Size wire for VFD

[paulengr]

Older VFDs did not do motor protection. Now it is Code and NRTL required.

The front end protection is to protect the power system against short circuits in the drive and to keep the drive components from launching out of the drive when they fail. Overload protection in the drive itself is that thermal temperature sensors shut it down if it overloads. Remember it is not a conductor…transistors are semiconductors with very little thermal mass. They melt and/or explode in milliseconds.

Some manufacturers have learned the hard way. They thought they could do short circuit protection in software and attempted it on large 20 Megawatt drive systems in Australia. To add insult to injury they also made a serious error by using RMS ratings of the transistors in a mostly DC applications. All the design errors were discovered 18 months later when the first drive transistors launched themselves through the cabinet doors. This happened on the CSIRO sponsored UDD project.

The drive protects the motor if you set it up properly. If you don’t the motor can be burned up. Usually the drive manufacturer makes you do this. They do things like set the FLA to 1 A and overload class to 10 and SF to 1.0. That way it trips on overload until you set it up properly. That’s for a Listed (not CE) VFD from somewhere other than China.

 

[Eddie702]

I agree. Size VFD input conductors at 125% of drive rating. Size conductors between motor and drive at 125% of Code motor amps. (most would use the same size as input conductors,

Dirive contains motor overload protection. Use 80A breaker on line side of drive. No fues or breaker between drive and motor

 

[ptonsparky]

I agree. Size VFD input conductors at 125% of drive rating. Size conductors between motor and drive at 125% of Code motor amps. (most would use the same size as input conductors,

Dirive contains motor overload protection. Use 80A breaker on line side of drive. No fues or breaker between drive and motor

When a mfg says 'fuse' it means fuse. Somewhere in the feed to that VFD a maximum fuse size and type stated by the mfg should be used. Not worried about warranty or parts coming loose? Use whatever you want.

 

[infinity]

Size VFD input conductors at 125% of drive rating. Size conductors between motor and drive at 125% of Code motor amps. (most would use the same size as input conductors,

Drive contains motor overload protection. Use 80A breaker on line side of drive. No fues or breaker between drive and motor

This is our approach as well. Input current * 125%, OCPD sizing as recommended by the drive manufacturer. Output side, follow Article 430 for conductor sizing.

 

[PowerdT]

Thank you all for answering and explaining. It’s been helpful

 

[kwired]

My understanding with VFD protects the thermal overload for the motor.
Am I required to have ocpd between the VFD and motor? Or is it optional this case

The drive's internal protection will trip on low level fault current that a fuse or circuit breaker in the output will not even be thinking about tripping for. Some the reasoning for this is for protecting components in the drive from high fault currents. If those low level fault currents are while starting the motor and it is still in low volts/low frequency at the time, the current will only rise as volts/frequency increases, but most fuses or breakers will not see this as objectionable current at this point.

Introduce a sudden fault while motor is at a higher voltage/frequency though and that current can be more damaging to drive components, though the drive internal protection possibly still much faster at responding and limiting damage than an external OCPD would be.

 

[kwired]

When a mfg says 'fuse' it means fuse. Somewhere in the feed to that VFD a maximum fuse size and type stated by the mfg should be used. Not worried about warranty or parts coming loose? Use whatever you want.

Many of them anymore will have instructions telling you when available fault current is over a certain level they suggest specific fuse type or at least a line side reactor.

Also like you said if not worried about warranty, like maybe a 5 or 10 hp drive or smaller do what you want. Larger more expensive drives...might not be bad idea to follow their recommendations, even if there is no electrical inspector going to be involved and pushing those instructions.