XFMR PRIMARY & SECONDARY WIRE SIZING

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jwcooke

Member
I AM INSTALLING A 75KVA DRY TYPE XFMR TO SUPPLY A LIGHTING AND BRANCH CIRCUIT PANELBOARD. IT WILL BE 480V/90A PRIMARY AND 120/208V-208A SECONDARY AS PER THE MANUFACTURES NAMEPLATE. I AM CONFUSED ON THE RULES REGARDING THE PRI & SEC CONDUCTOR SIZING. I AM USING A 100A BRANCH CIRCUIT BREAKER WITH #2 XHHW or THHN CU CONDUCTORS(BOTH 75 DEG RATING)FOR THE PRIMARY FEED ALONG WITH A #8 CU FOR EQUIP GRNDG. THE SECONDARY WILL BE A 200A MAIN DISCONNECT BREAKER IN THE PANELBOARD WITH 300MCM XHHW or THHN CU CONDUCTORS ALONG WITH A #6 CU FOR EQUIPMENT GRNDG. ALL WILL BE IN GREENFIELD (FMC). AM I RIGHT?????
 

bphgravity

Senior Member
Location
Florida
Re: XFMR PRIMARY & SECONDARY WIRE SIZING

The 300 kcmil conductors seem a little excessive for a 200A secondary, but all looks good to me.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: XFMR PRIMARY & SECONDARY WIRE SIZING

First, please turn off the CAPS. It gives the impression you are shouting.

If you leave the secondary breakers and conductor as they are, you will need to upsize the # 6 AWG EGC on the secondary in proportion to the amount you upsized the secondary conductors from 3/0 to 300 MCM per 250.122.
 

templdl

Senior Member
Location
Wisconsin
Re: XFMR PRIMARY & SECONDARY WIRE SIZING

I am one that likes to know what trouble I can get into be for I do an not be surprised by it.
I am to assume that it is a 75kva 480-208y/120. As a former transformer application engineer I'm not familiar with a 120/208-208.
Also, as a former applications engineer I have become accustomed to looking at the entire application such as the FLA would be 90a and the inrush current to energize a 150degC unit would be about 900a. If your 100a breaker were to have a magnetic pickup of about 10x (hopefully yours happens to be as some may be lower), that would place it at about 1000a and, as such, the transformer should be able to be energized with out nuisance tripping. But knowing that breakers are calibrated commonly +-20% that would place it between 800-1200a, which could create a conflict.
If you say that you have a transformer rated 115 0r 80gedC, or has a K factor that inrush will be much higher, anywhere from about 1100-1400a, which can pose to be a significant problem.
Remember Table 450.3(B) which allows you 125% (90x125%=112.5a) 112.5a is not a standard rating which you are allowed to go to the next higher standard rating but not over, which is 125a.
After having to field numerous complains about a 75kva transformer causing nuisance tripping and requests for warranty replacements because of a defect I find that the contractor has backed himself in a corner with a panel that can only accommodate a maximum of a 100at breaker.
 

kiloamp7

Senior Member
Re: XFMR PRIMARY & SECONDARY WIRE SIZING

If I need a plain jane 150 degree rise xfmr that large, generally I try to use a 125A pri. bkr. & #2. As templdl says, sometimes the inrush, even w/o a load on the sec., can trip a 100A bkr. Of course if it does, just reset the bkr. & close it again. Also, the 125A has capability of carrying continuously the 90A pri. FLA to take advantage of the 100% 75KVA rating.

To take advantage ot the 100% 75KVA on sec. you'd need a 250A bkr. & 250KCMIL.

Like all applications, there are many considerations. For example if you already have a 200A main breakered panel, then 3/0 is okay for sec. If you already have a 100A bkr. for the pri., then #3 is fine. Do you really need full 75KVA capacity?

Would not be unusual to see a 225A bkr. & 4/0 on the sec. 300KCMIL seems excessive unless there are extenuating circumstances.
 

rhn

Member
Location
N. Ft. Myers
Re: XFMR PRIMARY & SECONDARY WIRE SIZING

250.30 (A)(2)(a) A gec for a single separately derived system shall be sized in accordance with 250.66 for the DERIVED phase conductors.
If you use 300 kcmil your gec will be #2 cu
 

jwcooke

Member
Re: XFMR PRIMARY & SECONDARY WIRE SIZING

First of all, my apologie to dereckbc for the use of caps. Please attribute their use to my ignorance of forum etticut. Secondly I would like to sincerely thank you all for your responses. This was my first time using this forum and based on how helpful they were, it certainly won't be the last time I'll ask for help.
Best Regards to all
Joe
 
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