Paralleling Power cables calculations

overviev

is the load a motor ?

methods for motor loads are not the same as that for static loads.

is the application ug , directly mounted, in a duct, are there any other circuits parallel to the new cable/s to be installed ?

derate factors must be considered according to manufacturers charts according to above.

is the parallel section necessary, can you not use one cable to do this

the following need to be checked if parallel cables are to be employed

is there sufficient room for spacing of at least d/2 between the parallel sections,especially if you used this method on cable ladders or racks in the future and several circuits are used with large cable sizes on the tray fill.try to prevent cables from touching(d/2 is the overall external diameter of the cable to be used )

im not sure about the vds dont have manufacturers data sheets but rule of thumb by kirchhoffs voltage law, the sum of voltage drops in a parallel branch constitutes overal voltage dropand total current by Kirchhoffs current law at the point of entry into the branch equals sum of currents in each segment of the branch

step 1

estimate capacity of conductor to be selected

since you are using 3 in //

total current It=P/(3^.5)x 660volts x cos Θ

branch current In=It/3 and

using this,cross check for capacity of conductor cross section capacity from manufacturers data sheets.

step 2

use manufacturers data sheets mV/Ω/km

manipulate to and fill distance and conductor effective resistance to solve for volt drop of at recieving end

remember that the recieving end voltage should not be less than 95 % of the sending eend voltage.

hope this helps uou

the outpu voltage is found by losses.the losses are from resistance of the cable and

load.to decrease the losses we have to size the cable.

This is not my speciality, but some years ago I was testing high current power supplies such as aircraft starters and traction supplies. We used parallelled conductors on the test rigs. Under the overload and surge tests, the parallel conductors visibly repelled each other. You would need very strong fixings for the cables because the surge caused by a short circuit would create very strong magnetic repulsion which could tear cables from their mountings.

Chankley, you are right.

But in a 3-phase ac power application , to use parallel cables to increase current carrying capacity you must bundle them in groups of three : one from each phase.

If using a cable duct, these three cables will be placed in the same duct.

in this way the problem of repelling forces under fault and high loads do not arise.

If it tis a hifh voltage application, then it is eben more omportant since if you dont do that the screen current will be very high resultinmg is excessive heating of the cables

Stones,

3 runs of 750mcm for each phase is ok, but the load is at what distance?

If this load is say a 1000 metres away the 3 X 750mcm for each phase will not work

The distance matters a lot, as it drops the voltage.

A see this mistake done in construction sites a lot. I have seen workers using a cable with say 2.5 sq mm for a 2000 watts drill machine which works well within a room of say 10 metres long, but does has problems when they put a 100 metre extension to it.

To summarise, keep the length of cable in the picture.. as this is the most negelected parameter ( although it does not alter the ampacity, but alters the voltage drop).

Cheers.

Stones,

The numbers you give in the post do not calculate to anything I recognize, so I'll have to ask either for more details or let you solve the problem with a few general thoughts. Usually when doing these calculations, I would expect to be given the distance involved.

1. You usually want voltage drop in a feeder to be no more than 3% of the applied voltage, so at 600VAC, you want to lose no more than about 18V (line to line). Your stated 5.13V is well within this limit.

2. With larger conductor sizes, you must use the AC reactance of the cables instead of the DC resistance, which adds perhaps 20% to the typical tables of DC resistance for the larger cables. Your voltage drop with this added is still well within the typical design tolerances.

3. In sizing the cables you have to consider conductor temperature limits. This was implied in the post which mentioned minimum spacing between conductor sets in cable trays. If they pass through areas which are at an elevated temperature, then this needs to be taken into account, as this can decrease the allowable ampacity of the conductors. However, you can base the allowable ampacity on the highest allowable conductor temperature (example---THHN/THWN is rated for 75 degC in wet areas and 90 degC in dry ones, so if it is dry you can do the temperature derating with the 90 degC table's ampacity.)

4. Since most equipment is rated for conductor terminations at 75 degC, you should use this table for your ampacity values for the conductors. If cost dictates a smaller conductor and you size them from a higher temperature table, make sure that your termination points at each end are also rated for this higher temperature.

5. Keep in mind the general rules of paralleling conductors--keep them in sets so each of the parallel runs has one of each (ABC/ABC/ABC not AAA/BBB/CCC); for each phase being paralleled all the conductors must be the same type and length (to ensure equal division of the current among them); and conduit types must be the same.

6. Finally, you may want to consider other numbers of parallel runs, such as 4 or 5 sets. As the required conductor ampacity decreases, the individual size of the conductors decreases much faster, so the additional runs can become cost effective if the lowered materials costs are greater than the additional installation costs. (You get the same ampacity with 3-sets 1750mcm in 5", 4-sets 900mcm in 3.5", or 5-sets 600mcm in 3".)

Regards--JMM