# Transformer Bonding



## RePhase277 (Feb 5, 2008)

What kind of transformer are you using to step up? Is it just a standard step down used in reverse, or will it be a 208 delta primary and 480Y/277 wye secondary? This affects how the bonding is done.


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## CES (Jan 18, 2013)

It is a standard transformer used in reverse


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## RePhase277 (Feb 5, 2008)

In that case, you'll have a 480 delta secondary, so you need to ground one of the phases and identify it white. At the first transfromer, you'll bond the case to the primary EGC, and you bond building steel to one secondary phase, then from that phase bond the case again.

At the other end, all will be standard since you have an X0 terminal.


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## CES (Jan 18, 2013)

Ok, I'll get back with you when we are ready to terminate. I have no experience with this setup and just reading that sounds like it would explode.

Thanks


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## micromind (Aug 11, 2007)

Also, X0 at the step-up unit cannot be connected to anything. If the factory has a bonding jumper, remove it.


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## micromind (Aug 11, 2007)

CES said:


> Ok, I'll get back with you when we are ready to terminate. I have no experience with this setup and just reading that sounds like it would explode.
> 
> Thanks


Yes, it sounds like a bad idea to ground one of the 480 phases but remember, in a transformer like this, there's no electrical connection from the 120/208 side to the 480 side. 

Since the 480 side is a separately derived system, it needs to be grounded or have some sort of ground-fault detection system. Grounding (bonding) one of the phases (usually H2) is the easiest way. 

The end result is a 480 grounded B system. 480 phase to phase and 480 phase to ground.


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## telsa (May 22, 2015)

On the economics, the plan is screwy.

You can't justify the complications for such a short run and such a trivial voltage boost.

The usual solution is to go with hefty aluminum conductors ... perhaps paralleling them, too.

Stay away from copper -- it's so much more expensive -- and not a lot of fun to tug in.

Yes, there still are electrical contractors that don't price // bid for aluminum versus copper.

Once this is tried, their eyes bug out.


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## don_resqcapt19 (Jul 18, 2010)

InPhase277 said:


> In that case, you'll have a 480 delta secondary, so you need to ground one of the phases and identify it white. At the first transfromer, you'll bond the case to the primary EGC, and you bond building steel to one secondary phase, then from that phase bond the case again.
> 
> At the other end, all will be standard since you have an X0 terminal.


There is no requirement that a 480 volt system be a grounded system. However an ungrounded system does require ground detectors....see 250.21(B).


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## RePhase277 (Feb 5, 2008)

don_resqcapt19 said:


> There is no requirement that a 480 volt system be a grounded system. However an ungrounded system does require ground detectors....see 250.21(B).


This is true, but in my opinion it adds a layer if complexity to a job unnecessarily. I believe the grounded system is more reliable.


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## RePhase277 (Feb 5, 2008)

CES said:


> Ok, I'll get back with you when we are ready to terminate. I have no experience with this setup and just reading that sounds like it would explode.
> 
> Thanks


Why would it explode? Because a phase is grounded? That's not how electricity works.


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## telsa (May 22, 2015)

InPhase277 said:


> Why would it explode? Because a phase is grounded? That's not how electricity works.


I never cease to be amazed at seasoned electricians that are weak on transformers.

I'm also amazed at how many 'smart guys' don't have a copy of Ugly's. 

Sheesh.


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## don_resqcapt19 (Jul 18, 2010)

telsa said:


> ...
> I'm also amazed at how many 'smart guys' don't have a copy of Ugly's.
> 
> Sheesh.


I have never even looked at a copy of that book


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## zac (May 11, 2009)

don_resqcapt19 said:


> I have never even looked at a copy of that book


Well then you may be sarcastic or trying to prove something. Most people now don't use ugly's because of smart phone technology, but in the past it was a good resource to have. 
I also respect your knowledge of the trade and am not trying to disrespect you, just pointing out that you may be in the minority. 
I myself rarely used ugly's but have had a few copies over the years and some other "cliff note" type of pamphlets. My best resources were and are my general foreman I worked under over the years. 


