Bond Bushings, when necessary?

[jonny1982]

So I've always been trained to use bond bushings if there is a koncentric knock out and not all of it has been knocked out, and every raceway entering a transformer (standard transformer install, nothing out of the ordinary). I see in the code book about the koncentric knock out's, but are they really needed in transformers? Can't a regular plastic bushing be used?

Thanks

 

[romex jockey]

w/o looking, service entrance ,SDS's, and 480v work....~RJ~

 

[infinity]

So I've always been trained to use bond bushings if there is a concentric knock out and not all of it has been knocked out, and every raceway entering a transformer (standard transformer install, nothing out of the ordinary). I see in the code book about the concentric knock out's, but are they really needed in transformers? Can't a regular plastic bushing be used?

Thanks

Concentric knock out-maybe depending on voltage or other factors.
Every raceway entering a transformer-almost always not required except on GEC in a metal raceway

 

[GoldDigger]

Concentric knock out-maybe depending on voltage or other factors.
Every raceway entering a transformer-almost always not required except on GEC in a metal raceway

That is actually GEC in a ferrous metal raceway (i.e magnetic), not that aluminum or stainless are all that common.

 

[augie47]

It's definitely one of those scenarios where the devil is in the detail. Type of raceway, voltages, concentric & eccentric knockouts and GEC or not all play a part not to mention some inspector's outlook. For that reason, some folks just adopt an attitude of "use one regardless".
In your case the type raceway (ferrous vs non-ferrous, flex vs hard pipe) and the voltage of the conductors will play a part,
Generally speaking, < 250 volts-to-ground, a bond bush is likely not needed; >250v to ground, one well might be

 

[infinity]

That is actually GEC in a ferrous metal raceway (i.e magnetic), not that aluminum or stainless are all that common.

Yes I should have said magnetic raceway (although the NEC doesn't). Stainless steel can still be a ferrous metal.

 

[Carultch]

So I've always been trained to use bond bushings if there is a koncentric knock out and not all of it has been knocked out, and every raceway entering a transformer (standard transformer install, nothing out of the ordinary). I see in the code book about the koncentric knock out's, but are they really needed in transformers? Can't a regular plastic bushing be used?

Thanks

Service raceways and raceways with GEC's inside them are special cases that require them, where they otherwise would not be required. Service raceways would require at least one, GEC ferrous raceways require them on both ends. No special raceway bonding requirements for either the primary or secondary of the circuits connected to customer-owned transformers. The secondary conductors of a utility-owned transformer, are service conductors, and would require the bonding bushing for that reason.

As for the general case, it depends on if standard locknuts can get you proper electrical bonding to the enclosure. The over 250V nominal to ground threshold is specified that when ring knockouts remain, you cannot rely on the narrow metal connections between ring KOs to have sufficient continuity, and thus need a bonding bushing/jumper to provide an additional path. That is, unless the enclosure is otherwise listed for standard locknut bonding through the knockout at the voltage in question (and it will be a much more heavy duty knockout than a typical knockout). Nonmetal enclosures for obvious reasons, cannot bond the raceway to the enclosure wall, and therefore would require a bonding bushing if either not properly bonded on the other end, or you use the raceway as the EGC (instead of a separate wire).

Twice the voltage feeding a fault, means four times the power generated in a chokepoint like a ring KO connection. That is why higher voltages drive this requirement.

 

[infinity]

In many applications other devices such as bonding locknuts or bonding wedges can be used instead of bonding bushings where standard locknuts are not sufficient for bonding. Service raceways would be one example.