I have 65 VFD's mounted inside of a control panel. They are 25 A and the FLA for all of the motors is either 2.1 FLA or 4.2 FLA. What is a better raceway for these motors. I have always been told no more than 3 VFD loads in one conduit. Can i use a trough or something else so I don't have to run 22 separate conduits for these motors?
65 VFD's
- Thread starter rspeaks1
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- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Don't believe what you have "always been told," without at least making some effort to learn the reasons. So here is the reason for that bit of wisdom:
- If these are three phase loads, and if there is no neutral wire associated with each VFD, then with 3 VFDs served via one conduit you will have a total of 9 current-carrying conductors in that conduit.
- Table 310.15(B)(3)(a) tells us to derate the conductors by 70%.
- Article 110.14(C) tells us (if you can read between the lines) that when we do an ampacity adjustment for a 90C rated conductor, we can start with the value associated with the 90C column of table 310.15(B)(16).
- If you are using THHN, the 90C rating for #12 wire is 30 amps.
- Taking 70% of 30 amps gives us 21 amps.
- So if you need a 20 amp circuit to have every bit of 20 amps worth of conductor available, you can achieve that with #12 THHN, so long as you don't exceed 9 current-carrying conductors.
- That is the basis for the "max of 9 rule."
Welcome to the forum.
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
By the way, as a "homework assignment," see if you can tell us how many #10 THHN wires can go into the same conduit, assuming they are all "current-carrying conductors," also assuming each circuit serves a 30 amp non-continuous load, and still be able to use a 30 amp breaker to protect each circuit.
JFletcher
Senior Member
- Location
- Williamsburg, VA
How so? If you had a 65% derating (or 35% left), your 10.5A is correct, tho how would you be allowed to still put that on a 20A breaker? if you have to derate #10 to 70% capacity, you get 28A, and cannot then use a 30A breaker.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Charlie has given a thorough discussion of the ampacity adjustment factors involved, so I will not go into that.
Another factor for VFDs is that even though the current can be pretty smooth the voltage on the output lines is pulsed DC with an amplitude that is about the same as the peak voltage of the AC input.
Those pulses, coming from different VFDs at different frequencies, can induce noise voltage on the other wires in the same raceway unless shielded VFD cable is being used. This can, in some cases, cause problems for the other VFDs.
An important parameter to know to determine whether this could be a significant problem is the length of the longest run of wire. A long run of wire from VFD to motor can also cause problems with standing waves on the length of wire having peak voltages which may strain the insulation on the wires or cause problems with the motors.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Sorry, I was typing faster than I was thinking. You are right, of course. So let's go over that last part again.
Does the VFD manufacturer have a minimum size for the overcurrent protection? If not, and if you can use a 15 amp breaker (the lowest standard rating), then you can put up to 20 CCCs in the same conduit, and still use #12 conductors. That would serve 6 of the VFDs. Another solution would be to use #10 THHN and a 15 amp breaker. Then you can get up to 40 CCCs in the same conduit, and that would serve 13 VFDs.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I don't understand the 25 A reference. Are the VFDs all 25 Amp but operating motors with just 2.1 or 4.2 FLA? One VFD per motor? Multiple motors per VFD?
In any case, as a rule of thumb the rule you cited is not bad as such rules go, but there is nothing in the code that actually requires it. You have to run the numbers and see what comes out.
No reason you can't use cable tray and run tray cable, especially if the distances are fairly short. You might consider using VFD cable which is tray cable with special shielding and insulation with a higher voltage rating (among other things). It is a little pricey though.
The code might even allow you to run smaller conductors if you used cable of some sort, but that is dependant primarily on the OCPD rating of the circuit.
JFletcher
Senior Member
- Location
- Williamsburg, VA
Thanks for the clarification. If you ask my gf, I am seldom right, so "of course" is not a given, rather the exception.
Could the OP use one conduit serving 5A or 7.5A fused discos? Not that it would be the best solution, but would it be compliant?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Do NOT discount what GoldDigger is saying here. I don't know where you heard this "rule" on no more than 3 VFD outputs in a conduit, but that is not true. MAYBE if all of the VFD outputs are from the EXACT SAME VFD, i.e. you have multiple motors from one VFD, you can put multiple outputs in the same conduit, but NEVER put more than one VFD output in a conduit if not. Currents at different frequencies do NOT cancel each other out, they ADD. And the issue of the multiple pulses at different frequencies he mentioned is not just bad for the VFDs, its WORSE for the motor insulation.
Just last year I was party to a situation where a VFD output was in a separate conduit, except for 12ft in which they laid it in an open cable tray (going across a roadway) along side the INPUT cables of the same VFD. So the input cables were at 60Hz, the outputs were varying. Took out an inverter duty motor in 6 months due to winding insulation breakdown from standing wave spikes. The motor insulation was "Inverter Spike Resistant" rated for 2000V...
On another recent project, the contractor USED the shielded VFD cable, but didn't read the instructions. Laid them all in cable tray, no shield grounding at the motor end. out of 80 motors, they lost 36th of them in the first MONTH from insulation failure, ALL of them were Inverter Duty as well.
If you use SHIELDED VFD cable properly installed (i.e. grounded at BOTH ends), you can do anything you like really, as long as you satisfy fill ratios etc. But single conductors? No, you are cuizin' for a bruizin'.
Just last year I was party to a situation where a VFD output was in a separate conduit, except for 12ft in which they laid it in an open cable tray (going across a roadway) along side the INPUT cables of the same VFD. So the input cables were at 60Hz, the outputs were varying. Took out an inverter duty motor in 6 months due to winding insulation breakdown from standing wave spikes. The motor insulation was "Inverter Spike Resistant" rated for 2000V...
On another recent project, the contractor USED the shielded VFD cable, but didn't read the instructions. Laid them all in cable tray, no shield grounding at the motor end. out of 80 motors, they lost 36th of them in the first MONTH from insulation failure, ALL of them were Inverter Duty as well.
If you use SHIELDED VFD cable properly installed (i.e. grounded at BOTH ends), you can do anything you like really, as long as you satisfy fill ratios etc. But single conductors? No, you are cuizin' for a bruizin'.
Fulthrotl
~Autocorrect is My Worst Enema.~
- Occupation
- E
duh. read thread before opening mouth. duh.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I read it. What did I miss?
When I mentioned "other wires" I meant other VFD outputs, not unrelated wiring.
So it almost seems as though the safest correct answer would be to use VFD cable for all of the outputs? Cable tray is not an option as this is a food production facility and the spec everything in rigid conduit. As to Jraef's comment, does it mean more to keep the Line side of the VFD separate from the Load side or more important to keep the loads separated? This is just for future reference. The client " doesn't like a bunch of small conduits running around" which is why i was trying to figure this out.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I thought he maybe responded and then changed his post after seeing something he never read the first time?
Understandable with food production, more raceways means more places for bacterial growth to occur and more places to have to keep clean. But at same time they will want to know why this application goes through a lot of drives if you end up with problems like have been mentioned. Take both issues into account and maybe run multiple shielded drive cables in larger raceways for the "home run" portions of the runs.
I'd run SWAPVC, one per motor.
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