power cable rating

The main relation is the capacity to carry current. More sqmm = more capacity. Now if you are looking for a precise ampacities for certain cable sizes then please download programs which are available on net. ( google it..) The capacity to carry current (safely) depends on many variables, for example conductor material, ambient temperature, cable location,no of cables in a tray..etc. good luck, Chaterpilar

what is the thumb rule for determinig the ampacities of power cables (copper conductor)

considering normal ambient conditions and what are the precise derating methods

Like everything this is an apparently simple question but with a complex answer. The wiring Regulations in your locality will define the correct procedure in your country, however the laws of physics being (more-or-less?) universal the main current-carrying factors are:

1. material (basically copper or aluminium)

2. size (sq mm, AWG or even sq inches)

3. insulation type (PVC, XLPE, mineral, paper, lead etc)

4. mounting conditions (buried, clipped, exposed etc)

5. grouping (nos. of cables run together and how close)

6. ambient temperature

et alia.

It is important to have a 'feel' for a subject before embarking on intense calculations. For feel read 'rough idea', 'rule of thumb', 'experiece' and a variety of other phrases bounced around by professionals. The perfect analogy is to ask a golfer how they know what club to use. Spreadsheets and such are excellent tools for building this 'feel', especially if you design the sheet yourself. A free - but basic - cable calc program for New Zealand conditions can be found at

http://www.generalcable.co.nz/new_zealand/download_genCALC.asp

Pump numbers into this and get your 'feel'. Be warned however - undersizing cables can be catasrophic, expensive and deadly, and not necessarily in that order. Better to build a wall at the top of the cliff, than a hospital at the bottom.

See post:

http://cr4.globalspec.com/thread/11729/POWER-AND-SIGNAL-CABLE-SEPARATION

Cable Sizing Calculations? 01/15/2007 10:37 PM

http://cr4.globalspec.com/thread/4768/Cable-Sizing-Calculations?frmtrk=CR4digest

Difference Between Control And Power Cable <http://globalspec.ip09.com/rd/9z1ze620l9jf9njk2u3h009lf6njoo1brh30h567228> (Electrical Engineering)

See also:

http://www.interfacebus.com/Aluminum_Wire_AWG_Size.html

Don't forget the amount of voltage drop your application can tolerate. Sure, the cable and it's location may tolerate the temperature rise/heating, but your application on the far end might not like the voltage that's left over.

1. Circuit design Current (amperes ) is used to calculate the size of cables(mm2)

The Fators used to determine the Size of Cables are

1, Circuit Description

2 Circuit Type

3 Circuit From

4 Circuit To

5 . Length in metres

6 Circuit Design Current

first we will callculate the current carrrying capacity of the cable and from the relevant table cable size can be found out .

Ampere rating of the power cable can be used to check the voltage drop of the cable choosen and to see whether it falls within our constraints

mm square is the cross section of conductor of the cable

the amp rating depends on the current carrying capacity of conductor section.

The required current to carry by conductor is derated for various factors e.g laying in air / under ground, no of cables , disctance between cable, temerature etc. after applying the factors required current worked out shall be greater that the inteded current to carry by condcuctor.

The cable length is required to considered for limiting allowable voltage drop.

nascon

The current rating of a conductor is a function of the power dissipated in that conductor and the ability of that conductor to get rid of the heat.

An Aluminium conductor is typically rated for a maximum conductor temperature of 90 degrees C and a copper conductor for a maximum temperature of 105 degrees C.

Where an insulation is applied to the conductor, there can be an improvement in the ability of the cable to get rid of heat, but there can also be a limitation placed on the allowable temperature rise of the conducor. For example, pvc insulation can often limit the conductor temperature rise to 85 degrees C.

The power dissipated in the conductor is the product of the resistance of the conductor and the square of the current in the conductor. The resistance of the conductor is a function of the length of the conductor, the resistivity of the conductor and the cross sectional area of the conductor. Double the cross section and you halve the resistance.

The ability of the conductor to dissipate the heat, is proportional to the surface area of the conductor. If you double the diameter of a conductor, you increase the surface area by a factor of two, and reduce the resistance by a factor of four, so the power that can be dissipated in the conductor doubles with twice the diameter.
So, if you double the diameter of the conductor, you increase the current carrying capacity by 2 x rt(2). If you double the area, then the resistance is halved and the surface area increases by rt(2).

Adding a thick insulation can increase the effective cooling area without changing the resistance, and result in an increase in current carrying capacity.
If the air circulation around the cable is restricted, the cooling capacity is reduced and the current carrying capacity is reduced. Bunching cables reduces the cooling, - lower rating. Changing the shape of the conductor changes the ratio between the cross sectional area and the surface area and changes the current carrying capacity.

While current density is often used as an indicator of current carrying capacity, the temperature rise limit dictates that the current density changes with cross section shape, and the ratio between corss sectional area and surface area.

Best regards,
Mark Empson

http://www.LMPhotonics.com | http://www.LMPForum.com