Calculate the power loss in electrical cable

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Discussion Overview

The discussion revolves around calculating power loss in an electrical cable, specifically addressing discrepancies in calculations of power loss using given resistance values and current. The context includes considerations of AC versus DC resistance, the length of the cable, and whether the system is single-phase or three-phase.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents the system's specifications and initial calculations for power loss using the formula P=I^2*R, arriving at a result of 4902.1W.
  • Another participant questions whether the circuit is single-phase or three-phase, suggesting that this information is crucial for accurate calculations.
  • A different participant proposes that the resistance should be treated as series resistance rather than parallel, calculating a total resistance of 7.92 ohms for the full length of the cable, leading to a loss of 3466W for one direction and suggesting that the total loss in a three-phase system would be 10398W.
  • Clarifications are made regarding the nature of the circuit, confirming it is a three-phase AC system.
  • One participant emphasizes that resistance should be considered as either AC or DC, not both, and notes that in a balanced AC three-phase circuit, each conductor carries the same current.

Areas of Agreement / Disagreement

Participants express differing views on the correct approach to calculating power loss, particularly regarding the treatment of resistance in the circuit. There is no consensus on the correct method or the final power loss value.

Contextual Notes

Participants highlight the importance of distinguishing between single-phase and three-phase systems, as well as the type of resistance (AC or DC) being used in calculations. The discussion reflects uncertainty in how these factors influence the power loss calculations.

Xeno1221
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Hi

I'm trying to calculate the power loss over an electrical cable.

The information of the system is:
Maximum current of 20.92 A per unit at 34.5kV

The cable has the following properties:
DC Resistance at 25 deg C (0.1672 ohms/1000 ft)
AC Resistance at 25 deg C (0.1672 ohms/1000ft)
The length of the cable is 47370.1ft

The technical report paper I got the calculation from gives the maximum losses in the cable as 10398.8W

So far I haven't been able to get the same answer using P=I^2*R where R is the impedance of the cable R=sqrt(0.1672^2+0.1672^2)=0.2365 ohms/1000ft then (0.2365/1000)*47370.1=11.2 ohms as the impedance of the length of the cable. P=I^2*R=4902.1W

Could someone please tell me where I've gone wrong here?

Thanks in advance
 
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Xeno1221 said:
Hi

I'm trying to calculate the power loss over an electrical cable.

The information of the system is:
Maximum current of 20.92 A per unit at 34.5kV

The cable has the following properties:
DC Resistance at 25 deg C (0.1672 ohms/1000 ft)
AC Resistance at 25 deg C (0.1672 ohms/1000ft)
The length of the cable is 47370.1ft

The technical report paper I got the calculation from gives the maximum losses in the cable as 10398.8W

So far I haven't been able to get the same answer using P=I^2*R where R is the impedance of the cable R=sqrt(0.1672^2+0.1672^2)=0.2365 ohms/1000ft then (0.2365/1000)*47370.1=11.2 ohms as the impedance of the length of the cable. P=I^2*R=4902.1W

Could someone please tell me where I've gone wrong here?

Thanks in advance
Looks like you left out some important information about whether this is single phase or a 3 phase circuit. Is it an AC or DC voltage?
 
I think the problem will be that you take the forward and return resistance as parallel forward resistance. But it's series instead.

You already have the resistance, you have to multiply it with the length: 0.1672*47.370 => 7.92Ohm for full length.
For the forward line it gives 3466W loss. If the return path is similar, it'll be doubled: 6932W

The paper most likely calculates with a three phase system, where the loss will be tripled: 3*3466 = 10398W
 
Last edited:
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Apologies it's a three phase circuit with AC voltage
 
Xeno1221 said:
Apologies it's a three phase circuit with AC voltage
Ok. Resistance is either AC or DC, not both. For a balanced AC 3 phase circuit with the same conductor on each phase, each conductor of each phase carries the same current
 

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