Bonding Devices to Junction Boxes

mjc1060

Member
I am installing a raceway system that uses EMT and metal junction boxes. My question is when installing devices in particular 15A 120V receptacles and switches are these required to be bonded to the junction box if the EGC in this installation is the EMT raceway? Does the NEC accept the 6-32 device mounting screws as an effective means for bonding these devices to the EMT raceway system?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
See 250.146. If the boxes are in a wall, the screws on the receptacle are not acceptable as the required bonding. If you don't want to install a physical jumper, you can use "self-grounding" receptacles. Self grounding receptacles have a spring clip on one end of the yoke that improves the bonding connection and permits you to omit a wire type bonding jumper.
 

ArcLazerbeam

Member
Location
VT
See 250.146. If the boxes are in a wall, the screws on the receptacle are not acceptable as the required bonding. If you don't want to install a physical jumper, you can use "self-grounding" receptacles. Self grounding receptacles have a spring clip on one end of the yoke that improves the bonding connection and permits you to omit a wire type bonding jumper.

Forgot "self" a normal receptacle isn't the same as stated above.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
I am installing a raceway system that uses EMT and metal junction boxes.
It sounds like you are installing a flush mounted system where the pipe and boxes are recessed in the walls. However, if the system is surface mounted, there are more Code passages.

Break is down into the parts.

The EMT is the EGC and is bonded to the metal junction box by the EMT connector.

In a surface mounted the box, many times the next piece is a raised cover or a plaster ring, which needs to be bonded to the box. A plaster ring is bonded by the two box 8/32 screws holding the flats of the plaster ring tight to the box.

A raised cover, if it has flats for the box 8/32 screws like a plaster ring, also is bonded by those 8/32s. However, legacy raised covers that require a approx. 3/4" long cover screw are not bonded by the 8/32 screws.

Next, the device mounts to the cover or plaster ring. When held with two screws, the device is generally bonded to the cover or plaster ring. An interesting note: be sure to remove one of the 6/32 screw retaining fiber (or plastic) washers from between the yoke and the surface mounted box or plaster ring to effectively bond the device yoke to the metal box or ring. (Removal of the retaining washer does NOT help with flush mounted boxes and plaster rings.)
 

mjc1060

Member
Bonding devices to EMT raceways with separate EGC conductors

Bonding devices to EMT raceways with separate EGC conductors

The installation I am referring to is one in which the EMT raceway is concealed behind drywall. Thank you for the interpretation. I prefer to use a separate EGC conductor. This brings another question. Does the NEC require the EGC to be bonded to the junction box with a 10-32 grounding screw that is listed for this application? If this is not done and the EGC is terminated on only the devices and the panel board am I creating a parallel grounding path for potential fault current?
 

iwire

Moderator
Staff member
Location
Massachusetts
The installation I am referring to is one in which the EMT raceway is concealed behind drywall. Thank you for the interpretation. I prefer to use a separate EGC conductor. This brings another question. Does the NEC require the EGC to be bonded to the junction box with a 10-32 grounding screw that is listed for this application? If this is not done and the EGC is terminated on only the devices and the panel board am I creating a parallel grounding path for potential fault current?
If you run a wire type EGC or use the raceway is up to you. It is worth pointing out that the impedance of the raceway will be lower than the impedance of any wire EGC you will put inside it.

If you choose to run a wire type EGC 250.148 requires you connect it to the box.

The screw does not have to be listed or even green, it does have to comply with 250.8

As far as parallel paths for fault current that is a good thing.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
The screw does not have to be listed or even green, it does have to comply with 250.8
This is one of the more interesting anomalies in the materials we use in this trade. 250.8(5) does not call for "listed" machine screws, nor does it specify 10/32. However, the machine screw must engage at least two threads, or be secured with a nut.

If you go to the manufacturer's published literature for a "ground screw", it is extremely rare to find one that is actually tested and listed by a nationally recognized testing laboratory. Try it.

. . . am I creating a parallel grounding path for potential fault current?
As Iwire states, EGCs are required to be connected together, again and again. The effect, when there are multiple circuits in multiple raceways, when applying 250.144, 250.148, and especially 250.148(C), is to create a mesh, or "net", of EGC paths back to the "source" resulting in an effective fault clearing path that is harder to disrupt.
 
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