Packaged VFD OCPD Rating

[rancamEE]

I saw a previous closed thread on this issue (http://forums.mikeholt.com/showthread.php?t=159284) but it didn't seem like the original question was really cleared up.

When sizing the circuit breaker that will be feeding a packaged/enclosed drive is it sized as you would a normal load 125% of VFD input amps and not specifically a motor controller?

I've always thought that the oversizing of the overcurrent protective device in Table 430.52 was to account for high starting current. A VFD input does not have that significant of a starting current draw and therefore the breaker can be sized for the actual load.

430.122 is very clear on the conductor size being a minimum of 125% of the VFD input amperage. But technically since the VFD OCPD is not discussed and the VFD is by definition a controller Table 430.52 would allow you to way oversize the OCPD for the conductor size?

So, how does everyone size the VFD OCPD? I haven't seen or heard any issues sizing it at 125% of VFD input amps and rounding to next NEMA size up.

Thanks,

rancamEE

 

[petersonra]

I saw a previous closed thread on this issue (http://forums.mikeholt.com/showthread.php?t=159284) but it didn't seem like the original question was really cleared up.

When sizing the circuit breaker that will be feeding a packaged/enclosed drive is it sized as you would a normal load 125% of VFD input amps and not specifically a motor controller?

I've always thought that the oversizing of the overcurrent protective device in Table 430.52 was to account for high starting current. A VFD input does not have that significant of a starting current draw and therefore the breaker can be sized for the actual load.

430.122 is very clear on the conductor size being a minimum of 125% of the VFD input amperage. But technically since the VFD OCPD is not discussed and the VFD is by definition a controller Table 430.52 would allow you to way oversize the OCPD for the conductor size?

So, how does everyone size the VFD OCPD? I haven't seen or heard any issues sizing it at 125% of VFD input amps and rounding to next NEMA size up.

Thanks,

rancamEE

I'd be inclined to size it according to the rules in the code.

X. Adjustable-Speed Drive Systems
430.120 General. The installation provisions of Part I
through Part IX are applicable unless modified or supplemented
by Part X.

IMO this is a clear indication that the OCPD sizing requirements found for other motor applications also applies to VFD applications. Having said that, there is also the issue of the listing instructions. With modern drives they generally explicitly tell you the maximum size OCPD and it is usually 250%.

Personally, I do not like having to argue over such things so most of the time I size the input circuit conductors to at least 125% of the rated input current and with an ampacity greater than or equal to the rating of the OCPD. Most drives I use are small enough that it does not change all that much one way or the other. It looks a little strange to have to have full size conductors up to the OCPD and then go to something smaller so I avoid a lot of questions that way.

 

[Jraef]

Actually I personally think it was answered, then cleared up by the inclusion of the text of the new 430.130 by Smart $ in post 21. There was some further discussion parsing out details, but in reality, the issue raised in paragraph A) 2) is the thing that trumps all. If the VFD mfr states a size range, you cannot exceed it, and in my experience, most of them will state a size range. If they don't, there is language in 430.130 to deal with that, but that usually also means the drive is not UL listed, because UL requires it. So if you are in a situation where UL listing is required and someone wants you to use a drive that does not state the max OCPD in their technical data, it should be a red flag.

PS: I agree with Bob's approach, that's the way I have always done it and I have never seen that result in a conflict with the VFD mfr's instructions.

PPS: Oh wait, ONE exception, but it was highly unusual. I used a VFD that was exceptionally larger than the motor I was connecting to, simply because the VFD was on hand and the customer was hurting. The capacitor charging of the large VFD kept clearing the smaller fuses, something that rarely happens because there is always some sort of "pre-charge" circuit in the VFD to prevent this. But when you have a gross mismatch, in this case 10:1, even the current limited capacitor charging current was too much for fuses sized for the motor. So I ended up with fuses that, per the motor specs, would be too large, but per the drive specs were at the very bottom of it's range. Consider that an outlier though.

 

[rancamEE]

Continuation

Continuation

Yes, I understand the conductor size must be 125% and since no OCPD is described in Section X you default to a traditional motor OCPD and Table 430.52.

We have a cheat sheet (unfortunately) that gets passed around to new hires. It has for a 30HP VFD an input ampacity of 44A. Taking 125% of that would mean our conductors would need to be rated for 55A. Assuming no derating issues I would select a #6 which agrees with the cheat sheet.

The inverse time breaker on the cheat sheet is listed as a 100A/3P, which meets Table 430.52 requirements of <250% of FLC and the manufacturers MOCP.

The issue I have is the circuit conductors are a #6 on a 100A/3P breaker that feeds a VFD.

When I oversize the circuit breaker for a across the line starter I am doing it to handle starting inrush currents, the only device I have between the panel breaker and motor is the contacts for the MCP, contactor, and OLs. The OL's protect the cable upstream from long term overcurrent since they are in series. Nothing significantly between is drawing power.

For the packaged VFD the VFD is the load, not really the motor. Why would the NEC allow me to greatly oversize the VFD for starting current when its minimal? I would think I would have to protect the #6AWG with a 60A/3P breaker to ensure the conductors are protected for long term overcurrent.

UL508A seems to me to size it to 125% of the input amperage, "branch circuit fuse or inverse-time circuit breaker shall be used and sized in accordance with 31.3.1(a)" .

Thanks for the help on this by the way, I appreciate the discussion.

 

[petersonra]

Yes, I understand the conductor size must be 125% and since no OCPD is described in Section X you default to a traditional motor OCPD and Table 430.52.

We have a cheat sheet (unfortunately) that gets passed around to new hires. It has for a 30HP VFD an input ampacity of 44A. Taking 125% of that would mean our conductors would need to be rated for 55A. Assuming no derating issues I would select a #6 which agrees with the cheat sheet.

The inverse time breaker on the cheat sheet is listed as a 100A/3P, which meets Table 430.52 requirements of <250% of FLC and the manufacturers MOCP.

The issue I have is the circuit conductors are a #6 on a 100A/3P breaker that feeds a VFD.

When I oversize the circuit breaker for a across the line starter I am doing it to handle starting inrush currents, the only device I have between the panel breaker and motor is the contacts for the MCP, contactor, and OLs. The OL's protect the cable upstream from long term overcurrent since they are in series. Nothing significantly between is drawing power.

For the packaged VFD the VFD is the load, not really the motor. Why would the NEC allow me to greatly oversize the VFD for starting current when its minimal? I would think I would have to protect the #6AWG with a 60A/3P breaker to ensure the conductors are protected for long term overcurrent.

UL508A seems to me to size it to 125% of the input amperage, "branch circuit fuse or inverse-time circuit breaker shall be used and sized in accordance with 31.3.1(a)" .

Thanks for the help on this by the way, I appreciate the discussion.

OCPD protects against both short circuits and over current. The reason you are allowed to "oversize" the OCPD is because even the higher rated OCPD will still protect the conductors from short circuits while the VFD protects the conductors from over current, just as the overload protects against overcurrent in a magnetic starter.

If you look closely at UL508a 31.3.1 it refers you to table 31.1 which has the 250% number in it.

31.3.2 The branch circuit protection for a single-motor circuit provided with a variable-speed drive shall
be of the type and size specified by the manufacturer’s instructions provided with the drive. When the
instructions do not specify the type and size, a branch-circuit fuse or inverse-time circuit breaker shall be
used and shall be sized in accordance with 31.3.1(a) based upon the full-load motor output current rating
of the drive.

Note that UL508a explicitly refers to the manufacturer's instructions for the rating of the OCPD on a VFD. If it is not in the instructions, than it refers you back to 31.3.1.