UL508A VFD fuse rounding

[JovialBulge]

If I have a VFD that is rated HP=25.0, Output Amps=37.0, Max Input Amps=33.7, 480VAC and I am getting just a bit fuzzy on the Class J fuse sizing for the line side of the drive. The 25HP motor that the drive is controlling has a nameplate FLA of 29.50A @ 480VAC, using table 50.1 in the UL document that motor has an FLA of 34A. It's my understanding that I need to use the values within table 50.1 when doing any FLA calculations for the main incoming disconnect for the panel overall, however does that rule apply here as well?

In this case the manufacturers table shows a Min and Max fuse rating for non-UL applications, 45A and 70A respectively. Then they give a MAX rating for a fuse in a UL application of 70A. With my max input amps to the drive being 33.7, should I be using 33.7 x 1.25 = 42.125A and then rounding down to 42A and choosing the next common size of 45A on the fuse? Should I be using 33.7 x 1.50 = 50.55A and rounding up to 51A and then choosing the next common size of 60A on the fuse?

I know that the minimum ampacity of the wires feeding VFD is 125% of the rated input current of the drive as per 430.122(A).

First I read...5.2 in the UL508A doc "For calculations involving amperes, calculations resulting in a fraction of less than 0.5 shall be rounded down to the next whole number. Calculations resulting in a fraction of 0.5 or more shall be rounded up to the next whole number."

Then I read...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. Lower sizes, of the same type, can be used. 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.

 

[petersonra]

You should use whatever is in the manufacturers instructions. This has been a code requirement for some time now.

In your case you can use up to a 70 Amp fuse because the instructions tell you. Don't try to overthink this.

 

[JovialBulge]

The manufacturer only provides a MAX rating for a fuse in a UL application of 70A. So with my max input amps to the drive being 33.7 I assume I should be using 33.7 multiplied by some engineering factor and then rounding to the nearest fuse size, is that overcomplicating things? Should I just go with 33.7A as the max input amps next fuse size is 35A and go with that?

 

[petersonra]

The manufacturer only provides a MAX rating for a fuse in a UL application of 70A. So with my max input amps to the drive being 33.7 I assume I should be using 33.7 multiplied by some engineering factor and then rounding to the nearest fuse size, is that overcomplicating things? Should I just go with 33.7A as the max input amps next fuse size is 35A and go with that?

As long as the fuse is rated 70 amps or less you are ok.

There is no factor it needs to be multiplied by.

 

[JovialBulge]

So 33.7A max input at the drive, 35A is the next Class J sized fuse, 35A it is then. Is that the correct train of thought here?

Thank you for the responses.

 

[ptonsparky]

As I understand it those fuses are to protect against rapid disassemble of the drive under fault conditions.

Size and price would be other considerations.

 

[petersonra]

So 33.7A max input at the drive, 35A is the next Class J sized fuse, 35A it is then. Is that the correct train of thought here?

Thank you for the responses.

No it is not the correct train of thought. The instructions tell you that the max fuse rating is 70 Amps. That is the correct train of thought. Any fuse of the class rated 70 amps or less makes the grade. As a practical matter you need to consider how much current the VFD input takes.

 

[petersonra]

As I understand it those fuses are to protect against rapid disassemble of the drive under fault conditions.

Size and price would be other considerations.

I have been told that the one of the ways ul tests vfds now is to wrap cotton around the outside of the VFD and expose it to the short circuit. If the cotton does not catch on fire it passes.

 

[Jraef]

I have been told that the one of the ways ul tests vfds now is to wrap cotton around the outside of the VFD and expose it to the short circuit. If the cotton does not catch on fire it passes.

That is the old UL508C standard that has now been withdrawn. The new UL61800-5-1 standard that replaced it “harmonized” with the IEC standard (61800-5-1 by coincidence…), which has more stringent testing requirements. ONE OF THEM is now a minimum fuse size. So if the VFD instructions do not list a minimum fuse size in the UL compliance data, that means it is not yet listed under the new standard and is considered “grandfathered” into the old standard. That’s not a problem though, it’s fine for now.

But to that point, the negative of using a minimum fuse size is simply the risk of nuisance clearing. Even under the old standard, drives have been required to provide short circuit protection for the motor, so the fuses are only there to prevent, as Tom mentioned above, the drive experiencing a a R.U.D.E. (Rapid Unplanned Disassembly Event). I tend to use the maximum size to try to avoid nuisance clearing for simple short term overloads. But even then, the reality is that if a fuse clears on the front end of a VFD, the damage to the VFD has already happened, because that’s the only way to clear a fuse feeding a VFD. So the real purpose of the fuse is to limit COLLATERAL damage to other things around it and in that case, the difference between minimum or maximum sizing is irrelevant.

 

[ptonsparky]

...ONE OF THEM is now a minimum fuse size...

In a previous thread I looked at an AB525 and it did have min fuse size when used in IEC applications, but not for UL. Are they under an older standard yet or does that allow for no minimum fuse within UL installations?

