A multifamily home four stories high has one 40W fluorescent fixture on each landing of a stairwell. Three- and four-way switches at each landing can turn all the fixtures off or on. The switches are fed from the basement, and the grounded conductor is carried with the travelers all the way to the top floor where the switched conductor ties into the lights. The lights are daisy-chained together all the way back down to the basement where the circuit dead-ends.
For security reasons, the live-in landlord wanted the provision to the keep the lights illuminated regardless of the switch positions in the stairwell. So a second feed (from a switch in the landlords apartment) was connected to the dead-end of the daisy chained lights. This allows the lights to be supplied with power regardless of the position of the three- and four-way switches.
Each of the two feeds originate from the same junction box on the same circuit. The grounded conductors from each feed are tied together, essentially making one big loop.
The circuit that you describe is not in and of itself a code violation. Consider having two single pole switches in parallel, each capable of feeding a single lamp. The lamp is on if either or both switches are on. This is totally fine. What you describe is simply a multi-way switch arrangement in parallel with a single pole switch.
Creating the big loop violates the general principal that supply and return current should follow roughly the same path, however this isn't a code violation unless you are using metal raceway and boxes. I'm pretty certain that this would not be a problem with NM, though I would call it ugly because of the current loop.
You could fix this by converting the multi-way assembly into a switch loop, moving the lamp string dead end to the top floor, and then tying everything together (the two parallel switch loops, the supply, and the lamp string) on the bottom floor.
Re: Back Fed Lights#87768 04/15/0401:10 AM04/15/0401:10 AM
Re-reading 310.4, I believe that you are correct, but believe that violating 310.4 in this case would result in a safe installation. It seems to be 'common sense' (which is often wrong!!) that it should be okay to have parallel conductors where each _individual_ conductor could safely carry the entire load. In fact Exception 2 of 310.4 points to this very condition, though it is not applicable because it is for control instruments.
Additionally, one could argue that while these conductors are _electrically_ in parallel, meaning that some current will flow in each path, the fact that these conductors follow different paths through different components (different switches, etc) that these are in fact _not_ paralleled conductors. I agree that this is a _very_ weak argument
IMHO there should be an exception for 'switched branch circuit conductors in parallel where the _individual_ conductors are capable of supplying the load, and properly protected by the branch circuit OCPD.'
The normal rules about paralleling large conductors are there to make certain that the load is properly balanced between them, and that any variance in the balance will not damage one of the conductors.
If the conductors were arranged such that current in each of the parallel paths was restricted to the individual cable, to get rid of the current loops, then I believe that the installation would be safe, but as Bob points out the parallel conductor rule makes it a code violation.
An alternative approach would be to wire a single pole switch loop to short the two traveller terminals of one of the three way switches together. This would also result in a 'parallel path' situation, but it is now limited to the current flow path _inside the switch_ being in parallel with the entire switch loop.
Re: Back Fed Lights#87771 04/15/0408:05 AM04/15/0408:05 AM
1) The alternative connection that I described would not result in _any_ parallel path current flow. In the case that the three-way switches were 'on' then the 'shunting switch' would result in the other traveller leg being energized, but no current flow through this alternate path. In the case of the three-way switches being 'off', the current would flow through the shunting loop and one of the traveller legs, but again not the other.
2) The first paragraph of 310.4 ends with '....shall be permitted to be connected in parallel (electrically joined at both ends to form a single conductor).' As I now read 310.4, it does not seem to be a general prohibition against parallel paths for the flow of current, but instead a set of requirements for treating multiple separate conductors as a single conductor for the purpose of determining ampacity and OCPD rating. If you do not meet 310.4, then you can still have parallel paths, but each path individually must comply with code.
Re: Back Fed Lights#87772 04/15/0409:06 AM04/15/0409:06 AM
Is the stairwell done with a metallic wiring method?
If non-metallic, then 300.3(B)(3) would allow the use of only the hot in the feed to the landlord's master switch, and the use of only the switched leg in the cable between the switch and the end luminaire.
Re: Back Fed Lights#87773 04/15/0409:58 AM04/15/0409:58 AM
Attach a hot to the black screw and attach the switchloop to the red screw. Attach the black from the cable to the fixture to the white screw. Neutral and grounds can pass straight though the box unmolested with no looping or parallel conducters.
SD It is best for a leader to be both feared and loved. But since this usually cannot be done, it is safer to be feared.
Re: Back Fed Lights#87774 04/15/0402:46 PM04/15/0402:46 PM
I could not find a specific code reference but I think this is a possibly dangerous install. For now the circuits are the same phase, but for future trouble shooting or renovation work, no one can insure these two circuits will remain on the same phase. if the landlords hot leg is changed and becomes a different phase than the one feeding the lights you will have a 208v or 240v (depending on the service) short at the landlords switch when the lights are on and not knowing he turns them on. this has the making of a dead short across two phases. my solution, and i have used them before is a dual element fixture. one element controlled by tenant space, one by landlord. they work great for energy conservation, but would work in this application also.