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Voltage Drop Using Cables in Parallel

09/07/2014 9:16 AM

Dear

pls provide the voltage drop calculations for cables in SI units. To meet the voltage drop requirement how to calculate the voltage drop using 2 or more cables in parallel. specially when distance is more than 1000 meters.

for example distance 1500 meters- Current is 160Amps recomended cable is 240mm2 - voltage drop limit is 5%

one cable of 240mm2 can not meet the requirement so more than 2 cables have to be used - I assume that if voltage drop with one cable is about 12% then the voltage drop using 2 cables will be 12/2 = 6 % and using three cables in parallel voltage drop will be 12/3 = 4%

pls comment

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#1

Re: VOLTGE DROP USING CABLES IN PARALLEL

09/07/2014 9:40 AM

Did you include the skin effect in your calculations? Depending on the frequencies this length will also encounter transmission line effects.

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#2

Re: VOLTGE DROP USING CABLES IN PARALLEL

09/07/2014 9:53 AM

Use 20 cables and make your own power.

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#8
In reply to #2

Re: VOLTGE DROP USING CABLES IN PARALLEL

09/07/2014 4:10 PM

Yes but what good is negative voltage?, You'd have to run everything backwards...

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#11
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Re: VOLTGE DROP USING CABLES IN PARALLEL

09/08/2014 4:23 AM

Dear Mr.Solar EAGLE,

ENJOYABLE (OR enviable) Reply. Even a hard nut will be cracked.

Definitely by laughing, mind relaxed and I am sure, Blood Pressure will come down by 10 units, or more.

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#3

Re: VOLTGE DROP USING CABLES IN PARALLEL

09/07/2014 11:55 AM
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#4

Re: VOLTGE DROP USING CABLES IN PARALLEL

09/07/2014 12:13 PM

nice math, each drop is determined by the cables resistance and distance. it wont change when run in parallel

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#5
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Re: VOLTGE DROP USING CABLES IN PARALLEL

09/07/2014 12:39 PM

Man, if it did we would need to modify our entire methodology for distribution systems.

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#6

Re: VOLTGE DROP USING CABLES IN PARALLEL

09/07/2014 12:56 PM

Distribution, sub-transmission, transmission?

Likely makes no difference whatsoever.

At transmission, go HVDC and cuts your losses, so to speak.

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#7

Re: VOLTGE DROP USING CABLES IN PARALLEL

09/07/2014 1:31 PM

Voltage drop = current x resistance. Multiple cables share the current. 2 cables, each has half current, half voltage drop. 3 cables, each has 1/3 current, 1/3 voltage drop, etc.

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#9

Re: VOLTGE DROP USING CABLES IN PARALLEL

09/07/2014 6:51 PM

Treat them analytically the same way you'd treat parallel resistors in a circuit.

Alternatively, you can also look at it from a conductivity standpoint: if you double the cross-sectional area, you halve the voltage drop.

If, at the given cable current and length, the cross-sectional area of the conductor is such that the voltage drop is 12%, then you need at least 12%/5% more cross-sectional area, ie, a total of 576 mm2. This comes out to 2.4 ea, 240 mm2 cable cross-sections.

If your voltage-drop target is exactly 5%, then you could do it by paralleling two cables of 240 mm2 cross-section and one cable of 96 mm2 cross-section - or just use a single cable having a cross-sectional area of 576 mm2 (about 27.1 mm in diameter, sans insulation).

Btw, assuming your 12% figure for one cable is correct, then your voltage drop is 16.8 V and your applied voltage is 140 V, yes? At 5% your voltage drop will be 7 V.

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#10

Re: Voltage Drop Using Cables in Parallel

09/07/2014 11:58 PM

Go through current rating of cables by manufacturers,you will find current rating is not directly proportional to cross section due to skin effect.Check VD too from manufacturer's tables & select the cable allowing for derating if laid near each other,ambient temperature etc

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#13
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Re: Voltage Drop Using Cables in Parallel

09/08/2014 8:44 AM

Skin Effect is applicable to AC, but the OP did not say whether DC or AC, nor the frequency if AC. Even if AC, and with Skin Effect present, the voltage drop will reduce to 6% by using two cables, and 4% with three - *provided* the cables are not close together, else Proximity Effect comes into play.

