Ran into this today. I remember seeing a post some time ago where this was covered but I’ve searched and can’t find it, and I don’t recall what was mentioned. One of the service entrance conductors is cooper and the other is aluminum. Main breaker is 150 amps. Is there a problem with mixing them like shown?
Is
Not as far as I know, as long as the copper is sized correctly for the amperage and the aluminum is one size larger.
They are both copper, the one on the left has more antioxidant paste. As long as the installer used correct paste (for copper), nothing wrong there.
Thanks @lkage I was reading somewhere else exactly that, as long as they are both sized correctly shouldn’t be a problem just had never seen it done that way, and I’m not sure why they would do it that way either. And @srechkin im not sure if it’s not too clear from the picture but the one on the left is Aluminum and the one on the right is cooper.
How did you determine that it is aluminum and not copper (not cooper), from the pic you uploaded alone?
No, not just from the picture, the cooper conductor seems newer then the aluminum one (the panel didn’t seem original to the home so my thought was maybe when they replaced the panel they ran the new cooper conductor for some reason) the labeling was different on the two and the cooper was 1/0 while the aluminum was 2/0.
@srechkin what is leaning you towards thinking they are both cooper? From what I’ve been able to see from other posts I respect your opinion specially when it comes to electrical so just for the sake of me knowing for the future, what is your take on if they were one aluminum and one cooper? No problem as long as they are sized correctly? And why would something like that even be done?
They don’t have to match… just sized and installed correctly. From here it looked like they are both copper because of the antioxidant goop. 2/0 AL is good for 150amps Many reasons why a panel can end up with different conductors, but it’s not the norm.
Makes sense, I appreciate the help 
Is that cardboard around the conductors where they enter the panel?
Good eye !!
Yes it is ! I did point that out good catch
I am only commenting on a couple of things I experienced that may relate and limited info from Picture. I do not have an electrician’s license.
1st possibility is that the cable in question could possibly be tinned copper instead of Aluminum. I have run into this one with a home from the 50’s and had to correct a colleague. I mention it because the ground cable appears to be lighter than I would expect and the others seem to all be the same gauge.
2nd thing that seems missing is evidence of anti-oxidant compound that should be present if Copper meets Aluminum. (IMHO) Theo
Copper and aluminum should not be touching in a connector. Antioxidant paste is for use on aluminum conductors.
Antioxidant paste is not only for aluminum connections. Copper will corrode in corrosive environment and they make antioxidant for such applications:
https://www.gordonelectricsupply.com/p/Harger-Hcajc8-8Oz-Cu-Antioxidant/6095518
Tinned copper would not have Thermoplastic insulation so it is definitely aluminum. Although not required I do see anti oxidant paste on the copper and aluminum conductors. It does appear that the original aluminum conductor has been replaced with copper. There is some visible paint over spray on the surrounding conductors but not on the copper conductor.
There is a lot of overspray on the other conductor on its upper portion but what about the bottom of where they enter? what is that stuff on the copper conductor? Also, unless I’ve gone completely color blind 
, is that not a metal conduit of sort where you spotted the make-shift cardboard insulating bushing? where is the bonding jumper with a bonding bushing?
Why are you looking for a bond bushing? Is this over 250 volts?
250.92(B) eccentric knockout at service.
I don’t even see a locknut so I’m guessing that there isn’t even a connector. For a metallic service raceway the bonding can be on either side of the raceway so maybe it’s on the meter side.
