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#214593 12/24/14 02:59 AM
Joined: Feb 2006
Posts: 78
I have a 100 amp 480/277 volt service disconnect feeding a repair shop from power company transformer,service is feeding a 75 kva 480/120/208 3ph transformer bonding will be done in transformer,from transformer to a 200 amp load center, are ground bushings needed on line and load side all counduit is emt

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Joined: Jun 2004
Posts: 613
Bonding bushings are generally only required on 1 end. But with the bonding at the Utility owned transformer end, the AHJ may want 1 on the customer end also.

Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
If you are referring to the SDS, bond bushings required on one end of the Load Side Feeders, along with the Raceway for the Grounding Electrode Conductor (if Raceway is Metallic).

--Scott (EE)

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Mar 2003
Posts: 849
You might want to look at 250.97 .
Soares covers it with Illustrations.

Joined: Sep 2002
Posts: 131
There is no requirement to bond the conduit on the primary or the secondary of the transformer. Most electricians do it for some reason but there is nothing in the NEC that requires it. You don't need to use bonding bushings on the load side of the service disconnecting means except if there are concentric or eccentric KO's and greater than 150 volts to ground 250.97. If there is a broken ko you would need one too. That's it... Everyone seems to do it but it is not required.

Last edited by tsolanto; 02/01/15 08:53 PM.
Joined: Jun 2004
Posts: 1,273

The SECONDARY side is required to be bonded.

See SDS rules.

The reason is that under fault conditions an astounding spike of current is generated by the collapsing field in the secondary.

The bonding bushings provide 'choke elimination.'

The physics of ferrous materials causes them to 'choke' the current -- ramp the impedance in the circuit -- causing a total disruption of the equi-potential plane.

While this effect lasts but a moment or two, it's enough to stun or kill anyone touching metal while well grounded.

This effect is NEVER witnessed in normal operations. Any choking effect would require sensitive test gear to spot. (A 'scope.)

Bonding bushings are ALSO required IF the grounding conductor bond is routed back up the line towards the Service -- something that is COMMONLY done in commercial installations.

No choking effect is permitted along the GEC system -- at any point in the scheme.

Joined: Sep 2002
Posts: 131
Telsa, why not back that up with an NEC reference? No concentrics on transformers 250.97 and it is not a service 250.92 and article 100 definition of service.

Locknut is ok on both sides. Show me the code. NEC is specific on supply side bonding 250.92. Where is it specific on separately derived system bonding?

Joined: Mar 2003
Posts: 849
Since most Rough in to Utility Transformers just come underground into an opening And PVC might be used with RSC 90's even if not. There is no Physical connect between the pipe & the ground at the transformer. The utility end would for sure require a bonding bushing. '

Joined: Apr 2002
Posts: 7,383
Likes: 7
250.80 addresses RGS sweeps, with an 18" depth to any part of the sweep.

An instance where the depth could not be achieved, would result in bonding being required. Bonding bushing or clamp would be required, bonded to the transformer cabinet.

I'm still looking into the SDS question; Tesla has not responded with any further comments...

Joined: Jun 2004
Posts: 1,273
At my rate of pay I am not willing to do ALL of the homework for any query.

SDS = Secondarily Derived Service

= ALL transformers ( relaxed for dinky transformers )

= ALL on site Gen-Sets

= Solar PV type power has its own NEC section -- which is not quite the same as a typical SDS... typically a dry-type three-phase transformer Delta 480 to Wye 208Y120.

The SDS rules -- and the rationale for bonding across the secondary is in 230 or 232. Use the index at the back, hint.

As for the GEC system... it's in 230 or 250...

The essence of the scheme is that bonding bushings are REQUIRED along the ENTIRE GEC bonding run.

Since there is a common habit of running the GEC of the SDS right on back to the Service -- you'll find bonding bushings used in BOTH directions primary and secondary.

This requirement is limited to STEEL/ EMT raceways, underground or above ground.

If PVC ( non-metallic raceways) is used then the need to eliminate the choking effect is gone.

So many, many Services have no bonding bushings at all: they've been run underground.

The need for bonding bushings also is missing if the GEC system is not encased in EMT/ RMC.

So, it's common for most residential installations to NEVER have a bonding bushing.

The trivial situation of a Service riser -- for an overhead residential Service is excluded from the requirement -- since it's outside the structure -- most unlikely to be touched during a surge -- and it's almost impossible to attain a voltage differential at the riser because it's going STRAIGHT UP out of reach.

If any acronym is obscure -- just post so -- and many here will post a response.


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