet your 2 speed motor needs 6 conductors, the only way to disconnect the motor at the motor location is to

use a 6 pole disconnect. I doubt very much that these conductors are paralelled.

 

[Steve-R]

Thanks for the responses everyone.

Steve,

I,ll bet your 2 speed motor needs 6 conductors, the only way to disconnect the motor at the motor location is to

use a 6 pole disconnect. I doubt very much that these conductors are paralelled.

I'm sure you are right. In this case however, the motor is also being replaced with a "standard" motor with three terminals.

I'm presuming that you are referencing Table 430.250.

Where is the VFD being inserted ? Before or after the disconnect ?

I kinda doubt that the manufacture of the disconnect would agree that each "side" of the disconnect will open / close at the same time and therefore when paralleled up would increase the allowed amperage. Best to call them and ask.

For instance, a typical relay with bifurcated contacts, when compared to another in the same family & model, has only 1/2 the amp rating per contact.

Let's also look at the needs of a VFD since you are wanting to put one in. A typical one (Automation Direct DURAPulse for instance) for 60hp has an input amp rating of 90 amps. Your wire and upstream components must be able to meet the VFD supply specs + NEC rules. If you put in a larger VFD, no matter what the motor HP size is, your wiring and source needs to satisfy that demand.

Let me also throw this in: Many VFD's do not like to have their motor disconnected while under load. I have always provided an early-break Aux. contact on the disconnect wired to the VFD to insure it knows the motor circuit is being interrupted.

While 240.8 permits fuses and CB's to be connected in parallel (when factory assembled and listed as a unit), no mention is made of disconnects since they are not overcurrent protection devices.

As an engineer, I would not accept a design of what the OP describes since I consider the load on each pole to be that of the motor FLA when closing/opening, unless the mfg. allowed it.

The VFD is upstream from the disconnect.

As for opening and closing the disconnect contacts simultaneously, with a spring loaded, quick make/break disconnect, the contacts will not have more than a few tenths of a second difference between them, if that. It is more likely hundredths of a second where they would be overloaded by less than 50%. However this is far less overload than they must endure when a motor is starting across the line and inrush is 500% to 800% of FLA, and many times the disconnect's rating.