Sizing aluminum conductors to a 15kV Power Transformer

[bdn2004]

We've installed a specialty transformer that pulls 988 FLA at 15kV on the primary side per the nameplate. We are attaching the primary to some open bus that is rated 3000A and feeds other transformers as well. My question is about the size of these tap conductors. The transformer was somewhat oversized but will have a constant load of about 80% of it's rating. Do these tap conductors need to be upsized 125% of the FLA or can they be sized at the actual FLA?

 

[HornTootinEE]

If you aren't subject to NEC in this application, you can size however you want to size. Just know that if you overrate a cable you better have some justification (and probably a PE stamp to go with it) to show why you installed what you did.

Thats a big tub if 15 kV is your primary voltage.

 

[Marmite]

In my opinion, having invested a significant amount of money in your transformer, it would be false economy to limit its throughput my undersizing the primary connections. The cost differential will be minimal now but much higher if you have to uprate them later. I would size them for the cyclic rating.
Regards
Marmite

 

[PUDGUY]

I was recently reviewing the ratings of aluminum bus in one of our open-air substations. Our bus is 12.47kV nominal (15kV rated insulation).

I stumbled upon the Aluminum Electrical Conductor Handbook . It's waaay more than you'll ever need, but probably worth browsing through or having on your shelf as a nice reference guide. Wish I could get this in hardback for my collection:

http://www.aluminum.org/AM/Template...MPLATE=/CM/ContentDisplay.cfm&CONTENTID=30772

I was using Chapter 13 but that was for aluminum bus. It sounds like you were asking about aluminum conductor, you'll have to locate the appropriate chapter or chapters.

 

[HornTootinEE]

My post was pretty vague I guess. In general, I'd agree with Marmite. Don't limit your transformer capacity by saving a few dollars with the primary conductors. I wouldn't say you'd need to size at 125% (unless NEC applies??) but don't size below 100% of transformer rating.

 

[mesutphen]

In addition to what others have posted, think of the transformer inrush, which will be more than your FLA. Your conductor should be sized to handle your inrush current. Otherwise, you may damage your conductor the first time you energize the transformer. Also, if your conductor is undersized for the inrush current, even if it can withstand the current for that short period of time, too much cycling of this kind can cause eventual failure.

I definitely agree with Marmite. From a safety perspective, it is minimally more expensive to oversize the conductor.

I don't remember the NEC saying that conductors have to be oversized for the current, with the exception of applying correction factors. Article 310 says that conductors must be sized for the maximum current they will carry. I remember that breakers, unless they are rated as continuous current capable, must be rated at 125% of the maximum load current. Is this what you were thinking of?

 

[RLebel]

The Canadian Electrical Code, section 26-258, requires that the conductors supplying transformers shall have an ampacity rating of not less than 125% of the rated primary current of the transformer. The NEC and CEC are usually pretty close to each other, so I wouldn't be surprised if something similar was in the NEC.