Joined: 06 June 2007
Its been a while since my college days but I've been asked a question that I'm struggling to answer and was hoping for some assistance.
A colleague is involved with writing a specification for having some new cable installed that will feed a section of 1500V DC overhead line. He's noticed that the manufacturers data shows a significant drop in current carrying capacity of a single cable if you run more than one in a duct.
Can anyone offer a technical explanation as to why this is?
Joined: 07 August 2007
The passage of current through a cable results in heat.
If the cable is not to be damaged then temperature rise must be limited.
If a number of cables are grouped together, then the heat cant so readily escape, therefore the amount of heat produced by each cable must be reduced by restricting the current to a lower figure than for a single cable.
In the case of large or complex installations, detailed calculations on heat loss are sometimes required, with corrections for ambient temperature and thermal resistivity of the surroundings.
For simpler installations, guidance may be found in the IET regulations.