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So there are two types of 6 AWG cables:

  • PVC insulated cable with tiny copper strands inside. Official specs: 135A @ 100% max current, resistance 1.21 ohm, temperature from -40 to 85 degrees C.

  • Silicone insulated cable with tiny tin-plated copper strands inside. (Chinese) Specs: 300A max current, resistance 1.21 ohm/km, temperature from -60 to 200 degrees C.

Given the same dimensions, why does the max current differ so much? Is it because of the silicone insulation? Or are the specs wrong?

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    \$\begingroup\$ Actually, the max current specification is not usually provided by the manufacturer. If you are building a house, it is determined by the building code. A boat, by Lloyd's of London or the ABYC, etc. But the limiting factor is usually the insulation. I guess since your PVC insulation is good to 85C, it can't handle as much current. But your silicone wire is good to 200C. For your information, building code in the US only allows around 65-75 Amps for AWG 6 cable. Also, the wire itself will actually melt at around 670 Amps. Just for perspective. \$\endgroup\$
    – user57037
    Commented Mar 1, 2020 at 10:43

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The long term current specification for conductors is limited by the temperature rise, and its effect on the insulation. You would expect therefore wire insulated with a higher temperature rated insulation to be able to carry more current.

However, a key word is 'rated', what are the rated conditions? A cable will have a much higher current rating if it's to be used as a single wire in the open air, than if it's to be used with a bunch of similar wires in a closed conduit, because of the radically different cooling performance of the environment round them.

Dig into the conditions under which the two cables are rated. If you can't find those conditions spelled out anywhere, then you may assume that they've probably used a more 'favourable' environment than you would expect. Some unscrupulous suppliers may even rate wire for 'short term use', for instance 30 seconds in every 10 minutes would allow you to show a far higher current capability than continuous.

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  • \$\begingroup\$ The PVC wire is sold as welding cable. The silicone doesn't have a purpose listed, but could it be used as a 'twice as good welding cable' considering the higher current rating? \$\endgroup\$
    – Alex
    Commented Mar 1, 2020 at 10:59
  • \$\begingroup\$ @Alex I would not want my welding cable insulation to be hotter than 85C because that is hot enough to cause burns. Probably I would buy whatever was cheaper or came from a more reputable source. \$\endgroup\$
    – user57037
    Commented Mar 1, 2020 at 23:40
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As mentioned before, cable ratings are related to many factors beyond the cable dimension. To give an example, here is a text from ABB (European standards, no english version found) where cable ratings are related to specific conditions like (insulated/non insulated) in-wall and on-wall mounting, mounting in open/closed trays, distance to the next cable etc. See the graphics on page 4 ff.

Cable Loads

Another important point is the fact that some manufacturers/sellers in a big asian country are very over-optimistic with the specification of their products.

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