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 is the difference between eccentric and concentric knockouts?

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. Eccentric knockouts look like crescent moons, staggered around the outside of a circle.

Where are bond bushings required?

You need a bonding bushing if: Your raceway contains service conductors. Your raceway contains grounding electrode conductors. In fact, you may need one on both ends, for GEC performance.

What should be used to ensure continuity and proper conductivity at impaired connections within the service?

Bonding Jumper. Installing bonding jumper(s) is one method often used between metal raceways and metal parts to ensure electrical conductivity.

What is a bonding bushing?

Bonding bushings are used on metal raceways when the raceways have service conductors in them. Bonding bushings are also used when the grounding electrode conductor is in a metal raceway- both ends need to be bonded and one end often has a bonding bushing.

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

Overview. 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 volts have not been investigated for grounding and bonding, and would therefore require a bonding jumper to be installed.

Is a ground wire required in metal conduit?

2 Answers. Show activity on this post. Conduit (RMC, IMC or EMT) is an NEC-accepted ground path. So, if the wires are in steel conduit, no OTHER ground wire is required.

What is the difference between grounding and bonding?

Bonding is the connection of non-current-carrying conductive elements like enclosures and structures. Grounding is the attachment of bonded systems to the earth. Both are necessary to safeguard people and property from electric hazards.

What is the minimum length of a ground rod?

eight feet longGround Rod Length Ground rods come in both 8-foot and 10-foot lengths, with 8-foot being the most common size used in residential installations. As a rule, ground rods must be a minimum of eight feet long and should not be cut down.

Are grounding bushings required on Transformers?

Moderator. I agree bonding bushings are not required on a transformer.

What is a grounding locknut?

3/4“ grounding locknuts are used on 3/4” threaded conduits or connectors to bond locknuts when additional locking and grounding connections are required, such as when severe vibration is encountered.

What is the main bonding jumper?

Main bonding jumper is the connection between the grounded circuit conductor and the equipment grounding conductor at the service.

What is a bonding locknut?

A bonding-type locknut differs from a standard-type locknut in that it has a bonding screw with a sharp point that drives into the metal enclosure. Assured bonding one end of a service raceway provides the low-impedance path for fault current to return to the utility power supply.

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.

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.

Overview

Bonding at Services

Bonding For Over 250 Volts

Electrical Testing of Metallic Outlet Boxes

Other Types of Metallic Enclosures

Additional Code Considerations