To me, there are two issues in determining the wire size required to carry
a certain current. These are the voltage drop and the temperature rise.
Short runs, of course, have less resistance, so you can get away with a
smaller wire size without excessive voltage drop. The temperature rise,
though, is pretty much determined by the I2R loss per unit length of the
wire and how well the wire can radiate or conduct the heat away. You CAN
run a higher current through a wire if the insulation is rated for a
higher temperature. Here in the US, for residential/industrial wiring, TW
insulation has a lower temperature rating than THHN, so THHN is rated to
carry more current. Another issue is the wire environment. Wire in open
air is rated to carry more current than wire in a bundle in conduit.

Harold


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