what is a concentric knockout

A concentric ring knockout is defined by a concentric score or weakened line which extends in two almost semi-circular but unjoined arcs around the disc, adjacent ends of each concentric are being connected to the inner circle where the latter terminates at the nonweakened connecting portion.

Full Answer

What does a concentric knockout look like?

Concentric knockouts look like bulls-eyes... one circle indside of another with the inside and outside lines spaced equal distances apart. Each "ring" must be completely removed, all the way around the circle.

Do you need a jumper for a concentric knockout?

For bonding circuits over 250 volts, NEC Section 250.97 is applicable, and by excep- tion, allows concentric and eccentric knockouts to be used without bonding jumpers, provided the box or enclosure with concentric or eccentric knockouts is listed to provide a reliable bonding connection. This is where the UL White Book comes into play.

What is an AHJ inspection for concentric or eccentric knockouts?

When inspecting for proper grounding and bonding of electrical equipment that contains concentric or eccentric knockouts, the AHJ should verify that the integrity of the remaining concentric or eccentric rings has not been compromised during the installation process.

What is the a test current for a 4-second knockout?

A test current of 1180 A for 4 seconds (1/2 trade size) or 1530 A for 6 seconds (3/4 and 1 trade sizes) is passed from the conduit to the outlet box. As a result of the application of the current, the outer knockout is

What does concentric mean in electrical?

Where are bond bushings required?

Do you need bushings on PVC conduit?

At what voltage do I need to install a grounding bushing on concentric knockouts?

Why bond jumper for concentric knockouts?

If the integrity of the concentric or eccentric knockouts has been compromised during installation , a bonding jumper should be provided to ensure compliance with all of the requirements of the NEC.

What voltage is a knockout?

Concentric or eccentric knockouts that are part of a cabinet, cutout box, junction box, or pull box, installed on an electrical system that has a voltage over 250 volt s have not been investigated for grounding and bonding, and would therefore require a bonding jumper to be installed.

What is NEC 250.92 B?

NEC Section 250.92 (B) is applicable to bonding at services, and in part states, “Bonding jumpers meeting the other requirements of this article [Article 250] shall be used around concentric or eccentric knockouts that are punched or otherwise formed so as to impair the electrical connection to ground.” For services, this confirms that bonding jumpers are required unless all concentric or eccentric knockouts have been removed.

Is arcing and burning during a current test acceptable?

As a result of the application of the current, the outer knockout is required to remain in place, and continuity is required to be maintained between the box and the conduit. Arcing and burning during the test are common and are acceptable.

Can a concentric knockout be used without a jumper?

For bonding circuits over 250 volts, NEC Section 250.97 is applicable, and by exception, allows concentric and eccentric knockouts to be used without bonding jumpers, provided the box or enclosure with concentric or eccentric knockouts is listed to provide a reliable bonding connection. For the UL product category for Metallic Outlet Boxes (QCIT), the UL Guide Information indicates “All boxes with concentric or eccentric knockouts have been investigated for bonding and are suitable for bonding without any additional bonding means around concentric (or eccentric) knockouts where used in circuits above or below 250 V, and may be marked as such.” Please refer to UL Product iQ TM and enter “ QCIT ” into the search box.

How do you know if a knockout is concentric?

If you look closely at the way the concentric knockouts are stamped on the electrical load center you will see that they alternate between being punched in and being punched outwards.

What was the last concentric KO ring that I needed to remove?

The last concentric KO ring that I needed to remove was cut with my Knipex Diagonals.

How to get knockout rings off?

For subsequent knockout rings sometimes you can use your pliers to carefully pry the ring halves until they are up enough to squeeze them together and then move the pliers forth and back parallel to the anchor point until it breaks off.

Which panel is easiest to punch out first?

In the electrical panel above the inner 3/4″ knockout was the easiest to punch out first.

Do concentric knockout washers pop out?

Sometimes they pop out too easily and you have a much bigger opening then you need. Fortunately there are reducing washers available in all trade sizes, made to transition from the larger opening down to the size that you need.

Can you remove a knock out on a load center?

The knock outs on a load center cover tend to be much easier to remove to allow for each circuit break er. However there is always a possibility that the wrong knock out gets removed or a circuit breaker is relocated thereby leaving an open knock out on the load center cover. Fortunately every electrical load center manufacturer also has available blank filler plates to cover up the empty slots in the circuit breaker cover.

Can you make a hole with a knockout punch?

If no pre-punched factory made concentric knockouts are available, you will need to make your own hole using a hole saw or a knockout punch. You can find the correct size on my electrician’s hole saw post.

How much voltage does an Ecentric Ko have?

I was told by a foreman that a ecentric ko is rated up to 250V.

Do you need a bonding bushing on concentric KO?

The way I understand it the bonding bushind is not required at that voltage, unless it is at the service enclosure, on the LINE side. I don't see where you are even required to use a bonding bushing on concentric KO's when leaving the service enclosure with load side conductors.

What are the problems with concentric knockouts?

There are, however, problems with concentric knockouts having to do with proper grounding of the conduit to the box and also having to do with the ability of the electrician to remove the knockouts.

What is the eccentric relationship between knockouts 16 and 18?

The offset of these centers produces an eccentric relationship between knockouts 16 and 18 so that a wide region indicated generally at 23 and a narrow region indicated generally at 25 are formed in knockout 18, these regions having an area of maximum width and an area of minimum width along the common diameter of the two knockouts which passes through the two centers.

What is the diameter of knockout 16?

The center 19 of knockout 16 and of punch line 14 is offset from the center 21 of punch line 12 by a distance which is determined, in part, by the size of a standard lock nut which is employed to fasten conduit to a box. In the present embodiment which is designed for 3/4" and 1/2" conduit, the separation between the centers is 0.0495", the outer diameter of knockout 16 being 0.0860", and of knockout 18 being 1.109".

How to remove knockout 16?

As will be readily recognized, when a 3/4" conduit is to be connected to the wall, knockout 16 is removed by exerting force against the outer surface of the knockout, causing bridge member 28 to bend and break. Knockout ring 18 can then be easily pulled outwardly, since it is held by only one bridging member and since there is a relatively long lever arm between that bridging member and the opposite side of the ring by which force can be exerted on the bridge. The conduit is then inserted with the usual bushing and lock nut, or with two lock nuts where such is required, and securely attached to wall 10.

How does the punching operation displace knockout ring 18 from the box wall 10?

2, the punching operation displaces knockout ring 18 from the box wall 10 by a distance which is substantially equal to one-half the thickness of the wall and leaves knockout 16 with its surfaces substantially coplanar with the opposite surfaces of the wall.

What is an electrical box?

In this context, the term "electrical box" is used to include substantially any kind of metal box designed for housing the interconnection of wires from various cables or the mounting of electrical devices. The term includes relatively large housings such as metal cabinets as well as smaller housings such as switch boxes, masonry boxes, outlet boxes, extension rings and the like. It is known to produce concentric knockouts in several ways as shown by the following patents:

Can you use lock nuts in place of bushing 35?

It should also be mentioned that a lock nut can also be used in place of bushing 35. This technique is preferred by electricians in some circumstances and is required by codes in some areas and also with higher voltage systems. This, however, does not change the advantageous operation of the system but, instead, simply contributes further to the grounding of the conduit against the box wall directly.

Overview

Bonding at Services

Bonding For Over 250 Volts

Electrical Testing of Metallic Outlet Boxes

Other Types of Metallic Enclosures

Additional Code Considerations