We are in the process of wiring a house with a Homeworks system. The house is also going to have a utility interactive PV system with battery backup. They would like the lights on the ceiling fans in the bedrooms to be backed up by the the PV while keeping the actual fans on the standard utility panels. The potential problem that I see is that ceiling fan/lights usually only have one neutral for the fan and the light therefore I would have neutrals from two differnt systems tied together at a common point. I couldn't find anything in the code book that prohibits it, but it seems like a bad idea. I am wondering if I am just being paranoid or it could actually cause some problems? I am going to suggest that the fans and lights be connected to the PV system but it still has me curious.

Thanks for any thoughts.

Off hand I cannot think of a specific prohibition that says 'you cannot connect neutrals from two different branch circuits feed from two different panels'. But I am sure doing so will create code violations and cause problems with system functionality.

At a minimum, if you have an AFCI requirement, such a tied neutral would present as imbalanced return current and cause the AFCI to trip.

How do you properly size the neutral conductors? Remember that either could potentially carry current from _both_ circuits.

Is this tied neutral effectively a parallel conductor with the feeder neutral? What happens if there is a fault on the neutral of the PV system?

Don't even bother trying to figure out in what circumstances such a tied neutral would be permissible, except for the fun of finding holes in the code IMHO put the energy into getting fixtures without the shared neutrals, or convincing the customer not to go with this particular design.

-Jon

If every part of your system shares a common ground that is bonded to the neutrals of both systems you should not have any problems,think common reference point.?

Thanks Winnie. I didn't even consider the afci's. Those will certainly trip with this configuration. My other concern was the homeworks system. I have seen some weird stuff happen with neutrals and Homeworks systems before.

Code requires that all circuit conductors be run together. This is pretty hard to do with one neutral... and two panels !

Code also calls for neutrals of different systems, when run together, to be identified. While this is usually applied to systems of differing voltages, I think it would also apply here.

It may not seem necessary, but the "electric gods" are a tempermental bunch, and I find it best to keep things as simple, as separate as possible. Backfeeds and open neutrals are headaches enough when there's only one panel involved.

Well in this case Homeworks would be the origin of the neutral (you could keep them seperate there but they would still be coming together in the light) and each of the panels the origin of the CB... How would you handle tie the CB's? Lights and fan same system/circuit I'd say.....

But anyway, will this PV system be seperately derived? (invertor generated) If so, you need to have all of the neutrals be of two identifications as well due to 210.4D Maybe invest in red/white striped wire....

[This message has been edited by e57 (edited 11-09-2006).]

I have run into this problem in the past and the solution I came up with is having the fan and light on the same circuit. You need to do the following:

1. run one three conductor from each fan/light back to one PNL-8[homeworks switching panel]
2. Have the homeworks person spec a HWI-4FSQ [fan controller] for fan and a RPM-4U for the lights
3. Both units need to be next to each other, I typically put them in locations 7 and 8 in the PNL-8
4. Feed them with ONE circuit and your problem with neutral seperation is solved.
5. Mark the units as on same circuit so future work won't overload the circuit.

The homeowner will have to accept the fans on the same power system as the lights.

In my opinion you will run into 300.3(B) and or 300.4 violations.

I also do not think it is a stretch to say it is a direct violation of 250.24(A)(5) which prohibits 'grounding' a grounded conductor on the load side of the service.

By connecting the neutrals together you have in fact connected both to ground a second time.

Than as has been pointed out if the PV system is SDS which I imagine it is you have other issues.

Why not snake a second neutral down through the fan to the light?

If that fan is energy-star compliant, it should be drawing, at max, 70W or so on high and probably only 8W or so on low. The manufacturer may have more information.

Depending on how the homeowner plans to use this fan, it may have negligible impact on the PV system.