TTP (Theory To Practice)

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gwz2

Senior Member
Location
Indiana
Re: TTP (Theory To Practice)

Ed,

Those colored diagrams of your post's sure make the described installation vivid.

To meet NEC 250.30(A), the Neutral to transformer ( N - G ) Bonding Jumper would need to be removed and an Equipment Grounding Conductor (EGC) [ or should it be called an Equipment Bonding Jumper ( EBJ )] should be installed in the PVC between the Equipment Grounding Bus in the Distribution Panelboard and connected to the transformer enclosure.
Sections 250.5(A), 250.20(D), 250.32, 250.86, 250.96, 250.104(A)(4), 250.110, 250.118(1), 250.134(B) among others, should be checked for applicability

What size, per 250.30 ( ? ) should this particular EGC be ?
 

gwz2

Senior Member
Location
Indiana
Re: TTP (Theory To Practice)

Maybe ED could add another diagram showing a correct method, per the NEC 250.30(A), by adding a green EGC from the Secondary OCPD Neutral/Ground bus through the PVC and connect to the transformer enclosure and then remove the green jumper from the transformer X3 terminal to the transformer enclosure.

Of course, all of the grounding could be done in the transformer enclosure and then have an isolated Neutral ( grounded conductor ) bus in the secondary disconnect and have an EGC from the transformer Xo terminal through the PVC to a grounding bus in the secondary disconnect.

For this particular installation, it seems best to do all of the 250.30(A) grounding in/at the secondary disconnect enclosure.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: TTP (Theory To Practice)

gwz,
It appears that the installation as shown in Ed's drawing is in compliance with 250.30(A)(1) Exception #1.
Don
 

gwz2

Senior Member
Location
Indiana
Re: TTP (Theory To Practice)

Don,

That is the reason for adding all of the other Sections to check.

Since the total area for the 480V feeder to the transformer, the rack supporting the items and the Distribution Panelboard and the DSL/Phone cabinet are probably with-in a 10' circle, I would think a common bonding of the equipment by 800-40, 250.4(A), 250.50, 250.97, is 'in-place' and there is a parallel path for that neutral (Xo) conductor through the common grounding.

Is there any phone 'land-lines' in use ?

Just other items to consider.

*********

As for the 480V side to the transformer, the RMC should be the EGC if installed per 250.97
 

scott

Member
Location
Colorado
Re: TTP (Theory To Practice)

Thanks for the assist Ed. My descriptive skills are lacking, making this tougher than I would have liked.

Ed's picture is code compliant in my opinion, and was the way that the installation was intended to be. If everyone would ignore the neutral connection in the transformer, and locate it to case ground, then you can see the way the installation was done. The X2and X3 are connected to each other, and to nothing else.

What this created was an ungrounded secondary of the transformer. Not only was it ungrounded, but the neutral had no electrical connection to the transformer. The distribution equipment was designed for 120/240volt loads, but all of the installed equipment was only 240 volt. No line to neutral loads were served with the exception of the convenience outlet.

Once I plugged in the old laptop into the receptacle, the neutral current had no place to go. This caused wild fluctuations in the system voltage, and almost literally caused the surge protection to explode.

To answer my questions...

1) 95 volts to 170 volts depending on energized load

2) 240 volts

3) system overvoltage and non-working equipment (sparks and arcs if a surge protector is installed)

Enjoy your day!
 

Ed MacLaren

Senior Member
Re: TTP (Theory To Practice)

I have revised the sketch above (now Diagram 1) as per Scott's latest clarification.

Are the grounding/bonding methods shown in Diagrams 2 and 3 (below) NEC compliant?

I would prefer the method in Diagram 3.
In Diagram 2, the fault clearing path for a secondary winding ground fault is longer.

I believe the two grounding electrodes are required to be interconnected, are they not?





Ed
 

gwz2

Senior Member
Location
Indiana
Re: TTP (Theory To Practice)

Ed,

Great diagrams.

Per NEC 250.30(A), either diagram is correct.

I would think by the posts of the viewer of the site that diagrsm 2 would be the best for this particular installation, because:
1) Is the Ring Gounding Electrode (RGE) 250.52(A)(4), 250.53(F) connected to all of the structures which require bonding. It seems a 10' diameter circle would enclose all of the structures and the 20' minimum length of the RGE would only need to be increased a little more than 50 percent ( 30+ feet) to be the common grounding electrode.

When using non-metallic conduit between the SDS transformer case and the secondary disconnecting means on a system with a Grounded conductor, an additional conductor for grounding ( or is it called Bonding Jumper) will always be needed if there is any other parallel paths between the two structures.

Probably, in general, it would always be best to do all the necessary grounding with-in the transformer enclosure, as per diagram 3, but, either method is permitted.

Again, Ed, Great Diagrams.

[ March 03, 2003, 09:46 PM: Message edited by: gwz2 ]
 
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