Wire a tranformer for testing purposes

Phinster

Member
Location
Roanoke, VA
Hello, I have a 75KVA 480 Delta to 208/120 Wye transformer that I've obtained strictly for testing purposes of our control panels. The maximum power that we'll be using is less than 1KVA. This transformer will be connected only temporarily for now. So, my question is, can I safely wire it to a 480 three phase breaker using only 10awg wire. Will the inrush current, with no load, be significant enough to require larger gauge wire for the primary?
Thanks in advance.....
 

Eddie702

Licensed Electrician
Location
Western Massachusetts
Occupation
Electrician
Hello, I have a 75KVA 480 Delta to 208/120 Wye transformer that I've obtained strictly for testing purposes of our control panels. The maximum power that we'll be using is less than 1KVA. This transformer will be connected only temporarily for now. So, my question is, can I safely wire it to a 480 three phase breaker using only 10awg wire. Will the inrush current, with no load, be significant enough to require larger gauge wire for the primary?
Thanks in advance.....
With #10 wire you can get 19KVA continuously. I would protect the #10 @30 amps on the primary. On the secondary I would use #6 protected at 50 amps making sure everything is grounded with a #10 minimum and the wires are protected in raceways.
 

mike_kilroy

Senior Member
Location
United States
So, my question is, can I safely wire it to a 480 three phase breaker using only 10awg wire. Will the inrush current, with no load, be significant enough to require larger gauge wire for the primary?
Thanks in advance.....
To answer your question: Yes, absolutely with no worries.

Eddie went on to suggest proper size breaker for your input to protect the no. 10 wire.
 

gar

Senior Member
101107-2224 EST



Phinster:

For the most part inrush current won't determine wire size. Inrush is a tramsient event mostly occuring in 1/2 cycle. This will produce little heating in the wire. If you have a fairly long primary run, then a smaller wire might be better. Higher resistamce, thus, lower peak current. Your load current is very small so it is not a factor in wire size.

Full load primary line current might be around 10 A for the transformer rating, but much lower for your expected load. I would use a much smaller transformer.

Inrush current and primary side wire size will determine primary breaker size. With no primary side wire resistance I might guess at a peak inrush current of 150 A. Can a 15 A breaker tolerate this. Not likely. But, if in each primary line I put an appropriate negative temperature coefficient resistor in series, then it will probably work.

See my website plots P6 and P7 at http://www.beta-a2.com/EE-photos.html .

By use of a series thermistor I could tolerate this peak inrush current. By memory I believe it was a 3 A magnetic breaker with dashpot damping.

.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Hello, I have a 75KVA 480 Delta to 208/120 Wye transformer …

can I safely wire it to a 480 three phase breaker using only 10awg wire. … (and a 30A CB)

Will the inrush current, with no load, be significant enough to require larger gauge wire for the primary?.
No, this will not work reliably.

Inrush current for a 75kva, 480V can be as high as 11 x FLA = 990A.
The Instantaneous trip for a 30A cb is ~ 270A.
GE says the max inrush will happen 1 in six energizations. They don't say anything about minimum energization.

The connected load has no bearing on the inrush current - unloaded, loaded does not matter.

That being said, I have seen one case where a 225KVA was fed with temporary power, 100A CB. When energized, the xfm oozed on line, held the 50A load just fine. We never turned it off for the duration of the temporary power requirements.

For your installation, A long #10 feeder will help. The added impedance will reduce the inrush.

So, give it a try. Connect a peak reading clamp-on ammeter so you can see what you are doing to the CB.
Don't be surprised if the CB trips. Reset and try again.... and again ….. and again. You might get one to hold.
The installation is not particularly unsafe, although after 5 or 6 times of the CB tripping on the instantaneous, I'd be wondering what I was doing to the CB contacts.

For references, do a search on "transformer inrush". It is a statistical issue. It has to do with the residual flux in the core
I used a SquareD 30A FA for a typical time current curve for thermal magnetic 30A CB. That is pretty generic.
 

gar

Senior Member
191110-2356 EST

iceworm provided a much better estimate of peak inrush than I did. Apparently I lost a zero. I can remember thinking I should comment on my peak being the actual peak vs the RMS of the peak. So 1500*0.7 is 1050. That tends to correlate with iceworm's value.

.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
You can basically eliminate inrush if you are willing to use a precharge circuit.

You make sure there is no load on the transformer, energize through a set of resistors, then use a contactor to bypass the resistors.

-Jon
 
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