Do I have to bring the neutral to the disconnect?

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The project I worked on today was installing a step down transformer. Its a 480-208 delta/wye. feeds a 208 compressor in an area where only 480 is available. I know it would have been cheaper to get a delta delta transformer but this one was readily available and that mattered.

We have: 480v MCC bucket - unfused disconnect (existing) - 480/208 transformer - fused disconnect - air compressor

My question is the journeyman I was with said we need to bring the neutral to the disconnect. All the transformer will ever feed is this air compressor. No panel. No single phase circuits. So we have a full sized neutral wire coming up to the neutral point on the disconnect, bonding screw installed and from there its tied to building steel.

Is there something in the code that prevents not running the neutral to the disconnect at all and tying the neutral to building steel at the transformer? Its all piped in EMT and grounding conductors are installed.

Im about 10 weeks out from being able to take my test and have been trying to research things I question. Id love to hear some thoughts or code references I should visit.

Thanks guys!
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
The project I worked on today was installing a step down transformer. Its a 480-208 delta/wye. feeds a 208 compressor in an area where only 480 is available. I know it would have been cheaper to get a delta delta transformer but this one was readily available and that mattered.

We have: 480v MCC bucket - unfused disconnect (existing) - 480/208 transformer - fused disconnect - air compressor

My question is the journeyman I was with said we need to bring the neutral to the disconnect. All the transformer will ever feed is this air compressor. No panel. No single phase circuits. So we have a full sized neutral wire coming up to the neutral point on the disconnect, bonding screw installed and from there its tied to building steel.

Is there something in the code that prevents not running the neutral to the disconnect at all and tying the neutral to building steel at the transformer? Its all piped in EMT and grounding conductors are installed.

Im about 10 weeks out from being able to take my test and have been trying to research things I question. Id love to hear some thoughts or code references I should visit.

Thanks guys!

You could have installed the system bonding jumper at the transformer and not brought the neutral to the disconnect if you do not need the neutral. Study 250.30. If all is done correctly you would still have a compliant fault return path.
The requirements would differ if this was a service as opposed to a SDS. That may be what your journeyman was thinking of.
 
No, this is not a service. It is a SDS.
Also, there is no such thing as 250.24 5(C). I think you are referring to 250.24(C).

Yes. My bad.

The bucket is fused. It feeds an unfused disconnect that was the local disconnect for a compressor that’s no longer there. Building was built as a plastic plant as it closed up. New customer makes doors. So all new equipment is going in. The old disconnect just provided a convenient way to power the new transformer.

Ill read the code section you referenced in the morning. Thank you.
 

ActionDave

Chief Moderator
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Location
Durango, CO, 10 h 20 min from the winged horses.
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The project I worked on today was installing a step down transformer. Its a 480-208 delta/wye. feeds a 208 compressor in an area where only 480 is available. I know it would have been cheaper to get a delta delta transformer but this one was readily available and that mattered.

We have: 480v MCC bucket - unfused disconnect (existing) - 480/208 transformer - fused disconnect - air compressor

My question is the journeyman I was with said we need to bring the neutral to the disconnect. All the transformer will ever feed is this air compressor. No panel. No single phase circuits. So we have a full sized neutral wire coming up to the neutral point on the disconnect, bonding screw installed and from there its tied to building steel.

Is there something in the code that prevents not running the neutral to the disconnect at all and tying the neutral to building steel at the transformer? Its all piped in EMT and grounding conductors are installed.

Im about 10 weeks out from being able to take my test and have been trying to research things I question. Id love to hear some thoughts or code references I should visit.

Thanks guys!
Your journeyman is correct if you make the bond in the first disconnect. texie is also correct that you can make the bond in transformer and not bring the neutral out. I was taught to make the bond in the first disconnect and that's what I do.

If you have a copy of the NEC Handbook there are some very good illustrations in 250.30.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Yes. My bad.

The bucket is fused. It feeds an unfused disconnect that was the local disconnect for a compressor that’s no longer there. Building was built as a plastic plant as it closed up. New customer makes doors. So all new equipment is going in. The old disconnect just provided a convenient way to power the new transformer.

Ill read the code section you referenced in the morning. Thank you.

