area vs I

[PPC800]

Suppose a wire is carrying 3 phase ac current, as you increase the current, by what magnitude must the cross sectional area increase by?

Is this a linear relationship?


Thanks.

 

[geekEE]

It depends. If you want to maintain a constant voltage drop, it's a linear relationship as the resistance is inversely proportional to the cross-sectional area. If you want to comply with NEC ampacity (NFPA 70), it's not linear since the integrity of the insulation has to be taken into account as well.

Glenn

 

[davidbeach]

A wire won't be carrying 3 phase ac current. It might be carrying ac current, but not 3 phase current; 3 phase in a single conductor is called a bolted fault.

 

[7anoter4]

If what do you meant is the maximum admissible for one conductor steady state [balanced] current of three phased cable [maximum admissible or "rated" is the current- in steady state- which will heat the conductor up to maximum admissible insulation temperature-90 oC for XLPE for instance].
This value depends upon many factors as ambient temperature, ambient material -exposed to free air or concealed in conduits, buried in a certain depth in a certain ground and so on.
If two cables are in the same conditions- for small cross section of conductors- the required cross section will grow[lesser a bit] with the square of current ratio.
For instance, if for 1.5 sqr.mm permissible may be 24 A for 2.5 sqr.mm will be only 32A (32/24)^2*1.5= 2.67 sqr.mm
For instance, if for 2.5 sqr.mm permissible may be 32 A for 4 sqr.mm will be only 42A (42/32)^2*2.5= 4.3 sqr.mm