Home › Electrical Engineering Forum › General Discussion › cable size & material vs current capacity
 This topic has 8 replies, 5 voices, and was last updated 11 years, 6 months ago by Spir Georges GHALI.

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2011/05/04 at 2:56 am #10473tukang listrikParticipant
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.
2011/05/04 at 3:51 pm #12052electricalexpert65ParticipantUnfortunately, 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.
2011/05/05 at 1:04 pm #12054tukang listrikParticipanthello 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.
2011/05/06 at 2:48 am #12059erickenchParticipantHave 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
2011/05/06 at 11:18 am #12061tukang listrikParticipantyes, i have. but, still, it doesn't show the relationship between all the criteria mentioned above..
2011/05/06 at 3:33 pm #12063adminKeymasterHello 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,
2011/05/07 at 8:46 pm #12074erickenchParticipantWhat do you mean by purity of the material?
2011/05/25 at 8:35 am #12097adminKeymasterhei there i would like to learn some basics of grade three please
2011/06/01 at 10:30 am #12122Spir Georges GHALIParticipanttukang 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 60364552 “ 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.

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