Breaker size for 60 HP VFD and 60 HP Starter

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Shahzad

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Hello everyone,

I have SIEMENS Model 95 MCC, and I am trying to figure the correct required breaker sizes for my two loads as mentioned below.

Load-1: 60 HP VFD, that has input current of 88Amps. So as per NEC rule 430.122 (A): conductors need to be sized at 125% of VFD input current, thus 88x1.25=110A. So. I will be using 3C,#2 AWG having ampacity of 115Amp as I believe most of the terminal are rated at 75 deg C. Now the question is, do I need to use thermal mag breaker for short-circuit protection or instantaneous (Mag only). Should I use 125A thermal mag breaker or I should upsize it to 150A. Now, if I refer to VFD manufacturer, they recommend putting 125A fuse, but they have not mentioned anything about breaker. I am curious what size and what type of feeder breaker I should buy to feed this 88Amp, 480V, 60 hz drive?

Load-2: 60 HP across the line starter: The motor name plate is 66Amps, but if I go as per NEC table 430.250, the FLC (Full Load current) is 77Amps. I have received quote from SIEMENS vendor and he is using CED63A125 which is mag only breaker (IT) as it's a combination starter. I am curious why he has chosen 125A Inst breaker why not 2.5x66=165A (selecting 150A). I am wondering why some people use 250% Table 430.52 (inverse time) breaker selection criteria as it is confusing when it comes to starters breakers.

Thanks in advance for taking the time to respond my questions.
 
As far as your instantaneous breaker note from 430.52:
Instantaneous Trip Circuit Breaker. An instantaneous trip circuit breaker shall be used only if adjustable and if part of a listed combination motor controller so if you use a breaker if would have to be Inverse Time. If the manufacturer recommends "fuse" not having a fuse at some point in the system could lead to warranty issues and might be seen as a violation of 110.3b

The combination starter can normally be supplied with either type breaker.
(Note for sizing GFSC protection you use the Table value not the Nameplate so the allowable breaker would be 200 amp)
 
An MCC bucket would normally come with the correct overcurrent protection device already installed. You don't get to choose what you want. The MCC manufacturer will select that in accordance with their underwriters laboratory listing.
 
petersonra said:
An MCC bucket would normally come with the correct overcurrent protection device already installed. You don't get to choose what you want. The MCC manufacturer will select that in accordance with their underwriters laboratory listing.
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OP didn't say the the VFD was part of the MCC.
 
petersonra said:
He most certainly did.
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VFD is wall mounted outside the MCC, the drive part number is 22C-D088A103. I am trying to figure out if I have to use 125Amp thermal mag breaker inside the MCC bucket to protect the drive, or I need to use 150A. I asked Rockwell folks and they said since drive is not part of the package, it is standalone, they don't specify breaker size, all they say fuse need to be 125A. so If I think in this direction, meaning 125A thermal mag breaker should be ok because I need to protect the #2, 3C cable as well as per the NEC code 240.4, conductors need to be protected against overcurrent in accordance with their ampacities? But, when I ask SIEMENS because it is SIEMENS MCC Model 95, the vendor tells me that as per his chart, he says to use HFD63B150 which is 150 AMP FEEDER to feed the drive. I am kind of bouncing back and forth between drive folks and SIEMENS. So I was wondering any thoughts and good engineering practice to following in this case? If any one has any please advise.
 
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from the PF400 manual

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Interestingly, while it seems to say you can use a UL489 CB, it does not list what size so I would take this to mean need to use fuses, since there is not a 140 MPCB in the chart for the size VD you have.
 

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Bob is correct, there is no SCCR listing for PowerFlex 400 drives at that size with circuit breakers, only fuses. There is for a PowerFlex 750 class drive, but not the PowerFlex 400s because they are meant to ne HVAC drives sold as packaged units where they would always have fuses, not industrial drives.

If you don't care about UL listing (and you must not if you are putting a Rockwell VFD into an old Siemens MCC), then you can use the fuse size as a guide, so 125A. The NEC however still makes you responsible for making sure that the equipment SCCR is equal to or greater than the Available Fault Current. Without fuses, that VFD is only good for 5kA. So really, even if you want to have a breaker, you would need to also have fuses, in which case you can size the breaker any way you wish.

There is no option for an MCP (mag-only breaker) for almost all VFDs (and soft starters) by the way. UL stopped allowing that back in 2012, so since then, any drives listed in combination must use Thermal-Mag breakers. There are still a few old designs out there that are grandfathered in with MCPs, but they are falling by the wayside because VFD mfrs have all had to change their products recently, and new drives mean new series listings.

