Steve and the Forum,

There appears to be some confusion in several of the posts in response
to your original question, and I believe that I can clear up some of
that confusion.

Most of the "tables" or "calculators" available today have their basis
in the old MIL STD 275 tables which plotted "current" in amps against
"rise in temperature above ambient" in degrees C for a given "thickness"
or weight of copper on a PC Board.

These tables can be found today in IPC-2221 on page 38 as Figure 6-4.

There are numerous other versions of these "tables" and also many
"calculators" available today which offer basically the same

What appears to be the basic misunderstanding in the posts in reply to
your question is that they seem to be talking about a given amount of
current through a given conductor size of a given layer thickness at a
given temperature. This is not the correct application of the charts or

The results are not to be viewed at a specific temperature, but rather
viewed as generating additional heat and "adding" a certain amount of
heat to the "ambient" or normal temperature.

That means that if the normal temperature in a given area of the PCB is
25 degrees C, and you can tolerate an additional 10 degrees C
temperature rise in the copper conductors in that area due to current
being passed thru them, then such and such a current can be passed thru
such and such a width of such and such thickness of copper in that area.

In other words, passing X amount of current through a conductor of Y
width and Z thickness will cause the conductor to rise so much in

Please remember that only the thickness of the copper counts in these
current calculations, since solder is only about 16% as conductive as

There have been a number of recent related posts to the "PCDList"
listserver forum lately, and much useful related information may be
gleened by looking at some of those archives. The "list" can be accessed
via the PCDMag site at:  


Hopefully this clears up some of the confusion.

JaMi Smith
Optical Crossing Inc.

-----Original Message-----
From: Stephen Smith [mailto:[EMAIL PROTECTED]] 
Sent: Friday, November 16, 2001 6:07 AM
To: Protel EDA Forum (E-mail)
Subject: [PEDA] Copper Calculations

Does anyone know a simple way of calculating copper track size (in, mm),
when you know the amount of copper (in, ounces), and the current flow
(in, Amps)??
I've never had to do any high current circuits before, so any help much


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