# 75VKA Transformer



## MXer774 (Sep 1, 2014)

Here's how I do it. I will be using American voltage systems. 
75KVA XFMR
Primary: (75/831)*1000 = 90A 
Secondary: (75/360)*1000 = 208A

The formula I used is loaded in the front with 3Ø: 480*1.732 = 831. Same for Secondary. 

Canadian 3Ø: 600*1.732 = 1039
(75/1039)*1000 = 72A

That's as far as I will go. Your OCPD can vary dependent upon what your doing.


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## eddy current (Feb 28, 2009)

MOOZ said:


> I was wondering if anybody had any advice on transformer calculations, it's been about 10 years since I've been in school and my formulas are getting foggy.
> 
> I have a 200A, 208V 4 wire panel with a load of 160A that is coming off of a 75KVA 600V/208V Delta/Wye transformer. I have 90A fuses that are currently on the primary side of the transformer and needed to choke the transformer below 200A.
> 
> I need to find the size of fuses on the primary that can accommodate but can't remember the formula. Any Advice is much a appreciated. A little embarrassed to ask but more concerned with doing it correctly.


Not to be an a**, but this is why I think there should be mandatory training for journeyman every new code book.

Fuse the primary with a 60 amp, secondary will be limited to 200 amp.

See ESA bulletin 14-2-8


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## MOOZ (Jan 5, 2018)

*Bulletins*

I appreciate the answer and direction to the proper bulletin. I have gone to CSA and registered for bulletins to be sent to me from now on. While the process for these bulletins may take a day or so, could you please enlighten me on how you came to that answer. It would be a great help


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## eddy current (Feb 28, 2009)

You need the ESA bulletins, not CSA.


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## telsa (May 22, 2015)

Fuses don't choke, they merely blow.

Forget about throttling the primary coils.

You choke the system by limiting the loading in the Secondaries.

90A is typical for 480 to 208Y120 schemes.

It's a tad high for 600 to 208Y120 schemes... but it's not going to be a problem if the conductors match.

Keep in mind that dry-type transformers can go into overload for hours on end and survive nicely.

They are THAT robust// over-designed.


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## sbrn33 (Mar 15, 2007)

eddy current said:


> Not to be an a**, but this is why I think there should be mandatory training for journeyman every new code book.


You shut your whore mouth.


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## MOOZ (Jan 5, 2018)

No need for anybody to upset. Eddy Current was actually very helpful today. Appreciated his time and knowledge


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## eddy current (Feb 28, 2009)

sbrn33 said:


> You shut your whore mouth.


Typical response from you get from guys who haven’t seen a code book in decades :vs_laugh:

Every trade needs to upgrade, especially electrical


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## sbrn33 (Mar 15, 2007)

eddy current said:


> Typical response from you get from guys who haven’t seen a code book in decades :vs_laugh:
> 
> Every trade needs to upgrade, especially electrical


I can out code you with one hand behind my back. Actually probably not but I can find anything in my 93 edition in 10 seconds flat. 
CEU's are the biggest joke ever. I seriously sleep straight through mine and still know code better than 90%. I learn more proving stevie wrong on a topic than I do at code class. Just a $320 per year money grab.


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## daveEM (Nov 18, 2012)

No kidding. I hooked up a 75 KVA Transformer in, 1981 I think.

Those were the days. 

Then the economy crashed an I wired garages. Oh well.


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## tates1882 (Sep 3, 2010)

telsa said:


> Fuses don't choke, they merely blow.
> 
> Forget about throttling the primary coils.
> 
> ...


bingo! Being a SDS the secondaries should have an OCPD sized for the connected load.


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## brian john (Mar 11, 2007)

eddy current said:


> Not to be an a**, but this is why I think there should be mandatory training for journeyman every new code book.
> 
> Fuse the primary with a 60 amp, secondary will be limited to 200 amp.
> 
> See ESA bulletin 14-2-8


If you use it you know it if you don't normally use it you do not need to overcrowd your brain.

I am sure there is stuff I may know you do not and same for you know what I don't know.

I have worked with residential men that for them a transformer is for doorbells. And commercial electricians that think a 3-way is a sexual escapade.

In other words do not be so quick to judge what others should do, I doubt you know it all. 

In the states, I have never seen a question in the 9 masters I have taken asking to size primary and secondary current.


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## telsa (May 22, 2015)

In a 3-phase dry-type step-down transformer, fusing the primary is insufficient to limit current flow in the Secondaries. 

Should a L-N fault develop, the current flow goes crazy, not necessarily popping the Primary fuses.

This is why the NEC demands OCPD on the Secondaries.


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## telsa (May 22, 2015)

In Commercial practice, a 70A fusing would be expected, and the panel would be expected to be a 225A 3-phase 208Y120VAC affair.

THIS is what the NEMA players have standardized on. 

You just don't see EEs specifying 200A panels -- it's 225A every time.

This is not a Code requirement -- it's just the standard practice you keep running into.


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## cabletie (Feb 12, 2011)

In the NEC if you size the primary OCP at 125% of the primary current you don't need secondary OCP protection. 

Is 125% enough for inrush problems? I don't know. That's why they can be fused at 250% if secondary OCP is used. 

There's actually two problems. Protecting the transformer, and protecting the conductors with tap rules. I think that is why we see primary and secondary overcurrent protection used so much when following the NEC. 

I always wondered what was considered "choking" when referring to transformers. I assumed it was sizing the OCP smaller to match the panel it was feeding. Like a 45KVA transformer is good for 125 amps. It may be fused at 100 amps at the panel main breaker though. Is that "choking"? The same 45KVA transformer can be fused at the secondary at 125% of the secondary current. That would be 156 amps.


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## cabletie (Feb 12, 2011)

Here's the transformer schedule of the current job I'm on.


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## cabletie (Feb 12, 2011)

Maybe this is a clearer picture


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## telsa (May 22, 2015)

*


cabletie said:



In the NEC if you size the primary OCP at 125% of the primary current you don't need secondary OCP protection.

Click to expand...

*


cabletie said:


> *This is a FALSE belief for 3-phase transformers.*
> *
> It takes a while to bob around, but the primary fusing is wholly insufficient when 3-phase L-N loads are fed by a 3-phase delta primary.*
> 
> ...


It is essential that the myth of omitting OCPD for Secondaries end.


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## eddy current (Feb 28, 2009)

telsa said:


> *
> 
> It is essential that the myth of omitting OCPD for Secondaries end.*


*

Is it a requirement in the NEC to have OCPD on the secondaries?

Most applications as per the CEC, primary is fine, no secondary OCPD required*


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## eddy current (Feb 28, 2009)

brian john said:


> If you use it you know it if you don't normally use it you do not need to overcrowd your brain.
> 
> I am sure there is stuff I may know you do not and same for you know what I don't know.
> 
> ...


I agree 100%

I still believe in mandatory training every code cycle though.


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