I have been advise to convert old aluminum branch wiring with AlumiConn connectors and maybe also in the breaker box, unless the circuit breaker is AL & CO compatible. How do I determine this? Can I tell just by looking at the connectors?
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The breakers should be specifically marked as to what wire types and sizes the terminals are compatible with; they also should be aluminum wiring compatible if the house is wired with aluminum wire. But it's always good to check.
You may need to remove the breakers to read the markings on them.
You should be aware (before you buy) that Tyco "COPALUM" is the other approved repair/remediation method. The CPSC website seems to be out of order today - when it comes back up, look for "Publication 516" from them.
At the risk of stating the obvious, your neutral bus and your lugs are made of aluminum. Aluminum wires into them is going to work like chocolate frosting on a chocolate cake. If anything, putting copper wires on those should make you pause to think.
Seriously though, they're aluminum for a reason. When they change dimension due to thermal expansion differences, the aluminum is more resilient and will flex without distorting the copper. This depends on your screw-down torques being set correctly, which is why NEC 2017 now requires you to use a proper torque wrench when setting torques.
And that's why Alumiconns are also made of aluminum.
The reverse (Al wire on Cu termination) doesn't work so well, which was the source of all the trouble in the 80s.
Actually, aluminum is the best (practical) conductor on earth in almost every unit of measure. The problem is, SI units chose to focus on volume occupied as the standard unit for conductivity, and aluminum's very light weight gives it a disadvantage in that particular scale of measurement.