voltage drop calculation
voltage drop calculation
(OP)
Experts,
I'm new to my job.
How would you find the total voltage drop from a main swich yard(at 480V) to a load at 120V? The details are follows.
Conductor at 480V is 100ft(impecence= 0.000777),
then a transformer(480/120V, 3 kVA, 3.5% impedence)and
load(500 Watt) at a distance of 50 ft(impedence=0.0019)at 120V. I am assuming the transformer does not has taps.
Looking for your help.
Thank You for reading.
I'm new to my job.
How would you find the total voltage drop from a main swich yard(at 480V) to a load at 120V? The details are follows.
Conductor at 480V is 100ft(impecence= 0.000777),
then a transformer(480/120V, 3 kVA, 3.5% impedence)and
load(500 Watt) at a distance of 50 ft(impedence=0.0019)at 120V. I am assuming the transformer does not has taps.
Looking for your help.
Thank You for reading.






RE: voltage drop calculation
I was thinking of transferring the voltage drop on primary side of the transformer and the voltage drop on the 480V conductor to secondary side by the transformation ratio (120/480) And then to calculate the voltage drop on the 120V conductor, and add up total voltage drop.
Is this right?
I would appreciate any help.
Thanks
RE: voltage drop calculation
Why don't you give us at least an attempt...
RE: voltage drop calculation
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: voltage drop calculation
Current at Secondary side
Isec = 500 watts/120V = 4.17A (I've skipped the power factor)
Iprimary= (120/480)*4.17 =1.04A
Voltage drop on conductor at 480V
=I*Z*L
= 1.04*0.000777*100 (length is 100ft)
= 0.081V
For Transformer (3kVA, 3.5%Z)
At primary, Zbase = 480^2/3000 = 76.8 Ohm
Therfore, Z primary = 0.0035*76.8 = 0.27ohm
Voltage Drop=I*Z = 1.04*0.27 = 0.218V
Total Voltage Drop(VD) on Primary Side
= VD on conductor+VD on Transformer
= 0.081+0.281 = 0.362V
Now the VD refered to secondary (120V) side
= 0.362 * (120/480) = 0.091V
For VD on conductor at 120V
VD= 2*I*Z*L
= 2* 4.17*0.0019*50
= 0.79V
Total voltage drop on the conductor at 480V, Transformer and the conductor at 120V is
=0.091+0.79 = 0.881V
%VD = (0.881/120)*100 = 0.7% which meets limits set be NEA
This is what I have done.
I would appreciate anybody who would correct me, if wrong.I have been delaying on this matter so much and it is so embarrassing to ask such basic questions to my supervisor as well.
Thanks so much
RE: voltage drop calculation
Find your reactances.
Find your reactive current.
Calculate the reactive voltage drop.
Find your resistances.
Find your real current.
Calculate your in phase voltage drop.
Use Pythagoras' theorem to calculate;
Impedance.
Actual current.
Actual voltage drop.
Not good
X:R ratio??
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: voltage drop calculation
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: voltage drop calculation
RE: voltage drop calculation
Unfortunately these impedances are seldom at the same phase angle or power factor.
For a rigorous solution, you must add the impedances vectorily. A simple solution may show a greater voltage drop than actually exists. However, this is invariably a conservative solution.
Possibly the worst source of error will be the X:R ratio of the transformer which may show a calculated voltage drop of several times the actual voltage drop if impedace is used instead of regulation. A lagging power factor may lessen the error a small amount.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: voltage drop calculation
You can use the following formula to get a close approximation of the voltage drop:
VD = I·(R·cosØ + X·sinØ) where cosØ = power factor
I, R, and X are referred to the same side of the transformer.
RE: voltage drop calculation
Can you look your transformer up in the catalog and find a regulation value for it? We are missing R and X for the transformer. Other than that, use jghrist's method.
jghrist;
Thanks for the help.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: voltage drop calculation
I ask basic questions to my supervisor all the time. He knows I'm an idiot until I get some more basic real world experience under my belt.