VFD Input Breaker Size Scenario

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jason.durby

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Electrical Superintendent
This is an existing installation.

The rated input current on my VFD is 388A @ 460V.

I currently have a 400A feeder breaker and 2 - 3/0 per phase feeding this VFD. VFD recommended minimum for the input breaker was 600A.

I haven’t had any nuisance tripping of the breaker in the field. I have a feeling that I will have to increase the cable size based on 125% rule. What about the breaker? Do I need to upsize to 600A per NEC? What do you think?
 
Check out Part X of Article 430

Input cabling to be based on 430.122.. 125% of rated input
- This means at least 485A conductors

Disconnecting means to be based on 430.128.. 115% of rated input
- This means at least 447A rated disconnect (or breaker)


I'm not sure that you need to go all the way to 600A based on their "recommendation". It certainly wouldn't be a bad thing, but if it actually says "recommendation" and not "requirement" then you may be able to treat it as such.. just a recommendation.
 
Thanks for the response. So this is the confusing part because I don’t think having a 400A breaker would matter outside of nuisance tripping but does 430.128 mean that I can’t go any lower than 447A on the disconnect which means that the breaker needs to be at least a 450A. Could I just leave the 400A breaker in if I’m not nuisance tripping?
 
You are largely correct. Even if things are lower than they should be, the input conductors are protected at their ampacity, you're not exceeding any maximums (see 430.130), and you're not tripping. So what's the issue? Whether it's an NEC violation or not is another question.

One outcome of the way 430.128 is written is that you could put a 450A safety switch disconnect between your breaker and the VFD and then claim that this is your disconnecting means and is meeting 430.128.

Another thing to consider is when was this installed? NEC VFD requirements have been a little odd at times and changing for a while now, so there's always the chance that it may have been compliant at time of install.
 
I have not seen VFDs that give you a MINIMUM breaker size, they usually give you a MAXIMUM breaker size that they are listed for, because that's how UL works when you want to list a VFD behind a breaker. Some import brands however don't provide adequate information on that issue, somethimes because their drives are NOT UL listed for use behind a circuit breaker at ALL, they only listed them behind fuses. So installers sometimes took the lack of information to mean they could do what they wanted...

The NEC rules on conductor sizes are however MINIMUM, so bwat is correct about the minimum conductor ampacity being 485A, so your 2 x 3/0 come up short in that aspect. The first rule on that 125% conductor requirement and the 115% disconnect requirement was in the 2002 NEC though (430.22 and 430.2 at that time, respectively), so if it is older than that, it may have been compliant given the 400A breaker. If installed after 2002, it was never compliant.

2002 NEC, article 430.2 for the disconnect size;
...
The disconnecting means shall be permitted to be in the
incoming line to the conversion equipment and shall have a
rating not less than 115 percent of the rated input current of
the conversion unit.
Click to expand...

2002 NEC, article 430.22 for the conductor size:
Exception No. 2: Circuit conductors supplying power conversion
equipment included as part of an adjustable-speed
drive system shall have an ampacity not less than 125 percent
of the rated input to the power conversion equipment.
Click to expand...

The article numbers have changes over the years, but the requirements have not.
 
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Jraef said:
I have not seen VFDs that give you a MINIMUM breaker size, they usually give you a MAXIMUM breaker size that they are listed for, because that's how UL works when you want to list a VFD behind a breaker.
Click to expand...
Not that Jraef ever needs tactical air support, but just evaluated a chiller / VFD combination in which I was originally was told the VFD would have a bypass and the max overcurrent would be 1000A. To CYA, I specified a 1000A CB along with the required feeder. Submittal comes in with no bypass and a sizable credit that the client accepts and a 600A CB as an input disconnect mounted within the VFD enclosure by the manufacturer. So since there will be no inrush to deal with, and mostly a varying load, they manufacturer decides they can handle the whole shooting match with a 600A input CB and feeder, so we reduce everything upstream.
 
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