Powering a motor from two separate disconnects

[Allen Bradley]

Hey Guys,

We have a 480v 3000A switchgear with two separate 400A breakers each supplying power to a separate 400A breakers in panels in different parts of the building. These then supply power to two separate 400A fusible disconnect switches in a test bay. Neither of these 400A disconnects is large enough nor rated to power a 350HP motor with an FLA of 399A individually. Since this is a test bay and the connection to the motor is temporary, some here think it is ok to power this motor from both of these 400A disconnects at the same time and essentially providing up to 800A. To me this seems completely wrong and unsafe even for temporary testing purposes. What say the experts?

 

[Dennis Alwon]

I you parallel the conductors and use an 800 amp ocpd then you would have 800 amps. It is not compliant to do what you plan to do. You wouldn't have e 800 amps either.

 

[Allen Bradley]

@Dennis Alwon Exactly what I was thinking. Thank you for the confirmation.

 

[ptonsparky]

Of course, each source will be the same distance to the load. No impedance differences.

 

[petersonra]

Since it is temporary, run it off one of the 400 A cbs.

 

[ggunn]

Hey Guys,

We have a 480v 3000A switchgear with two separate 400A breakers each supplying power to a separate 400A breakers in panels in different parts of the building. These then supply power to two separate 400A fusible disconnect switches in a test bay. Neither of these 400A disconnects is large enough nor rated to power a 350HP motor with an FLA of 399A individually. Since this is a test bay and the connection to the motor is temporary, some here think it is ok to power this motor from both of these 400A disconnects at the same time and essentially providing up to 800A. To me this seems completely wrong and unsafe even for temporary testing purposes. What say the experts?

You are correct; it is completely wrong.

 

[Allen Bradley]

Since it is temporary, run it off one of the 400 A cbs.

This would seem like the simplest solution but the 400A time delay fuses will not survive the inrush current of the 350HP motor typically 5 to 6 times the motor FLA or in this case (2000A - 2400A), not to mention the maximum HP rating of the 400A disconnect at 480v is 250HP. We simply need a larger circuit breaker and disconnect. 600A would work.

 

[ptonsparky]

You are doing an ATL start?

 

[Allen Bradley]

You are doing an ATL start?

ATL?

 

[Allen Bradley]

ATL?

Across The Line....Not in every case. Still the single 400A disconnect is not large enough for the 350HP motor.

 

[Allen Bradley]

So, this story gets better. I opened up one of the 400A disconnects to see what size conductors were being used and discoved this:

600A fuses in a 400A disconnect. Is this possible?

So, I happened to discover that the same guys that wanted to run a 350HP motor from two separate 400A disconnects had previously put 600A fuses in one of the 400A disconnects. https://forums.mikeholt.com/threads/powering-a-motor-from-two-separate-disconnects.2580335/ Notice how the long blades...

forums.mikeholt.com

 

[ptonsparky]

Across The Line....Not in every case. Still the single 400A disconnect is not large enough for the 350HP motor.

No, it is not. Everything would need to split evenly across both breakers. Won't happen.

 

[ptonsparky]

So, this story gets better. I opened up one of the 400A disconnects to see what size conductors were being used and discoved this:

600A fuses in a 400A disconnect. Is this possible?

So, I happened to discover that the same guys that wanted to run a 350HP motor from two separate 400A disconnects had previously put 600A fuses in one of the 400A disconnects. https://forums.mikeholt.com/threads/powering-a-motor-from-two-separate-disconnects.2580335/ Notice how the long blades...

forums.mikeholt.com

Problem solved!

 

[kwired]

Is this for just testing an uncoupled/unloaded motor for short time? If so you likely can get by with the single source as long as overcurrent protection will hold during starting. Reduced voltage starting methods could be necessary.

 

[Allen Bradley]

Is this for just testing an uncoupled/unloaded motor for short time? If so you likely can get by with the single source as long as overcurrent protection will hold during starting. Reduced voltage starting methods could be necessary.

Yes this is for testing but the duration could be several hours a day for several days. The main problem is a single 400A disconnect is only rated up to 250HP so the whole setup would need to be revamped to include at least a 600A disconnect with 600A fuses and the appropriate conductors to handle the load.

 

[ggunn]

Yes this is for testing but the duration could be several hours a day for several days. The main problem is a single 400A disconnect is only rated up to 250HP so the whole setup would need to be revamped to include at least a 600A disconnect with 600A fuses and the appropriate conductors to handle the load.

Be that as it may, you cannot connect those discos in parallel per 240.8.

 

[augie47]

There are probably a number of ways you can"rig" a way to perform the test but, as noted in posts above, the methods listed are not allowed by the NEC and could pose a danger.

 

[Allen Bradley]

Be that as it may, you cannot connect those discos in parallel per 240.8.

Agreed. That is why I stated we would need a single 600A disconnect to run the motor instead of trying to parallel two 400A disconnects which would be insane not to mention a clear safety hazard and NEC violation.

 

[kwired]

Yes this is for testing but the duration could be several hours a day for several days. The main problem is a single 400A disconnect is only rated up to 250HP so the whole setup would need to be revamped to include at least a 600A disconnect with 600A fuses and the appropriate conductors to handle the load.

Is it driving any load or are you just spinning the rotor that isn't coupled to anything? If not driving anything I wouldn't be surprised at all if it draws under 400 amps and your bigger concern would be can you start it without tripping the 400 amp device. A majority of the current it does draw will be reactive current from low power factor and adding corrective capacitors could lower that current even more, soft starter may allow you to start it on 400 amp device, if it isn't already using reduced voltage starting method of some kind. Many POCO's want motors over 100HP to not be started across the line, some may even say anything over 50 or 60 HP.

 

[Allen Bradley]

Is it driving any load or are you just spinning the rotor that isn't coupled to anything? If not driving anything I wouldn't be surprised at all if it draws under 400 amps and your bigger concern would be can you start it without tripping the 400 amp device. A majority of the current it does draw will be reactive current from low power factor and adding corrective capacitors could lower that current even more, soft starter may allow you to start it on 400 amp device, if it isn't already using reduced voltage starting method of some kind. Many POCO's want motors over 100HP to not be started across the line, some may even say anything over 50 or 60 HP.

Yes, this is direct coupled to a rather large compressor. This is running off a VFD so the inrush is not as high as it would be starting across the line, but probably still too much for the 400A disconnect but not the 600A fuses somebody jammed into it. (See below)

600A fuses in a 400A disconnect. Is this possible?

So, I happened to discover that the same guys that wanted to run a 350HP motor from two separate 400A disconnects had previously put 600A fuses in one of the 400A disconnects. https://forums.mikeholt.com/threads/powering-a-motor-from-two-separate-disconnects.2580335/ Notice how the long blades...

forums.mikeholt.com