Looked at this panel today - single strand AL wire for a 45-amp circuit which is labelled for the “A/C”. I normally see AL wire, but it is not single strand. This a Siemens panel in a house built in 2000. I was not expecting to find this…
…and, the AL gauge looks small for a 45 amp circuit, eh, Andy?
Well, it is hard to tell from MI. But, it doesn’t look like at least 8 gauge:
I can only figure it may be AA-8000 series aluminum alloy (sometimes referred to as “new technology” aluminum wiring) as specified by industry standards such as the NEC.
However only for a 15 or 20 amp circuit, not that 45A breaker that Larry identified.
Also if it was the smaller size, then you run into that the Siemens breaker may not be designed for AL.
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Check the data plate on the AC, it will tell you what you need.
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Step #1?..
Thanks, Marcel. I missed that it was for the AC…my bad. 
Should have mentioned - data plate calls for 45… I cant remember ever seeing single strand aluminum for this before…
I have seen copper wire look like aluminum its called tinning only way to tell is to pull a wire from the breaker and look at the core I believe this is what you have here based on the year built.
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It is nice to see you on our forum again, Bill!..Enjoy! 
Since the MCA is 26 amps you need #8 Al NM cable.
Does it have to be NM type?
No but I’m guessing that the cable in the OP is NM.
Oh okay, I thought it had to be NM in general. Thanks!
Sorry for the confusion. For NM cable the conductor ampacity is based on 60° C column in T310.15(B)(16) in the NEC. That lists #10 aluminum as 25 amps, since the MCA is 26 amps the aluminum conductor would need to be a minimum of #8 (35 amps @ 60° C) with a 45 amp Max OCPD. Further I don’t believe that they ever made solid #8 aluminum in a cable so one could assume that the conductor in the photo is smaller than #8.
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Tinned copper would pre-date a house from 2000 by many years.