Fused Disconnect switches for motors

[mshields]

If you are feeding a motor, lets say a 30HP motor @ 480V. And lets say you're using a FVNR motor starter (which you don't don't see alot of these days but lets just say). You run a feeder that is 125% of the FLA and you use a 100A circuit breaker (i.e. 250%). At the motor, concerned about fault current you provide a fused disconnect. What size fuse would you provide.

I would only be providing the fuse to give the disconnect a withstand rating larger than the 7kA or so they are rated for without the fuse. Still you want to make sure that the fuse doesn't go before the breaker. It seem obvious to me that you'd need a 100AS/100AF.

Do you agree?

If it's a drive (clearly more likely in this day and age), then I would go with 50A wire, a 50A Circuit breaker and a 60AS/50AF. It seems to me that we should be running more motor circuits like this. For some reason, our standard is to provide breakers that are typically 175% of FLA. You end up need to have a 100AS rather than the 60A you ought to be able to use.

 

[adamscb]

I'm from an industry where pretty much all I see is FVNR. I'm not a huge supporter of disconnects in the field in addition to the breaker in the bucket. One more thing to check when troubleshooting, they eat up space, cost money, etc. You should have all the protection you need in the bucket with a magnetic breaker and overloads. If the MCC you're feeding this motor from is underrated from a short circuit current perspective, installing a bunch of fused disconnects for the motors won't change the fact that the MCC is still underrated.

However, if you decide to use a field disconnect with a fuse, it doesn't have to be at the same rating as the circuit breaker in the bucket to my knowledge. I would go with a 50 amp fuse. But it really depends on what kind of fuse you're using, and how it coordinates with the upstream breaker. The possibility of miscoordination is another reason why I'm not a fan of fused disconnects with MCC buckets. If you have a magnetic breaker with a set of overloads, that's all you need.

 

[tom baker]

I like to size my fuses about 125% of the motor FLC. OL are set at 115%, this way the fuses back up the OLs. I used dual element time delay fuses, EG fustrons.

 

[Jraef]

When looking at the SCCR of the Non-Fused Disconnect, don't forget that because you will have a fuse / CB in the motor starter, you have a SERIES circuit with regard to the rating of the disconnect. So if the starter has a fused disconnect, the local disconnect would carry SCCR as if it had those fuses inside of it. They just need to be IN the circuit, they don't have to be in the box with it.

Also, some of them are going to be series rated for higher fault currents if used with the same brand of breaker, i.e. an Eaton NF disconnect is series listed at 25kA if protected by a Eaton breaker (specific series). Then don't forget that the NF disconnect at the motor has all that cable impedance between it and the starter as well. It's not at all unheard of for you to have 40kA at the switchgear, 30kA at the MCC and and under 15kA at the motor terminals.

If it's a drive (clearly more likely in this day and age), then I would go with 50A wire, a 50A Circuit breaker and a 60AS/50AF.

NEVER put a fuse on the output of a VFD by the way... that's a good way to blow your transistors. You don't have to worry about the Available Fault Current down stream of the VFD, upstream energy cannot get to it through the drive conversion, so fault energy down stream is limited by the rating of the VFD.

 

[paulengr]

This seems crazy. The contactor does not itself have an SCCR but a rule of thumb is that they tend to weld shut at around 30x rated current. If you are worried about the disconnect you are in that range. NEMA gives appropriate ratings for RK-1 (identical package to RK-5) and CC fuses. This makes it a type 2 (no damage) starter and much more reliable from a protection point of view than a MCP. Then you will have your lower SCCR via the fuse and thus your nonfused secondary disconnects aren’t really a problem.

Not sure what you mean by contactors being rare. If you don’t need a drive or a soft start they are more reliable, simple, and readily available.

 

[mshields]

When looking at the SCCR of the Non-Fused Disconnect, don't forget that because you will have a fuse / CB in the motor starter, you have a SERIES circuit with regard to the rating of the disconnect. So if the starter has a fused disconnect, the local disconnect would carry SCCR as if it had those fuses inside of it. They just need to be IN the circuit, they don't have to be in the box with it.

