You are here:

# Physics/Relation with current carrying capacity and cross section area of conductors.

Question
Hello Sir,

I have a big confusion.
As per an authorized chart, the current carrying capacity of a 1sq.mm copper conductor is 11 Amps. But the same of a 4sq.mm conductor is 24 Amps only. Not 44 Amps.
It is confusing. If we use 4 separate conductors of 1sq.mm each, we can deliver 44 Amps current. But, We can pass only 24 Amps when we use a single conductor with combined cross section area of these 4 conductors. What is the reason?

Regards
Prakash

Hello Prakash,

The resistance of the wire is directly proportional to the cross-sectional area. (Other things being equal.) But it is true that the current carrying capacity is not.

The published current carrying capability is the limit beyond which the conductor, or insulation, will melt due to I^2*R heating. As the wire heats up, the wire dissipates more of the heat produced by I^2*R heating and eventually the temperature reaches an equilibrium temperature. The rate of I^2*R heating equals the rate of dissipating heat from the wire to the environment. The amount of current that produces so much heat that the equilibrium temperature is equal to the melting point is then considered the maximum current carrying capability.

As the wire diameter increases, the wire's ability to dissipate heat decreases. That is the reason that the 4sq.mm conductor is not capable of conducting 4 times as much current as 1sq.mm wire.

I hope this helps,
Steve

Physics

Volunteer

#### Steve Johnson

##### Expertise

I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

##### Experience

I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

Education/Credentials
BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University