occupancy switch says neutral not needed

[RustyShackleford]

This Lutron Meastro occupancy-sensing switch boasts that no neutral is required. In addition to the line and load wires, the switch has a bare wire and a green one, and the instructions say to connect both to the box's ground.

http://pdf.lowes.com/installationguides/027557983136_install.pdf

This seems most odd to me. I thought the whole point of 404.2C (requiring a neutral in every switch box, with some exceptions), is that when "smart" devices such as this are installed, that no currents are introduced into the EGC system. Is the operating current of the logic in the switch so low that it's been deemed acceptable to have such currents flowing through EGC ?

Odder still is that they don't even suggest to use a neutral if there happens to be one in the switch box.

 

[LarryFine]

Is the operating current of the logic in the switch so low that it's been deemed acceptable to have such currents flowing through EGC ?

Certainly been deemed acceptable by the manufacturer; not so sure about the NEC.

 

[kwired]

Can you tell us if there is continuity between that green and bare pigtail?

I'd bet the green is actually the "neutral" for the smart device, and it won't work if that lead is not connected to something grounded.

I have heard there are some devices similar to what you describe but you are supposed to connect the insulated lead to the neutral if present, otherwise EGC is acceptable to the instructions. Can't say I have seen them though.

 

[Jraef]

You can make a capacitive power source without the need for a connection to neutral, so long as your power needs are just a few milliwatts. That's likely what they are doing here. The load is in series with a cap which doesn't really affect it, but the cap creates a small reactance, that reactance then is exploited as a pseudo voltage drop to power up the micro-electronics. You can't get much out of that without causing more problems, but it's often enough for very low power circuits like timers, sensors, etc.

 

[RustyShackleford]

Certainly been deemed acceptable by the manufacturer; not so sure about the NEC.

Lutron is a pretty major manufacturer. Can't quite feature them suggesting actions that violate NEC. I guess putting this device into older construction, that is not compliant with current code, would not conflict (or would it ?). But not specifying to connect to neutral in newer work (compliant with current code), would seem awfully odd.

Can you tell us if there is continuity between that green and bare pigtail?

Will check when I can. I doubt it. Green goes to PCB inside, whereas bare goes to the device yoke.

I'd bet the green is actually the "neutral" for the smart device, and it won't work if that lead is not connected to something grounded.

I'd bet you're correct.

 

[kwired]

You can make a capacitive power source without the need for a connection to neutral, so long as your power needs are just a few milliwatts. That's likely what they are doing here. The load is in series with a cap which doesn't really affect it, but the cap creates a small reactance, that reactance then is exploited as a pseudo voltage drop to power up the micro-electronics. You can't get much out of that without causing more problems, but it's often enough for very low power circuits like timers, sensors, etc.

But NEC now thinks we need a neutral conductor to supply a proper return path.

 

[RustyShackleford]

You can make a capacitive power source without the need for a connection to neutral, so long as your power needs are just a few milliwatts. That's likely what they are doing here. The load is in series with a cap which doesn't really affect it, but the cap creates a small reactance, that reactance then is exploited as a pseudo voltage drop to power up the micro-electronics.

The cap will eventually get discharged and a new potential cannot be developed without some current flowing. Unless somehow it's poaching current from the hot line-to-load connection somehow.

 

[kwired]

The cap will eventually get discharged and a new potential cannot be developed without some current flowing. Unless somehow it's poaching current from the hot line-to-load connection somehow.

In an AC circuit current doesn't stop flowing, it reverses every half cycle. DC circuit the current stops flowing once the capacitor is charged.

 

[RustyShackleford]

But NEC now thinks we need a neutral conductor to supply a proper return path.

Yeah, that's the rationale behind 404.2(C) I believe. So if somehow Lutron has developed a smart switch that does not use any current for the logic - which I believe violates the laws of physics - then maybe that NEC update shouldn't have been approved.

