Voltage Drop Calculation 1

[luiscunha1978]

Hey!

This week I found this circuit in an industrial facility (at Europre). There is a Main Distribution Board that connects 4 Partial Boards as you can see in the figure below. In the Main Distribution Board there is an MCB (32A) dedicated to the heating of the Partial Boards. The circuit consists of a 4x2.5mm2 cable, and on each Partial Board they connect only one phase, as you can see below. How can I calculate the voltage drop across the entire circuit, to check if it is ok?

Cable characteristics:
0,6/1kV
Cable 4x2,5mm2 (cross-section)
Current carrying capacity (25 amps)three charged conductors
Impedance per unit length = 7,89 ohm/meter with cos(fi) =0,9

 

[waross]

Calculate the voltage drop from the source to the farthest panel R on both the line conductor and the neutral conductor..
Add the voltage drop from the source to the closest panel R.
The other neutral currents will almost cancel from the source to closest panel R.
The voltage drops on phase S and phase T will be less than the drop on Phase R.
The voltage drop from the double load on phase R will be great enough that errors due to phase shifts caused by the neutral currents on the other phases may be ignored in most cases.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[luiscunha1978]

Hi waross, I don't think I fuly understand you kind explation. I will try to do it. But what you said is that the R voltage dropp will be greater than the S an T phases,so it's not necessary to sum the R + S + T because they are diferent phases, becasuse they are distribute in the diferent boards. But the N voltage drop is always the same. Sorry if this is a stupid question but this is nor my academic bsckgroung. Thank you in advanced.

 

[waross]

You have three phase wires and a neutral wire.
There will be a voltage drop on each wire.
Assuming that your loads are equal;
The greatest voltage drop will be the longest wire.
In addition to the longest wire feeding the furthest load on R phase, that phase also feeds the panel with the shortest wire.

Figure the voltage drop on the phase wire from the source to the closest R panel, remembering that this will be at twice the current of the other phases. (Two loads.)
Now add the voltage drop on the r phase wire from the close R panel to the far R panel.
Add the two drops together for the drop at the far panel.
(WE are not calculating the voltage at the far panel, we are calculating the voltage drop in the wires that feed the panels.)
The neutral currents from the first three panels will cancel each other on the run from the panel to the close R panel.
Calculate the voltage drop on the neutral wire from the close panel to the far panel.
Add the three voltage drops together.
This is your total voltage drop to the far panel.
Subtract the total voltage drop from the source voltage to determine the voltage at the far panel.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[luiscunha1978]

Hi Waross,

I don't know if I do the math correctly like you pointed maybe not (I did not fully understand its indications again) but I've tried.

I found a manual on the web and I try to compute the voltage drop:

"Electrical installation handbook Volume 2 1SDC010001D0204 ABB SACE Electrical devices 4th edition"

Could you please check if the attached file (voltage drop.xlsx) is ok?

+ I've assumed that this is a single phase circuit. I'm correct?
+ I've assumed that the "THERMOSTAT STEGO KTO 011# does not consume any current. I'm correct?

Thank you in advanced,
Luis

 

[waross]

Those heaters are only 30 Watts.
They will draw about 0.12 Amps each.
The cable is rated for 25 Amps.
The voltage drop will be negligible.
Forget this and do something more important.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

Use a temperature gun and compare the temperatures. All that matters is that the heaters are functional.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[luiscunha1978]

This is an important issue for me. I spend several hours doing the spreedsheet, because I would love to understand if I'm doing the things right. This is not my academic background and I need help do understand this matter.

Thank you
Luis

 

[FacEngrPE]

If your loads were 3000W instead of the 30W shown on the drawings, doing a rigorous solution would likely be warranted. The reason is that with long runs and higher currents, the wire size must be adjusted for voltage drop.
As the drawings stand, the thing I would check is that a bolted fault at the end of the line will trip your protection in a reasonable amount of time.

 

[waross]

If you are trying to learn to calculate voltage drops:
1. Go to the student forum.
2. Pick an easier problem.
A rigorous solution of voltage drop involving distributed line to neutral loading involves shifted voltage drop phase angles on the neutral that add a layer of difficulty.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[luiscunha1978]

Ok waross, thank you for your time.

Luis