I'm installing additional light bulbs in my basement. I have an older house with aluminum wiring.
Can I do this:
[new light bulbs]<--copper-->[old light bulb junction box]<--aluminum-->[main circuit]
Is it safe / does it comply to code ?
Ignoring expensive crimpers, Ideal Twister Al/Cu (purple) wirenuts are NEC compliant. (Your jurisdiction might vary.) They are not for use on aluminum to aluminum connections (which is a little baffling to me, but I digress). You can find some controversy on the net about the safety of these, so read up and decide if they're appropriate for you.
Another compliant option is the King AlumiConn, which can connect both aluminum to copper and aluminum to aluminum. These require a precise torquing of the screw connectors.
Depending on the wire going to your light, you might have to pigtail copper to aluminum in the above manner and then wirenut the new copper to the light.
I've heard people say that a squirt of no-alox in a standard wirenut is fine, but it is not code compliant in my neighborhood. (And probably nowhere.) And it should go without saying that a normal wirenut is absolutely not compliant anywhere.
Straight out of the National Electric Code 2014
Splicing wire connectors are required to be marked for the material of the conductor and for their suitability where intermixed. Splicing wire connectors, such as twist-on wire connectors, are not suitable for splicing aluminum conductors or copper-clad aluminum to copper conductors unless it is so stated and marked as such on the unit container or an information sheet supplied with the unit container. The required marking is “AL-CU (intermixed-dry locations)” where intermixing (direct contact) occurs.
Ref: 110.14 Electrical Connections (B) Splicing
I bought a house with aluminum wiring so I had to educate myself on what was so different. The only way considered safe to connect copper and aluminum is through a splice connector. Specifically, you have to connect the wires individually so they are not prone to corrosion.
The Ideal connectors (purple, for aluminum) are not considered a good fix because all they do is contain a paste that is supposed to help prevent corrosion. This can be deceptive that you've "fixed" the problem and lull you into a false sense of security
The effectiveness of “pigtailing” using twist-on connectors has been evaluated by CPSC staff. In CPSC-sponsored laboratory testing and life tests, substantial numbers of these connectors overheated severely.
What the pigtails fail to address is the other danger of aluminum/copper joining: thermal expansion. The problem here is that wires under load will heat up some. Because copper and aluminum expand at different rates, this allows your pigtails to move some. The wires can work themselves loose inside the nut, creating an arcing situation, and eventually become a fire hazard, as demonstrated by this picture (link defunct)
So why doesn't every copper-to-aluminum pigtail fail like this? Mostly it's because, at lower amps, the wires produce very little heat. As such, you can "whistle past the graveyard" and install these, hoping you never modify the circuit to where the wires can produce enough heat to work themselves loose into an arc fault.
Splicing is the only true safe solution, as it allows the wires to expand at different rates.
I went with the AlumiConn splicing product. (Copalum, the other splice for this situation, is for electricians only, as it requires special tools). They not only splice, they contain anti-oxidant jelly already added. It's relatively easy to find (a local big box and a local electrician store both carry them, although they're pricey) and can be installed as DIY as long as you get the lugs tightened to the torque specs (I bought a torque screwdriver since I had so many to replace). You can use a non-torque method if you want. Also, make sure the wires are actually connected by giving them a tug. I was able to replace several places where previous electricians (and even the previous owner) had simply used regular pigtails and crossed their fingers. So far, they have been very effective in letting me splice copper in.
You can find WAGO 273 with contact paste inside from factory, at least my nearby stores have them. Any WAGO have tinned coper core which can be safely connected to both aluminum and copper wires. Contact paste protects from moisture (which can connect wires and they will corrode) and also have some grain material to remove surface oxidation from wire.
Working with aluminum wire requires knowledge of it properties:
Be aware of that properties and you can safely
get rid of handle aluminum wires.
The aluminum issue is an interesting one.
In part because you need to not only understand the problems that occur in aluminum connections, but also how code is handled.
Problems with aluminum wire arise in Al-to-cu splices, aluminum-to-device connections and al-to-al splices probably in that order greatest-to-least.
The National Electric Code dictates that products are used in their listed capacity and only so. Further, if the product is "listed" that doesn't mean you may use it, as the "Authority Having Jurisdiction" will have the final say.
The Ideal purple wire-nuts are listed, and must be used according to the products instructions. Some of the failures arise from placing to many conductors in the unit. I'd say to hire an electrician to ensure proper installation and use, but I've seen some over-stuff these connectors, among other issues.
The alumiconn connector is the best middle-of-the road option. You would need a torque-screwdriver.
Finally, there is copalum which is by far the best process, but you have to be a licensed electrician, factory trained and have the crimp tool leased to you (They do this to prevent improper installations)
Hiring a professional: I'm seeing the aluminum wiring issue come up more and more during home-sales. Lenders and insurance companies are requiring the wire to be remediated by a Licensed Professional, and receiving a signed letter including the contractor's name, their license and other pertinent information.
I mention this because if you try to DIY this, you may get stuck hiring someone such as myself to come and verify that it's correct. If I were to receive such a request, I would have to inspect every device and it would take me nearly as long as doing the remediation.
Stuff like splicing connectors or push wire connectors are for that purpose. Both solve the problem of connecting two different metals which normally cannot be done directly. You need to apply special contact paste to aluminum wires before inserting them into connector. (Another source.) The two types somewhat differ in usage.
Splicing connectors are more expensive but can be easily used also with stranded wires or braided wires. Latter ones are cheaper and are suitable only for solid wires.