"K" factor transformers and IGC

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marcosgue

Member
Location
Tampa
Occupation
Electrician
Question: When we use transformers building with K factor to handle the effects of harmonic of nonlinear load is not necessary run the isolated ground wire with the branch circuits?
Thanks
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
IMO no. The K-factor rating has nothing to do with an IG system although in the past they were typically used together.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
If you are using isolated grounds grounds on branch circuits the isolated EGC must be run with the branch circuit conductors whether you have a K factor transformer or not.
 

ron

Senior Member
Isolated grounds are an old concept that are not longer used. The reason for isolated grounds were concerns about common-mode voltage and it has turned out that IG has made it worse in some applications. IG is not a requirement of having non-linear loads.
K-Factor rated transformers or oversized transformers are however a requirement of having non-linear loads to accommodate the excess heating.
 

marcosgue

Member
Location
Tampa
Occupation
Electrician
I'm sorry for the terminology that I was used, my point is we've a school project with 30 kva step down 480V-120/208V k rating transformer in every electrical room, but the panel they fed no use isolated EGC in any branch circuits, is this design correct?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I'm sorry for the terminology that I was used, my point is we've a school project with 30 kva step down 480V-120/208V k rating transformer in every electrical room, but the panel they fed no use isolated EGC in any branch circuits, is this design correct?
Sounds like a modern design which is correct because IG systems are a thing of the past. You may still see them with point of sale equipment but in reality IG systems are obsolete.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I'm sorry for the terminology that I was used, my point is we've a school project with 30 kva step down 480V-120/208V k rating transformer in every electrical room, but the panel they fed no use isolated EGC in any branch circuits, is this design correct?
There is never a requirement for IG, except as a consequence of a design decision to use them. IMO, such a decision is generally not a very good decision, although it can eb a lucrative one for the people doing the work.
 

marcosgue

Member
Location
Tampa
Occupation
Electrician
I got some confused about why the NEC don't have any information about the characteristic of K rating transformer, I was looking but I couldn't find any information
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I got some confused about why the NEC don't have any information about the characteristic of K rating transformer, I was looking but I couldn't find any information
The use of a K rated transformer is strictly a design issue...not a safety or code issue. The code requirements for a K rated transformer installation are the same as for any standard transformer installation.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
As Ron said, transformers with various K-factor ratings are specified to handle different levels of harmonic currents.
Assuming that the K rated transformer was specified because such harmonic load currents are present, then that could mean that the neutral conductor capacity cannot be reduced as stated in 220.61(C)(2). That would depend on the results of load calculations.

By the way, what is the K factor of the transformers being utilized?
 

marcosgue

Member
Location
Tampa
Occupation
Electrician
that's should be sizing the neutral secondary up to 200% of the load connected between any ungrounded conductor and the grounded conductor, is that correct?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
that's should be sizing the neutral secondary up to 200% of the load connected between any ungrounded conductor and the grounded conductor, is that correct?
Yes with a K-13 you would typcially use a 200% neutral but it's not a written in stone rule just a design option.
 

ron

Senior Member
that's should be sizing the neutral secondary up to 200% of the load connected between any ungrounded conductor and the grounded conductor, is that correct?
For a typical mixed linear / non-linear load install, many designers try to reduce the neutral (<100%). When harmonics are expected, a K rated transformer gets specified and a full sized neutral gets used.
I haven't seen oversized (>100%) neutrals specified in several years, even in data centers.
 
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