Adequacy of reducing washers questioned
When removing concentric or eccentric knockouts from a sheet-metal enclosure, it sometimes happens that a larger size than desired comes out or the only available knockout is larger than needed. This is where reducing washers, one inside the enclosure and one outside, fit the bill. However, the adequacy of using reducing washers in certain circumstances for equipment-grounding means is now in question.
If an enclosure is painted, 250.12 requires the removal of paint around the opening where a reducing washer is to be used. However, this can often be difficult. In fact, when the knockout is removed, bare metal is exposed around the opening, and the reducing washers contact this exposed edge, making the removal of paint unnecessary.
Furthermore, the Underwriters Laboratories (UL) subject all listed reducing washers to a resistance test and a current test. These tests are in the standard UL 514B, and also in UL 467. Nevertheless, at least one manufacturer recommends that reducing washers be bonded around.
The UL White Book paragraph about reducing washers reads, “Metal reducing washers are considered suitable for grounding for use in circuits over and under 250V and where installed in accordance with ANSI/NFPA 70, National Electrical Code.”
I cannot find any instructions in the NEC on the installation of reducing washers, except for the general requirements that equipment shall be installed and used in accordance with any instructions included in the listing or labeling [110.3(B)] and the requirement that installations shall be in a neat and workmanlike manner [110.12].
As an inspector, I would accept reducing washers only with a locknut inside and a locknut outside, or with an EMT connector on the outside and a locknut on the inside. Only locknuts can be tightened sufficiently to, in my opinion, ensure the equipment-ground continuity required. I would not accept a chase nipple on the inside and a coupling on the outside, for example, because the chase nipple does not bite into the metal of the reducing washer as a locknut does.
For conduit entries into cast metal enclosures with threaded hubs, where the hub is larger than the conduit size, reducing bushings are used. They are also used where it is necessary to change the size of the conduit run. (Sometimes part of a run will be larger than necessary to allow for future growth or for ease in pulling. Although it is not prohibited in the NEC, it is poor practice to change the conduit size in the middle of an underground run. Fishing may be difficult, and there is the possibility of damage to the conductors when being pulled in, especially when pulling the wire from the larger raceway through the smaller raceway. It also makes troubleshooting difficult when tracing a run if it disappears underground as one size and appears at another location as another size.)
In the 1968 National Electrical Code, the conduit fill for the more commonly used wire insulations was 25 percent for new work and 40 percent for rewiring. In the 1971 NEC, the difference between new work and rewiring was removed, and the conduit fill for three or more conductors was set at 40 percent, where it remains to this day. This decision was based on the reasoning that if 40 percent fill was safe for rewiring, then it must be safe for new wiring. At that time, the UL standard for sealing fittings used in hazardous locations was based on the 25 percent fill, and it was not until the 1990 NEC, 20 years later, when it was realized that these sealing fittings were undersize for the 40 percent fill then in effect.
In order to use a sealing fitting of a larger size, reducing bushings were necessary, so reducers were added to the list of things permitted between the sealing fitting and the explosion-proof enclosure. In the 1996 NEC, the requirement was added that the reducers had to be explosion-proof, and a new Section 501-5(C)(6) was added, which reads, “The cross-sectional area of the conductors permitted in a seal shall not exceed 25 percent of the cross-sectional area of a conduit of the same trade size unless it is specifically approved for a higher percentage of fill.” By now, all of the sealing fittings have been redesigned and are suitable for 40 percent fill.
If the reducers meet the NEC and there is no logical problem with using them, there should be no question of adequacy. EC
SCHWAN is an electrical Code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org.