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Current carrying capacity of copper

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Q. Can somebody explain to me precisely the relation between area of copper, the current it carries and the temperature rise.

In short the question would be: What is the area of cross section of copper I am to use if I were to pass 1 amp of current for x hours. Assume that I permit a temp rise over ambient of 40 °C. Also assume that the copper is 99% pure.

Hariharan Rajgopal
- Bangalore, India


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A. The answer is complicated by the fact that there is some thermodynamics involved in the solution, and unless you can specify wire resistivity, ambient conditions including insulation, humidity, etc. for heat loss, an expert mechanical engineer will not be able to do the simulation job for you, and you may still have a theoretical answer of no use. So I would simplify things this way:

1) According to wire gauge, each wire/strand is rated for a certain current limitation. Just look at the table. Somebody has done this work for us a century ago. You may extrapolate information you need from this table.

) For your specific ambient and insulation conditions, do an experiment. Get a variable current power supply and pass increasing current through the wire of interest. Attach a thermocouple to the wire and measure the temperature. An engineering college lab could help you in the case you do not have the set up.

3) A typical 0.25" (6.35 mm) dia. pure copper rod can handle up to 150 amp current without getting hot at 75 °F ambient and no insulation. See if this helps extrapolate.

Mandar Sunthankar
- Fort Collins, Colorado

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A. As far as I know, (I heard it from someone working in my company):
First you should know the current density of the conductor; let's take, for example, copper has 4.02 Amp/sq. mm. as current density. So you want to pass a 2A current thru a copper conductor then you need a conductor of area:
area=current in Amps/current density.
area= 2/4.02
area=0.5 sq.mm
They use this formula for calculating the thickness of the busbar that they use in big electrical panels.

Rajiv
- Mumbai, India

February 1, 2013

A. Copper current density allowable is 1.6 A/mm2

Swamulu Saluvadi
- Hyderabad, Andhra pradesh, India

March 5, 2013

A. The allowable current carrying capacity is actually defined as the amount of current that can be carried without melting the conductor or the insulation. Googling "current capacity of copper conductors" will immediately produce a dozen sites with the same definition and same charts.

But in many cases we don't want to put nearly that much current through the conductors. In the electroplating industry we have always used an allowable current of 1000 A / square inch for bare DC busbar, which equates to 1.55 A/mm2 -- essentially the same as Swamulu's number. This is empirical, based on the recognition that we don't want a large voltage drop across the conductors because it would interfere with the plating process, and waste a lot of energy, and a hot bare bus bar would introduce the possibility of operators getting burned.

I think Mandar's & Rajiv's numbers may be valid for some applications but are much too high for low voltage DC bus bars. Please see letter 50556, "Copper Bus Bar Sizing for DC low-voltage high-amperage" for further discussion of low voltage bus bar runs.

Regards,

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Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey

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