As far as the OP, a 60 amp fuse is about $37 cheaper than a 70. Size of the fuse holder is another thought.

 

[JovialBulge]

No it is not the correct train of thought. The instructions tell you that the max fuse rating is 70 Amps. That is the correct train of thought. Any fuse of the class rated 70 amps or less makes the grade. As a practical matter you need to consider how much current the VFD input takes.

I appreciate the responses here, but I'm just having a hard time wrapping my head around all of this for some reason and seem to be stuck in the paralysis of analysis.

I understand that 70A, in this case, is the max fuse size listed for a UL application. The IEC options have a min and max of 45A and 70A respectively. I also understand what Jraef outlined above about the newer UL standard also now listing a min and max fuse size to align with the IEC standard. However if the drive was made and tested prior to this new UL standard the listing for max only fuse size for the drive is ok. If the VFD maximum input amperage is listed at 33.7A and as discussed above there is no 'engineering factor' to multiply by in the past I have just reverted to using the MIN size on the IEC chart, which usually equates to somewhere around the PF525s max input amps x 1.25. Now that does fall within the UL limit of 'less than 70A' so is my traditional method the proper way to go? Is it a better option to say grab a fuse that's right in the middle of the 45A and 70A and go that route?

 

[ptonsparky]

I appreciate the responses here, but I'm just having a hard time wrapping my head around all of this for some reason and seem to be stuck in the paralysis of analysis.

I understand that 70A, in this case, is the max fuse size listed for a UL application. The IEC options have a min and max of 45A and 70A respectively. I also understand what Jraef outlined above about the newer UL standard also now listing a min and max fuse size to align with the IEC standard. However if the drive was made and tested prior to this new UL standard the listing for max only fuse size for the drive is ok. If the VFD maximum input amperage is listed at 33.7A and as discussed above there is no 'engineering factor' to multiply by in the past I have just reverted to using the MIN size on the IEC chart, which usually equates to somewhere around the PF525s max input amps x 1.25. Now that does fall within the UL limit of 'less than 70A' so is my traditional method the proper way to go? Is it a better option to say grab a fuse that's right in the middle of the 45A and 70A and go that route?

It's your choice. If those fuses blow the damage to drive has already happened.

 

[JovialBulge]

It's your choice. If those fuses blow the damage to drive has already happened.

Is that an absolute certainty? I ask because I have seen where smaller 0.5hp PF525s that were sized with 3A fuses on the line side would blow 1 of the 3 fuses during its first initial startup, the fuse would be replaced and then the drive would operate as normal.

 

[ptonsparky]

Is that an absolute certainty? I ask because I have seen where smaller 0.5hp PF525s that were sized with 3A fuses on the line side would blow 1 of the 3 fuses during its first initial startup, the fuse would be replaced and then the drive would operate as normal.

They were to small then or the wrong kind. Read the manual. One of the drives I have looked at can supply 1.6 times it's rated current for one second. Look at the time current curves of the fuses you use. If those currents and times intersect than you have a problem.

Typically the codes don't care if your equipment does not start. They are concerned more about failures. RUDE.

 

[petersonra]

I appreciate the responses here, but I'm just having a hard time wrapping my head around all of this for some reason and seem to be stuck in the paralysis of analysis.

I understand that 70A, in this case, is the max fuse size listed for a UL application. The IEC options have a min and max of 45A and 70A respectively. I also understand what Jraef outlined above about the newer UL standard also now listing a min and max fuse size to align with the IEC standard. However if the drive was made and tested prior to this new UL standard the listing for max only fuse size for the drive is ok. If the VFD maximum input amperage is listed at 33.7A and as discussed above there is no 'engineering factor' to multiply by in the past I have just reverted to using the MIN size on the IEC chart, which usually equates to somewhere around the PF525s max input amps x 1.25. Now that does fall within the UL limit of 'less than 70A' so is my traditional method the proper way to go? Is it a better option to say grab a fuse that's right in the middle of the 45A and 70A and go that route?

The proper way to go is any fuse of the class rated 70 Amps or less. The exact fuse rating is a design decision. I would probably go with 60 Amps because it is a lot cheaper than a 70 Amp fuse and fuse holder.

 

[JovialBulge]

That is the old UL508C standard that has now been withdrawn. The new UL61800-5-1 standard that replaced it “harmonized” with the IEC standard (61800-5-1 by coincidence…), which has more stringent testing requirements. ONE OF THEM is now a minimum fuse size. So if the VFD instructions do not list a minimum fuse size in the UL compliance data, that means it is not yet listed under the new standard and is considered “grandfathered” into the old standard. That’s not a problem though, it’s fine for now.

But to that point, the negative of using a minimum fuse size is simply the risk of nuisance clearing. Even under the old standard, drives have been required to provide short circuit protection for the motor, so the fuses are only there to prevent, as Tom mentioned above, the drive experiencing a a R.U.D.E. (Rapid Unplanned Disassembly Event). I tend to use the maximum size to try to avoid nuisance clearing for simple short term overloads. But even then, the reality is that if a fuse clears on the front end of a VFD, the damage to the VFD has already happened, because that’s the only way to clear a fuse feeding a VFD. So the real purpose of the fuse is to limit COLLATERAL damage to other things around it and in that case, the difference between minimum or maximum sizing is irrelevant.

