Are Bonding Bushings Required?

[Watt4]

I am running a new feeder (480V 3Phase, 100 A) to a new electric closet in an existing office building by tapping into switchgear bus, through a fused disconnect into the new closet, through a meter into a transformer, which then feeds the distribution panels for the tenant. I am pulling 3 1/0 feeders and a # 6 ground. Do I need to install bonding bushings at the switchgear cabinet,fused disconnect, pull box, meter and transformer and bond the #6 through them?

 

[jcole]

I dont think that 250.92 applies because I wouldnt consider it service equipment but are there concentric or eccentric knockouts involved? If so than 250.97 would require bushings. I think.

 

[augie47]

bond bushings

bond bushings

you have to take it step by step with 250.92 in mind. Look at each segment (box, sections of conduit, etc) and ask youslef how each is bonded. Bonding bushings would probably not come into play unless you have concentric or eccentic kockouts and then only if that component isn't listed for grounding purposes.

 

[infinity]

Is your service tap ahead of the service disconnect? If so, the raceway between the service bus and the new fused disconnect would require bonding beyond the use of standard locknuts. It could be bonding locknuts, bonding bushings or bonding wedges on one side of that new service raceway. Beyond that point, if your KO's are punched for the size of the raceway or the concentric or eccentric KO's are listed for grounding, than you will not be required to use bonding bushings.

 

[Watt4]

There are concentric knockouts in the fused disconnect so I understand I will have to install bonding bushings there, it that the only place where it will be required?

 

[danickstr]

I thought he would need a bond bushing at the new sub panel.

 

[infinity]

I thought he would need a bond bushing at the new sub panel.

Any particular reason why you would think that?

 

[stickboy1375]

Q11. When are bonding bushings required on metal electrical raceways?

A11. Bonding bushings are one of the methods that can fulfill the NEC requirements for bonding of service raceways, for bonding raceways of 277/480V systems, and for bonding in hazardous (classified) locations. Here are some details:



When a metal service raceway terminates to an enclosure with a ringed knockout, a listed bonding device, such as a bonding wedge or bonding bushing, must bond one end of the service raceway with a bonding jumper sized in accordance with Table 250.66 [250.92(B)(4) and 250.102(C)]. If ringed knockouts are not encountered, a bonding locknut can be used instead of a bonding wedge or bonding bushing.



Metal raceways or cables, containing 277V or 480V circuits, terminating at ringed knockouts must be bonded to the metal enclosure with a bonding jumper sized in accordance with Table 250.122, based on the rating of the circuit overcurrent protection device [250.97, 250.102(D)].



Because of the explosive conditions associated with electrical installations in hazardous (classified) locations, electrical continuity of the effective ground-fault current path (metal parts of equipment and raceways) must be ensured by one of the methods specified in 250.92(B)(2) through (4). A bonding bushing is one of the acceptable methods [250.100].

 

[danickstr]

I guess I have always put a bond bushing inside every panel or subpanel I have installed. A habit. But feeder panels evidently do not need them.

 

[tom baker]

I recommend using a 250.92 bonding method at one end of every raceway containing a 277/480V circuit, this recommendation from the IEEE Green Book and Soares. A 277 ground fault, at one ohm, develops 75,000 watts of heat.
Good workmanship is critcal on 277V circuits.

 

[augie47]

There are concentric knockouts in the fused disconnect so I understand I will have to install bonding bushings there, it that the only place where it will be required?

In my opinion, it's possible that you don't even need bond bushings there.
If your #6 Eq. ground is terminated to the disconnect it will serve as gorund for the disconnect. If the conduit to the disconnect is bonded at the other end by whatever approved means, then there is no mandatory call for bonding at the switch end. That's why I said earlier, take it one step at a time.....how is the 1st enclosure bonded, then the 1st section of conduit, then the 2nd enclosure, etc. Unless you are encountering concentric k.o.s on both ends and neither enclosure is approved as grounding, you may not be required to use any bond bushings, although, as Tom points out, using them is a good safety step.

 

[infinity]

Also don't forget that there are concentric and eccentric KO's that are listed for bonding above 250 volts. These wouldn't require anything beyond a standard locknut.

 

[jcole]

How do you tell if the knockouts are listed for grounding? Will it have a certain UL listing or are all boxes or enclosures that have a rating over 250 volts listed for grounding? Basically, how would an inspector know?

 

[infinity]

How do you tell if the knockouts are listed for grounding? Will it have a certain UL listing or are all boxes or enclosures that have a rating over 250 volts listed for grounding? Basically, how would an inspector know?

I'd guess that you could start here:

www.ul.com