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Can I twist copper on legacy aluminium wire using regular wire nuts or do I need to procure aluminium wire? The wires coming into my switch box are too short and I would need to extend them.

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  • Ugh. Not really sure what you should do -- installing Alumiconns or any other AlxCu lug is going to be a pain, and the COPALUM crimps require a specially trained/equipped electrician to install – ThreePhaseEel Aug 13 '17 at 22:16
  • Is putting in another box to house the splices not an option? – ThreePhaseEel Aug 13 '17 at 22:39
  • Did you put in a new load center and cut off the wires outside the old box? Why are the wires too short in the new box? – Jim Stewart Aug 14 '17 at 0:50
  • Never splice aluminum to copper with ordinary splices. Only use Alumiconns which are specifically made for this purpose. The purple wirenuts are legal, but don't work. Also consider an AFCI circuit breaker on the circuit, to avoid the worst problems with aluminum wire on copper terminations. – Harper Aug 14 '17 at 2:56
  • @ThreePhaseEel -- no -- it would involve breaking a plaster wall. I need to install new switches in a box that is close to 100 years old and has aluminum armored cable coming in. – amphibient Aug 14 '17 at 14:17
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Jumbo Pigtails are the answer here

Ordinarily, I'd recommend using AlumiConn lugs torqued to spec for a copper to aluminum pigtail job like yours; however, since your wires are already too short as-is, installing any sort of lug is going to be a pain in the arse.

So, your best bet is to install another (a single gang old work box should suffice, but run the box fill numbers to be sure) box "upcable" from where the culprit cable comes into the box, pull the old cable out of the old box, install it into the new box, strip back the jacket a bit, and terminate the ends of the old cables into AlumiConns. You can then use some regular copper NM as a "jumbo pigtail" from the box with the AlumiConns in it to the existing switch box. Put a blank cover on the new box, and you're set.

  • The 2-port AlumiConns would be what you would use to make this sort of splice to both make a good connection and to satisfy the code for up to 10 AWG wire. Absolutely do NOT use the UL certified Ideal purple wire nuts--they are expensive, they are prone to failure, and they occupy a large volume. If you don't care about the code and want something that will make a good connection, and is cheap, then I think you can use a good quality wire nut (of the right size) like the 3M Scotchlok or the new 3M version, and use a good quality antioxidant like Burndy Pentrox, used properly. – Jim Stewart Aug 13 '17 at 23:38
  • @JimStewart -- actually, if you can't get AlumiConns, what you want to do is use a Al9Cu rated lug (even though the smallest AlxCu lugs around are 14-4 range, which makes them far chunkier than an AlumiConn) – ThreePhaseEel Aug 13 '17 at 23:43
  • The aluminum conductors of 30 A, 40 A, and 50 A circuits will not fit in an AlumiConn connector. For connecting these to copper wire you will have to use wire nuts (grey or blue with antioxidant compound such as Penetrox which may not be approved by code) or the larger set screw splice connectors which are approved by code for aluminum and I think come preloaded with antioxidant. – Jim Stewart Aug 13 '17 at 23:44
  • @JimStewart yeah -- the set screw connectors are what I'm talking about when I say "AlxCu lug" -- there's an off chance I'll be running some brand new aluminum in the future depending on what the inspectors turn up... – ThreePhaseEel Aug 13 '17 at 23:47
  • If you are running brand new aluminum, will this be for branch circuits to wall receptacles and will this be 10 AWG or even 12 AWG? If so, what receptacles and switches are approved for this? – Jim Stewart Aug 13 '17 at 23:49

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