Motor disconnect and sizing

[Therealcrt]

I’m supplying power to a three phase 3 hp 230 V motor that is connected to the output side of a VFD I intend to run number 12 wire on a three pole 20 amp breaker and I plan to set a disconnect on the column next to the equipment and then flex into the VFD from the disconnect question is do I have to have a fused disconnect or can I have an infused disconnect and also the VFD controller has a spot for a neutral, but I’m wondering if that’s just if it was a single face motor it’s a three phase motor. I typically don’t need a neutral.

 

[petersonra]

The manual will tell you exactly what you need. I have not seen a three phase drive with a N power input but it's possible there is one. Perhaps you could post a link to the manual.

A lot of lower cost drives these days require fuses for SCCR reasons.

 

[Master Nater]

book puts 3hp 3ph 230v motor at 9.6a. you can go down to #14, if you like. #12 is just fine. inverse time breaker, @ 250%, can put on up to 20a breaker, even if #14. so #12 is also good. conductors protected. therefore, i see absolutely no need to install fused disconnect, unless vfd is calling for it. correct vfd settings will provide correct overload protection based upon motor nameplate ratings.

 

[infinity]

I’m supplying power to a three phase 3 hp 230 V motor that is connected to the output side of a VFD I intend to run number 12 wire on a three pole 20 amp breaker and I plan to set a disconnect on the column next to the equipment and then flex into the VFD from the disconnect question is do I have to have a fused disconnect or can I have an infused disconnect and also the VFD controller has a spot for a neutral, but I’m wondering if that’s just if it was a single face motor it’s a three phase motor. I typically don’t need a neutral.

Find the VFD input current rating and multiply by125% to size the conductors. As petersonra stated the drive manual will tell you what size OCPD you need ahead of the drive.

 

[kwired]

The manual will tell you exactly what you need. I have not seen a three phase drive with a N power input but it's possible there is one. Perhaps you could post a link to the manual.

A lot of lower cost drives these days require fuses for SCCR reasons.

Most drives I have used from Automation Direct in recent years only require specific fuse types if available fault current is over certain levels whether it be their lower cost drives or their more expensive ones.

 

[4-20mA]

And not necessarily do you need a fused-disconnect, but you do need the drive protected by fuses.

 

[kwired]

And not necessarily do you need a fused-disconnect, but you do need the drive protected by fuses.

read drive manual, not always true. If you do it tells you what type fuses are acceptable. They usually not RK5 either.

At same time should the front end rectifier of the drive fail, it is still toast, and for drives less than 10 maybe up to 25 hp they aren't worth repairing and you need to replace whole drive anyway. The type of fuse used may lessen how spectacular that failure is though.

 

[petersonra]

book puts 3hp 3ph 230v motor at 9.6a. you can go down to #14, if you like. #12 is just fine. inverse time breaker, @ 250%, can put on up to 20a breaker, even if #14. so #12 is also good. conductors protected. therefore, i see absolutely no need to install fused disconnect, unless vfd is calling for it. correct vfd settings will provide correct overload protection based upon motor nameplate ratings.

You can't size the ocpd for a vfd based on the motor ocpd ratings found in the code. You are required to use what is in the manual. A fair number of cheap drives have surprisingly low ratings.

 

[kwired]

You can't size the ocpd for a vfd based on the motor ocpd ratings found in the code. You are required to use what is in the manual. A fair number of cheap drives have surprisingly low ratings.

You are supposed to size per the input capacity of the drive according to code. Thing is output has similar power factor as if running direct from the line where the input isn't going to have displacement power factor though it could have some distortion power factor plus you do have some heat losses from the drive itself that you won't have directly running from the line.

 

[Master Nater]

You can't size the ocpd for a vfd based on the motor ocpd ratings found in the code. You are required to use what is in the manual. A fair number of cheap drives have surprisingly low ratings.

code is bare minimum. code must be met, as well as manufacturer specs.

if vfd requires (not recommends) more protection per manufacturer spec, then so be it.

 

[petersonra]

code is bare minimum. code must be met, as well as manufacturer specs.

if vfd requires (not recommends) more protection per manufacturer spec, then so be it.

The code actually says you have to use the ocpd ratings found in the instructions. It's not optional, nor is it more or less protection. There is no code for VFD input ocpd input ratings beyond that unless it is a very old drive.

 

[Master Nater]

The code actually says you have to use the ocpd ratings found in the instructions. It's not optional, nor is it more or less protection. There is no code for VFD input ocpd input ratings beyond that unless it is a very old drive.

you're telling me it's possible for a vfd to ask for less-stringent requirements than the NEC requires for standard motor calculations? i'd like to see an example of that.

 

[kwired]

you're telling me it's possible for a vfd to ask for less-stringent requirements than the NEC requires for standard motor calculations? i'd like to see an example of that.

A VFD is a power conversion device. An induction motor started across the line has a high starting current. VFD's won't hit the motor with full voltage like ATL starter will, they likely can't take that starting surge, but they will hit it with a reduced voltage typically within similar V/F ratio and ramp that v/F up during acceleration which also keeps ramp up current lower than it would be for across the line starting.

This lower power requirement during starting will allow lower short circuit/ground fault protection setting. Also motor retains it's operating power factor for the most part but the resulting kVAR is between the motor and the drive and not the drive input which only sees the actual kW plus whatever losses occur within the drive.

So yes the drive input could have less current when drive is operating at rated output than what the current on the motor leads is.

The fact NEC wants us to select input conductor ampacity and overcurrent protection for the most part results in nearly the same conductor ampacity needed presuming the motor rating is the same as drive output rating- like 2 HP motor on a 2 HP drive.

A vfd that is converting single phase input to three phase output will often need larger input conductors than output conductors.

 

[petersonra]

If

you're telling me it's possible for a vfd to ask for less-stringent requirements than the NEC requires for standard motor calculations? i'd like to see an example of that.

If the VFD is listed it will follow the UL rules for such things that will be NEC compliant. So if the VFD is listed there will not be an example to show you.

I don't recall but does the code allow an unlisted VFD? It's a motor controller and I am pretty sure motor controllers are required to be listed.

 

[williamshober]

You are supposed to size per the input capacity of the drive according to code. Thing is output has similar power factor as if running direct from the line where the input isn't going to have displacement power factor though it could have some distortion power factor plus you do have some heat losses from the drive itself that you won't have directly running from the line.

Could you share where you find that in the NEC code book and what code cycle you’re on? I’m still working out of the 2017. This is for my knowledge, not doubting you. Thanks