current carrying

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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.
--  
Cheers,
Paul Saccani
We've slightly trimmed the long signature. Click to see the full one.
Re: current carrying
FMurtz wrote:
Quoted text here. Click to load it

**  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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