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Voltage Drop ( Nec)


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Hi Chaterpilar,


Selection of cable for motor is dependent on many factors. I want to explain these factors according to IEE regulations i.e.

(1) Starting current of motor and its duration.

(2) Route length

(3) Maximum temperature rising of surrounding

(4) 1 core, 2 core …….

(5) Grouping factor (single cable or multiple cables)

(6) Laying in ground, in cable tray or through duct.


These are the most common factors which de-rate the cable.

Here are the examples of above;

If starting current duration is too long (above 20sec) then you have to over size the cable so that it may not get too hot at the end of starting.

If cable is laying less than 100 meter length, then there is no problem but exceeding the length beyond 100 meters, voltage drop problem would be faced.

25-35Cº is normal temperature. But at 45Cº, you have to multiply the current rating of cable as x0.87.

50mm² single core cable’s current rating at 100 meter length is 207A and voltage drop 0.97mv/meter. And at 4 cores, it de-rates at 157A and voltage drop 0.82mv/meter.

Up to 4 cables/phase, the cable de-rates as x0.9. But exceeding this limit, you have to multiply the current rating of cable with x0.7.


Now in your case, (I am assuming 4 core single cable, ambient temperature 35Cº and laying on cable tray), we would calculate the voltage drop under 2.5% voltage drop of terminal voltage. At 380V, maximum permissible voltage drop is 9.3.


VD = VD max limit * 1000/route length * load current

0.22 = 9.3 * 1000/300 * 150

VD = 0.21mv/Amp/meter

Now we can check the cable manufacturer data sheet that which cable is suitable for our application as voltage drop must not exceed the limit 0.21. When I check, and found 240mm² suitable which VD is 0.21mv/amp/meter at 4 core. If we choose 185mm² which VD is 0.25, then

VD max limit = 0.25 * 300 * 150/1000 = 11.25

Terminal voltage = 380-11.25 = 368.75

Hope this will help you in selection of proper size of cable for your application.

"Don't assume any thing, always check/ask and clear yourself".

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The rating of the cable is dependent on the power dissipated in that length of cable and the thermal resistance of that cable to the surrounding environment.

If you can keep the cable cool, you can dissipate more power and therefore pass more current. If you have very poor cooling on the cable, then the cable rating goes down.


A single cable clamped to a cable tray, will have a higher rating due to the free air movement around the cable and this provides some cooling effect. If the cable is bunched with other cables, the cooling is greatly reduced and the rating must also reduce.


If you bury the cable, the rating change is dependant on what the cable is buried in. A cable buried in damp soil will have cooling by the damp soil, but a cable buried in very dry soil or sand, will have very poor cooling and will need to be derated as the dry soil or sand will provide very poor thermal conductivity.


You need top be very careful with "Rules of thumb"


A few years ago, a major city in New Zealand lost power for an extended period of time because they did not derate the feeder cables that were buried in dry soil. The assumption was that the original rating still applied.


Best regards,

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Hello Marke,


I am also agreed with you. I have just neglected one thing which you mentioned. I am assuming that cable will be single, armored and bury in the damp soil which depth may not beyond 01 meter. Under normal conditions, if we install the cable 4C x 185mm² in air where surrounding temperature must not increase 35c, the ampacity of cable will be 374A (according to IEE regulations). If we lay the cable direct in ground as above mentioned method, its ampacity increases at 401A


"Don't assume any thing, always check/ask and clear yourself".

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