# Ab vfd



## DesignerMan (Jun 13, 2008)

If you refer to the VFD manual at AB's website it will show you what CB or fuses to use.


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## MDShunk (Jan 7, 2007)

Correct. An inverse time circuit breaker only. For a 500HP load, I'm guessing that will be a 2000 amp breaker. The breaker's gonna cost near as much as the drive.


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## EC2253 (Mar 7, 2008)

I was thinking a 700. The VFD requires 510 x 1.25=638. What am I missing?


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## MDShunk (Jan 7, 2007)

Inverse time breakers you do 250%


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## MDShunk (Jan 7, 2007)

Table 430.250 shows 500hp at 590 amps *2.5 = 1475 amps. Next standard breaker size is 1600 amps. I was close in my guess. It'll likely be a 2000 amp frame breaker with a rating strip.


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## JRaef (Mar 23, 2009)

Just for your future reference. You cannot ever use an instantaneous-only breaker for any purpose in the field, except as an exact part number to part number replacement of one that came in a factory assembly. There are no allowable uses for that breaker as far as an electrician is concerned. No exceptions. 

The AB installation manual for the drive will provide you with a maximum breaker size with which that drive has been UL listed. You are welcome to use anything smaller if you think you can, but not larger. Start there. The 250% rule is intended to allow over sizing on Across-The-Line started motor circuits so that you avoid nuisance tripping of the breaker on starting current. But you will not have a starting current surge with a VFD, so that is unnecessary and a waste of money. Not illegal, just pointless. 430.122 dictates that the supply conductors must be no SMALLER than 125% of the maximum input current rating of the DRIVE, not the motor. So read that in the manual, size your conductors accordingly, then size your breaker to protect those conductors. Save your money for something more important.

If you don't have the manual yet, post the drive series (PowerFlex 700 or 750) and I'll tell you what it says.


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## EC2253 (Mar 7, 2008)

Thanks to everyone, I should have mentioned this 500 hp motor is for a blower. The plant has an exiting 400 hp blower motor, vfd fed by a 600 amp breaker so the starting current must not be excessive.


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## MDShunk (Jan 7, 2007)

The manual for a PowerFlex 700 actually says to use a 2000 amp inverse time breaker for a 500hp motor.


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## JRaef (Mar 23, 2009)

MDShunk said:


> The manual for a PowerFlex 700 actually says to use a 2000 amp inverse time breaker for a 500hp motor.


I see 1800A max on mine, and the PF750 says the same thing. And that's MAX, not exact. Note 8 in that column in the PF700 guide:


> (8) Maximum allowable rating by US NEC. Exact size must be chosen for each installation.


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## KennyW (Aug 31, 2013)

I would avoid just choosing the largest protective device allowed and calling it a day. 

For the us, I would follow JRaefs advice. 125% of the rated input current. 

In Canada we don't have the rule and often will size things smaller than that, depending on the application. 

I would expect 1800 or 2000 amps is insane overkill and isn't really providing any meaningfull coordination to anything on a drive that size. Off the top of my head, I would not expect a breaker larger than about 1200a on a drive that size.


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## triden (Jun 13, 2012)

JRaef said:


> I see 1800A max on mine, and the PF750 says the same thing. And that's MAX, not exact. Note 8 in that column in the PF700 guide:


How do you best determine an adequate size for a VFD application?

edit: 125% of rated current, thanks.


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