No primary protection required?

[Isaiah]

It's no problem to reverse feed a transformer. 450.11(B) makes this clear. Besides, it's only been recently that the code had anything to say about it at all and I don't think the manufacturers did anything different to their transformers other than add a note to the instructions that says, "It's ok if you want to backfeed this".

Manufacturers vary on this topic. GE says their transformers are capable of stepping up as long as the OCPD is set high enough to handle the inrush current and meet the NEC. Hammond on the other hand states that it would require a special order transformer - same for Eaton Cutler-Hammer.

http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White%20Papers%7CTransformer2%7Cgeneric

 

[jumper]

Manufacturers vary on this topic. GE says their transformers are capable of stepping up as long as the OCPD is set high enough to handle the inrush current and meet the NEC. Hammond on the other hand states that it would require a special order transformer - same for Eaton Cutler-Hammer.

http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White%20Papers%7CTransformer2%7Cgeneric

That is why I did not want to get into the design. It can get tricky real quick.

 

[Isaiah]

That is why I did not want to get into the design. It can get tricky real quick.

What are your thoughts regarding the grounding aspect for secondary side at 480V?

 

[ActionDave]

Manufacturers vary on this topic. GE says their transformers are capable of stepping up as long as the OCPD is set high enough to handle the inrush current and meet the NEC. Hammond on the other hand states that it would require a special order transformer - same for Eaton Cutler-Hammer.

http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White%20Papers%7CTransformer2%7Cgeneric

It's a hunk of iron with wingdings around it. If it gets to the level of design needed to meet NASA specs I'll worry a little bit more about it. We back fed them for years and years without any added instructions or problems.

 

[LarryFine]

What are your thoughts regarding the grounding aspect for secondary side at 480V?

I think an option would be to use a transformer with a center-tapped 480v winding for the first transformer, and ground the center tap, creating two 240v-to-ground conductors. This would, among other things, reduce the electrical stress on the insulation.

 

[Isaiah]

I think an option would be to use a transformer with a center-tapped 480v winding for the first transformer, and ground the center tap, creating two 240v-to-ground conductors. This would, among other things, reduce the electrical stress on the insulation.

Yes - and would also provide a reference point to ground. The client's specification calls for a grounded system vs ungrounded w/detectors.
I'd have to pull a white, grounded conductor (not a neutral) with the two phase conductors, through the second transformer primary OCPD and ground the 2nd tranny center tap on the primary. Of course the secondary side would also be grounded, 120V (ph and neutral) to the Control Panel CB.

Basically both transformers (120-480V and 480-120V) would be grounded on both the primary and secondary windings. Is this the way you see it?

 

[ActionDave]

Yes - and would also provide a reference point to ground. The client's specification calls for a grounded system vs ungrounded w/detectors.
I'd have to pull a white, grounded conductor (not a neutral) with the two phase conductors, through the second transformer primary OCPD and ground the 2nd tranny center tap on the primary. Of course the secondary side would also be grounded, 120V (ph and neutral) to the Control Panel CB.

Basically both transformers (120-480V and 480-120V) would be grounded on both the primary and secondary windings. Is this the way you see it?

You are mixing up terms.

Grounded = physically connected to the earth

Neutral = the conductor we usually ground

Equipment grounding conductor = a conductor used to clear a fault

You would pull two hots and an equipment ground ,ie, green, from the first transformer to the second, you derive a "neutral" at the secondary of the second transformer and ground it and bond it to the transformer case or first disconnect and also connect the EGC from the first transformer.

 

[electrofelon]

For this install I believe that 240.4(F) allows the primary protection for the first tranny to protect the primary and secondary of the tranny and secondary conductors also.

I'm more concerned about the primary windings of the second tranny.

240.4 is about the conductors, not the transformer protection requirements in 450.3. 450.3 doesnt have an allowance to protect a transformer through another transformer - but does it need to? It seems we could just use common sense, electrical theory, and the allowance in 240.4(G) and 408.36(B) exception as supporting evidence :angel:

OP said the wire was already run, but the best way to do this is use the loophole in 250.30(A)(1) exception to save a conductor.

 

[Isaiah]

I like your idea of using 250.30(A)(1) but I still have to agree with Jumper - OCPD needs to be there for primary windings of second transformer in accordance with 450.3(B). This same overcurrent device would also serve to protect the secondary conductors of the first transformer secondary in accordance with 240.21(C)(4).


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[Isaiah]

Electro felón. Yes the conductors have already been run between transformers, they pulled a 3/C with ground. Going by your post the white (Grounded) conductor could be eliminated and simply use the EGC to bond the center tap of each 480V winding. In essence two phase conductors with an EGC to provide fault path - and coil/tape the ‘extra’ conductor - correct?


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[jumper]

I agree with Larry’s design and Izzy’s code argument.

 

[Isaiah]

I agree with Larry’s design and Izzy’s code argument.

I'm going with this - thanks for your help.
I should have joined forum a long time ago; some very smart people here.

 

[LarryFine]

Basically both transformers (120-480V and 480-120V) would be grounded on both the primary and secondary windings. Is this the way you see it?

No. I see grounding the center tap on the secondary of the first transformer, and carrying that as an EGC with the 480v conductors to the second.

At the second transformer, I see grounding one secondary conductor, creating the typical hot-and-neutral 120v supply source we all know and love.

