Jdb

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JDB3

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I have difficulity understanding 250.92 , as to when & how grounding bushings are required. I have been in the trade for some time, [perhaps it is partially due to some utility companies requiring them, and others don't] {some inspectors want them, and others don"t} ??? Would someone please help.
 
Fairly often I get inspectors wanting bonding bushings where they are not required. Basically the bonding of any raceway containing service conductors cant done with standard locknuts. Pick any point on the cabinets and raceways containing service conductors: Can fault current get back to the neutral terminal without having to travel through a standard locknut?

1. RMC mast with threaded meter hub - extra bonding not required
2. RMC nipple or raceway - bonding bushing or locknut required on one end
3. Nipple or raceway section made from EMT - bonding bushing or locknut required on one end
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
Basically 250.92 states that all non current carrying metal parts associated with the service conductors must be bonded.
(B) gives you methods of assuring that bond. The key to bonding bushings may lay in the last paragraph:
Bonding jumpers meeting the other requirements of this article shall be used around concentric or eccentric knockouts that are punched or otherwise formed so as to impair the electrical connection to ground. Standard locknuts or bushings shall not be the sole means for the bonding required by this section.
If you encounter concentric or eccentric knockouts you must somehow bond around them. A bonding bushing is a common method of doing so.
I find it best to look at each component, the riser, the meter socket,, the service disconnect, and all interconnecting raceways and ask "is this properly bonded". In the case of the raceways, if one end is not bonded as as noted in (B)(1)-(4) and you have concentric/eccentric knockouts then a bonding bushing may be in order.

You also need to note 250.97 references you back to 250.92 for >250 volts to ground, however, the exception makes that far less of an issue.
 

infinity

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Location
New Jersey
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Journeyman Electrician
Metallic service raceways require more than just standard locknuts for bonding. If there are no eccentric or concentric KO's then a bonding wedge, bonding locknut or bonding bushing can be used. If there are concentric or eccentric KO's then the bonding bushing would be required. Similar rules apply to eccentric or concentric KO's when the voltage is greater than 250 volts.
 
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