Also, some of them are going to be series rated for higher fault currents if used with the same brand of breaker, i.e. an Eaton NF disconnect is series listed at 25kA if protected by a Eaton breaker (specific series). Then don't forget that the NF disconnect at the motor has all that cable impedance between it and the starter as well. It's not at all unheard of for you to have 40kA at the switchgear, 30kA at the MCC and and under 15kA at the motor terminals.


NEVER put a fuse on the output of a VFD by the way... that's a good way to blow your transistors. You don't have to worry about the Available Fault Current down stream of the VFD, upstream energy cannot get to it through the drive conversion, so fault energy down stream is limited by the rating of the VFD.

Great input but tell me this. if the fault current can't make it through the drive conversion, why do I care about the series rating of a fused switch at the drive or a breaker that is series rated with the safety switch again with the drive between the two?

 

[Jraef]

Great input but tell me this. if the fault current can't make it through the drive conversion, why do I care about the series rating of a fused switch at the drive or a breaker that is series rated with the safety switch again with the drive between the two?

You don't really. The output fault current of a VFD is roughly around 1.8 times the VFD rating. So lets say you have a big VFD rated for 1,000A output current (i.e. 700HP @480V). The maximum output short circuit current would be just 1800A FAR below even the lowest of SCCR for a load side disconnect.

The examples I gave about series ratings were for anything OTHER THAN a VFD, like Across-the-Line or Reduced Voltage starter.

 

[peterphamee]

I'm from an industry where pretty much all I see is FVNR. I'm not a huge supporter of disconnects in the field in addition to the breaker in the bucket. One more thing to check when troubleshooting, they eat up space, cost money, etc. You should have all the protection you need in the bucket with a magnetic breaker and overloads. If the MCC you're feeding this motor from is underrated from a short circuit current perspective, installing a bunch of fused disconnects for the motors won't change the fact that the MCC is still underrated.

However, if you decide to use a field disconnect with a fuse, it doesn't have to be at the same rating as the circuit breaker in the bucket to my knowledge. I would go with a 50 amp fuse. But it really depends on what kind of fuse you're using, and how it coordinates with the upstream breaker. The possibility of miscoordination is another reason why I'm not a fan of fused disconnects with MCC buckets. If you have a magnetic breaker with a set of overloads, that's all you need.

You might forget the MCC breaker is located way in basement the motor is located on the rooftop. I think your thought is so simple.

 

[Shahzad]

If you are feeding a motor, lets say a 30HP motor @ 480V. And lets say you're using a FVNR motor starter (which you don't don't see alot of these days but lets just say). You run a feeder that is 125% of the FLA and you use a 100A circuit breaker (i.e. 250%). At the motor, concerned about fault current you provide a fused disconnect. What size fuse would you provide.

I would only be providing the fuse to give the disconnect a withstand rating larger than the 7kA or so they are rated for without the fuse. Still you want to make sure that the fuse doesn't go before the breaker. It seem obvious to me that you'd need a 100AS/100AF.

Do you agree?

If it's a drive (clearly more likely in this day and age), then I would go with 50A wire, a 50A Circuit breaker and a 60AS/50AF. It seems to me that we should be running more motor circuits like this. For some reason, our standard is to provide breakers that are typically 175% of FLA. You end up need to have a 100AS rather than the 60A you ought to be able to use.

(1) Regarding VFD Conductors:
For the VFD, as per NEC rule 430.122 (A): conductors need to be sized at 125% of VFD input current, I am not sure what is input current of drive, but let's say if it is 42Amps, then 42x1.25=52.5A, you should size your wire accordingly which in this case might be #6,3C, assuming 75deg C column at 30deg C ambient without derating factors.

(2) Regarding VFD and cable protection:
Now, coming to the starter to feed the VFD, since you are feeding a drive not motor directly, so you need to size the over current protection device that would not only protect the drive but also protect the conductors as per per the NEC code 240.4, conductors need to be protected against overcurrent in accordance with their ampacities, I would use the thermal mag breaker in the bucket without overloads because thermal mag breaker offers both thermal protection and short-circuit protection and also is good from coordination point with upstream breaker, so in this case for drive protection I would use a 60A thermal mag breaker as per the value given in NEC table 240.6(A).

