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LZW31-SN non-neutral wiring theory

Help me clear up my understanding of the wiring needed for the LZW31-SN dimming switch.

My understanding of a neutral wire is that it’s a return path to the power source. Therefore, many smart switches require a neutral wire because this allows the switch controller to stay on even when the light is off, because the switch controller and the light are on different paths.

My confusion is about the situation in which there is no neutral wire. How does the LZW31-SN work in this case? Does it remain on continuously even when the lights are off? And why does the lack of a neutral wire mean that a bypass should be installed at the light fixture when the draw is less than 25W? What does neutral/non-neutral at the switch have to do with power consumption at the fixture? As long as the bulb can work with 120V, what does it matter how little current it draws?

And one technical question: if there are multiple bulbs, does each one need a separate bypass?

Thanks in advance for your help!

@alcorn, @JohnRob has a great explanation or maybe it’s @Ma2J , but basically the dimmer allows the voltage needed to power up the switch through the load. For some loads, the minimal voltage that leaks by doesn’t excite the bulb enough so it won’t illuminate and thus passes through to the neutral connected at the bulb. However, some LEDs will get excited and illuminate.

You only need one bypass per switch. I have 1 by pass connected in the first under cabinet light strip I have installed with 3 lights and have 3 strips daisy chained. I put it in my first light, not sure if it matter where in line, but I wanted to send the voltage back to neutral before it hit any bulbs in case it caused the bulbs to illuminate.

Thanks for the reply: it makes sense to me now. Is this the explanation you refer to? Non-Neutral Wiring v Neutral Wiring

Based on your information, it seems that in a non-neutral setup you could try installing the switch and LED bulb without a bypass first. If that works, great, but if not, and the LEDs are illuminating erroneously, then install a bypass. Is there any risk or downside to trying that approach?

Relatedly, in order to choose between having an electrician install a bypass vs. bring a neutral wire to the switch, what should be taken into consideration? Is there likely to be a big difference in labor, materials, risk of failure, overall cost?

Here’s my opinion with some facts thrown in. :slight_smile: If you can use a neutral, by all means, do so: the dimmer will support all features in that case — any three-way setup you want, power metering, etc. — plus you won’t have to worry about what kind of load you have or if you’d need a bypass. However, if you don’t have a neutral there already, it’s likely to be much more expensive to have one run (and some electricians might wonder why you want one…) than just installing the bypass, which is easily a DIY project (and probably still much cheaper if you hire). Do keep in mind that the bypass still has its own requirement for the load (I think it brings it down from 25W to 6W, though in my case that still did not work well and I had to increase the “wattage” of the LEDs I was using — none of which would have been an issue if I had a neutral).

@alcorn - If you’re capable of installing the switch, you’re capable of installing the bypass. The cost will all depend on how much effort the electrician will have to put into it to pull another 14/2 romex. Sometimes drywall will need to be cut/repaired/painted so I’d consider that as well.

You can definitely try without having the bypass. Sometimes it works, sometimes not; so just don’t be surprised.

I have all neutrals connected, and I still required one bypass for my under cabinet lights.

Thanks for the advice everyone. I don’t have a neutral and it seems like having one brought will be expensive in our case since it will almost certainly require getting through the drywall, so I will go with a bypass.

Just to check my understanding: would a simple in-line resistor achieve the same effect as the bypass (with the downside of wasting energy when the lights are on)?

@BertABCD1234 How did you increase the wattage of your LEDs? Did you just add more or do they have some configurability?

I just meant that I replaced the bulbs. It’s a two-bulb fixture and I was using 60W equivalents, which I had to bump up to 100W equivalents to make work well here. (Do note the maximum rating on your fixture — usually different for LEDs and incandescents but usually similar in terms of watt-equivalents — and don’t exceed that. I was well within mine.)

I’m pretty sure the Aeon (and Fibaro) Bypass is just a resistor inside, so this would probably be about the same effect, though I’d probably stick with the commercial “bypasses” that are ETL listed (and have been tested with the LZW31/LZW31-SN; others have said Lutron’s Minimum Load Capacitor did not work) instead of putting a random resistor in my box if that is the alternative. :slight_smile: