Dimmer switch install with no "Bond/ground" in box

Status
Not open for further replies.

powerplay

Senior Member
I got a call today regarding replacing an existing dimmer switch with a new one. The comment wa that there was no wire attached to the screw in the box, and only black and white conductors. the dimmer came with a green wire and two black ones. If it was a choice of leave the green off the Dimmer disconnected, or connect to the screw... which would be the better choice? I realize connecting to the screw would leave the faceplate screws the same potential as the box metal strap. Is there an issue with the performance of the Electronic dimmer without a good path to ground?...as i doubt he is going to bring bonding/ground into the existing box.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Some dimmers work with only two wires, line in and load out. It is relatively easy to make such a dimmer work well with incandescents, but harder with odd non linear impedances in the load path. A two wire dimmer may still have a green wire to bond exposed metal, but it is not part of the circuit.
Other dimmers use a three wire design in which either a neutral or ground wire provides a zero volt reference independent of the load. For one of these if you do not connect the ground it will not work.
 

cadpoint

Senior Member
Location
Durham, NC
About quite a few years ago, maybe more than three a member here and a representative of a major manufacture made a post stating that in fact with their device a dimmer with two lines w/EGC was in fact putting current on the EGC. I believe that's where the code article requiring a neutral conductor at a switch came from!
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
PS: if there is no EGC to the box it will be hard to stay code compliant when installing a new device.
A plastic cover plate and nylon screws might help.
 

rlundsrud

Senior Member
Location
chicago, il, USA
Perhaps I am misunderstanding this, but doesn't the yoke of the dimmer provide a ground path and the ground wire is only used when installing it in plastic boxes used with romex? I am assuming this must be a metal box as it has a point to land a ground wire and that there is some form of EGC wether it was a metallic raceway that provides a legal ground path or an actual wire that provides the same.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
Perhaps I am misunderstanding this, but doesn't the yoke of the dimmer provide a ground path and the ground wire is only used when installing it in plastic boxes used with romex? I am assuming this must be a metal box as it has a point to land a ground wire and that there is some form of EGC wether it was a metallic raceway that provides a legal ground path or an actual wire that provides the same.
There is a lot of old Romex with two imsulated wires and no EGC installed in metal boxes still in service all over the country.

I see your location is Illinois so it may not be something you would come across.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
140103-1548 EST

One way I classify phase shift dimmers is into two categories, two wire and three wire.

A two wire dimmer is strictly a series element and must derive the energy to operate from voltage across and current thru the dimmer. Requires a minimum load.

A three wire dimmer gets energy to operate from the input hot and neutral. It also gets timing information (voltage zero crossing) from hot input to neutral. Should not require any load to operate.

A three wire dimmer should have at least a black hot input, a white neutral, and a red output hot. None of the internal circuitry should connect to any exposed conductive parts of the dimmer. For example the yoke. It is possible there were some three wire dimmers made that used the EGC (green) for their neutral to power the dimmer. I never saw one, and if made they should never have been approved, a bad design.

I believe Lutron is making dimmers that will operate in either a two wire or three wire mode. This is determined by whether or not the white wire is connected or left open. The minimum load requirements are different for the two modes.

If a dimmer is well designed, then a green wire should only serve the purpose of being an EGC wire. It should not connect to any internal circuitry.

.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
We know that a large number of occupancy sensors were made to use the green wire for power. Not sure whether it happened for dimmers too.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
If I were replacing a dimmer that only needed 2 wires and there was no equipment grounding conductor available-this means the box is not grounded- then I would just replace it and not loose sleep over it. Not sure where the code stands on it but if ever switch that had to be replaced had to be grounded with none available then it would be quite expensive. In some case it would not even be practical.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
The NEC is okay with replacement

404.9(B) said:
Exception No. 1 to (B): Where no means exists within the
snap-switch enclosure for connecting to the equipment
grounding conductor, or where the wiring method does not
include or provide an equipment grounding conductor, a snap
switch without a connection to an equipment grounding conductor
shall be permitted for replacement purposes only.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
If I were replacing a dimmer that only needed 2 wires and there was no equipment grounding conductor available-this means the box is not grounded- then I would just replace it and not loose sleep over it. Not sure where the code stands on it but if ever switch that had to be replaced had to be grounded with none available then it would be quite expensive. In some case it would not even be practical.
True, but I would still take steps to avoid having exposed metal surfaces tied to the yoke. In particular plastic plates and maybe even nylon screws.
JMO.
I suspect that a two wire dimmer is more likely to fault to the yoke than a simple snap switch.
Note also that the exception that Dennis cited applies only to a snap switch, not a dimmer, occupancy sensor switch or RF remote switch.
 
Last edited:

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Note also that the exception that Dennis cited applies only to a snap switch, not a dimmer, occupancy sensor switch or RF remote switch.

I agree but I doubt the intent was just for a snap switch. Too late for a proposal for 2017..:D I will ask this question at the state meeting in March
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I got a call today regarding replacing an existing dimmer switch with a new one. The comment wa that there was no wire attached to the screw in the box, and only black and white conductors. the dimmer came with a green wire and two black ones. If it was a choice of leave the green off the Dimmer disconnected, or connect to the screw... which would be the better choice? I realize connecting to the screw would leave the faceplate screws the same potential as the box metal strap. Is there an issue with the performance of the Electronic dimmer without a good path to ground?...as i doubt he is going to bring bonding/ground into the existing box.

If the box is grounded then the dimmer will be grounded via the two 6-32 screws just like a snap switch.
 
Status
Not open for further replies.
Top