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I just noticed that you mentioned the cold.

I use a compact fluorescent on my porch light and it works just fine in the cold.

Sure it starts dim but it does warm up in no time.

Todays popular T-8 fluorescents start dim even at room temp and take a minute or two to brighten up.

Another idea is low wattage metal halide which is what would be used if this was a commercial job.

You will pay more up front but the lamps last thousands of hours longer than incandescent lamps.

Another advantage is they can be operated at 240 volts directly.

Bob,

A I read PDH's description, there are no parallel conductors as defined in 310.4. The 'white' conductor is the neutral feeding the string of lamps. The 'red' conductor is the hot leg feeding the string of lamps. The 'black' conductor is the hot conductor that feeds the 'red' conductor, but from the _far_ end of the string. Net result is that the path for current through all of the lamps is equally 2400 feet round trip, so all lamps would be equally dimmed by voltage drop.

This trick could even be extended to multi-wire circuits by using 4 conductors. One hot leg and the neutral fed at one end of the string, the other hot leg fed from the other end of the string.

Another possible use of this trick is that you end up being able to measure the voltage drop at the 'head end' of the circuit. I suppose that one could use this measured voltage to properly adjust a variable buck/boost transformer, and provide a nice fixed 120V all along the circuit. I don't know if there are any listed systems that would do this.

It is unfortunate that something similar to the 'Lightbulb Voltage Regulator' http://www.cs.indiana.edu/~willie/lvr.html is not made for mains use for incandescent lamps. I know that there are voltage regulating transformers and the like, but an incandescent lamp does not require smooth sinusoidal power. Something as simple as a common triac dimmer, but measuring the voltage and maintaining a _fixed_ output voltage, would permit a system to tolerate quite a bit of voltage drop with constant light output.

-Jon

Jon, I see that as a ring circuit.

We have many threads about ring circuits and IMO they do volatile 310.4.

Bob

I agree that a ring circuit violates 310.4, but I don't see this as a ring circuit. In a ring circuit, the supply conductors go out from the source, feed various loads, and then loop back and are electrically connected to the source a second time.

In this circuit the supply conductors go out from the source and then turn back, but are _not_ electrically connected. There is no electrically continuous loop or ring that you can trace in either the grounded or ungrounded conductors.

I've sent a diagram that shows what I think PDH's approach is, as compared to a normal circuit or a ring circuit.

-Jon

Jon, Thanks, I am not sure I understand the advantage of this method as drawn it looks like a major increase in circuit lenght and cost.

I will think on it some more.



Bob

Bob,

I believe that the advantage is to provide the _same_ voltage drop to all of the lamps, with the disadvantage of having to run an additional conductor, and having all the lamps at the _maximum_ voltage drop for the circuit.

With PDH's circuit, all of the lamps are on a circuit 2400 feet long. With the normal approach, the first lamp's current only goes a couple hundred feet, and the last lamp has 2400 feet of circuit.

I happen to think that it is an interesting and legal circuit, and addresses the issue of having the lamps at the end of the chain dimmer than the lamps at the beginning.

I still suggest a multi-wire circuit with 5 lamps per side and #8 conductors

-Jon

If you are comparing the cost of heavying up the wire to minimize the voltage drop DIFFERENCE between the first and last light, to the cost of running the extra red wire at a somewhat normal wire size, then I believe the latter might even be the more economical. Compare this with #6 wire in the normal circuit diagram, and #10 or even #12 in my alternate circuit diagram.

You might get 3% drop at the last light with heavy wire, and 12% drop on all lights with normal wire plus a third normal wire. The 12% drop might even be tolerable on its own with higher wattage bulbs. And if that is not the case, at least the drop is uniform so that other approaches to correct the voltage will work evenly.

It's also possible to drive my alternate circuit at a higher voltage, as others have suggested for the normal circuit, and get the advantages of both (then all the mini transformers would have primaries connected between white and red instead of the lights directly). The circuit principle would work even if fed line-to-line 240 volts (just mark the white wire blue everywhere).

Voltage drop is not some evil daemon that has to be excised everywhere just because it exists. But it is inconvenient and we are justified in removing it in a lot of places. Perhaps the most common problem people notice is lights dimming at times when heavy loads start (and getting brighter when they stop). But that won't be the case in this situation unless someone puts some big intermittent load on the end of the string of lights. A less common problem is just uneven light intensity. That could be a problem in this situation, and my alternate circuit corrects that.

I just don't see the need (in this case) to make sure all the lamps operate at the same intensity.

We end up with all lights dim and a lot of 'wasted' conductor. $$

The lamps will be to far apart to compare one from the other.

I suggest the multiwire circuit and point to point VD calcs.

PDH brought up another issue of lamps dimming when other loads are applied.

In my own house I installed a circuit for a 12,000 BTU AC unit, it could have been wired with 14 AWG on a 15 amp circuit.

Being the overkill electrician I used 10 AWG on a 20 amp breaker with a 20 amp duplex.

When the compressor starts it dims the lights momentarily.

I probably should have used 14 AWG and the start up current seen at the panel would have been less although the compressors start up time would be longer.

Bob

[This message has been edited by iwire (edited 04-09-2006).]

I don't know if you ran your PVC yet, but would your answers change if you ran the feed pipe to light 5 first. Then from post 5, you went to post 8 and 3. Then from 8, you went to 9 and 7. From 7 to 6, and 9 to 10. Sane with the first 5 posts...

More Pvc, but less cost on wire? NO???


Dnk...

pauluk:

I'm neither a code expert, nor an electrician (I do specify electrical needs in data centers I design, so I have reason to be up to date on the code), but my interpretation summary goes like this:

210.6(A)(1) does not apply because it is not inside the dwelling unit.

210.6(B)(1) gives permission for circuits not exceeding 120 volts between conductors, but does not address higher voltages.

210.6(C)(2) would allow ordinary Edison base lights to be powered from a transformer 120 volt secondary, when the primary power does not exceed 277 volts to ground, and could be as much as 554 volts line to line.

210.6(C)(3) and 210.6(C)(4) would permit higher voltages on other types of incandescent luminaires designed for such voltages. This could be up to 277 volts line to ground on screw bases (see below) and up to 554 volts line to line on other kinds (like bi-pin).

200.10(C) would apply to any screw base prohibiting a line-to-line connection (hence maximum of 277 volts on the mogul base).

Of course, all other rules must always be considered where they apply, such as 410.3 and 410.18(A).

In my opinion (I can't find a specific rule on this, yet), these voltages should be considered open circuit voltages. So driving a 120 volt Edison base light at the end of a circuit with a lot of voltage drop using something higher like 144 volts from a buck-boost transformer may violate the spirit and/or letter of the rules. The Japanese 100 volt bulbs could come in handy in some of these cases.