 

[LarryFine]

.. . simply use the EGC to bond the center tap of each 480V winding.

I would not ground a center tap on the primary of the second transformer.

 

[Isaiah]

I would not ground a center tap on the primary of the second transformer.

Objectionable currents? So I suppose bond to 2nd tranny case only with EGC after bonding to primary OCPD...


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[kwired]

What are your thoughts regarding the grounding aspect for secondary side at 480V?

I think an option would be to use a transformer with a center-tapped 480v winding for the first transformer, and ground the center tap, creating two 240v-to-ground conductors. This would, among other things, reduce the electrical stress on the insulation.

240.4(F) has already been mentioned - it allows two wire to two wire transformer to have secondary overcurrent protection on the primary. If you center tap secondary you can't use this rule. Nothing wrong with two wire 240 volts with one conductor grounded. That conductor will need to be white or gray. A separate EGC needs to be run regardless. If feeding a separate building, one may be able to use the grounded conductor for equipment grounding. See 250.32(B), there is several possibilities covered in that section.

Also some have questioned if you need additional overcurrent protection on the second transformer primary. IMO your feeder from the first transformer is already protected by the primary device. If you have same VA rating on the second transformer why wouldn't it still protect the second transformer, you wouldn't necessarily need additional protection if you had utilization equipment there instead of a second transformer.

 

[GoldDigger]

If a ground fault occurred between the two xfmrs how would it be cleared if there is no overcurrent device? The primary device, i.e. CB on the first transformer would not see the fault.

Manufacturers vary on this topic. GE says their transformers are capable of stepping up as long as the OCPD is set high enough to handle the inrush current and meet the NEC. Hammond on the other hand states that it would require a special order transformer - same for Eaton Cutler-Hammer.

http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White%20Papers%7CTransformer2%7Cgeneric

The biggest single issue happens when the transformer is wound asymmetrically (one winding directly on the core and the other winding entirely on top of the first, creating a partial air core.
The inrush (even after allowing for turns ratio) can be as much as ten times as high for one winding compared to the other. If the two windings are wound at the same time, with similar distances from the core, then the percent inrush current will be similar for both windings. In that case the manufacturer will usually be happy to specify use in either direction.

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[Isaiah]

240.4(F) has already been mentioned - it allows two wire to two wire transformer to have secondary overcurrent protection on the primary. If you center tap secondary you can't use this rule. Nothing wrong with two wire 240 volts with one conductor grounded. That conductor will need to be white or gray. A separate EGC needs to be run regardless. If feeding a separate building, one may be able to use the grounded conductor for equipment grounding. See 250.32(B), there is several possibilities covered in that section.

Also some have questioned if you need additional overcurrent protection on the second transformer primary. IMO your feeder from the first transformer is already protected by the primary device. If you have same VA rating on the second transformer why wouldn't it still protect the second transformer, you wouldn't necessarily need additional protection if you had utilization equipment there instead of a second transformer.

As long as everything functions normally, there would be no problem using one OCPD to protect both Xfmrs, but what about when a fault occurs between the two? Not sure how much fault current would occur (20A source and 3kVA xfmr, cable is #6) but a ground fault would just circulate between transformers indefinitely until it goes phase to phase - then the first OCPD would trip.
If we leave it ungrounded we'll need to add ground detectors which is more cost - and the client wants to see a grounded system, hence the need to for the center tap/grounded conductor.

 

[kwired]

As long as everything functions normally, there would be no problem using one OCPD to protect both Xfmrs, but what about when a fault occurs between the two? Not sure how much fault current would occur (20A source and 3kVA xfmr, cable is #6) but a ground fault would just circulate between transformers indefinitely until it goes phase to phase - then the first OCPD would trip.
If we leave it ungrounded we'll need to add ground detectors which is more cost - and the client wants to see a grounded system, hence the need to for the center tap/grounded conductor.

Fault between the two? If you ground one conductor on the secondary of the first transformer, and fault the other conductor to ground, then you will get a high current on the secondary. Primary current will increase proportionally and primary OCPD will trip. If you don't ground it, first fault simply makes the secondary a grounded system, second fault will do exactly what I described before and trip primary OCPD.

That second situation should likely have fault monitoring, but same thing still happens if one ignores the fault indication and a second fault eventually occurs.

 

[Isaiah]

Fault between the two? If you ground one conductor on the secondary of the first transformer, and fault the other conductor to ground, then you will get a high current on the secondary. Primary current will increase proportionally and primary OCPD will trip. If you don't ground it, first fault simply makes the secondary a grounded system, second fault will do exactly what I described before and trip primary OCPD.

That second situation should likely have fault monitoring, but same thing still happens if one ignores the fault indication and a second fault eventually occurs.

Since its a separately derived system, a ground fault on the secondary of the first transformer will circulate indefinitely via grounded conductor from step up to step down xfmrs - it will not increase proportionally through the windings (1st xfmr primary side) until it goes phase to phase - then, the primary OCPD of will trip. Section 240.21(C)(1) explains when a single primary OCPD can be used to protect both primary and secondary conductors.
This link from ECM magazine shows how a typical fault will flow on 3-wire secondary with grounded center tap:

https://www.ecmweb.com/code-basics/grounding-and-bonding-separately-derived-systems