No primary protection required?

[electrofelon]

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

Still not following you here. The fact is, an ocpd on the primary of a 2 wire transformer will protect the second transformer. I agree with kwired it is protected per 450.3. The code is permissive. If they want to micromanage that protection then they need to tell us. Yes they do do that in other sections. Such as the transformer conductors, but IMO that should be taken out and rewrithen in an informational note.

 

[Isaiah]

Still not following you here. The fact is, an ocpd on the primary of a 2 wire transformer will protect the second transformer. I agree with kwired it is protected per 450.3. The code is permissive. If they want to micromanage that protection then they need to tell us. Yes they do do that in other sections. Such as the transformer conductors, but IMO that should be taken out and rewrithen in an informational note.

Its not a two-wire system: Its a three wire secondary. You cannot protect both with a single OCPD on the primary.

 

[Adamjamma]

saw in the graphics over the last week the same described that the ocpd on primary did protect the secondary. so confusing.

 

[Isaiah]

saw in the graphics over the last week the same described that the ocpd on primary did protect the secondary. so confusing.

Yes; but, section 450.14 requires a disconnecting means for the second transformer anyway, so why not make it a fused disconnect and remove all doubt?

 

[kwired]

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

You have pretty much the same thing with what we are talking about. A fault to ground on secondary side of the first transformer, simply gives the secondary a ground reference and not much else happens. If you intentionally ground one of the secondary leads, then fault the other lead to anything grounded - this gives you a short circuit on that secondary, current will rapidly rise to levels that may be damaging to components. That current doesn't magically appear, it is powered by the primary coil, so current in the primary will raise proportionally to the current in the secondary. If secondary current increases by a factor of 5 then primary current increases by a factor of 5.Properly selected overcurrent device in the primary will protect both. When you have a multiwire secondary you need additional secondary protection because it is possible to overload a portion of the secondary but not the primary. Example 1kva transformer with 120/240 secondary, each half of the secondary is only rated for .5 kVA. If you put a .75kVA load on one half - that half is overloaded, but primary still sees less than it's 1 kVA rating so a primary only protection device would never catch this condition.

Its not a two-wire system: Its a three wire secondary. You cannot protect both with a single OCPD on the primary.

This discussion is about a two wire system. If one had a three wire secondary then you are correct.

Yes; but, section 450.14 requires a disconnecting means for the second transformer anyway, so why not make it a fused disconnect and remove all doubt?

If two wire system throughout the setup, I don't see why disconnect for first transformer can't also be the disconnect for the second transformer. If you are feeding a second building with second transformer at second building you have to comply with art 225 and should need a disconnect but it is because of the separate building more so than because there is a transformer there.

 

[Isaiah]

You have pretty much the same thing with what we are talking about. A fault to ground on secondary side of the first transformer, simply gives the secondary a ground reference and not much else happens. If you intentionally ground one of the secondary leads, then fault the other lead to anything grounded - this gives you a short circuit on that secondary, current will rapidly rise to levels that may be damaging to components. That current doesn't magically appear, it is powered by the primary coil, so current in the primary will raise proportionally to the current in the secondary. If secondary current increases by a factor of 5 then primary current increases by a factor of 5.Properly selected overcurrent device in the primary will protect both. When you have a multiwire secondary you need additional secondary protection because it is possible to overload a portion of the secondary but not the primary. Example 1kva transformer with 120/240 secondary, each half of the secondary is only rated for .5 kVA. If you put a .75kVA load on one half - that half is overloaded, but primary still sees less than it's 1 kVA rating so a primary only protection device would never catch this condition.

This discussion is about a two wire system. If one had a three wire secondary then you are correct.

If two wire system throughout the setup, I don't see why disconnect for first transformer can't also be the disconnect for the second transformer. If you are feeding a second building with second transformer at second building you have to comply with art 225 and should need a disconnect but it is because of the separate building more so than because there is a transformer there.

