Ask Helena & Leo

Question: We work in cable assemblies and had a question about Circular Mil Area (CMA) buildup. We find ourselves frequently building up our conductors when using a crimp style contact, using the Rule of 3’s (28 AWG folded in half would be 25 AWG). What is the rule when it comes to a solder style contact that is using a conductor smaller than the contact, for example, a 12 AWG contact using a 16 AWG wire? Is buildup required or does the solder fill make a good enough connection?

Answer: This is a difficult answer to provide due to not knowing all the elements or parameters involved.

Working through the numbers from Manko’s Book Solders and Soldering, first edition and using the formulas he provided and doing some research on the tensile strength of the wire in your example.

An estimated solder joint strength can be calculated as follows:

• A 12 awg connector should be able to accept a wire of 0.080” dia
  a 16 AWG wire has a diameter of 0.0508”
• Assigning a solder joint length of .25 inches, such as a solder cup.
• Going on line and finding the tensile strength of the 16 AWG copper wire, the tensile strength is between 75 and 86.55 pounds.
• Soldering the 16 awg wire in the 12 awg cup the .25” solder joint can support 104 pounds.

This implication is that the wire would break before the solder joint would fail. To answer your question the solder fill of the cup does make a good enough connection.