From time to time I get to work in a house that has original aluminum wiring. Seems to me that back in the day, you were supposed to apply this substance called Noalax (not sure about the spelling) whenever aluminum met copper. Something about corrosion resistance. But I have seen cases of where somebody pig-tailed copper directly to some aluminum with no adverse effects. Is it a codebook thing? Is it a practical sense thing? Does it matter if it's only a 15- or 20-amp circuit?