Cable length estimation 1

[calabala]

Hi All,
Is there any method to calculate or estimate cable length in a building based on the panel amps or transformers they connect? For example, I do assume cable length inside a panel are about 20-30 ft.

 

[dpc]

No.

 

[7anoter4]

Hi calabala
The dpc answer is very laconic and very correct.
But for desperate situation [ as my self I was sometime] and only for information I'll try to sketch something:
The LV cable crossection is selected on basis of 2 main criteria:
Cable heat
cable voltage drop
The MV cable crossection is selected on basis of 3 main criteria:
Short circuit thermal stability
Cable heat
cable voltage drop
For LV[low voltage] cables:
If we should neglect other criteria and we should consider only the voltage drop We could appreciate the length of the cable.
cable length=DU%/100*Uvolt/sqrt(3)/Irated/Z
We have to know the permitted voltage drop[DU%] , the cable impedance [Z=SQR(R^2+XL^2] ohm/ft and the cable Irated[and Uvolt of course!]
R the conductor resistance[depends on material-copper or aluminum and maximum temperature of the insulation material]
R depends also upon current frequency for AC [cycle/s=Hz] [in order to appreciate skin effect and proximity effect]
XL is the inductive reactance of the cable.
DU% depends on type and size of consumer.
If the consumer is a motor then DU% =5-20% [for smaller motor the bigger DU%]
Irated also depends on type and size of consumer.
For example for a motor supplied directly from the LV bus bar the start current may be
6-8 time the rated motor current[FLA]
If the motor is VFD controlled then 1-1.5*Irated may be the start current.
For good Z appreciation see:

http://www.okonite.com/engineering/index.html

For DU% at motor start see EPRI-Pwr.pl.ref.series vol4 pp4-59 or IEC34-12
An extract from IEC34-12 see table
Upto [hp]: 12.5
DU%=30.00%
Upto[hp]: 50
DU%=25.00%
Upto[hp]: 75
DU%=19.70%
Upto[hp]: 200
DU%=16.33%
Upper=9.63%
For DU% of other consumer from my experience :
LV distribution panel [MCC or LV Switchgear] Istart =1-3 depends if the consumer supplied from this panel are VFD controlled or direct supplied.
LV distribution panel [MCC or LV Switchgear] DU%=10% .
For MV cable there is no possibility to appreciate the length as the short circuit current depends very slightly on cable length.
By the way, in my opinion the length of cable inside a panel may be 12-15 feet[see the attachment]
Best Regards

 

[calabala]

Hi 7anoter4,
Thanks for your valuable and detailed information.
BTW, cable resistance (R) should be then measured, also voltage drop too (in case for example we have a cable which is connected to a transformer).
This means that still cable length estimation is really hard. Am I right or wrong?

 

[dpc]

It would be much easier to look at a floor plan and estimate based on that.

 

[burnt2x]

And besides, figures arrived at from the plans usually end up short! That's why we call them "estimates".

 

[7anoter4]

Hi calabala
I am very sorry but if your transformer is supplied on Medium Voltage[6-15 kV] no possibility to appreciate the cable length as the cable cross section is calculated merely according with short circuit current and cable length is not involved in this calculation. The method suggested by dpc is only possible, except the R measurement [if it is possible by means of a DC source]
The average temperature should be measured in order to correct the data.
R=ro*length/cross section*(1+k*(Tmeasured-To))
Length=Rmeasured/ro/(1+k*(Tmeasured-To))*cross section.
[If the measured loop is 2 cores of the cable this result shall be divided by 2, of course]
K=is the thermal resistance factor
To is the temperature at which ro [specific resistance] was measured.
This method has also a lot of errors as cross section [=/- 10%], measurement itself [the bridge error], average temperature .So the accuracy of this method is not more beyond the measurement on the lay-out drawing.
Best Regards

 

[jghrist]

I like dpc's original answer. The idea of estimating how much cable it takes to get from a panel to a motor by using voltage drops instead of knowing the distance between the panel and the motor is batty.

 

[7anoter4]

Batman returns.
As I said that is not a correct way to know the length of a cable.
But there are a lot of situations you have to sense the order of this length.
The correct way is to extract the length from the Cable Tabulation. But in the Cable Tabulation you have the designed length and the actual length declared by Contactor. The Contractor tends to exaggerate .You are the Costumer and you have to pay it. You don't have the distance from manhole to manhole and the entire length may be 3-4 time the linear distance measured on the layout drawing from one end to the other end.
Let say, another case, you have to order power cables for a retrofitting. The old Power Station was designed 40 years ago and only the load and the cable cross section is available.
Another case: you are the cable designer. The general design is in progress and what you have is only the schematics or Master Diagrams. The schematic diagram designer [you don't know how] appreciated the length of the cables and stated the cross section. There are hundreds of cables and you have to order cables, cable trays, ducts and appurtenances in a short time. How can you proceed? I did so, and never failed up today.All right, is a little expensive, but was just in time and for a Power Station is more important.
Best Regards