Also note that the NEC rule on sizing the input conductors is based on the rated INPUT current of the VFD. That VFD is rated for 88A, but that is the OUTPUT current. The input current of that drive is actually 90A, so the conductors must be rated for 112.5A minimum. That probably doesn't change your wire size in this case, it's just something everyone should pay attention to because those values are never the same.
 
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Jraef said:
Bob is correct, there is no SCCR listing for PowerFlex 400 drives at that size with circuit breakers, only fuses. There is for a PowerFlex 750 class drive, but not the PowerFlex 400s because they are meant to ne HVAC drives sold as packaged units where they would always have fuses, not industrial drives.

If you don't care about UL listing (and you must not if you are putting a Rockwell VFD into an old Siemens MCC), then you can use the fuse size as a guide, so 125A. The NEC however still makes you responsible for making sure that the equipment SCCR is equal to or greater than the Available Fault Current. Without fuses, that VFD is only good for 5kA. So really, even if you want to have a breaker, you would need to also have fuses, in which case you can size the breaker any way you wish.

There is no option for an MCP (mag-only breaker) for almost all VFDs (and soft starters) by the way. UL stopped allowing that back in 2012, so since then, any drives listed in combination must use Thermal-Mag breakers. There are still a few old designs out there that are grandfathered in with MCPs, but they are falling by the wayside because VFD mfrs have all had to change their products recently, and new drives mean new series listings.

Also note that the NEC rule on sizing the input conductors is based on the rated INPUT current of the VFD. That VFD is rated for 88A, but that is the OUTPUT current. The input current of that drive is actually 90A, so the conductors must be rated for 112.5A minimum. That probably doesn't change your wire size in this case, it's just something everyone should pay attention to because those values are never the same.
Click to expand...

So, if I use 125Amp or 150Amp thermal mag breaker in the MCC bucket and feed the wall mounted Rockwell drive directly without fuses, will it be a non compliance? I am not sure how, because the breaker I will be using is rated for 42kA (SSCR), and my available fault current at SIEMENS MCC bus is 35kA, so using breaker alone in this case should still be be ok as I am meeting or exceeding the SCCR rating and also protecting the drive. By the way, I received a reply from local vendor from whom I am buying this rockwell drive, he said that Rockwell has told him that I could use 125A thermal mag breaker to protect the drive. So, any thoughts about this?
 
nspirzada1346 said:
So, if I use 125Amp or 150Amp thermal mag breaker in the MCC bucket and feed the wall mounted Rockwell drive directly without fuses, will it be a non compliance? I am not sure how, because the breaker I will be using is rated for 42kA (SSCR), and my available fault current at SIEMENS MCC bus is 35kA, so using breaker alone in this case should still be be ok as I am meeting or exceeding the SCCR rating and also protecting the drive. By the way, I received a reply from local vendor from whom I am buying this rockwell drive, he said that Rockwell has told him that I could use 125A thermal mag breaker to protect the drive. So, any thoughts about this?
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You need appropriate fuses if your available short circuit current exceeds 5 kA by the time the juice reaches the VFD. Did you calculate what the available short circuit current will be at the VFD yet? That is the important issue. If it exceeds 5 kA you need the fuses.

The manual is poorly written. It mentions UL489 circuit breakers but only lists MPCB in the chart that are not UL489. It is possible that the actual listing allows for UL489 circuit breakers even if the manual does not explicitly say so.

The NEC is sort of vague on this point as well.

1618494374857.png
 
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petersonra said:
You need appropriate fuses if your available short circuit current exceeds 5 kA by the time the juice reaches the VFD. Did you calculate what the available short circuit current will be at the VFD yet? That is the important issue. If it exceeds 5 kA you need the fuses.

The manual is poorly written. It mentions UL489 circuit breakers but only lists MPCB in the chart that are not UL489. It is possible that the actual listing allows for UL489 circuit breakers even if the manual does not explicitly say so.

The NEC is sort of vague on this point as well.

View attachment 2556198
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If I have to calculate short circuit at drive level, can you please provide some info or guideline how can I do it? I had previously used Etap in my old company, but in new one, we don't have any software, so I don't have a model built in for the site as don't have a software and manually might take long time to do it. So, is the any quicker way to find out the short circuit level at drive input?
 
petersonra said:
Incidentally, I had not noticed but you seem to be in Canada. The rules may be different there.
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Yes, but this is for the US client.. Bye the, way the Max Short circuit rating of the drive is 100kA for frame C and 200kA for frame D, so not sure how it was 5kA as mentioned earlier.
 
nspirzada1346 said:
Yes, but this is for the US client.. Bye the, way the Max Short circuit rating of the drive is 100kA for frame C and 200kA for frame D, so not sure how it was 5kA as mentioned earlier.
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The default rating for a motor controller according to UL508a of 60 HP is actually 10 kA.

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To get beyond that you have to have a UL approved combination of fuses or circuit breakers.
 
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