cable size & material vs current capacity

[tukang listrik]

hello there, i have a problem and need some solutions from all fellows here.

are there any formulas that can show the relationship between the area of cable, its purity of the material, and the current capacity? until now i mostly find just the table of the ampacity, but don't know how to calculate it.

[electricalexpert65]

Unfortunately, there is no short cut formulae for calculating the current rating of cables. May be you will be able to calculate the DC Current rating of cables with respect to the cross sectional area fairly linearly, as the DC resistance would vary linearly with the cross section. But, in AC the resistance is affected by SKin & Proximity Effect and also reactances play a rule. Reactances do not vary in linear proportion with the cross section. So, best is to follow the manufacturer's rating tables.

[tukang listrik]

hello electricalexpert65,

what you explained is already right. i had already worked with all formulas mentioned above. however, it takes too much time just to calculate ampacity through all of those criteria (DC resistance, ac resistance, skin effect, etc.). when we work on planning some systems, i think there is an easier way to calculate the ampacity from the cross area section vice versa so that we don't spend much time calculating and need not to carry the rating table all the time.

i would like to know what formula it is.

[erickench]

Have you ever heard of this formula:

Resistance = K x L/A

 

The length L is in feet and the area A is in circular mils. K has two different values for copper and aluminum.

Copper K = 12.9

Aluminum K = 21.2

[tukang listrik]

yes, i have. but, still, it doesn't show the relationship between all the criteria mentioned above..

[admin]

Hello Sir,

It is recommended to finalise Ampacity for Cable (Single core / Multic core) by manufacturers recommendation, hence as a designer we should consider where we are using cables i.e Ambient temperature and area (Under ground, Over hanging, inside control panel  or Control panel to field wiring).

For all these case we should consider derating factor and Factor of safety.

For E.g – if manufacturer specified as Current rating for 95 sq.mm cable @ 30 Deg C as 292 A, based on our working condition we should fix actual current capacity of this cable. Barely fix FOS as 77% – then this cable appicable for current rating of upto 224.8A.

B.Karthikeyan, 

[erickench]

What do you mean by purity of the material?

[admin]

hei there i would like to learn some basics of grade three please

[Spir Georges GHALI]

tukang listrik said:

hello there, i have a problem and need some solutions from all fellows here.

are there any formulas that can show the relationship between the area of cable, its purity of the material, and the current capacity? until now i mostly find just the table of the ampacity, but don't know how to calculate it.

---

Dear Tukang ;

 

First, the calculation methode of the nominal current for any cable, accordingly the the section & material, is very complicated, so, it's better to refer not the tables of manufacturers but to the table of ” IEC 60364-5-52 “ where you can find the nominal currents for each section accordingly to the methods of installation and the type of cable either for Cu  or  Alu. Noting that these values are refered to the standard installation conditions, so, you should know : how the cable will be isntalled, the ambiant temperature around the cable, the number of circuits, then you can calculate the Correction Fcator, by which you can calculate the current of the cable accordingly to the installation conditions.

 

By the way, there's now a new version of this standard.

 

Regards.

[Author]

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