How does a manufacturer specify the current rating of a cable they have manufactured?

Below are good websites with answers to your questions.

Please note that some manufacturers are selling Aluminum or Copper Clad Aluminum conductors in place of Copper. 

Others are selling Copper Clad Steel wires.

There are valid applications for these Copper Clad conductors, but overall they have higher resistances (lower ampacity) than do Copper wires or cables.

http://www.powerstream.com/Wire_Size.htm

http://en.wikipedia.org/wiki/American_wire_gauge

http://us.tdk-lambda.com/lp/about/university_2007-02.htm

Hi Malcolm,

I understand your confusion, but there are many parameters associated with cable selection. A great deal depends on the maximum temperature rating of the insulation on the wires/cables and the overall cable length. These parameters vary a great deal.  As you noticed, in the first link a 18AWG wire for chassis wiring (short lengths) is rated at 16A (the cable insulation temp rating in not mentioned), but for longer electric transmission lines it's only rated at 2.3A due the resistance of the cable and the associated voltage drops.  In the second, link a 18AWG wire is rated at 14A with a insulation that is rated at up to 90 degrees C.  And, in the last link it shows that an 18AWG cable (with 90C insulation) when connected from the output of a power supply to its load, should not carry more than 5A.  In this latter case, the chart is taking into consideration the "allowable voltage drop" for which the "Remote Sense" feature of the power supply can compensate for the voltage drop (e.g., 0.5V) in fairly long interconnect cables. The voltage drop factor is discussed in this last link which is a power supply application note. Hope this helps clarify the many parameters that need to be considered when deciding on which size wire or cable to use for various applications.

Best regards,

Mel

Hi Again Malcolm,

I see your point and here is the answer.  It is a mistake to use the manufacturer's "wire datasheet" as an obsolute guide for the wire's current rating.  The manuacturer's datasheet only lists the "maximum" current rating for the wire based on its maximum insulation temperature rating and the "Safety Agency" specs for which the wire was designed. I have attached a copy of the "spec details" regarding the "Safety Agency Specs" and the max. insulation temps for the Farnell wire P/Ns you mentioned previously.  But, in the end, the designer must consider the maximum "voltage drop" he can tolerate for any given length of wire, and he must adhire to any Product Category codes (e.g. Medical Devices), as well as National/Regional Electrical Codes.  It is true that all the wires you listed above have a conductor area of about 1mm sq, which is approximately equal to a 18AWG wire. Therefore, they all have an approximate resistence of about 6.31 ohms/1000 feet, at 77 degrees F.  So, a 100 foot-length of 18AWG wire would have a resistance of about 0.631 ohms.  If, for example, you needed to run 10 Amps through a 100 ft length of 18AWG wire, the voltage drop would be: 0.631ohms x 10A = 6.31V.  In this case, if you were trying to use a 12V power supply to power a 12V device that was located 100ft away, this would not work.  The wire itself would be "okay", but your 12V device would not operate, since only 5.7V would reach it!  In this example you would probably have use a 6 or 8 gauge wire to minimize the voltage drop from the power supply to your device. Here is a web link to assist with calculating voltage drops with various gauges and lengths of wires.

http://www.stealth316.com/2-wire-resistance.htm

Please remember that the "wire datasheet" only lists the maximum current the wire is rated for before reaching its maximum insulation temperature or exceeding the Safety Codes for which it was designed to meet.  These datasheets do not take into account the voltage-drop and other parameters the engineer must consider when designing the end product.  I hope this solves the "datasheet mystery" for you.

All the Best,

Mel