Cable Sizing

You have three phases to supply - that means you need three (mutually insulated) conductors - at least ie if you don't want to carry the Neutral.

Each of these phases must be rated for the required ampere - 204 for you (assuming your calculation is OK)

It is leftt to you that whether you want to carry 3 (or 4 ) single core cables of 204A each or one three/four core cables, each of the cores rated to that ampere.

If you are confused, usual practice- unless the company deliberately tries to mislead- is that the current rating of the cable is for each core (ie phase) and not the added up.

_{Anyway (not my answer ) PWS- respond.}

You calculate the rated current .

Then you have to calculate the voltage drop....to calculate the voltage drop it is necessary to estimate the length of the cable or assume it 1 KM

once you calculate the voltage drop....then go for IEC standard tables from where you can find the size of the cable. since you have 3PH you require 3C/4C(with N)

In the table you can see whether the calculated voltage drop within the voltage deop mentioned in the table and check out relevant size from the table

hope this works out

check here http://cr4.globalspec.com/search/sitesearch?do=show&sort=textmatchrank&srch=cable%20rating&srchobjs=t%2Cc&fs=12&order=asc

British Standard 7671.

Dont be confused.U can use a 3 core cable of appropriate current rating.U can also use a single core cable of appropriate rating as long as the cable current carrying capacity can serve the motor.But what about yr earth cable for earthing yr motor,and yr neutral cable to supply yr motor control components coils(eg contactor)? Maybe u hv neutral/earth already in yr panel or mcc.But then u still need extra cable to earth yr motor from the panel or mcc.U may need 4 core or run a seperate earth cable.

However,i dont seem to agree with yr motor current calculation based on the given data or is that yr cable current carrying capacity calculation?.What is O.8 and O.85 ? Are they power factor and what? Well If i must use use O.8 as pf,then yr motor current would be 173.9Amps. If i use O.85 as pf,then motor current would be 163.7Amps.

Well check yr table of values for the appropriate cable sizing or calculate for it and if you hv already done that,pls go for a 3 core cable rather than single core of 3 runs.It makes a neat installation and more cable protection from external contacts apart from other advantages.

Note that if yr motor full load current is 204Amps for example,each 3 core cable must be able to handle the 204Amps,and they should be of same size except for neutral which could be halfed in some cases.

Patrick Whowha

I agree, very nice information.

Its more of an installation problem than one of engineering. Each installation was created to serve a purpose and if you have an option, weight the following.

How much will the material cost?

How much will the labor cost?

Which one is easier to install?

A single motor must be calculated at 125% of rated amperes(including service factor).

All three phase must carry a ground wire.3 Phase Y must carry a neutral and ground wire,even if the neutral is not used at the motor.

Per NEC.

The current for multicore is per conductor.

Single conductors are usually cheaper

You can use a conductor rated for higher amps, but not lower.

Each core of 3 or cores cable... say if 3C x 120mm2 = 204A then each rated the same amperage. Note that if this cable rating is given by the cable manufacturer's then is the maximum current carrying capacity of the said cable. To determine the approriate size of power cable/s it is important consider the voltage drop within the line at given distance or cable length, then after getting the voltage drop you can decide whether you need to reduce the size of the cable or increase it into the next higher commercial size. If the cable is directly laid on the ground or direct burial, it needs to derate the ampacity of a selected cable size at least 83% of its ampacity rating.

For Voltage drop NEC allows at maximum of 5% voltage drop of supply/nominal voltage from the power source to the farthest point of the circuit. Means that max. of 5% voltage regulation from receiving end(Er) up to Sending end(Es).

Voltage Drop Formula:

Single Phase : Vd = 2 x L x K x I / 1000 or CM(circular mil area of conductor)

Three Phase : Vd = 1.73 x L x K x I /1000 orCM(circular mil area of conductor)

Where L = Lenght of conductor in feet.

I = Current inconducor or connected load in ampere.

K = value changes with temperature. See NEC Code chapter 9, Table 8.