I suspect the problem is academic; a homework problem? The numbers seem a bit 'too tidy' for actual engineering work...

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#14
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Re: Voltage Drop Using Cables in Parallel

09/08/2014 8:59 AM

Agreed. That's why I brought up skin effect and transmission line concerns. A student should be exposed to more theory than what is in only one textbook.

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#15
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Re: Voltage Drop Using Cables in Parallel

09/08/2014 9:30 AM

Not a perjorative to the OP, but the question is so basic that I suspect mention of SE and PE would probably do more to cloud matters at this juncture than illuminate the concept the OP is trying to grasp: identical parallel resistances. It is why I deliberately limited the scope of my answer to just this concept. Once the very Basics are well in hand, then it makes sense to add more layers of Reality. This can come later and will come later as part of the OP's coursework - if this is what it is...

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#16
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Re: Voltage Drop Using Cables in Parallel

09/08/2014 9:38 AM

Yes but the OP already appears to grasp the basics by the change in the drop percentages proffered for two and three parallel cables. Showing the OP the theory names when this approach is inadequate would hopefully be eye opening.

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#17
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Re: Voltage Drop Using Cables in Parallel

09/08/2014 9:44 AM

Well, now he/she knows of the existence of both (presuming the OP returns to this forum. Some of the initial 'answers' may have put him/her off, so maybe not...)

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#12

Re: Voltage Drop Using Cables in Parallel

09/08/2014 5:37 AM

Apply British Standard 7671. Please comment.

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#18

Re: Voltage Drop Using Cables in Parallel

09/08/2014 10:18 AM

The voltage drop across multiple cables will depend on the source impedance of the supply. For low source impedance supplies, the voltage drops can be obtained from tables. For high impedance sources, the voltage drops will be independent of the number of wires/cables.

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#20
In reply to #18

Re: Voltage Drop Using Cables in Parallel

09/08/2014 4:58 PM

Yes, the voltage drop across the cable(s) - when measured in absolute terms, ie, Volts - does depend on the source impedance, but that is not what the OP is doing here.

Rather, the OP is expressing the voltage drop as a ratio, as a percentage of applied voltage. As the voltage drop, in volts, is proportional to the applied voltage, the ratio of the two voltages remains constant, regardless of the source impedance.

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#19

Re: Voltage Drop Using Cables in Parallel

09/08/2014 12:07 PM

are you not going to take temperature into consideration?

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#21
In reply to #19

Re: Voltage Drop Using Cables in Parallel

09/08/2014 5:08 PM

I suspect this is a homework problem...

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#22

Re: Voltage Drop Using Cables in Parallel

09/11/2014 10:14 AM

Let's say the system voltage is 415 V and the frequency 50 Hz. Let's say the cable is 3*240+120 mm^2 copper conductor XLPE insulated [0.6/1 kV] not armored-as per IEC 60502-1.If we'll take maximum d.c. resistance for 20oC from IEC 60228 and multiply with 1+0.00393*(90-20) for 90oC and calculate the skin and proximity effect factors according to IEC 60287-1-1 ch. 2.1.2 and 2.1.3 we shall get the r as per IEE Regulation Table4E4B [divided by sqrt(3) of course].r=0.175 mohm/m

If we shall calculate inductive reactance of the same cable using formula:

X=2*pi*f*(0.2*ln(2*s/dcond)+K)/10^3 ohm/km using K=0.0554 for 19 strands we shall get the x=0.125 mohm/m from the same table of IEE.

Now, using simplified formula of voltage drop DV=sqrt(3)*[R*cos(fi)+X*sin(fi)] and cos(fi)=0.9

we shall get 50.18 V for 160 A and 1500 m.[approx.12.09%].

However, we can put two [or 3] cables in parallel any way without any alteration of the parameters [except ampacity of course] keeping the temperature limit. Since the cores are twisted they change their position in space continuously then no proximity effect or current unbalance will bother-if the length will be the same, the temperature close and the contact resistance negligible.

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