I believe it's more likely that Lutron developed logic that uses so little current that they convinced UL to approve it.

 

[kwired]

Yeah, that's the rationale behind 404.2(C) I believe. So if somehow Lutron has developed a smart switch that does not use any current for the logic - which I believe violates the laws of physics - then maybe that NEC update shouldn't have been approved.

I believe it's more likely that Lutron developed logic that uses so little current that they convinced UL to approve it.

I think (grounded) current in such devices was considered fairly negligible before the code change, but then there started to be more of them in use and they maybe were more concerned about the additive effects.

 

[Electromatic]

Though 404.2(C) says we have to provide a neutral to the switch location, it doesn't say we have to use it.

 

[iceworm]

This Lutron Meastro occupancy-sensing switch boasts that no neutral is required. ...

That suggests Lutron is making this device for the pre NEC 2011 market. Since 2011 on should have a neutral. I mean who on here would not have followed the code?

Or
Jraef's response is really good. A cap can definitely give a few volts drop with no appreciable heat loss. The OFF position would have to interrupt any bleed current. There has been sufficient micro-energy-magic developed in the last 7 years to make this feasible.

Just curious, does the Lutron have a listing mark?

 

[JFletcher]

If the instructions say to connect both wires to ground, you have no choice. 110.3 B

 

[K8MHZ]

I looked at the instructions and I didn't see any sort of listing mark or logo.

 

[RustyShackleford]

I posted this at stackexchange and got some interesting answers. I think the longest answer "There is more than one way to skin this cat" nailed it.

https://electronics.stackexchange.c...this-occupancy-sensor-switch-not-need-neutral

 

[Adamjamma]

the one that said slip a white over the green and connect it to neutral?? should work, but let us know...

 

[Jraef]

I looked at the actual user manual, not the installation guide, and it is UL listed.
http://www.lutron.com/TechnicalDocumentLibrary/369488.pdf

The capacitive power supply would be just to power up the logic, a few milliwatts of power. Once the logic is alive and it commands the unit to switch the load on, it likely has an inductive coupler or a little CT to parasitically provide any added power needed via the current flowing through it. No laws of physics need be violated. This type of technology for powering microchip based devices has been around for decades, it's nothing new. You just likely didn't know it was happening in some of the stuff you have used. The little neon bulbs used in lighted toggle switches have worked on a similar principal for years; there has never been a neutral needed for those.

 

[don_resqcapt19]

The previous UL standard permitted the EGC to be used for a grounded conductor for the power to the electronics in a switch like that. Both the standard and the code prohibit that effective 1/1/2020.

404.22 Electronic Lighting Control Switches
Electronic lighting control switches shall be listed. Electronic lighting control switches shall not introduce current on the equipment grounding
conductor during normal operation. The requirement to not introduce current on the equipment grounding conductor shall take effect on January 1, 2020.

 

[kwired]

I looked at the actual user manual, not the installation guide, and it is UL listed.
http://www.lutron.com/TechnicalDocumentLibrary/369488.pdf

The capacitive power supply would be just to power up the logic, a few milliwatts of power. Once the logic is alive and it commands the unit to switch the load on, it likely has an inductive coupler or a little CT to parasitically provide any added power needed via the current flowing through it. No laws of physics need be violated. This type of technology for powering microchip based devices has been around for decades, it's nothing new. You just likely didn't know it was happening in some of the stuff you have used. The little neon bulbs used in lighted toggle switches have worked on a similar principal for years; there has never been a neutral needed for those.

My first thought when I saw lighted toggle was those that have the light across the contacts and the light is in series with the load when switch is open and had me scratching my head on what you were saying, but then got to wondering if you weren't talking about a switch with light that indicates the load is "on". Never seen one that didn't require a neutral connection, but I guess it is possible.

 

[Dennis Alwon]

I have seen a few of these electronic devices that use the equipment grounding conductor as a neutral. I assume at some point they will disappear from stock.