pkardok
New member
- Location
- norwell, ma, USA
I have a 50A AC motor driven by a VFD with a 150A rating. What size should the breaker feeding the VFD be? 150A or 60A? What are the code instructions?
Breaker size for highest or lowest draw?
- Thread starter pkardok
- Start date
- Status
- Location
- Massachusetts
From the 2011 NEC
- Location
- New Jersey
- Occupation
- Journeyman Electrician
So does this assume that the OCPD is sized according to the conductor size? The way I read it only the conductors need to be sized according to the ampacity of the VFD. How would you size the OCPD?
- Location
- Massachusetts
Good question and I don't know the answer.
They specify the disconnect size ...
But I see nothing about the breaker or fuse size.
Perhaps 110.3(B) and the VFD instructions?
- Location
- Connecticut
- Occupation
- Engineer
The OCPD should be sized per 430.52, the same as other motors.
430.120 say "The installation provisions of Part I through Part IX are applicable unless modified or supplemented by Part X." Like Bob, I don't see anything in Part X that modifies the earlier provisions.
430.120 say "The installation provisions of Part I through Part IX are applicable unless modified or supplemented by Part X." Like Bob, I don't see anything in Part X that modifies the earlier provisions.
- Location
- Massachusetts
I agree but find it odd that both the conductors and the disconnect must be sized per input current yet the breaker sized per the motor.
I am missing the point. :?
- Location
- Connecticut
- Occupation
- Engineer
I don't understand the logic either. I don't think they're considering that you would have a 50A motor on a 150A rated drive.
I think it would make more sense if they said the conductors needed to be 125% of the rated input current, but not required to be greater than the OCPD feeding the equipment.
i think (and I could be wrong) the point is that in the future, someone might increase the motor size without changing the rest of the installation.
So if they do that, the disconnect won't be overloaded, and the wires won't be overloaded. But if the breaker is sized too small, it should simply trip. And if they get tired of the breaker tripping, and install a larger breaker, then everything else is already sized for the larger breaker.
renosteinke
Senior Member
- Location
- NE Arkansas
All these 'what if's' do get wearying .... especially when you see, with hindsight, that the issues almost always arise from one of two situations. Either the original design is poor, or somone is trying to 'finess' the code to do the least possible.
Let's get back to basics. I'm sure you've heard it before: the breaker at the panel is there to protect the wire, and not the load. With motors, the heaters are there to protect the load.
Let's also look at what we are trying to do with an electronic drive. We're trying to change the behaviour of the motor by altering the electricity it receives. It's that simple.
Where does that leave us when we have a 'typical' motor / VFD situation? Well, we've broken that motors' branch circuit into two segments: the part that feeds the drive, and the part that feeds the motor.
What we have feeding the drive needs to have the wire sized and overcurrent protection selected based upon what the drive requires. "Requires" as what it might call for within the limits of the drive. If your drive is oversize (say, a 10-hp drive controlling a 3-hp motor), you still need to supply the drive according to its' nameplate.
Treat the circuit between the drive and the motor as if it were just supplying the motor. Size wires to the NEC table for the motors' horsepower and the fuses to the nameplate. Size your heaters to what the motor actually draws under its' "normal" load.
Does it sound like I'm calling for more than the 'minimum' number of fuses and disconnects? Absolutely. Keep it simple ... break the question into its' parts, and tackle them one at a time. Never confuse 'good design' with the 'minimum for safety.'
That's the basic recipe. Sure, there are details that can be improved upon .... but the first 'detail' is to use the right equipment. Power a 3-hp load with a 3-hp motor and a 3-hp drive. Bigger isn't always better, and one size never fits all. Amazingly enough, a lot of your 'power quality' issues seem to disappear when you use the right equipment, and operate it within its' limits.
Let's get back to basics. I'm sure you've heard it before: the breaker at the panel is there to protect the wire, and not the load. With motors, the heaters are there to protect the load.
Let's also look at what we are trying to do with an electronic drive. We're trying to change the behaviour of the motor by altering the electricity it receives. It's that simple.