Just as an FYI, your delta to wye xformer was the correct choice. A delta to delta would be a poor choice for this and something I would not even consider.
I have to wonder why in the world would one have a compressor operating at 208 when a 480 source is available? A terrible waste of material and energy loss.
 

ActionDave

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Just as an FYI, your delta to wye xformer was the correct choice. A delta to delta would be a poor choice for this and something I would not even consider.
I have to wonder why in the world would one have a compressor operating at 208 when a 480 source is available? A terrible waste of material and energy loss.

Everybody is always afraid to change the motor.
 
Just as an FYI, your delta to wye xformer was the correct choice. A delta to delta would be a poor choice for this and something I would not even consider.
I have to wonder why in the world would one have a compressor operating at 208 when a 480 source is available? A terrible waste of material and energy loss.

Whats your reasoning on a delta delta being a poor choice? The fact that you’d have to either float it or corner ground it?

The compressor is a large screw compressor that is being moved from the company’s current location where they didn’t have 480 available.
The documentation we have is not real great, but lists the main motor as 208v only. From the way it’s wired it looks to me like it’s a wye start/delta run.
Several of us looked at it and we decided the customer was aware changing it to 480 wasn’t promised as possible and we didn’t want the liability of damaging something.
 

mivey

Senior Member
The compressor is a large screw compressor that is being moved from the company’s current location where they didn’t have 480 available.
The documentation we have is not real great, but lists the main motor as 208v only. From the way it’s wired it looks to me like it’s a wye start/delta run.
Several of us looked at it and we decided the customer was aware changing it to 480 wasn’t promised as possible and we didn’t want the liability of damaging something.

like Dave said:

Everybody is always afraid to change the motor.
 
I can somewhat understand about not wanting to change the motor. It depends who is doing the changing. As an electrician, I dont really want to change a motor on a large compressor: Just the weight of it is not something I am thrilled about wrestling around, changing pullys, dealing with potentially different mounting holes, etc. Now if there are a few guys at the plant that do this sort of thing all day, then sure it seems like it would be cheaper for them to do that than write a check to the EC for the transformer and labor.
 

ActionDave

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Occupation
Licensed Electrician
I can somewhat understand about not wanting to change the motor. It depends who is doing the changing. As an electrician, I dont really want to change a motor on a large compressor: Just the weight of it is not something I am thrilled about wrestling around, changing pullys, dealing with potentially different mounting holes, etc. Now if there are a few guys at the plant that do this sort of thing all day, then sure it seems like it would be cheaper for them to do that than write a check to the EC for the transformer and labor.

It's not that big of a deal. If weight is an issue that can easily be overcome by using brain over brawn. Motors are standardized by the frame size, so shaft size, height to center, mounting holes, and most other dimensions are all consistent.
 

ActionDave

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Whats your reasoning on a delta delta being a poor choice? The fact that you’d have to either float it or corner ground it?

Why would you choose to increase your voltage to ground? It gains you nothing.

The compressor is a large screw compressor that is being moved from the company’s current location where they didn’t have 480 available.
The documentation we have is not real great, but lists the main motor as 208v only. From the way it’s wired it looks to me like it’s a wye start/delta run.

That would require a special starter so it should be obvious if it's a wye start. How many leads are there? Everything you need to know would be on the nameplate on the motor.

Several of us looked at it and we decided the customer was aware changing it to 480 wasn’t promised as possible and we didn’t want the liability of damaging something.

It is highly likely that the motor can be reconnected for 480V. It's not any more difficult to hook a motor up correctly than it is to hook up a transformer correctly.
 
like Dave said:

i took that as rewire the motor to 480v when I read it this morning. Not physically get a different motor.

Not to argue, and this is far from what I posted here to ask, but the compressor draws 140 some amps total. I think the big motor is like a 50 horse. It’s not something small or easy to get to. Bolts directly to the veins/screw/blower whatever the term is for the compressor part, so it’s not going to be some easy to get in two days sort of thing most likely. And what would the cost be compared to the route we went? You’d have to change the starter set up and overloads too. Do we as electricians do that? Do they hire a factory tech to come do it? Fly one in? If we do it what sort of warranty do we void? It’s not like they said 6 months ago they wants this done, it was more like hey, we moved this in place, we need it going in two weeks. In my experience that’s common. 10000% agree having it run at 480 would be ideal, but stuff never seems to go that way in this trade for me. Don’t you all run into that too?
 

kwired

Electron manager
Location
NE Nebraska
Whats your reasoning on a delta delta being a poor choice? The fact that you’d have to either float it or corner ground it?

