*New to this* Motor Panel Main Feeder sizing OCPD Question

[Ezburn]

Hello,
I am tasked with upgrading our equipment from starters to VFDs on a system. Also doing this process we are going to over size the VFD for future project to upsize a couple motors.

I have hit a brick wall trying to figure out how and why a Motor Feeder Disconnect OCPD was sized with this particular rating as well as how to size my new OCPD for this feeder if needed.

Current
We have 600 AMP Time delay Fuses used as OCPD for the feeder. This Feeder feeds
(9) 50 HP Motors FLC per table 430.250 = 65A With Motor Branch circuit short circuit/Ground fault protection =100A MCCB each

(3) 10HP Motor FLC per table 430.250 = 14A With Motor Branch circuit short circuit/Ground fault protection =50A MCCB each

(3) 1.5HP Motor FLC per table 430.250 = 3A With Motor Branch circuit short circuit/Ground fault protection =7A MCCB each

(12) 20kW Heaters = 24A per calcs With Motor Branch circuit short circuit/Ground fault protection =30A MCCB each

First question, Should the original Feeder OCPD have a rating of 125%*100A MCCB+8*65A FLC+3*14A FLC+3*3A FLC +12*24A = 984 or 1000A???? per 430.62
I do not understand where we came up with 600A Fuses?

My second question is really dependent on if I am correct with my thinking in the first question.
I am replacing(6) of the across the line starters with VFDs which documentation states input 140A recommended OCPD 200A rating.
Should my Feeder OCPD = 1.25*200+5*140+3*65+3*14+3*3+12*24 = 1484 or 1500A??

Does it matter if we will have 3 separate panels with a 400AMP breaker feeding (1) VFD, (1) 50HP, (1)10Hp, (1)1.5HP and (4)20kW heaters
and 3 separate panels with a 200Amp Breaker feeding (3) VFDs each?

Any help and understanding is much appreciated. Thank you.

 

[Smart $]

Welcome.

H...
First question, Should the original Feeder OCPD have a rating of 125%*100A MCCB+8*65A FLC+3*14A FLC+3*3A FLC +12*24A = 984 or 1000A???? per 430.62
I do not understand where we came up with 600A Fuses?

Note 430.62 establishes a maximum OCPD rating... not minimum like in other articles.

My second question is really dependent on if I am correct with my thinking in the first question.
I am replacing(6) of the across the line starters with VFDs which documentation states input 140A recommended OCPD 200A rating.
Should my Feeder OCPD = 1.25*200+5*140+3*65+3*14+3*3+12*24 = 1484 or 1500A??

Does it matter if we will have 3 separate panels with a 400AMP breaker feeding (1) VFD, (1) 50HP, (1)10Hp, (1)1.5HP and (4)20kW heaters
and 3 separate panels with a 200Amp Breaker feeding (3) VFDs each?

You only need comply with the documents at the branch level. Feeder level OCPD is just like any other motor application.

 

[Jraef]

Also to Smart $'s point it's possible, if not likely, that whomever originally designed that panel knew that all of the motors would not be running at the same time. That's perfectly legitimate. You CAN design it around a 1500A feeder, or assume the original design was correct and replicate it, or break it up into smaller chunks, that's up to you.

PS:

Also doing this process we are going to over size the VFD for future project to upsize a couple motors.

Be aware that many VFDs have a limit on how much you can over size them, because at some point you move out of the acceptable range of the current sensors inside. Check with the VFD mfr's tech support (not their sales people) on what you want to try to do here.

 

[petersonra]

Welcome.


Note 430.62 establishes a maximum OCPD rating... not minimum like in other articles.

Only if you want to use the minimum conductor size allowed. You can alternately size the feeder conductors to the OCPD rating and make the OCPD any higher rating you want.

 

[Smart $]

Only if you want to use the minimum conductor size allowed. You can alternately size the feeder conductors to the OCPD rating and make the OCPD any higher rating you want.

True... but you can't make the feeder conductors smaller than 125% the largest motor plus the FLA of other motors plus 125% continuous load plus the noncontinuous load. For the case considered here, even that is greater than 600a. This is quite likely the situation cited by jraef, i.e. noncoincident loads.

 

[Ezburn]

Also to Smart $'s point it's possible, if not likely, that whomever originally designed that panel knew that all of the motors would not be running at the same time. That's perfectly legitimate. You CAN design it around a 1500A feeder, or assume the original design was correct and replicate it, or break it up into smaller chunks, that's up to you.

PS:

Be aware that many VFDs have a limit on how much you can over size them, because at some point you move out of the acceptable range of the current sensors inside. Check with the VFD mfr's tech support (not their sales people) on what you want to try to do here.

Thank you all for your responses!!! Jraef, This makes a lot of sense given the system. This is what was done. I also verified the over size limit on the VFDs beforehand. BUT it was with a rep so I will have to check the tech documents myself just to be on the safe side.

I was overthinking some things. When I saw Smart $ reply
"Note 430.62 establishes a maximum OCPD rating... not minimum like in other articles."

This is where the confusion lies. I was overthinking but not really thinking if that makes any sense.

 

[kwired]

Even if the original selections were marginal - if they allowed the motors to start across the line you won't have any overcurrent device problems starting them with VFD's.

 

[Ezburn]

Welcome.


Note 430.62 establishes a maximum OCPD rating... not minimum like in other articles.


You only need comply with the documents at the branch level. Feeder level OCPD is just like any other motor application.

In the above example I should have used 430.63 since my loads are motors and heaters? This case would have led me to a rating of Not less than so it would have been a minimum vs maximum?
Is this correct or am I over thinking again?

 

[Smart $]

In the above example I should have used 430.63 since my loads are motors and heaters? This case would have led me to a rating of Not less than so it would have been a minimum vs maximum?
Is this correct or am I over thinking again?

Actually, 430.63(3) would make it a combination... 'not greater than' for the motors plus 'not less than' the other loads. Talk about confusing. :blink:

Still amounts to the only way you can supply all these loads _compliantly_ with a 600A rated supply is by having noncoincident loads [220.60].

 

[Ezburn]

Even if the original selections were marginal - if they allowed the motors to start across the line you won't have any overcurrent device problems starting them with VFD's.

This makes sense but we are "planning" to upgrade 6 of the motors from 50HP to 100 HP so now I am planning ahead with 100HP VFDs. Given we now have (6) 100 Hp Drives @ 140A each VS 50 HP rated at 64A FLC each, things "may" or "may not" change load wise.

Thank you to everyone for your responses.


Smart $ Yes It is confusing, and I agree the only way "compliant wise" a 600A OCPD would work is if we used 220.60