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Cable sizing for VFD application

Cable sizing for VFD application

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?


RE: Cable sizing for VFD application

You missed article 430.122; the cables feeding a VFD must be sized at 125% of the VFD rated input current not the NEC table current for the motor HP. So you must obtain the VFD input current from Eaton, it's often not listed on the nameplate of the VFD. Although one would think that given this NEC requirement they would list it, the problem is most VFDs, Eaton's included, are made overseas so they don't bother tailoring the drive nameplate to our installation requirements. Then because these drives are typically designed around 400V 50Hz standard motor sizes and sold to the nearest NEMA equivalent HP, you often end up with an amp rating slightly higher than needed, which results in an input current rating higher than you want, forcing you to use larger feeder conductors than you expect.

" 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

Excuse my lack of clarification. I was referring to the conductors to the motor. Also, about the VFDs, they can be set up as normal or heavy duty, which corresponds to two different current ratings. You actually have to look into the drive settings to know what the "nameplate" current is.

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

For the load side, size is per normal NEC tiles. 125% of the FLC values in table 430.250 for the motor HP.

" 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

They probably provided you with the largest cables the lugs can accommodate. It may be based on how long the cables can be before you need load filters. It is hard to say. The phase wire size is determined the same way as a non-vfd motor.

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

Quote (OP)

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.
Eton is using a 75 deg rating and you are using a 90 deg rating.
Part of the discrepancy is a result of different temperature ratings.

"Why not the best?"
Jimmy Carter

RE: Cable sizing for VFD application

Good catch Waross.

RE: Cable sizing for VFD application

If the drive terminals are marked as 75*C wiring, then you have to use the 75*C column to pick the wire.

RE: Cable sizing for VFD application

I don't agree with Lionel on this one. You can use the 90C wire/column for ampacity calculations if you are using 90 degree wire: derated due to temp or multiple conductors in raceway.
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

You may have to use short 90C pigtails to connect the 90C wire to the 75C rated terminals if the current is greater than the 75C rating.

"Why not the best?"
Jimmy Carter

RE: Cable sizing for VFD application

buzzp - you don't have to agree, but if talking about wiring in North America then you are wrong assuming you are going to put the 90*C sized wire into a 75*C marked terminal. I'd like to know what you meant in your second part saying to check the 75*C column to ensure the terminals are OK because I'm not following what that means.

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

I agree with buzzp regarding NEC requirements.


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

My comments are all related to the NEC. My comment about recalculating at 75C is only for the terminals. This verifies the terminals are rated to carry this current and no correction factors applied. If everything is good there, then you can use the 90C wire in the ampacity calculations to size the wire. So the terminal rating of 75C is used to make sure the terminals can carry the current (no correction factors) and the 90C column is used to size the wire (with correction factors applied).

RE: Cable sizing for VFD application

I understand what you are saying now. Once you de-rate then there are cases where it works out using both 90*C and 75*C for the different sizing requirements.

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

I agree with Lionel. It's the same under the Canadian Code.

"Why not the best?"
Jimmy Carter

RE: Cable sizing for VFD application

I'll third that.

RE: Cable sizing for VFD application

That is correct Lionel.
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

FYI, UL testing of devices says that at least 4' of wire has to be used in the terminal. So, you could assume 4' is required between the conduit where you used the 90*C number to rate the wire and the terminals where the wire has to meet the 75*c number.

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