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480/277 volt service bonding #88770 07/21/04 07:51 PM
Joined: Jun 2004
Posts: 613
shortcircuit Offline OP
What size bonding conductor is required to connect the bonding bushings installed on the steel conduit on the line side of a 200 amp 480/277 volt service disconnect with 3/0 copper service entrance conductors?
Also,the service will be fed with a underground latteral in 2" pvc with a section of 2" ridged conduit up the power company's pole. What is the method for bonding that ridged riser conduit.Is a ground clamp at the top of the conduit all that is needed for the poco to connect to?

2017 / 2014 NEC & Related Books and Study Guides
Re: 480/277 volt service bonding #88771 07/21/04 08:47 PM
Joined: May 2004
Posts: 162
cpal Offline
Equipment Bonding Jumpers are sized per section 250.102 in this case 250.102 (C) Size — Equipment Bonding Jumper on Supply Side of Service. I would size the jumper per table 250.66 (#4CU or 2 AL).

The metal riser at the pole should be bonded in accordance with section 250.92 (A) using a method described in 250.92 (B). Part (A) requires that the metal raceway enclosing service conductors be bonded. And the method in part (B) States that electrical continuity at service equipment, service raceways, and service conductor enclosures, shall be ensured by one of the following methods, ( 1-4):
(1) Allows bonding to the grounded service conductor, (2) allows connections utilizing threaded couplings etc, made up wrench-tight. The third (3ed) provision recognizes threadless couplings and connectors, and the fourth (4th) requires approved devices, such as bonding-type locknuts and bushings
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.

250.97 will require one of these four methods be employed where the voltage to ground exceeds 250V. (There is an exception)

I believe if you read the Code 250.102 (C) establishes the size of the bonding conductor on the line side of the service equipment, which would require a No. 4 CU or No. 2 AL to the metal raceway (yes a bonding bushing will work but so would a pipe clamp, etc.). I will stand by to read other opinions.

Re: 480/277 volt service bonding #88772 07/21/04 09:09 PM
Joined: Jun 2004
Posts: 613
shortcircuit Offline OP
thanx charlie...
OK...does the #4 copper bond to the metal riser pipe have to run out from the service equipment location to the riser or can the section of pipe be bonded out at the pole to the power companys grounded conductor or pole ground wire?

Re: 480/277 volt service bonding #88773 07/21/04 09:58 PM
Joined: May 2004
Posts: 162
cpal Offline
250.92 (B) allows the raceway to be bonded to the grounded conductor (neutral)
250.142 allows the use of the gounded conductor (neutral) to ground on the supply side of the main disconnect. the connection from the raceway to a neutral is a bonding wire. I think Its ok touse a No. 4 Cu or 2 Al from , say a bonding bushing to the neutral at the pole.
In our area most leave a bonding bushing on the riser and the utility will catch it with their No. 6 that they run to the rod at the base of the pole. No one seems to argue with this. But I think the Code requires a full size bond wire per 250.66 to the neutral.

any way thats how I read it!!

Re: 480/277 volt service bonding #88774 07/22/04 05:48 PM
Joined: Jun 2004
Posts: 613
shortcircuit Offline OP
An associate of mine told me that an inspector told him to drive a seperate ground rod at the pole to ground the steel riser pipe up the side of the POCO pole...this doesn't seem right though because there is no connection to the service grounded conductor?

Re: 480/277 volt service bonding #88775 07/22/04 06:04 PM
Joined: Jan 2003
Posts: 4,391
iwire Offline
Shortcircuit you are correct this ground rod would be worthless.

The only way a fault to this raceway would clear is with a connection to the neutral in some way not through the earth.

Bob Badger
Construction & Maintenance Electrician
Re: 480/277 volt service bonding #88776 07/22/04 06:42 PM
Joined: Jun 2004
Posts: 613
shortcircuit Offline OP
Thanx iwire...the groundrod didn't seem like the correct method for bondind the riser conduit...
Grounding and bonding is confusing even for some inspectors I guess.


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