Cable Sizing of Motor w VFD Control

H

hshaker

Member
Location
UK
Occupation
Electrical Engineer
Hello everyone,

I have a confusing situation where we might need to increase cable size.

Initial wire size calculations were based on distance and motor amps. We are using ACS880 105A drive from ABB for a 55kW/100A motor. We have a circuit breaker on the input side of the VFD.

However ABB is using their recommended circuit breaker in their panel for this drive which is 250A for a 105A VFD. When redone the calculations, you can choose the certain circuit breaker, with same distance and motor amps the cable size jumps drastically. from 150mm to 240mm.

So why is the recommended circuit breaker is 2.5 times higher than VFD/motor rating ? Is it something to do complying with short circuit current ? They are recommending the same 250A breaker between 105A and 206A which is confusing. How does circuit breaker rating affect the cable this much when it's put in the software ?

Thanks in advance to all
 
What national code are you using? Could it be that 250% is the max value you can use on that setup? The manufacturer would prefer that higher value to avoid nuisance tripping.
 
Dennis Alwon said:
What national code are you using? Could it be that 250% is the max value you can use on that setup? The manufacturer would prefer that higher value to avoid nuisance tripping.
Click to expand...
Yes. Here in North America, our Codes allow up to 250% of the motor FLC for an inverse time (thermal-magnetic) circuit breaker on motor circuits. However, the Codes make no allowances for changing the breaker size/motor size relationship when there is a VFD involved. Drive manufacturers therefore typically stick with the maximum sizes to avoid nuisance tripping, because in reality the VFD is providing short circuit protection for the motor which will act faster than the breaker would anyway. So the breaker is really only there for the protection of the circuit feeding the drive.

None of that, however, would change anything with regard to the VFD-to-motor conductor sizing. Again, here in North America those conductors are sized at 125% of the FLC value in a look-up table in the code, based on the rated HP of the motor. The only thing that is different with regard to conductor sizing when there is a VFD involved is that the INPUT conductors (feeding the VFD line terminals) must be 125% of the rated INPUT CURRENT of the VFD, which can result in differences in the conductor sizing between line and load.

This forum is based in North America and is decidedly biased toward our Codes, but is frequented by many people from around the world. So if you state what country you are in, someone may be able to help you with specifics that are relevant to your situation.
 
Thanks for your replies. Apparently 250A was the max load breaker supplier was recommending. They've confirmed it can be adjusted to lower amps with 125% value like you suggested.
 
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