Sent from my SGH-M919 using Tapatalk


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## cabletie (Feb 12, 2011)

I am curious why there is no mention of secondary over current protection on the first transformer (the corner grounded delta). 

Since one phase becomes the grounded conductor, that phase should not be fused.


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## don_resqcapt19 (Jul 18, 2010)

cabletie said:


> I am curious why there is no mention of secondary over current protection on the first transformer (the corner grounded delta).
> 
> Since one phase becomes the grounded conductor, that phase should not be fused.


The secondary of the transformer may not require protection (primary protection rated at 125% or less of primary current).
The secondary conductors will require protection based on one of the rules in 240.21(C).


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## RePhase277 (Feb 5, 2008)

don_resqcapt19 said:


> The secondary of the transformer may not require protection (primary protection rated at 125% or less of primary current).
> The secondary conductors will require protection based on one of the rules in 240.21(C).


Being a wye-delta, can it be protected by the primary only? I think the code says it has to be delta-delta for primary-only protection.


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## don_resqcapt19 (Jul 18, 2010)

InPhase277 said:


> Being a wye-delta, can it be protected by the primary only? I think the code says it has to be delta-delta for primary-only protection.


That would be for using the primary transformer protection to protect the transformer secondary conductors. 

Table 450.3(B) permits primary only protection for almost all transformers. It is often not used that way as sometimes the inrush will trip a primary protective device rated at 125% or less of primary current.

There are two sets of overcurrent protection rules involved when working with transformers. The ones in Article 450 that cover the protection of the transformer primary and secondary conductors, and the ones in 240 that cover the protection of the primary and secondary (240.21(C)) conductors. Often a single OCPD can be used to satisfy rules from both sections.


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## bobbarker (Aug 6, 2015)

Very good stuff here. I normally just do your straight forward delta-wye transformers and have never come across this scenario so this is informative to me because it is such a different set up than I am use to


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## RePhase277 (Feb 5, 2008)

bobbarker said:


> Very good stuff here. I normally just do your straight forward delta-wye transformers and have never come across this scenario so this is informative to me because it is such a different set up than I am use to


Personally, I live in the non-standard. I get too bored with the same ol', same ol'. Finding interesting solutions to tough problems makes the world a brighter place.


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## Cow (Jan 16, 2008)

What ampacities are we talking about here that it's cheaper to set two transformers and the associated OCPD's just to drop the wire/pipe size down?

Let's see some specifics, I'd like to know the math of it. Length, overhead, underground, etc??


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## RePhase277 (Feb 5, 2008)

Cow said:


> What ampacities are we talking about here that it's cheaper to set two transformers and the associated OCPD's just to drop the wire/pipe size down?
> 
> Let's see some specifics, I'd like to know the math of it. Length, overhead, underground, etc??


Now you're taking the fun out of it.:laughing:


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## V-Dough (Jul 22, 2014)

InPhase277 said:


> This is true, but in my opinion it adds a layer if complexity to a job unnecessarily. I believe the grounded system is more reliable.


Actually the ungrounded system is more reliable. If any phase shorts to ground everything still runs and a light goes off to show you that. Thats why they used to be common in industrial places. That and 3 wires are cheaper than 4


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## RePhase277 (Feb 5, 2008)

V-Dough said:


> Actually the ungrounded system is more reliable. If any phase shorts to ground everything still runs and a light goes off to show you that. Thats why they used to be common in industrial places. That and 3 wires are cheaper than 4


I get it, but it is still more complex and requires supervision. Just ground it and forget it.


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## Cow (Jan 16, 2008)

InPhase277 said:


> Now you're taking the fun out of it.:laughing:


Yep, I like to inject a little reality and common sense into most of my decisions.:whistling2:


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## mbednarik (Oct 10, 2011)

Transformer cost went up a bit since the first of the year, new DOE regs.


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