Air Compressor Fuse and Overcurrent Sizing

wattup

Member
We are in the process of installing a circuit for an air compressor and there has been some discussions and debate concerning the installation.
The customer supplied the following package ampacity and we determined the rest:
Package Amps 234.3 x 125% = 292.87 amps

Circuit Breaker in Gear supplying Disconnect = 300 amp thermal magnetic trip standard interrupt

Feeder conductors ran are 350 kcmil THHN Rated 310 Amps @ 75*C

Manufacture requests max 3x550 170M Style fuses. Our 400A heavy duty Square D fusible disconnect of course will not accept these fuses as they are rated 690V.
Our questions are:
1) What would be the recommended breaker in the gear as we haven't terminated the wire yet and could still swap it out if we should go up to a larger size.
2) What would be the best way to handle the fuse requirement or request?
3) Shouldn't the drive protection be done in the compressor and not be our responsibility to be provide in the feed circuit? We have typically always installed a service disconnect outside the compressor based on circuit requirements. There is a drive controlling the motor.
 

PhenixFord

Member
Location
Cabot, AR.
Occupation
Industrial Systems and Controls
I can make suggestions on the fuses and disconnect.

Our units all have Non-Fused disconnects. The fuses are usually located inside the cabinet containing the VFD / Soft Start (usually installed on a simple fuse holder).
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
170M fuses usually means it has a VFD, those are "high speed" specialty fuses. If that's the case, you need to check to see if the VFD UL listing REQUIRES this specific fuse and if it does, you have no choice, unless having it be UL listed is irrelevant to you.
 

kwired

Electron manager
Location
NE Nebraska
We are in the process of installing a circuit for an air compressor and there has been some discussions and debate concerning the installation.
The customer supplied the following package ampacity and we determined the rest:
Package Amps 234.3 x 125% = 292.87 amps

Circuit Breaker in Gear supplying Disconnect = 300 amp thermal magnetic trip standard interrupt

Feeder conductors ran are 350 kcmil THHN Rated 310 Amps @ 75*C

Manufacture requests max 3x550 170M Style fuses. Our 400A heavy duty Square D fusible disconnect of course will not accept these fuses as they are rated 690V.
Our questions are:
1) What would be the recommended breaker in the gear as we haven't terminated the wire yet and could still swap it out if we should go up to a larger size.
2) What would be the best way to handle the fuse requirement or request?
3) Shouldn't the drive protection be done in the compressor and not be our responsibility to be provide in the feed circuit? We have typically always installed a service disconnect outside the compressor based on circuit requirements. There is a drive controlling the motor.
Seems logical, but not always how it works.

Reality is not every installation will have same available fault current and because of that can have different needs.

Manufacturer can still refuse to warranty anything that isn't installed to their recommendations.

They can also leave some things up to field installer, this can help sell units at what appears to be lesser price then place the extras they left out on the installer.
 

PhenixFord

Member
Location
Cabot, AR.
Occupation
Industrial Systems and Controls
Manufacturer can still refuse to warranty anything that isn't installed to their recommendations.
I wouldn't call myself a VFD / Soft Start expert. But I have "serviced" very many.

The installation "always" consists of the recommended fuses.
 

kwired

Electron manager
Location
NE Nebraska
I wouldn't call myself a VFD / Soft Start expert. But I have "serviced" very many.

The installation "always" consists of the recommended fuses.
Larger drives most always have what is recommended.

Relatively cheap small drives, you will see omissions of some those accessories. especially when it comes to the somewhat expensive input fuses. But at same time with those smaller drives careful reading of instructions you can often find your available fault current is low enough you don't need those fuses anyway, unless maybe the distance to the source is pretty short in length.
 

paulengr

Senior Member
Larger drives most always have what is recommended.

