Number of VFD motor circuits in a pipe?

[electricmason86]

I have a question for the hive mind, how any VFD motor circuits in can you run in a conduit, I have 4 circuits to run and was thinking of running 2 circuits per pipe (rgs) regular THHN, or can I possibly run all four in one pipe?

Edit Thank you in advance

 

[infinity]

Take a look at the manufacturer's instructions. Many do not recommend more than one VFD output circuit in a raceway.

 

[going_commando]

If you follow the instructions from the VFD manufacturer, it will tell you to only run 1 VFD load per ferrous metal conduit using XHHW insulated conductors, or using a VFD cable. You could conceivably run multiple VFD cables in 1 conduit, but they are basically a shielded tray cable for the more affordable options so the conduit needs to be bigger.

All that being said, its done every day contrary to manufacturer's instructions. I won't do it the way you described, as I always run 1 conduit per VFD load, but finding XHHW cu locally for simple stuff isn't typical practical so thhn it is. You can get all kinds of weird feedback and faults on VFDs from all kinds of strange things. On a new install I always install a line reactor, and if the distance is more than ~75' I install a load reactor as well. I've had weird VFD faults from running a motor 125' away from the drive, using VFD cable that was rectified by installing a load reactor. It ran fine without one for a full year, and has now run for 6 years with one without an errant fault.

 

[Jraef]

When you run multiple cables in a conduit, all of the surrounding magnetic fields are expanding and collapsing at the same frequency, cancelling each other out, so there are negligible mutual induction effects. But when you involve VFDs, their outputs are NOT at the same frequencies, so they do NOT cancel. It gets real ugly real fast, resulting in motor winding damage, conductor insulation breakdown and possible transistor damage. So the answer to your question as to how many circuit, the answer is 1, unless you use fully shielded BFD cable for each individual motor circuit, with the shielded grounded at both ends.

 

[ptonsparky]

When you run multiple cables in a conduit, all of the surrounding magnetic fields are expanding and collapsing at the same frequency, cancelling each other out, so there are negligible mutual induction effects. But when you involve VFDs, their outputs are NOT at the same frequencies, so they do NOT cancel. It gets real ugly real fast, resulting in motor winding damage, conductor insulation breakdown and possible transistor damage. So the answer to your question as to how many circuit, the answer is 1, unless you use fully shielded BFD cable for each individual motor circuit, with the shielded grounded at both ends.

We followed up an EC that had put, IIRC, six all in the same conduit. Motors were always burning up but they were in a very high ambient and not knowing better, we just replaced motors as they died.