Solid Core Aluminum Wiring—Single source to A/C Unit

Brian - did you say Rabbit Hole? :slight_smile: I’m all the way in!

Here’s a chart showing the coefficient of thermal expansion of various metals (in laymen’s terms that means how much it expands when it gets hot - I remembered just enough from Physics class to find this):

Aluminum rates at 21-24, Copper is 16-16.7 (I’m not sure what the units are but it’s probably not important for our purposes). Basically, aluminum moves a lot more than copper. Hence, its problems with wiring since it will work its way loose at connection points as it heats/cools.

What I’m trying to wrap my head around is if/why larger gauge wires aren’t as affected? I’m thinking it has something to do with the way cross-sectional area is calculated and the amount of material inside the circle. A loose analogy I would give is flow rates through pipes. The squaring in the formula makes the area get exponentially larger as the radius gets larger and I’m thinking (maybe?) that keeps the outside of the conductor from moving as much (%-wise) as the aluminum conductor gets larger.

My sister teaches college Physics so this is right up her alley… I’ll try to remember to ask her next time I talk to her if no one here knows for sure if my suspicion is accurate.


Hmmm… So heat is created by resistance. Larger the wire, less resistance, less heat, less expansion. Cross section inversely effects resistance directly. Aluminum does not conduct as well, so larger gauge needed. Au Branch wiring is likely smaller out of practicality, therefore will naturally heat up causing expansion and contraction. That’s my humbled opinion.


That looks like SE cable.

Bottom line…aluminum wire is allowed, whether braided or solid. (Two years ago I inspected a new construction that was all aluminum). Because there is a stigma with aluminum wiring, we (and the InterNACHI SoP requires it) should note the presence of aluminum wire, not as a defect, but just an observation.
Aluminum wiring noted for 120v circuits Comments *
*Solid aluminum wiring has special installation instructions.
In my boiler plate disclaimer section, I note that stranded aluminum is allowed for 240v circuits.

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I am agreeing with Matt. If this is an allowable exception to the “no solid conductor aluminum branch circuit wiring” rule, then we should ask InterNACHI to clarify. Anyone know the best way to make that happen?

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Yes, it looks like a SER and the 2 phase + the bare neutral/ground is likely used for an exterior A/C condenser. Comparably, using aluminum SER over copper NM to the disconnect for the exterior A/C condenser is much cheaper and probably why many electricians are wiring it with stranded aluminum SER today.

See the price comparison below for range wiring:

With the material shortage, supply chain issues, and everything costing more today. It’s no surprising contractors are turning to aluminum more.