OCPD for VFD Circuit

[Smart $]

Look at the new 430.130 in the 2014.

Here it is...

430.130 Branch-Circuit Short-Circuit and Ground-Fault Protection for Single Motor Circuits Containing Power Conversion Equipment

(A) Circuits Containing Power Conversion Equipment.
Circuits containing power conversion equipment shall be protected by a branch-circuit short-circuit and ground-fault protective device in accordance with the following:

(1)The rating and type of protection shall be determined by 430.52(C)(1), (C)(3), (C)(5), or (C)(6), using the full-load current rating of the motor load as determined by 430.6.

(2)Where maximum branch-circuit short-circuit and ground-fault protective ratings are stipulated for specific device types in the manufacturer?s instructions for the power conversion equipment or are otherwise marked on the equipment, they shall not be exceeded even if higher values are permitted by 430.130(A)(1).

(3)A self-protected combination controller shall only be permitted where specifically identified in the manufacturer?s instructions for the power conversion equipment or if otherwise marked on the equipment.

Informational Note: The type of protective device, its rating, and its setting are often marked on or provided with the power conversion equipment.

(B) Bypass Circuit/Device.
Branch-circuit short-circuit and ground-fault protection shall also be provided for a bypass circuit/device(s). Where a single branch-circuit short-circuit and ground-fault protective device is provided for circuits containing both power conversion equipment and a bypass circuit, the branch-circuit protective device type and its rating or setting shall be in accordance with those determined for the power conversion equipment and for the bypass circuit/device(s) equipment.

 

[petersonra]

Here it is...

I do not see a change that requires one to size the OCPD to the wires feeding the VFD.

 

[Smart $]

I do not see a change that requires one to size the OCPD to the wires feeding the VFD.

I never did even in earlier editions. The 2014 edition just embellishes that SC/GFP limits can be set by manufacturer instructions.

 

[GoldDigger]

I never did even in earlier editions. The 2014 edition just embellishes that SC/GFP limits can be set by manufacturer instructions.

Also notice that the title of the section specifies short circuit and ground fault protection, but it wavers into the area of sizing for overload too.

 

[Smart $]

Also notice that the title of the section specifies short circuit and ground fault protection, but it wavers into the area of sizing for overload too.

I'm not seeing it... :huh:

 

[GoldDigger]

I'm not seeing it... :huh:

Well, to the extent that the sizing rules specified for use by reference to other sections include sizing for overload purposes.
The protection for a #14 wire for short circuit purposes only would arguably be perfectly fine at 60A or more as long as it was a bolted short capable of drawing enough fault current to trip the breaker.

On a secondary note, my concern is the tacit assumption that the VFD cannot possibly (under other than bolted fault conditions) draw more power from the branch breaker than the motor it is feeding.
That is a reasonable assumption when all that is between the branch breaker and motor is wires and contactors. But it fails when there is active high power circuitry in between.

 

[Smart $]

Well, to the extent that the sizing rules specified for use by reference to other sections include sizing for overload purposes.
The protection for a #14 wire for short circuit purposes only would arguably be perfectly fine at 60A or more as long as it was a bolted short capable of drawing enough fault current to trip the breaker.

On a secondary note, my concern is the tacit assumption that the VFD cannot possibly (under other than bolted fault conditions) draw more power from the branch breaker than the motor it is feeding.
That is a reasonable assumption when all that is between the branch breaker and motor is wires and contactors. But it fails when there is active high power circuitry in between.

Still appearing highly speculative on my end... :happyyes:

 

[GoldDigger]

Still appearing highly speculative on my end... :happyyes:

I think that is an accurate characterization, which I will not dispute :angel:

As the scorpion said to the frog: "It is my nature."