Cable sizing for VFD application
Cable sizing for VFD application
(OP)
Hello,
I am working on a 300 HP VFD application, running a pump. The breaker upstream of the VFD is 500A. The drive is rated at 385A. The question I have is about cable sizing. My motor nameplate is 330A, and following NEC I can use 2 runs of 4/0, with #2 for a ground wire. If I use 90C cable, derated for 6 cables in a conduit, I get 125% of the motor nameplate, and I'm allowed to use the nameplate instead of the table because this is a variable speed motor.
I normally size the conductors based on NEC, not based on the drive manufacturer recommendations. This time I checked, and the Eaton manual says for power wiring to use 2 runs of #300 with a #300 ground wire. They recommend 75C wire.
Eaton is two sizes above me with the current carrying conductors, and they are exceptionally oversized with respect to the ground. This is so far different from my calculations that now I'm concerned about why they would recommend this.
Has anyone experienced this? Any ideas?
EE
I am working on a 300 HP VFD application, running a pump. The breaker upstream of the VFD is 500A. The drive is rated at 385A. The question I have is about cable sizing. My motor nameplate is 330A, and following NEC I can use 2 runs of 4/0, with #2 for a ground wire. If I use 90C cable, derated for 6 cables in a conduit, I get 125% of the motor nameplate, and I'm allowed to use the nameplate instead of the table because this is a variable speed motor.
I normally size the conductors based on NEC, not based on the drive manufacturer recommendations. This time I checked, and the Eaton manual says for power wiring to use 2 runs of #300 with a #300 ground wire. They recommend 75C wire.
Eaton is two sizes above me with the current carrying conductors, and they are exceptionally oversized with respect to the ground. This is so far different from my calculations that now I'm concerned about why they would recommend this.
Has anyone experienced this? Any ideas?
EE





RE: Cable sizing for VFD application
" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
RE: Cable sizing for VFD application
Anyway, back to my question, maybe Eaton is only referring to input wiring. That would make sense, although the ground wire is still ridiculous. Table 250.122 says I can use a #2.
RE: Cable sizing for VFD application
" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
RE: Cable sizing for VFD application
The manufacturer's don't care if they recommend a larger wire, it cost's them nothing.
Getting load side current draw can be a nightmare for the reasons jraef pointed out. The Altivar 71 500HP from SqD is one such creature. The nameplate shows the dual rating but only current draw at the lower rating. The catalog shows another, contradictory number. I tried to find out the answer and have 4 emails back and forth to Germany?, I gave up. They just couldn't seem to provide me with what should be an easy answer. So I sized the wire using the max current I seen between the nameplate and catalog, which is probably higher than it really is.
Good luck
RE: Cable sizing for VFD application
Part of the discrepancy is a result of different temperature ratings.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable sizing for VFD application
RE: Cable sizing for VFD application
RE: Cable sizing for VFD application
You do have to check the 75C column to make sure the terminals are up to the task (without any adjustments).
RE: Cable sizing for VFD application
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable sizing for VFD application
According to the CEC, you can't use 90*C sized wire in a 75*C (or 60*C) marked terminal. I would expect the NEC is the same. Since this is commonly misunderstood, the involved parties in this area have been making a point to highlight the wiring sizing requirements during CEC courses and the electrical inspectors have been specifically looking for it. If I could be bothered looking, I'd try to find the actual section number that covers this.
The wording of the 75*C temperature marking on devices was twisted by many people who simply didn't know and didn't actually read or understand the code to mean you have to use at least 75*C rated wire so 90*C rated and sized wire is fine in a 75*C terminal. This is not the case. If the terminal only supports up to 75*C of wire temperature, then you can't put in wire that could run hotter than 75*C.
It's like Bill posted. You'd have to use a pigtail of heavier gauge wire that meets the 75*C rating column between a 90*C rated wire run and the 75*C marked terminals. I don't know the length of the pigtail required, but it's probably specified in the code somewhere.
FYI, during evaluation testing that terminals marked 75*C can't exceed 65*C.
RE: Cable sizing for VFD application
Example:
100A continuous load
Use 100A/0.8=125A breaker with 75C marked terminals
Wire must be:
Insulation rating of 75C or higher
Ampacity of 125A or higher at 75C
Copper conductors size 1AWG and larger not subject to derating meet this requirement.
If the cables were in a conduit with 4 to 6 current carrying conductors all of the conductors must be derated by 80%.
110.14(C) allows the following calculation (i.e. using the higher temperature rating for ampacity adjustment/correction):
170A (1/0 @ 90C) x 80% = 136A > 125A so this is acceptable
Note that 150A (1/0 @ 75C) x 80% = 120A
110.14(C)(1) requires that you verify that the ampacity of the wire at the temperature corresponding to that marked on the equipment is sufficiently high.
150A (1/0 @ 75C) > 125A so this is acceptable.
Personally I am not especially fond of this method as you don't know if the conductors have been in a 'normal' environment for sufficient length to cool back down to a temperature appropriate for the equipment terminals.
RE: Cable sizing for VFD application
RE: Cable sizing for VFD application
Still, you can not size the wire any higher than what the 75*C column says due to the terminals. 2awg @ 90*C is good for 125A when using 3 conductors in a raceway, but you can't use it for 125A due to the terminals.
RE: Cable sizing for VFD application
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable sizing for VFD application
RE: Cable sizing for VFD application
The terminal ampacity is easily looked up in the tables. There is no correction factors to apply. If the wire size you use for the current your carrying does not exceed the value in the table, the terminal temp is fine. The ampacity adjustments applied to the wire are applied but you still can't use a smaller wire if the 75C terminal temp is exceeded.
As far as different temperatures, the NEC addresses the change in temperatures and which temps to use. So if running raceway UG then earth temp is 20C and where it emerges above ground is say 50C. I think the rule is if higher temp is only 10% or less of total run, then the lower temp can be used. Something like that anyway.
Here in the US you can use 90C wire on a 75C terminal, if it is sized properly. They simply don't want the terminal rating exceeded by way of using higher temp wire.
RE: Cable sizing for VFD application