I wouldn’t feel comfortable with a fault travelling nearly two thousand feet, through secondary/primary windings and finally to the primary OCPD – seems like a lot of impedance. The disconnecting means for the second tranny primary is necessary from a maintenance perspective.

 

[kwired]

I wouldn’t feel comfortable with a fault travelling nearly two thousand feet, through secondary/primary windings and finally to the primary OCPD – seems like a lot of impedance. The disconnecting means for the second tranny primary is necessary from a maintenance perspective.

Two thousand feet yes you might want disconnect even if not required.

Was that kind of distance mentioned in OP? My replies have been general in nature.

How much impedance over that distance depends on size and type of conductors, but one must also consider the OCPD setting. If you have a fault near the far end of the circuit, it may not draw 20kA, and get into instantaneous trip curve, but if you have a low level OCPD setting and still get a few hundred amps of fault current, it should trip just might take a little bit longer.

 

[tom baker]

I have done this using a Sq D Mini Power zone. Primary, transformer, secondary panel, all in one box.
Wire it, hang it, install two ground rods and you are done!

 

[Isaiah]

Two thousand feet yes you might want disconnect even if not required.

Was that kind of distance mentioned in OP? My replies have been general in nature.

How much impedance over that distance depends on size and type of conductors, but one must also consider the OCPD setting. If you have a fault near the far end of the circuit, it may not draw 20kA, and get into instantaneous trip curve, but if you have a low level OCPD setting and still get a few hundred amps of fault current, it should trip just might take a little bit longer.

Yes, the distance was mentioned in my initial post along with wire size. Youve made some good points but I prefer to err on the conservative side and add disconnecting means / primary overcurrent protection for the second transformer. We're not looking at huge cost since the loads are so small.

Sent from my LM-X212(G) using Tapatalk

 

[jumper]

Yes, the distance was mentioned in my initial post along with wire size. Youve made some good points but I prefer to err on the conservative side and add disconnecting means / primary overcurrent protection for the second transformer. We're not looking at huge cost since the loads are so small.

Sent from my LM-X212(G) using Tapatalk

I just back to this thread.

I disagree with some who have said that the primary OCPD of first tranny can protect the second tranny. Every tranny needs a disco and a primary OCPD.

A primary tranny OCPD can protect the secondary depending on how you use 450.3.
Under certain conditions this primary OCPD can also protect the secondary conductors, but that is it.

 

[electrofelon]

I just back to this thread.

I disagree with some who have said that the primary OCPD of first tranny can protect the second tranny. Every tranny needs a disco and a primary OCPD.

A primary tranny OCPD can protect the secondary depending on how you use 450.3.
Under certain conditions this primary OCPD can also protect the secondary conductors, but that is it.

As I said, I think the first ocpd might be able to serve as protection for the second. We can agree to maybe disagree on that one, but regarding the transformer dosconnect, that can be remote if lockable, it's done all the time. I can't look up the code section but I'm 97% sure it's there.

 

[jumper]

As I said, I think the first ocpd might be able to serve as protection for the second. We can agree to maybe disagree on that one, but regarding the transformer dosconnect, that can be remote if lockable, it's done all the time. I can't look up the code section but I'm 97% sure it's there.

Yes, disco can be remote as long as there is info/plaque stating where it is.

450.14 Disconnecting Means. Transformers, other than Class 2 or Class 3 transformers, shall have a disconnecting means located either in sight of the transformer or in a remote loca- tion. Where located in a remote location, the disconnecting means shall be lockable in accordance with 110.25, and its location shall be field marked on the transformer.

 

[jumper]

As I said, I think the first ocpd might be able to serve as protection for the second. We can agree to maybe disagree on that one, but regarding the transformer dosconnect, that can be remote if lockable, it's done all the time. I can't look up the code section but I'm 97% sure it's there.