The compressor is a large screw compressor that is being moved from the company’s current location where they didn’t have 480 available.
The documentation we have is not real great, but lists the main motor as 208v only. From the way it’s wired it looks to me like it’s a wye start/delta run.
Several of us looked at it and we decided the customer was aware changing it to 480 wasn’t promised as possible and we didn’t want the liability of damaging something.
If it is wye start delta run, then you are likely stuck with only one input voltage for that motor. If it has a VFD, you are stuck with one input voltage, plus many VFD are not intended for delta systems period - they often have surge protection on input that wants to see equal voltage from each line to a neutral point and will be set up for voltage you see to neutral in a wye system.

I can somewhat understand about not wanting to change the motor. It depends who is doing the changing. As an electrician, I dont really want to change a motor on a large compressor: Just the weight of it is not something I am thrilled about wrestling around, changing pullys, dealing with potentially different mounting holes, etc. Now if there are a few guys at the plant that do this sort of thing all day, then sure it seems like it would be cheaper for them to do that than write a check to the EC for the transformer and labor.
If motor is a NEMA standard general purpose motor - it should have same shaft sizes, mounting hole arrangements, etc. If it is an OEM designed motor, it is possible there is some aspect that isn't normally found on a NEMA general purpose motor, often special shaft size, shaft length, shaft extension from back end of the motor.... making that general purpose motor impractical. Though you maybe can get an OEM with the necessary feature - at a bigger price than a general purpose motor. But you also need to look at cost of transformer and installation of it as well.
 
Why would you choose to increase your voltage to ground? It gains you nothing.
Fair point. For some reason I have been under the impression that delta/delta transformers are cheaper, but now i am questioning that. I'll have to do some reading on their merits. I've not run across many, but I don't have the experience that some of you here do.

That would require a special starter so it should be obvious if it's a wye start. How many leads are there? Everything you need to know would be on the nameplate on the motor.

Yep, it actually has 3 separate contactors on it. 6 leads going down to the motor.

If it is wye start delta run, then you are likely stuck with only one input voltage for that motor. If it has a VFD, you are stuck with one input voltage, plus many VFD are not intended for delta systems period - they often have surge protection on input that wants to see equal voltage from each line to a neutral point and will be set up for voltage you see to neutral in a wye system.

If motor is a NEMA standard general purpose motor - it should have same shaft sizes, mounting hole arrangements, etc. If it is an OEM designed motor, it is possible there is some aspect that isn't normally found on a NEMA general purpose motor, often special shaft size, shaft length, shaft extension from back end of the motor.... making that general purpose motor impractical. Though you maybe can get an OEM with the necessary feature - at a bigger price than a general purpose motor. But you also need to look at cost of transformer and installation of it as well.

No VFDs on this one actually.

Im not sure if its a standard motor honestly. Gut level I am going to say no, its like a C face, but to me looked a little unique. The base also was a little different if I am remembering it correctly. If I get back up there and the thing isn't running i'll take a picture of it and the name plate as I am curious now after all the criticism. Honestly in a perfect world I think they would have been best off selling this compressor and buying something equivalent designed for 480v, but thats never how this stuff works :angel:
 

ActionDave

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. If I get back up there and the thing isn't running i'll take a picture of it and the name plate as I am curious now after all the criticism.

I don't see any criticism. Your original question was answered in the first few posts and then you got some good feedback from the membership here. None of it was personal.

Honestly in a perfect world I think they would have been best off selling this compressor and buying something equivalent designed for 480v, but thats never how this stuff works :angel:

Happens to all of us all of the time. You play the hand you are dealt.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
Is the compressor mounted on a skid with a control panel, etc? I know a lot of the compressors we work on have boards and other electronics, well beyond, "just change the motor to 480v." But, it may be an older simpler style with nothing more than maybe a control transformer and a few starters it sounds like.

Second, I'd want to verify the OCPD in the MCC wasn't a repurposed starter bucket or similar with an instantaneous only breaker feeding the transformer.
 
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