Where does that leave us when we have a 'typical' motor / VFD situation? Well, we've broken that motors' branch circuit into two segments: the part that feeds the drive, and the part that feeds the motor.
What we have feeding the drive needs to have the wire sized and overcurrent protection selected based upon what the drive requires. "Requires" as what it might call for within the limits of the drive. If your drive is oversize (say, a 10-hp drive controlling a 3-hp motor), you still need to supply the drive according to its' nameplate.
Treat the circuit between the drive and the motor as if it were just supplying the motor. Size wires to the NEC table for the motors' horsepower and the fuses to the nameplate. Size your heaters to what the motor actually draws under its' "normal" load.
Does it sound like I'm calling for more than the 'minimum' number of fuses and disconnects? Absolutely. Keep it simple ... break the question into its' parts, and tackle them one at a time. Never confuse 'good design' with the 'minimum for safety.'
That's the basic recipe. Sure, there are details that can be improved upon .... but the first 'detail' is to use the right equipment. Power a 3-hp load with a 3-hp motor and a 3-hp drive. Bigger isn't always better, and one size never fits all. Amazingly enough, a lot of your 'power quality' issues seem to disappear when you use the right equipment, and operate it within its' limits.
- Location
- Connecticut
- Occupation
- Engineer
Do you have a code citation to support that? It appears to me that Article 430 requires that the overcurrent protection be based on the motor horsepower, per 430.52.
- Location
- Massachusetts
I would suggest it could possibly be a 110.3(B) issue depending on the inverter instructions.
Most ASD's have maximum OCPD's listed in their instructions. In my area of WA the ASD "supply" wiring is a feeder to a piece of equipment, and you cannot oversize the OCPD unless it is allowed in the installation instructions. However, if I cross state lines into OR the local AHJ is completely fine sizing the wiring and OCPD's off the motor info. (which most of us know is incorrect)
- Location
- Connecticut
- Occupation
- Engineer
I could see how the inverter instructions may list a maximum OCPD. But if you're putting a 50A motor on a 150A vfd, the vfd's maximum OCPD would exceed the max OCPD for the motor per 430.52. I think per 430.120, the OCPD supplying the vfd can't exceed the max allowed for the motor :?
renosteinke
Senior Member
- Location
- NE Arkansas
David, you raise a good point, and highlight the need for 'good design,' or simply using the right equipment.
You'll note that, in my description, I was advocating separate OCPD's for the motor and the VFD portions of the circuit. If you want to use just one, then you're caught in a contradiction unless the drive is matched to the motor.
The circuit is powering not the motor, but the drive. It is the drive that powers the motor. I suggest we look at the most common situation -where the motor is matched to the drive- as the exception to the rules, a unique situation that allows us to streamline things.
Let's look at another variation: one drive controlling several motors. I see that as analogous to a tap situation, where separate OCPD's are required for each motor. Simply put, if you oversize the drive, you cancel out any protective features the drive might have for the motor.
Looking at the opposite situation - an undersize drive- if you try to fuse the entire circuit to the motor nameplate, you'll let the smoke out of the drive. This, of course, leads us to the familiar situation where folks think it's the fuse that's the problem, rather than the overload situation that was created.
You'll note that, in my description, I was advocating separate OCPD's for the motor and the VFD portions of the circuit. If you want to use just one, then you're caught in a contradiction unless the drive is matched to the motor.
The circuit is powering not the motor, but the drive. It is the drive that powers the motor. I suggest we look at the most common situation -where the motor is matched to the drive- as the exception to the rules, a unique situation that allows us to streamline things.
Let's look at another variation: one drive controlling several motors. I see that as analogous to a tap situation, where separate OCPD's are required for each motor. Simply put, if you oversize the drive, you cancel out any protective features the drive might have for the motor.
Looking at the opposite situation - an undersize drive- if you try to fuse the entire circuit to the motor nameplate, you'll let the smoke out of the drive. This, of course, leads us to the familiar situation where folks think it's the fuse that's the problem, rather than the overload situation that was created.
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