Bad Transformer

[kalanjeya]

Re: Bad Transformer

could you explain how the 35KVA transformer would draw 83.3A. something wrong here.

Jeyakumar

 

[kalanjeya]

Re: Bad Transformer

I am sorry there is 30 KVA transformer.please clarify me adavey .

 

[ramsy]

Re: Bad Transformer

Originally posted by kalanjeya:
1.2 factor for 277/480. @ 30Kva max load is 30*1.2= 36A

Thats too simple. Are there any more transformer factors?

[ January 05, 2006, 02:20 AM: Message edited by: ramsy ]

 

[spsnyder]

Re: Bad Transformer

I was confused too on where the 1.2 came from. He must have a table with "factors" for various voltages. For 480V....1/(1.73*480) = 0.0012. He got rid of the kVA and used 30 so 0.0012 became 1.2. If you do the math using S=1.73*V*I you get the same answer.

In order to answer the 83 amp question look at the secondary. I = 30,000/(1.73*208) = 83 amps.

The "factor" for the 208V equation would be 2.8.

[ January 05, 2006, 06:02 AM: Message edited by: spsnyder ]

 

[kalanjeya]

Re: Bad Transformer

Please watch any movie from the title slide.

As adavey said
Indoor Dry type. 277/480 - 208 V 3 phase (step down). I will be making a site visit early next week to gather more info. ,it is not a 120/208V transformer.

How u got 83A Here!

Jeyakumar

 

[adavey]

Re: Bad Transformer

heres my calcs :

30 KVA transformer - stepdown
Primary: 480V 3phase
secondary: 208 3phase - 83A

30,000 / (208*1.73) = 83A for secondary

Currently protcted by OCPD of 100A 3poles

My solution - since my secondary current is over 83A w/ the heaters ON. I'am going to move the heater circuits to the 480V Panel which will handle the current draw (heaters are dual voltage. Please let me know if i'am wrong here ????????? I'am still learning.

 

[templdl]

Re: Bad Transformer

I'am still learning.

I love it. We all are still learning. At least I think we all are learning. Welcome to the club.
I think you're on the right track by identifying those heaters

 

[ramsy]

Re: Bad Transformer

Originally posted by kalanjeya:
1.2 factor for 277/480. @ 30Kva max load is 30*1.2= 36A

Originally posted by spsnyder:
look at the secondary. I = 30,000/(1.73*208) = 83
The "factor" for the 208V equation would be 2.8

These factors seem limited to line-side Y's, since they show max current for the lowest voltages in a full boat H-N. 83A x 3= 30Kva/120

Isn't the phase 208Y/120 I=30Kva/208 or 48A each x 3, or phase-to-phase 96A max for a parallel load.

For a 30Kva 480Y/277 I-phase= 21A each x 3, and phase-to-phase = 42A max for a parallel load.

I'd like to see a factor for Delta current, since its vectored w/Sqrt(3).

Maybe these factors are part of an incomplete set, and got mixed up between phase and TF connections?

 

[spsnyder]

Re: Bad Transformer

The formula S=1.73*I(line)V(line-line) holds true for delta or wye configurations. In a Wye configuration I(line) = I(phase). In a delta configuration V(line) = V(phase). Hope that helps. We're interested in the I (line) in this question.

[ January 06, 2006, 10:55 AM: Message edited by: spsnyder ]

 

[ramsy]

Re: Bad Transformer

Originally posted by spsnyder:
Hope that helps. We're interested in the I (line) in this question.

Yes, my bad. We can't keep line voltage once its phase-to-phase.