Number of VFD motor circuits in a pipe?

E

electricmason86

Member
Location
rochester, mn
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
 
Take a look at the manufacturer's instructions. Many do not recommend more than one VFD output circuit in a raceway.
 
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.
 
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.
 
Jraef said:
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.
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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.
 
ptonsparky said:
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.
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That’s one approach. I had a gravel pit that had run 16 2HP drive circuits in one 4” conduit out to the trap feeders at the bottom of the piles, with breakouts for each one. They decided, after I pointed out the problem, that it was cheaper to keep replacing the motors than to dig new trenches to redo it correctly or to pull all the wires out and replace them with VFD cables. It only had to last until the mine ran out, so likely less than 10 years. I couldn’t argue with the logic…
 
Only one run of VFD output per conduit. Or if you can shield with MC or AC cable that would be ok in one conduit.
 
Jraef said:
That’s one approach. I had a gravel pit that had run 16 2HP drive circuits in one 4” conduit out to the trap feeders at the bottom of the piles, with breakouts for each one. They decided, after I pointed out the problem, that it was cheaper to keep replacing the motors than to dig new trenches to redo it correctly or to pull all the wires out and replace them with VFD cables. It only had to last until the mine ran out, so likely less than 10 years. I couldn’t argue with the logic…
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Now that I think about it, it was one drive feeding those motors. Each with separate overload.
 
ptonsparky said:
Now that I think about it, it was one drive feeding those motors. Each with separate overload.
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That's a very different situation. I deal with lumber drying kilns that are setup that way. 8 7.5hp fans all run off 1 big drive, all running through the same conduit. Some of the motors are original to the kilns, dating back to the early 90s and chug along just fine.
 
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