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Old 04-03-2016, 01:22 PM   #1
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Default Transformer Bonding

I have a 120,208 3 phase service bonded and grounded including building steel. I need to set a step up transformer to 480v go several hundred feet and set another transformer to step back down to feed a 120/208 panel, as far as bonding/grounding is all I need at each transformer a bond to the steel. I will have a SBJ between first panel and transformer and no neutral until leaving second transformer to panel. The wiring I was going to use the conduit as a ground between transformers and bond at second transformer not the panel
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Old 04-03-2016, 02:49 PM   #2
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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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Old 04-03-2016, 03:05 PM   #3
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It is a standard transformer used in reverse
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Old 04-03-2016, 03:39 PM   #4
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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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Old 04-03-2016, 04:14 PM   #5
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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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Old 04-03-2016, 07:10 PM   #6
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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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Old 04-03-2016, 07:16 PM   #7
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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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Old 04-03-2016, 07:29 PM   #8
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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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Old 04-03-2016, 07:42 PM   #9
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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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Old 04-03-2016, 08:13 PM   #10
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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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Old 04-03-2016, 08:15 PM   #11
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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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Old 04-03-2016, 08:32 PM   #12
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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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Old 04-03-2016, 08:45 PM   #13
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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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Old 04-03-2016, 09:11 PM   #14
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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.


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Old 04-04-2016, 01:13 PM   #15
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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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Old 04-04-2016, 01:50 PM   #16
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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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Old 04-04-2016, 06:35 PM   #17
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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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Old 04-04-2016, 07:06 PM   #18
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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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Old 04-04-2016, 09:08 PM   #19
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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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Old 04-04-2016, 09:23 PM   #20
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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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