So say I go with a fuse size of 60A, I agree with the sentiment where the Class J fuse holder max is 60A and in going with 70A would add cost. Table 28.1 gets me 6AWG copper wire, I follow that so long as I am reading the code correctly here.

I appreciate all that have chimed in here.

Now my last, but equally confusing part I need to determine here for this panel is the fused main disconnect and the difference in using either or, or a combination of both 30.2 and 32.3

Motor FLA according to 50.1 = 34.0A
Largest BCPD is on that VFD which is = 60A
Input current rating on said VFD = 33.7A
750VA 480/120 control transformer ( 750/480 ) = 1.5625A
3 phase to 24VDC PSU = 1.3A

I have gathered reading through other posts on here that, 30.2 gives you the minimum rating of the disconnecting means, but I cannot determine if that is in reference to say a non fused disconnect or a MCCB. 32.3 gives you the maximum rating of the over current device. So if I were to go with a MCCB, the numbers from 30.2 and 32.3 would correspond to the same device. But if I were to go with a fused disconnect switch would I need to comply with 30.2 or 32.3 or both?

Using UL 30.2.2c c) Not less than 115 percent of the full load current ratings of all motors, in accordance with Table 50.1 OR the input current rating of a variable speed drive plus the full-load currents of all other loads.

Input current of the VFD + Control transformer FLA + 3 Phase PSU FLA
33.7 + 1.6 + 1.3 = 36.6A
36.6 x 1.15 = 42.09
Using 31.3.8 that gets me to a 45A MCCB or fused disconnect?

Using 32.3.1 The size of the overcurrent protection shall not exceed the ampere value determined from (a) and (b), whichever is larger:
a) The rating of the largest branch circuit protective device in the circuit plus 125 percent of all heater loads plus the full-load currents of all other motors or other loads in the group that could be in operation at the same time; or
b) The ampacity of the conductors or bus bars on the load side of the overcurrent device.

Largest BCPD is for the VFD + Control transformer FLA + 3 Phase PSU FLA
60 + 1.6 + 1.3 = 62.9
Using 31.3.8 that gets me to a 70A MCCB or fused disconnect?

 

[TwoBlocked]

I don't think you can go wrong with 1.5 times the input amps for the OCPD on a VFD. Same factor for an across the line motor controller.

 

[JovialBulge]

So say I go with a fuse size of 60A, I agree with the sentiment where the Class J fuse holder max is 60A and in going with 70A would add cost. Table 28.1 gets me 6AWG copper wire, I follow that so long as I am reading the code correctly here.

I appreciate all that have chimed in here.

Now my last, but equally confusing part I need to determine here for this panel is the fused main disconnect and the difference in using either or, or a combination of both 30.2 and 32.3

Motor FLA according to 50.1 = 34.0A
Largest BCPD is on that VFD which is = 60A
Input current rating on said VFD = 33.7A
750VA 480/120 control transformer ( 750/480 ) = 1.5625A
3 phase to 24VDC PSU = 1.3A

I have gathered reading through other posts on here that, 30.2 gives you the minimum rating of the disconnecting means, but I cannot determine if that is in reference to say a non fused disconnect or a MCCB. 32.3 gives you the maximum rating of the over current device. So if I were to go with a MCCB, the numbers from 30.2 and 32.3 would correspond to the same device. But if I were to go with a fused disconnect switch would I need to comply with 30.2 or 32.3 or both?

Using UL 30.2.2c c) Not less than 115 percent of the full load current ratings of all motors, in accordance with Table 50.1 OR the input current rating of a variable speed drive plus the full-load currents of all other loads.

Input current of the VFD + Control transformer FLA + 3 Phase PSU FLA
33.7 + 1.6 + 1.3 = 36.6A
36.6 x 1.15 = 42.09
Using 31.3.8 that gets me to a 45A MCCB or fused disconnect?

Using 32.3.1 The size of the overcurrent protection shall not exceed the ampere value determined from (a) and (b), whichever is larger:
a) The rating of the largest branch circuit protective device in the circuit plus 125 percent of all heater loads plus the full-load currents of all other motors or other loads in the group that could be in operation at the same time; or
b) The ampacity of the conductors or bus bars on the load side of the overcurrent device.

Largest BCPD is for the VFD + Control transformer FLA + 3 Phase PSU FLA
60 + 1.6 + 1.3 = 62.9
Using 31.3.8 that gets me to a 70A MCCB or fused disconnect?

With a bit better reading comprehension it seems as if I am incorrect in my above thoughts.

30.2.2.c) seems to give me the minimum rating for the main feeder protection. 45A.
32.3.1.a) gives me the maximum rating for the main feeder protection. 60A.

Is this more in line with what I should be doing?