250.30 (1) Bonded at first disconnecting means

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Jerry Albertelli

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Vidor
Here it goes.....I have a transformer where I have bonded the XO on the secondary (supply- side) with the case & grounding electrode. Now when I get to my First means of disconnect, then it becomes a separately derived system, right?

I am aware that I have to bond (unspliced) my XO with the ground and case-ground at the first disconnecting means, but can someone please tell me why I would have to bond at the transformer if I have to bond at the first disconnect or vise-versa??
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Jerry Albertelli said:
Here it goes.....I have a transformer where I have bonded the XO on the secondary (supply- side) with the case & grounding electrode. Now when I get to my First means of disconnect, then it becomes a separately derived system, right?

I am aware that I have to bond (unspliced) my XO with the ground and case-ground at the first disconnecting means, but can someone please tell me why I would have to bond at the transformer if I have to bond at the first disconnect or vise-versa??
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IMO, it just is an SDS. it does not become an SDS after the first disconnect.

I don't believe you are required to bond in both places. In fact, I think it is discouraged.

(1) System Bonding Jumper. An unspliced system bonding
jumper shall comply with 250.28(A) through (D). This
connection shall be made at any single point on the separately
derived system from the source to the first system
disconnecting means or overcurrent device, or it shall be
made at the source of a separately derived system that has
no disconnecting means or overcurrent devices, ...
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jazer

Senior Member
Location
Gibsonia, Pa
you definately don't want to bond at both the x-former and the panel/first disco, you've now parallelled your supply side bonding jumper and made it a current carrying conductor....no no
 
Dennis Alwon

Dennis Alwon

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Chapel Hill, NC
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Here is something from mike holt- taken from here

CAUTION: The neutral-to-case connection for a separately derived system cannot be made at more than one location if so doing would result in a parallel path for neutral current flow [250.30(A)(1) Exception No. 1]. Such multiple neutral current return paths to the grounded (neutral) conductor of the power supply can create fire and shock hazards, as well as power quality problems from electromagnetic interference. See 250.6 and 250.142(A).
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jazer

Senior Member
Location
Gibsonia, Pa
also note that wherever you decide to make the system bonding jumper connection, that you must also make the grounding electrode conductor connection in the same place, either the x-former or the first disco/panelboard.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Jerry Albertelli said:
Maybe I said it wrong. How about the "low-side" "source"? I guess what I meant is "IT" is the side that I will be using the Neutral (XO)
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Sounds like you need to learn or need a refresher in some terminology.

Primary side is the (normal) "supply side". In under 600 volts applications it is often the higher voltage side, but can be the lower voltage side, and will be lower voltage side in "step up" applications. Sometimes primary and secondary are same voltage when the transformer purpose is for isolation reasons only.

Separately derived is the entire secondary side of the transformer and is for a system that does not depend on any of the primary conductors to function as a part of the secondary system - most common example of "not separately derived" system from a transformer is in a instance where using an autotransformer, where one system conductor (typically a grounded conductor) is carrying both primary and secondary current. A conductor from the source that is for equipment grounding only is not counted as a functional conductor for the secondary that has a grounded conductor, though improper bonding can cause objectionable current to flow through such a conductor.
 
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