Relatively cheap small drives, you will see omissions of some those accessories. especially when it comes to the somewhat expensive input fuses. But at same time with those smaller drives careful reading of instructions you can often find your available fault current is low enough you don't need those fuses anyway, unless maybe the distance to the source is pretty short in length.
You have that backwards. On small drives the SCCR is very low. 5 kA is pretty common. If the transformer feeding it is about 10 times the kVA of the VFD or larger then you need a line reactor to prevent damage in the event of a short, or those obscenely expensive semiconductor fuses. Short circuit protection is always required even in less extreme cases. One of the issues at about ten years old is that the drive front end electronics can fail dead shorted and the drive cannot shut this down,

On larger drives the SCCR rating and general ability to withstand shorts often gets better and the size gets closer to the transformer anyways. On a 200 HP VFD for instance you’d be up over 1500 kVA to require line reactors.
 

kwired

Electron manager
Location
NE Nebraska
You have that backwards. On small drives the SCCR is very low. 5 kA is pretty common. If the transformer feeding it is about 10 times the kVA of the VFD or larger then you need a line reactor to prevent damage in the event of a short, or those obscenely expensive semiconductor fuses. Short circuit protection is always required even in less extreme cases. One of the issues at about ten years old is that the drive front end electronics can fail dead shorted and the drive cannot shut this down,

On larger drives the SCCR rating and general ability to withstand shorts often gets better and the size gets closer to the transformer anyways. On a 200 HP VFD for instance you’d be up over 1500 kVA to require line reactors.
I don't have it backwards. On small drives you will have a smaller branch circuit feeding it. The resistance of those supply conductors reduces available fault current at the drive terminals. Often you will already have less than 5kA available at drive terminals was what I was getting at unless maybe you are pretty close to the source to begin with.

Other thing is with a small enough drive, and especially some of the cheap ones, when the front end of drive fails it don't matter, the thing is not worth repairing and you will be replacing the entire drive anyway so we don't care if those additional fuses and/or line reactor would have made the failure a little less spectacular because it limited the current some.
 

wattup

Member
Thank you all for the replies. I am just getting back to office. It looks like we will be installing the high speed
fuses specified by Compressor company. Yes we are in the US and Yes the Compressor is UL listed. In past
years it was acceptable to use the Type J Low peak fuses that will fit in our service disconnects. The 170M Fuses
will not fit in any disconnect per manufacturer. We will have to add a box and fuse blocks between our service
disconnect and the termination in Compressor.
That being said, due to possible in rush on start-up even though they have a drive are we ok going to a larger
breaker in gear say 400 amp so as not to nuisance trip the 300 amp breaker and still be code with the 350 MCM
Conductors
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Thank you all for the replies. I am just getting back to office. It looks like we will be installing the high speed
fuses specified by Compressor company. Yes we are in the US and Yes the Compressor is UL listed. In past
years it was acceptable to use the Type J Low peak fuses that will fit in our service disconnects. The 170M Fuses
will not fit in any disconnect per manufacturer. We will have to add a box and fuse blocks between our service
disconnect and the termination in Compressor.
That being said, due to possible in rush on start-up even though they have a drive are we ok going to a larger
breaker in gear say 400 amp so as not to nuisance trip the 300 amp breaker and still be code with the 350 MCM
Conductors
There is no "inrush" when you have a VFD. In the VFD manual, there will be a listing for the maximum size of OCPD you can use, you cannot go larger than that.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I don't know if I see anywhere that you must install the max size fuse. The type of fuse may be a problem but as long as you meet the minimum ampacity requirements and it runs why install larger? Put the type of fuse and required disconnect at another location and feed it with the 400.
 

kwired

Electron manager
Location
NE Nebraska
Thank you all for the replies. I am just getting back to office. It looks like we will be installing the high speed
fuses specified by Compressor company. Yes we are in the US and Yes the Compressor is UL listed. In past
years it was acceptable to use the Type J Low peak fuses that will fit in our service disconnects. The 170M Fuses
will not fit in any disconnect per manufacturer. We will have to add a box and fuse blocks between our service
disconnect and the termination in Compressor.
That being said, due to possible in rush on start-up even though they have a drive are we ok going to a larger
breaker in gear say 400 amp so as not to nuisance trip the 300 amp breaker and still be code with the 350 MCM
Conductors
That is what I have come across in the past. Their recommended fuse does not fit your typical fused disconnect, is usually marked higher amp rating than maximum overcurrent for the drive, but is supposed to be what you need for high available fault current, and your other overcurrent device is for general purpose branch circuit protection reasons.