(3)Regarding field disconnect and motor protection:
You don't need to use fuses in the field as your VFD provides both overload and short-circuit protection, all you need is just a manual disconnect switch in the field which should be sized at 1.15% of the FLA as outlined in NEC code 430.110, Ampere Rating and Interrupting Capacity, (see Rule 2), the ampere rating of the disconnecting means shall not be less than 115% of the sum of all currents at full load condition determined in accordance with 430.110 (C) (1).

 

[david luchini]

(2) Regarding VFD and cable protection:
Now, coming to the starter to feed the VFD, since you are feeding a drive not motor directly, so you need to size the over current protection device that would not only protect the drive but also protect the conductors as per per the NEC code 240.4, conductors need to be protected against overcurrent in accordance with their ampacities,

I'm not sure this is correct. Section 430.130 provides specific direction for Branch-Circuit Short-Circuit and Ground-Fault protection for single motor circuits containing power conversion equipment. It seems to suggest that the rating would be based on the full-load current rating of the motor, just as a non-vfd supplied motor, unless there is a maximum size listed for the vfd.

 

[Shahzad]

I'm not sure this is correct. Section 430.130 provides specific direction for Branch-Circuit Short-Circuit and Ground-Fault protection for single motor circuits containing power conversion equipment. It seems to suggest that the rating would be based on the full-load current rating of the motor, just as a non-vfd supplied motor, unless there is a maximum size listed for the vfd.

Rule 430.130 (A): Circuits containing power conversion equipment shall be protected by branch circuit short circuit and ground fault protective device in accordance with:
(1) The rating and type of protection shall be determined by 430.52 (C) (1), (C) (5) or (C) (6). using full load current of the motor load as determined by the 430.6.
So in this case again, if motor FLC is as per NEC Table 430.250, for 30 HP is 40 Amp, so I normally used 125% factor so in this case 50 Amp breaker is recommended size however then we have to reduce the wire size to #8, else increase the breaker size to 60A if you use #6 wire as per VFD input current. Let me know your thoughts.

 

[david luchini]

Rule 430.130 (A): Circuits containing power conversion equipment shall be protected by branch circuit short circuit and ground fault protective device in accordance with:
(1) The rating and type of protection shall be determined by 430.52 (C) (1), (C) (5) or (C) (6). using full load current of the motor load as determined by the 430.6.
So in this case again, if motor FLC is as per NEC Table 430.250, for 30 HP is 40 Amp, so I normally used 125% factor so in this case 50 Amp breaker is recommended size however then we have to reduce the wire size to #8, else increase the breaker size to 60A if you use #6 wire as per VFD input current. Let me know your thoughts.

For the 30HP motor supplied from a starter, the conductor could be #8 and the circuit breaker could be 100A.

If the 30HP motor was supplied from a VFD with a rated input current of 50, then the conductor would need to be #6 but the circuit breaker could still be 100A.

 

[Shahzad]

For the 30HP motor supplied from a starter, the conductor could be #8 and the circuit breaker could be 100A.

If the 30HP motor was supplied from a VFD with a rated input current of 50, then the conductor would need to be #6 but the circuit breaker could still be 100A.

I see, well let's say if it is 60 HP motor fed by stand alone 60 HP VFD having input current of 88amp, what size of thermal mag breaker you would use in the bucket to feed the drive, remember #2,C is being used to feed the drive, would 125A be ok? or you would up size it to 150A?

 

[david luchini]

I see, well let's say if it is 60 HP motor fed by stand alone 60 HP VFD having input current of 88amp, what size of thermal mag breaker you would use in the bucket to feed the drive, remember #2,C is being used to feed the drive, would 125A be ok? or you would up size it to 150A?

I would imagine 125A would be OK. From a Code perspective, 200A would be allowable.

 

[petersonra]

You don't really. The output fault current of a VFD is roughly around 1.8 times the VFD rating. So lets say you have a big VFD rated for 1,000A output current (i.e. 700HP @480V). The maximum output short circuit current would be just 1800A FAR below even the lowest of SCCR for a load side disconnect.

So where does UL say this?