UL 508A: Sizing of branch circuit protection for single motor circuit

[LMAO]

I am not sure how familiar you guys are with UL 508A. I am designing VFD cabinets and need to size the breaker feeding the VFD running the motor. According to UL 508A 31.3.2, to size the branch protection for single-motor circuit provided with a variable-speed drive, you should follow "manufacturer?s instructions".
I do have the VFD manufacturer recommendations. My question is: Is there a wiggle room? Can we oversize the breaker provided that conductors are sized accordingly?

 

[petersonra]

I am not sure how familiar you guys are with UL 508A. I am designing VFD cabinets and need to size the breaker feeding the VFD running the motor. According to UL 508A 31.3.2, to size the branch protection for single-motor circuit provided with a variable-speed drive, you should follow "manufacturer?s instructions".
I do have the VFD manufacturer recommendations. My question is: Is there a wiggle room? Can we oversize the breaker provided that conductors are sized accordingly?

The short answer is "No".

The main comment I would have is why would you want to? Most modern VFDs allow for a CB that is 250% of rated input current. Is that not enough for you? There are some low end old VFD designs still out there that are limited to 125%.

 

[Jraef]

I am not sure how familiar you guys are with UL 508A. I am designing VFD cabinets and need to size the breaker feeding the VFD running the motor. According to UL 508A 31.3.2, to size the branch protection for single-motor circuit provided with a variable-speed drive, you should follow "manufacturer?s instructions".
I do have the VFD manufacturer recommendations. My question is: Is there a wiggle room? Can we oversize the breaker provided that conductors are sized accordingly?

Maybe, maybe not. Usually the recommendations they use are MAXIMUM sizes, so you need to pay close attention to the wording.
But you especially need to pay VERY close attention to the details of the mfrs recommendations with regard to the SCCR you can assign to the VFD. What you may find is that although they give you a recommended size for the MCCB feeding the VFD, the SCCR may require fuses, EVEN IF you have a CB. So in other words if you want to put a UL508A label on a control panel with a VFD inside, part of that will now mean including the SCCR, and the VFD without fuses will leave you with the default 5kA rating which becomes the maximum rating of your entire panel, which then means the end user basically can't install it in a typical industrial environment.

The tricky part of this is that for some of the companies that are not regularly engaged in having to deal with SCCRs and such, i.e. low cost Asian drives, the info on the SCCR is difficult to get or find. They'll toss out the recommended MCCB sizing, but leave out the details as it relates to SCCR, then you find out later when UL asks for it that you needed to add fuses. Also, it's not universally the same within each product line of each mfr, so you cannot assume anything. One product line might be fine, but that does not mean another line, or even a specific frame size WITHIN a product line, is the same.

 

[LMAO]

The short answer is "No".

The main comment I would have is why would you want to? Most modern VFDs allow for a CB that is 250% of rated input current. Is that not enough for you? There are some low end old VFD designs still out there that are limited to 125%.

How about "under-sizing" the feeder? Can we size the breaker lower than recommended?

About your question (why I'd want to do that?): We are standardizing our VFD cabinets and would like to use a breaker for a "range" of VFDs...

 

[LMAO]

Maybe, maybe not. Usually the recommendations they use are MAXIMUM sizes, so you need to pay close attention to the wording.
But you especially need to pay VERY close attention to the details of the mfrs recommendations with regard to the SCCR you can assign to the VFD. What you may find is that although they give you a recommended size for the MCCB feeding the VFD, the SCCR may require fuses, EVEN IF you have a CB. So in other words if you want to put a UL508A label on a control panel with a VFD inside, part of that will now mean including the SCCR, and the VFD without fuses will leave you with the default 5kA rating which becomes the maximum rating of your entire panel, which then means the end user basically can't install it in a typical industrial environment.

The tricky part of this is that for some of the companies that are not regularly engaged in having to deal with SCCRs and such, i.e. low cost Asian drives, the info on the SCCR is difficult to get or find. They'll toss out the recommended MCCB sizing, but leave out the details as it relates to SCCR, then you find out later when UL asks for it that you needed to add fuses. Also, it's not universally the same within each product line of each mfr, so you cannot assume anything. One product line might be fine, but that does not mean another line, or even a specific frame size WITHIN a product line, is the same.

OK, l am not worried about SCCR because that info is specified by the end user and if they require high SCCR we just give them fuses.

So you are saying UL would allow breakers smaller than recommended by manufacturer?

 

[Jraef]

OK, l am not worried about SCCR because that info is specified by the end user and if they require high SCCR we just give them fuses.

So you are saying UL would allow breakers smaller than recommended by manufacturer?

In most cases, yes. Remember, your CONDUCTORS must be sized at 125% of the VFD input current rating, but there is nothing saying that the CB cannot be smaller than the conductor size.

But it still has to work... If the CB trips from capacitor charging current when you first apply power, your users will not be happy. Generally the maximum size recommended by the VFD mfr will be a tested value. Smaller than that and you might run into unforeseen problems.

 

[LMAO]

In most cases, yes. Remember, your CONDUCTORS must be sized at 125% of the VFD input current rating, but there is nothing saying that the CB cannot be smaller than the conductor size.

But it still has to work... If the CB trips from capacitor charging current when you first apply power, your users will not be happy. Generally the maximum size recommended by the VFD mfr will be a tested value. Smaller than that and you might run into unforeseen problems.

OK, seems like I have some wiggle room. Breaker cannot be bigger what specified by manufacturer or too small to draw enough current at full load. But I still have to double check with "local UL" to make sure if that's OK.