Reality still is if that fuse is called upon to open, you probably will still be seeing front end components on the drive that need replaced, it just responds faster than other devices and probably results in less spectacular to watch failure of the drive components.
 

wattup

Member
Thanks again for replies. In this application they are calling for 550 amp High Speed fuses. This will have to be installed between the 400 amp disconnect and the Line supply terminals at the compressor. My next issue I would like input on is the only Fuse blocks available to house these fuses
come with provisions for Parallel conductors not single termination.
This will require me to run parallel conductors from my Fused disconnect to feed this fuse block. This disconnect is fused with 300 amp time delay fuses. From the load side of this fuse block that will be protected by the 550 amp high speed fuses we will have to run parallel conductors in a single conduit out to the air compressor. Can these parallel conductors still be sized by the FLA of the air compressor X 125% after derating .80 for the 6 current carrying conductors in one conduit.
 

kwired

Electron manager
Location
NE Nebraska
Thanks again for replies. In this application they are calling for 550 amp High Speed fuses. This will have to be installed between the 400 amp disconnect and the Line supply terminals at the compressor. My next issue I would like input on is the only Fuse blocks available to house these fuses
come with provisions for Parallel conductors not single termination.
This will require me to run parallel conductors from my Fused disconnect to feed this fuse block. This disconnect is fused with 300 amp time delay fuses. From the load side of this fuse block that will be protected by the 550 amp high speed fuses we will have to run parallel conductors in a single conduit out to the air compressor. Can these parallel conductors still be sized by the FLA of the air compressor X 125% after derating .80 for the 6 current carrying conductors in one conduit.
If after you apply adjustments to conductor at 90C ampacity you still can not go smaller than whatever 75C conductor is required.

FLA x 1.25 is your minimum required ampacity
 

wattup

Member
If after you apply adjustments to conductor at 90C ampacity you still can not go smaller than whatever 75C conductor is required.

FLA x 1.25 is your minimum required ampacity
Ok, I just wanted to make sure that was still based on the FLA x 1.25 for conductors or parallel conductors and would not need to go up because of the 550 amp High speed fuses. Thanks again that should be all.
 

kwired

Electron manager
Location
NE Nebraska
Ok, I just wanted to make sure that was still based on the FLA x 1.25 for conductors or parallel conductors and would not need to go up because of the 550 amp High speed fuses. Thanks again that should be all.
didn't look back too far, but I think you needed 300 amp fuses for the branch circuit fuses, those are general protection for the branch circuit, these 550 amp specialty fuses are there to satisfy short circuit withstand rating issues.

I have run into this myself before, those specialty drive fuses have higher amp rating then the branch circuit device, but completely different trip curve as well. They don't fit in standard fused switches either.
 

wattup

Member
Kwired you are correct. We have added an additional $1500.00 to get this accomplished to meet warranty for this install.
We contacted bussman they do not even MAKE a Disconnect for the fuses, all the more reason they should be provided and housed in the
air compressor cabinet. I digress glad to get this one figured out and done.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Generally, those fuses are used by OEMs, such as VFD mfrs, INSIDE of their units in order to pass UL listing requirements, not as part of a disconnect and I have never seen them required outside of a VFD before. A-B and ABB use them on a lot of their newer larger drives, but they are inside of the drive cabinets as they come from the factory. In your case I think someone got lazy and/or cheap when supplying the VFD, pushing it off into you to figure out how to do it.
 

kwired

Electron manager
Location
NE Nebraska
Generally, those fuses are used by OEMs, such as VFD mfrs, INSIDE of their units in order to pass UL listing requirements, not as part of a disconnect and I have never seen them required outside of a VFD before. A-B and ABB use them on a lot of their newer larger drives, but they are inside of the drive cabinets as they come from the factory. In your case I think someone got lazy and/or cheap when supplying the VFD, pushing it off into you to figure out how to do it.
My guess also. Buy some the cheaper drives out there and you must buy those fuses and fuse holders separately.
 
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