Today's post will be short and sweet. The problem I'm addressing today is double tapped neutral lugs in electric panels. A 'double tapped neutral' is a slang term for saying that one of the screws on the neutral bus bar in the panelboard has more than one neutral wire feeding to it. This is a no-no, and has been for a long time.
One reason that two neutral wires can't be connected to a single terminal in a panelboard is so that the circuit can be isolated if it needs to be worked on. There is a great explanation of this at the Schneider-Electric web site.
The other reason two neutral wires can't be connected under a single lug is because the connections might come loose, which could lead to a fire. While panel manufacturers my allow for up to three grounding conductors under a single lug, they typically only allow one neutral wire. The grounding conductors aren't going to normally carry current for extended periods of time, so they have a very slim possibility of coming loose. This isn't the case with the neutral conductors.
Despite what many seem to think, this didn't become a new requirement in 2002. This has been a requirement of UL 67 for a long time, but was spelled out in the NEC in 2002.