# K13, 200% Neutrals



## vinister (Apr 11, 2012)

I'm working on a job with these details:

- engineers spec'd a 500KVA K-13 Transformer (600 to 120/208)

- Primary load is a 320KV DC rectifier bank for telecom. Other loads are 4x 3 phase AC units, a small lighting circuit, small alarm and fire panels, and a few general receptacles. 

Since they spec'd a K13, and with the nature of the equipment, I assumed that the load must be highly non-linear, and proceeded specing the rest of the gear at 200% neutral, including all the wiring. 

The engineers are balking at my design, saying that the rectifiers are power factor corrected. In looking further into the design of their rectifier equipment, I can see that they are actually phase to phase connected, the neutral is just there for the monitoring equipment. 

So, my questions are:

1. When loads are connected phase to phase, is there still a concern with harmonic currents and voltage distortion? 

2. Are power factor corrected loads completely immune to these effects?

3. Why would they spec a K13 rated transformer with 200% neutrals, but balk at the thought of connecting it to 200% rated gear?

4. What is the cost difference between a K4 and K13 transformer, roughly? What about size?


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## HARRY304E (Sep 15, 2010)

vinister said:


> I'm working on a job with these details:
> 
> - engineers spec'd a 500KVA K-13 Transformer (600 to 120/208)
> 
> ...


Bump.


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## ohmontherange (May 7, 2008)

vinister said:


> I'm working on a job with these details:
> 
> - engineers spec'd a 500KVA K-13 Transformer (600 to 120/208)
> 
> ...


 *Not sure.*

I assume they spec'd a K rated transformer to allow for the additional heating caused by potential harmonic current flow, though this seems contrary to using rectifiers with active input filtering. 

Assuming you are putting a subpanel downstream from the transformer to feed the rectifiers, with all the loads (individual rectifiers) being connected L-L, the engineers may have felt it wasn't necessary to uprate the neutral to that panelboard.


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## brian john (Mar 11, 2007)

vinister said:


> I'
> So, my questions are:
> 
> 1. When loads are connected phase to phase, is there still a concern with harmonic currents and voltage distortion?


Yes



> 2. Are power factor corrected loads completely immune to these effects?


PF corrected I am guessing they mean the loads have input harmonic filters?



> 3. Why would they spec a K13 rated transformer with 200% neutrals, but balk at the thought of connecting it to 200% rated gear?


The load may be such that they feel they need a higher K rated transformer or they are getting a percentage of the installation cost or a good slaesman convinced them they needed it.



> 4. What is the cost difference between a K4 and K13 transformer, roughly? What about size?


Cost and size no idea but google can be your friend.


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## scottcsexton (May 16, 2010)

If a person who has hardware(stainless steel) inside of there body. Ie broken pelvis repair with wires and a chain... What will happen when said person becomes the path to ground. Will said hardware become like the internal wires of a toaster? Master electrician who may never work again. So give it to me real fellas. If there are any engineers out there can answer this would be great.


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## chicken steve (Mar 22, 2011)

that doesn't sound so good Scott.....



~CS~


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## micromind (Aug 11, 2007)

scottcsexton said:


> If a person who has hardware(stainless steel) inside of there body. Ie broken pelvis repair with wires and a chain... What will happen when said person becomes the path to ground. Will said hardware become like the internal wires of a toaster? Master electrician who may never work again. So give it to me real fellas. If there are any engineers out there can answer this would be great.


Stainless doesn't have enough iron in it to cause inductive heating.


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## micromind (Aug 11, 2007)

vinister said:


> I'm working on a job with these details:
> 
> - engineers spec'd a 500KVA K-13 Transformer (600 to 120/208)
> 
> ...


If the rectifiers are line-to-line (almost all of them are), it'll have no effect on neutral current, since the neutral is not involved. From the description of the system, the neutral could actually be reduced in size. Quite a bit, since there are very few line-to-neutral loads. 

There could be trouble with the A/Cs, because the rectifiers will cause a distorted waveform in the 208 system. When they begin to conduct, the voltage will stop rising and falling for an instant until the transformer can convert more magnetism into electrical current. 

Since a motor is basically a transformer, it will also tend to convert some of its magnetism into current. It will become a generator, just for an instant. The rotor will slow slightly, which means less counter EMF in the stator, which means more current to speed it back up. 

The net result will be a stator running hotter than if it were supplied by a pure sine wave. Refrigeration compressors are pretty high-strung, and not very tolerant of distorted waveforms. 

Odd as this may sound, a load reactor typically found on the output of a VFD will clean up the waveform to some degree.


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## scottcsexton (May 16, 2010)

micromind said:


> Stainless doesn't have enough iron in it to cause inductive heating.


Think of this....the hardware is enveloped by conductive tissue...would the resistivity of the different mediums cause excessive harm during a possible 277v jolt...as has happened to allot of us unfortunately.


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## ohmontherange (May 7, 2008)

I'm not sure if your question is purely academic, but you should be working on systems with a 277V potential de energized or with appropriate PPE if energized.


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## Big John (May 23, 2010)

EDIT: Nevermind. Sidetracking OPs thread with nonsense.


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## piperunner (Aug 22, 2009)

scottcsexton said:


> Think of this....the hardware is enveloped by conductive tissue...would the resistivity of the different mediums cause excessive harm during a possible 277v jolt...as has happened to allot of us unfortunately.


Well were made of water what elements of resistance of water & blood to stainless and what path to ground does it take plus if conducting not much current would actually flow which to me is offset by contact of your hands or feet skin and bone its a series circuit low current stainless has no direct contact with phase A to ground your skin does its control of the flow bro..
your only conducting what enters your body . So your not going to heat that stainless up because your body resistance will control the flow .
Now your going to die so who cares what happens . Think of this your in the box are you worried at the funeral there going to say hey Billy had the stainless knee joint heat up when he got electrocuted that must of hurt .

I think current flows into your whole skin & bone structure it does not follow a straight path but flows out branches out our whole body looking for ground .


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## ohmontherange (May 7, 2008)

Big John said:


> EDIT: Nevermind. Sidetracking OPs thread with nonsense.


:laughing::laughing:

I'm gonna start referring to you as Dr John.


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## brian john (Mar 11, 2007)

ohmontherange said:


> :laughing::laughing:
> 
> I'm gonna start referring to you as Dr John.


Did not know he played the blues.


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## ohmontherange (May 7, 2008)

:thumbup:


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## vinister (Apr 11, 2012)

thanks for the on-topic replies. 

I pushed back to the engineering consultants and they have been quiet for a few days now... likely arguing this point internally. 

I brought up the question of voltage waveform distortion due to harmonic currents, and their answer was "the rectifiers are power factor corrected". To me, this clearly shows their lack of understanding on the matter. 

We'll see what they come back with - I will post my results. 

Long story short if I can save on the K-rated transformer, it frees up some of their budget for work with higher profit margin. Either that, or I want them to upgrade the whole system to 200% neutral downstream from the transformer. In my view, having one without the other is senseless - if the system is going to fail because of overloaded neutral, it will fail at the weakest point.


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## Semi-Ret Electrician (Nov 10, 2011)

vinister said:


> thanks for the on-topic replies.
> 
> I pushed back to the engineering consultants and they have been quiet for a few days now... likely arguing this point internally.
> 
> ...


I believe, by asking about larger neutrals throughout the installation you've got the proponents of the initial 200% neutral requirement, back in the "justification hot seat".


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