Looking at Texie’s diagram in this thread, I still disagree about the first OCPD can be used to protect both primaries on the two transformers.

http://forums.mikeholt.com/showthread.php?t=175538

 

[Isaiah]

Looking at Texie’s diagram in this thread, I still disagree about the first OCPD can be used to protect both primaries on the two transformers.

http://forums.mikeholt.com/showthread.php?t=175538

I agree. But now, engineering is saying there's not enough available short circuit current to worry about damage - either to the windings or cable therefore sections 450.3(B) and 240.21 do not apply. This may be true, but regardless of the amount of fault current involved, its still not code compliant in my opinion. If I remember correctly, where code interpretation is an issue one must consult the AHJ, in accordance with Section 90.4 before moving forward with an installation.

 

[jumper]

I agree. But now, engineering is saying there's not enough available short circuit current to worry about damage - either to the windings or cable therefore sections 450.3(B) and 240.21 do not apply. This may be true, but regardless of the amount of fault current involved, its still not code compliant in my opinion. If I remember correctly, where code interpretation is an issue one must consult the AHJ, in accordance with Section 90.4 before moving forward with an installation.

Unless “engineering” wants to draw, stamp, and sign off-stick to your guns.

I have certain tranny gurus that I completely trust and not one has called me out, so I stand by my posts until proven/told else wise.

 

[electrofelon]

I agree. But now, engineering is saying there's not enough available short circuit current to worry about damage - either to the windings or cable therefore sections 450.3(B) and 240.21 do not apply. This may be true, but regardless of the amount of fault current involved, its still not code compliant in my opinion. If I remember correctly, where code interpretation is an issue one must consult the AHJ, in accordance with Section 90.4 before moving forward with an installation.

Unless “engineering” wants to draw, stamp, and sign off-stick to your guns.

I have certain tranny gurus that I completely trust and not one has called me out, so I stand by my posts until proven/told else wise.

If you had some sort of "feeder extension" like from subfeed lugs out of a panelboard, or a "tap" that was actually a "splitting splice" not a tap, you wouldnt require another OCPD would you? IMO the only argument than could be made for not allowing the discussed situation is deduction from extension of the fact that they DO give permission for conductor protection and panelboard protection in those respective articles, but they do not in 450. IMO those articles should remove that permission as the NEC is supposed to be permissive - just let the electrical theory determine when you can do it. I dont see the NEC delving into electrical theory for other stuff like why we bond both sides of a metal raceway containing an EGC, parallel conductor grouping, etc. Maybe just put in an informational note for when you can protect through a transformer since it is a bit on the not so intuitive side.

 

[jumper]

If you had some sort of "feeder extension" like from subfeed lugs out of a panelboard, or a "tap" that was actually a "splitting splice" not a tap, you wouldnt require another OCPD would you? IMO the only argument than could be made for not allowing the discussed situation is deduction from extension of the fact that they DO give permission for conductor protection and panelboard protection in those respective articles, but they do not in 450. IMO those articles should remove that permission as the NEC is supposed to be permissive - just let the electrical theory determine when you can do it. I dont see the NEC delving into electrical theory for other stuff like why we bond both sides of a metal raceway containing an EGC, parallel conductor grouping, etc. Maybe just put in an informational note for when you can protect through a transformer since it is a bit on the not so intuitive side.

You get David Lucini or similar to agree and I will step down. Else, I am sticking to my guns till proven else wise.

 

[electrofelon]

You get David Lucini or similar to agree and I will step down. Else, I am sticking to my guns till proven else wise.

very hurt, suing....

 

[jumper]

very hurt, suing....

Give me a valid code section and I will back off.

I have never seen an install that allows a primary tranny OCPD protect a downstream primary tranny also.

 

[electrofelon]

Give me a valid code section and I will back off.


I have never seen an install that allows a primary tranny OCPD protect a downstream primary tranny also.

Where does it say you cant

Yeah, I haven't either, and I wouldn't actually try it - not worth a likely fight with the inspector More of an academic discussion really.