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As per this resource, a single core 10 AWG (5.3 mm2) copper cable is rated for up to 52 amps, while the same cross section cable of 43 cores and more is only allowed to carry up to 15 amps.

Why is that? Why do multiple cores make it more dangerous to carry the same current?

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    \$\begingroup\$ heat removal is less, for the multi-core. \$\endgroup\$ – analogsystemsrf Apr 28 at 7:12
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    \$\begingroup\$ @analogsystemsrf That would make sense if the cores were individually insulated. But they are normally just bare metal wires in contact with each other. \$\endgroup\$ – Greendrake Apr 28 at 7:22
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    \$\begingroup\$ See if this can help: Circle packing \$\endgroup\$ – Vladimir Cravero Apr 28 at 7:27
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    \$\begingroup\$ @Greendrake: we recommend that you wait a day or two before accepting so that you give the whole of humanity a chance to answer your question. It encourages other answers which may give you other insights into your question. You can unaccept and re-accept if you want. \$\endgroup\$ – Transistor Apr 28 at 12:56
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It's important to distinguish between two different concepts:

  1. Single-core vs. multi-core cables.
  2. Solid conductors vs. stranded conductors.

A single-core cable contains one electrical conductor, which may be either solid-core or stranded. The picture below shows stranded conductor. Stranded conductor is the norm for all decently-large cables, as solid conductor is very difficult to unroll off the drum and does not withstand flexing or vibration in service.

enter image description here

A multi-core cable contains multiple electrical conductors. The cores are electrically insulated from each other. The cores themselves may be solid-core or stranded. The picture below shows a multicore cable with three cores (two power core and one earth). Again, these cores are stranded.

enter image description here

As a final example, a multicore cable may have many conductors - I have personally installed multicore control cabling with 20 cores (plus earth), similar to that shown below. Typical cross-sectional area is 2.5mm² per core, consisting of 7 strands each 0.67mm diameter. This would be similar to the "43 or more cores" quoted in the table you have linked to.

enter image description here

The table you have linked to refers to ratings for single-core and multi-core cables.

enter image description here

In this case, the de-rating for multi-core cables would be because the insulated cores at the centre of the cable cannot dissipate heat as easily as the insulated cores on the outside of the cable.

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  • \$\begingroup\$ So, are you saying that air gaps in single-core stranded cables do not actually de-rate them and I can safely assume 52amps for 10AWG single-core cable of 43+ strands? \$\endgroup\$ – Greendrake Apr 28 at 20:20
  • \$\begingroup\$ @Greendrake: Follow the golden rule; refer to the manufacturer's data. The manufacturer's data will take into account any de-rating that applies due to stranding of the conductor. Also note that ratings of cables depend on the installation environment - a cable that is good for 53 amps in open air is not necessarily good for 53 amps when installed in a enclosure. \$\endgroup\$ – Li-aung Yip Apr 29 at 23:30
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The limitation of the amount of current carried by a wire is about its ability to remove heat from the current carrying areas.

In a single strand wire the wire is heated by the current flowing through it. This heat is removed through solid copper to the outside of the wire.

In a central strand of a multi strand wire the heat has to go through the contact area between the strands or the air gaps between the strands to get out of the wire.

A greater effect will apply to a bundled multi core cable. The electrical insulation between the wires will add to the thermal insulation and the ratio of heating to overall surface area will increase hence increasing the temperature.

If you look at the circle packing link suggested by Vladimir Cravero you will find that the ratio between number of cores hence heating effect and circumference of the enclosing circle hence heat dissipation has approximately the derating ratio given in your table.

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  • \$\begingroup\$ Makes perfect sense, though this does not address whether the air gaps affect heat removal so badly that amps rate goes down from 52 to 15, OR that de-rate only applies to multi-core (individually insulated strands) not multi-strand. \$\endgroup\$ – Greendrake Apr 28 at 23:23
  • \$\begingroup\$ You are quite correct. I suspect that the table refers to bundled multi-core cables. In which case the same effect applies with the electrical insulation also adding to the thermal insulation. Editing my answer to address this. \$\endgroup\$ – RoyC Apr 29 at 0:00
  • \$\begingroup\$ @Greendrake I can totally believe that lots of strands can affect heat dissipation from the center strand. Remember that house insulators are mainly using air trapped inside another material. Also, I think the heat dissipation from the center is important. At 50A, 1m of copper 10 AWG is dissipating around 10W of heat. The thing is, even though copper can withstand a high temperature in the center, at 300°C (550+°F), the copper resistance is already doubled, which in turn doubles the heat. \$\endgroup\$ – ElderBug Apr 29 at 9:39

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