Does 208V 100A 3ph input approx. equal 480V 30A 3ph output for a step-up transformer?

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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
True, false, or somewhat true and/or false?
Multiply each side's voltage x current and compare the results.

Hint: P = E x I
(W = V x A)


We don't like to simply answer questions like this; we want you to figure it out.

We don't like to answer anything simply here. :D
 

EEEC

Member
Location
Benicia, CA, USA
The sub panel is a 100A 120/208V 4 wire. I am trying to determine how many available amps at 480V I'd have. I did some research and it said that I'd have one-third of the current available on the secondary side. But didn't know how they came up with that figure, so...

P = 208V (E) x 100A (i)
P = 20,800

20,800 = 480V (E) x X (I)
I = 20,800 (P) / 480V (E)
I = 43.333

So I'd have 43.3A?
 

oldsparky52

Senior Member
The sub panel is a 100A 120/208V 4 wire. I am trying to determine how many available amps at 480V I'd have. I did some research and it said that I'd have one-third of the current available on the secondary side. But didn't know how they came up with that figure, so...

P = 208V (E) x 100A (i)
P = 20,800

20,800 = 480V (E) x X (I)
I = 20,800 (P) / 480V (E)
I = 43.333

So I'd have 43.3A?

Just an FYI, when dealing with calculating power on a 3-phase system, the voltage is (208*1.732=360) and (480*1.732=831). Since this is a basic equation where the 1.732 is influencing both side of the equation, you come up with the same answer as if you didn't use it.

You could also take 480/208=2.30769; 100(amps at 208) / 2.30769 = 43.33 (amps at 480). If voltage goes up by a % then the the current goes down by the same % if the power is the same.

Like Larry alluded too, you have not allowed for transformer losses (don't know how important this is).
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Multiply each side's voltage x current and compare the results.

Hint: P = E x I
(W = V x A)


We don't like to simply answer questions like this; we want you to figure it out.

We don't like to answer anything simply here. :D
VA, not W
And all you need to do is multiply the current by the voltage ratio.
That gives you the 43.3A
 
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