250.30 - Separately Derived System, Re: Supply-Side Bonding Jumper (SSBJ)

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cdcengineer

cdcengineer

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I have been reading back in the forums about verbiage changes to the 2011 NEC. I want to make sure I understand the installation of SSBJ. Let's say we have a 480p:208s xfmr which sits next to (indoors) the switchboard (SWBD) it serves. The feeders are (6) sets of 500 AL. The xfmr will have a system bonding jumper installed inside the cabinet. Let's assume this installation is per 2011 NEC.

If the feeders from the xfmr to the SWBD are run in PVC, than a SSBJ can be installed in each raceway sized for the 500 AL per 250.66. There would be no equipment grounding conductor (EGC) installed correct - as the SSBJ is installed in lieu of EGC.

What if the feeders are installed in EMT? Does the non-flexible metal raceway act as the SSBJ?

I have been looking back at engineered drawings received over the years, and typically the engineer will provide a xfmr schedule which shows high and low side feeders and grounding electrode conductor sizing. The secondary feeder is usually sized with an EGC per 250.122. I'm beginning to think that this is being sized incorrectly by many in the industry. Anyone care to provide some input?

Thanks
 
cdcengineer said:
I have been reading back in the forums about verbiage changes to the 2011 NEC. I want to make sure I understand the installation of SSBJ. Let's say we have a 480p:208s xfmr which sits next to (indoors) the switchboard (SWBD) it serves. The feeders are (6) sets of 500 AL. The xfmr will have a system bonding jumper installed inside the cabinet. Let's assume this installation is per 2011 NEC.

If the feeders from the xfmr to the SWBD are run in PVC, than a SSBJ can be installed in each raceway sized for the 500 AL per 250.66. There would be no equipment grounding conductor (EGC) installed correct - as the SSBJ is installed in lieu of EGC.

What if the feeders are installed in EMT? Does the non-flexible metal raceway act as the SSBJ?

I have been looking back at engineered drawings received over the years, and typically the engineer will provide a xfmr schedule which shows high and low side feeders and grounding electrode conductor sizing. The secondary feeder is usually sized with an EGC per 250.122. I'm beginning to think that this is being sized incorrectly by many in the industry. Anyone care to provide some input?

Thanks
Click to expand...

You are correct, 250.122 is not the correct table for this. In the 2011 edition this would be Table 250.66.
As for using a metal raceway, you could use that as the supply side jumper but you would have to use bonding jumpers and bushings at the connection to the enclosure. See 250.30(A)(2)- Note that a non flexible metallic raceway is allowed to be used.
 
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texie said:
You are correct, 250.122 is not the correct table for this. In the 2011 edition this would be Table 250.66.
As for using a metal raceway, you could use that as the supply side jumper but you would have to use bonding jumpers and bushings at the connection to the enclosure. See 250.30(A)(2)- Note that a non flexible metallic raceway is allowed to be used.
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Texie, In the '08 and '11, I dont see the need for bonding jumpers/bushings. Can you enlighten me ?
 
Thanks Tex

What is I run non-flexible metallic raceway with a properly sized SSBJ inside each conduit? Would this be an un-approved installation method?
 
cdcengineer said:
Thanks Tex

What is I run non-flexible metallic raceway with a properly sized SSBJ inside each conduit? Would this be an un-approved installation method?
Click to expand...

What 250.30(A)(2) is saying is that if the raceway were metallic and non flexible then you could use that raceway as the SSBJ and would not need a wire type SSBJ. If you have PVC an SSBJ in each raceway is required and sized according to the conductors within that raceway in accordance with T250.66. What size SBJ did you use?
 
infinity said:
What 250.30(A)(2) is saying is that if the raceway were metallic and non flexible then you could use that raceway as the SSBJ and would not need a wire type SSBJ. If you have PVC an SSBJ in each raceway is required and sized according to the conductors within that raceway in accordance with T250.66. What size SBJ did you use?
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SBJ should be a #400 AL. Based on the sum of (6) 500 AL, per 250.66 at 12.5%
 
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Bonding Bushings

Bonding Bushings

Augie is right -according to Grounding and Bonding DVD-no bonding bushings for transformers and generators treated as SDS's. We also run conduit directly into transformers.
 
bms137 said:
Augie is right -according to Grounding and Bonding DVD-no bonding bushings for transformers and generators treated as SDS's. We also run conduit directly into transformers.
Click to expand...

I see bonding bushings on transformers all of the time and they're actually installed on FMC which doesn't even qualify as an EGC in larger sizes. IMO that's equivalent to putting a bonding bushing on PVC. :roll:
 
I gotta dig this thread back up.

Can anyone help me understand Exhibit 250.13 in the 2011 NEC? Is the black colored wire connecting the center tap of the Wye (I assume this center tap would be referencing terminal Xo) going to the neutral bar considered the neutral conductor? It seems to me that what I am calling the neutral bar is actually Xo and the SSBJ should be connected to the enclosure, terminating at the system bonding jumper.

It seems clear in Exhibit 250.14 how the connections are made. In reading 250.30(A)(1)(a), there seems to be a conflict with Exhibit 250.13. The SBJ does not appear to connect the grounded (neutral) and the SSBJ. This connection between N & SSBJ takes places on the terminal bar. Is the terminal bar considered a SBJ?

I'm not sure why am I making this so difficult?

Thanks for the comments.
 
You either bond your neutral at the transformer or at the first disconnect, GEC has to be the same place as the bond.

If you bond at the first disconnect it is just like the service coming to the building. If you bond in the tranny you'll probably need to buy some lugs.

FWIW I usually bond at the disconnect. It is the way I was taught. No reason other than that.
 
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