current carrying

I am getting befuddled, I asked google about current being constant for a given cable for differing voltages it goes on about tables and derating etc Which I can find for myself. It all started at a non electrical NG.. In regards to a question about a 20 metre cable for a 20 watt LED

I said

2.5 sq mm is plenty for one 20 w lamp at that distance. in 240v house wiring 2.5 twin and earth can be be rated at 20 amps.

Next poster.

240v being the key comment there, and one which is completely irrelevant to the question that was asked. 12V changes the landscape completely.

Then I said.

Stick to mechanics, current carrying capacity of cable is the same at different voltages and as long as you comply with ohms law you will be all right.

Then a boffin butted in and said, (at this time he was under the impression that it was 20 amps not watts)

No, it is *not* the same at different voltages, not by a long chalk.

The voltage drop for a given current at a given length *is* the same. Neglecting the issue of waste heat, this means that the voltage drops to zero at a much shorter distance with a lower voltage.

Example : 60A through 65mm^2, you are down to zero before 400m at 12V (ie 100% wasted as heat in the cable), at 220kv, you are less than 3% down at 200 km with the same cable.

the current will be higher at 12 volt but still much much less than
20 amps

2.5mm^2 is unacceptable if someone is paying you to do the job.... The loss exceeds the 3% maximum allowable loss. But it is unlikely to be hazardous.

Paul Saccani 
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** The safe current carrying capacity of a given cable depends on the temp rise during operation. Many things affect this, like the type and thickness of insulation and any nearby heat sources, the ambient temp etc.

Copper wire itself has a very high melting point, but most flexible insulation materials do not. Silicone and Teflon being notable exceptions.

.... Phil

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Phil Allison

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