7 Replies Latest reply: May 6, 2010 6:15 PM by mberman RSS

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

Malcolm
How does a manufacturer specify the current rating of a cable they have manufactured? Why the question? There seems to be a large difference in the current ratings of cables with a similar cross sectional area and wondered why this is (eg 10amps to 18amps for a 1mm squared) Anyone any ideas?
  • 2. Re: How does a manufacturer specify the current rating of a cable they have manufactured?
    mberman

    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

  • 3. Re: How does a manufacturer specify the current rating of a cable they have manufactured?
    Malcolm
    Hello Mel Thanks for the links, the first one is very impressive, I would recommend anyone reading this post have a look.  I still think there is a discrepancy between the various current ratings given, for instance the first link shows 18 AWG (.82 sq mm) as about 16 amps but the last link shows it as 5 amps (both copper). I am beginning to believe there is a major difference depending on country and the body in that country overseeing the regulations! Anyone else like to comment?   Anyway thanks again for the links, I will look at them in more detail. Regards Malcolm
  • 4. Re: How does a manufacturer specify the current rating of a cable they have manufactured?
    mberman

    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

  • 5. Re: How does a manufacturer specify the current rating of a cable they have manufactured?
    Malcolm
    Mel thanks for your reply, it is a very helpful post, I now fully understand the reason that a PSU manufacturer specifies a particular size conductor. I also recognise how the insulation temperature rating will affect it’s current rating. There still seems to me to be some untold reason that some parts have a higher manufacturer rating than others, let me specify three that are confusing. Farnell part number 117-8287    1mm sq    “Tri-rated switchgear” 32/0.2mm  105degC  18amp                           Farnell part number 146-5873  1mm sq    silicone      32/0.2mm  180degC    10amp                            Farnell part number 320-8837  1mmsq  Tufsil  7/0.4mm    180gegC      25amp                                                                          What a spread, from 10amps to 25amps, same temperature rating, same cross sectional area. Still confused somewhat. Thanks Mel for your comments Regards Malcolm
  • 6. Re: How does a manufacturer specify the current rating of a cable they have manufactured?
    mberman

    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

  • 7. Re: How does a manufacturer specify the current rating of a cable they have manufactured?
    Malcolm

    Hi Mel Great answer.... "Safety Agency" specs...."Safety Agency Specs"...."must adhere to any Product Category codes (e.g. Medical Devices)"....,National/Regional Electrical Codes, all must be considered. It explains why different manufacturers with similar cables state different current values.    Thanks for your answers and time Mel, Best regards

    Malcolm