Reducing washers

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augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
We have had discussions here previously on reducing washers and their effect on grounding paths.
Apparently there was a change in 250.92 in the '11 Code regarding these.
I was privileged to attend a seminar conducted by a UL engineer and learned a few things about reducing wishers which I thought I might share:

The information for reducing washers is located in QCRV;[FONT=&quot][/FONT]

GROUNDING[FONT=&quot][/FONT]
Metal reducing washers are considered suitable for grounding for use in circuits over and under 250 V and where installed in accordance with ANSI/NFPA 70, "National Electrical Code." Reducing washers are intended for use with metal enclosures having a minimum thickness of 0.053 in. for non-service conductors only. Reducing washers may be installed in enclosures provided with concentric or eccentric knockouts, only after all of the concentric and eccentric rings have been removed. However, those enclosures containing concentric and eccentric knockouts that have been Listed for bonding purposes may be used with reducing washers without all knockouts being removed.[FONT=&quot][/FONT]
[FONT=&quot][/FONT]
REQUIREMENTS[FONT=&quot][/FONT]
The basic standards used to investigate products in this category are ANSI/UL 514A, "Metallic Outlet Boxes," ANSI/UL 514B, "Conduit, Tubing, and Cable Fittings," ANSI/UL 514D, "Cover Plates for Flush-Mounted Wiring Devices," and ANSI/UL 651, "Schedule 40 and 80 Rigid PVC Conduit and Fittings."[FONT=&quot][/FONT]
[FONT=&quot][/FONT]
See 250.12;[FONT=&quot][/FONT]
Clean Surfaces. [FONT=&quot][/FONT]
Nonconductive coatings (such as paint, lacquer, and enamel) on equipment to be grounded shall be removed from threads and other contact surfaces to ensure good electrical continuity or be connected by means of fittings designed so as to make such removal unnecessary.[FONT=&quot][/FONT]
[FONT=&quot][/FONT]
The following information comes from UL 514B;[FONT=&quot][/FONT]
8.34 Reducing washers resistance test[FONT=&quot][/FONT]
8.34.1 ? reducing washers shall be assembled to threaded conduit and unpainted steel plates as illustrated in Figure 16, and then subjected to the resistance test described in Clause 8.9. As a result of the test, the voltage drop shall not be greater than 10 millivolts. The samples shall then comply with the current test described in Clause 8.10.[FONT=&quot][/FONT]
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Gregg Harris

Senior Member
Location
Virginia
Occupation
Electrical,HVAC, Technical Trainer
We have had discussions here previously on reducing washers and their effect on grounding paths.
Apparently there was a change in 250.92 in the '11 Code regarding these.
I was privileged to attend a seminar conducted by a UL engineer and learned a few things about reducing wishers which I thought I might share:

The information for reducing washers is located in QCRV;

GROUNDING
Metal reducing washers are considered suitable for grounding for use in circuits over and under 250 V and where installed in accordance with ANSI/NFPA 70, "National Electrical Code." Reducing washers are intended for use with metal enclosures having a minimum thickness of 0.053 in. for non-service conductors only. Reducing washers may be installed in enclosures provided with concentric or eccentric knockouts, only after all of the concentric and eccentric rings have been removed. However, those enclosures containing concentric and eccentric knockouts that have been Listed for bonding purposes may be used with reducing washers without all knockouts being removed.

REQUIREMENTS
The basic standards used to investigate products in this category are ANSI/UL 514A, "Metallic Outlet Boxes," ANSI/UL 514B, "Conduit, Tubing, and Cable Fittings," ANSI/UL 514D, "Cover Plates for Flush-Mounted Wiring Devices," and ANSI/UL 651, "Schedule 40 and 80 Rigid PVC Conduit and Fittings."

See 250.12;
Clean Surfaces.
Nonconductive coatings (such as paint, lacquer, and enamel) on equipment to be grounded shall be removed from threads and other contact surfaces to ensure good electrical continuity or be connected by means of fittings designed so as to make such removal unnecessary.

The following information comes from UL 514B;
8.34 Reducing washers resistance test
8.34.1 ? reducing washers shall be assembled to threaded conduit and unpainted steel plates as illustrated in Figure 16, and then subjected to the resistance test described in Clause 8.9. As a result of the test, the voltage drop shall not be greater than 10 millivolts. The samples shall then comply with the current test described in Clause 8.10.


250.92 Services.
(A) Bonding of Equipment for Services. The normally non-current-carrying metal parts of equipment indicated in 250.92(A)(1) and (A)(2) shall be bonded together


(B) Method of Bonding at the Service. Bonding jumpers meeting the requirements of this article shall be used around impaired connections, such as reducing washers or oversized, concentric, or eccentric knockouts. Standard locknuts or bushings shall not be the only means for the bonding required by this section but shall be permitted to be installed to make a mechanical connection of the raceway(s).

This 2011 change can ad to your post in reference to knockouts and reducing washers.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Is the factory protective coating/finish on electrical equipment non-conductive? Generally, does UL require it to be removed? We know that loadcenter ground bars are routinely supplied with instructions that do not mention removing the protective finish. I do not think I have ever seen bonding connection in switchboards where the factory has 'ground off' the coating.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I can't think of a factory bonding connection that does not include a threaded screw which seemingly would be a different connection than a reducing washer that depends solely on surface contact.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I can't think of a factory bonding connection that does not include a threaded screw which seemingly would be a different connection than a reducing washer that depends solely on surface contact.
Most ground bar mounting screws (as opposed to the 'green bonding screw') are not the conductive path, they simply provide the pressure to hold items together for proper surface contact. Have you ever seen instructions for selecting lug mounting hardware that dealt with ampacity?

I know what is common practice and what I specify. And, I have an opinion on reducing washers.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Is the factory protective coating/finish on electrical equipment non-conductive? Generally, does UL require it to be removed? We know that loadcenter ground bars are routinely supplied with instructions that do not mention removing the protective finish. ...
I would expect the finish to be non-conductive. The load center ground bars are connected to the panel with a thread cutting screw that will remove the finish from the area where the threads contact the enclosure. The screw itself and not the bar to enclosure contact would be the bonding connection.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I have not but I would like to. Have a copy of link available ?
To my knowledge, one does not exist for general applications. If the hardware was the conductive path, we would have all sorts of information (e.g. the pluses and minuses) of materials and dimensions, instead we only have stuff on mounting torques.
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Every exterior panel I have seen is painted and I have never seen anyone sand off the paint to make an electrical contact. Normally we use pvc so that is not an issue but occasionally metal conduit is used. It is hard to believe the paint is not conductive but that is what I have been told here a number of times.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Every exterior panel I have seen is painted and I have never seen anyone sand off the paint to make an electrical contact. Normally we use pvc so that is not an issue but occasionally metal conduit is used. It is hard to believe the paint is not conductive but that is what I have been told here a number of times.
The locknuts used with metallic conduit cut through the finish as long as you tighten them. If we bolt a lug, that was not supplied by the manufacturer, to a panel we will remove the finish under the lug.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
We call them "uhoh" rings...
Would you really depend on them for bonding?
 
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