Question: Knockouts on panelboards
Are concentric and eccentric knockouts on panelboards Listed for bonding at over 250-volts? How about on outlet boxes?
No. Enclosures for panelboards as well as enclosed switches are evaluated using requirements in the UL Standard for Safety for Enclosures for Electrical Equipment, UL 50. Requirements in UL 50 do not include an evaluation of concentric and/or eccentric (also known as tangential) knockouts for bonding. UL did have requirements in place at one time, which detailed construction, and performance requirements for evaluating concentric and/or eccentric knockouts for this purpose. Subsequent to actions taken by the UL 50 Standards Technical Panel (STP) in 2005, these requirements were withdrawn.
It should be noted that there are enclosed switches in the field, which were manufactured prior to action taken by the UL 50 STP in 2005 that were investigated by UL and marked for this purpose. These enclosed switches were evaluated for the suitability of using the knockouts in the bonding path and continue to be Listed for this purpose. However, subsequent to the 2005 UL 50 effective date, newly manufactured enclosures evaluated using requirements in UL 50 are no longer permitted to be marked indicating the use of concentric and/or eccentric knockouts for bonding purposes.
Concentric and eccentric knockouts on all Listed metallic outlet boxes are Listed for bonding above and below 250 V. Metallic outlet boxes are Listed under the product category Metallic Outlet Boxes (QCIT), located on page 247 of the 2007 UL White Book. The Guide Information for QCIT states: 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.