SDS Bonding question

[augie47]

A 208Y/120 transformer is added to a 480v distributon system. The transformer is added to a rtail shop on the 2nd floor of a precast concrete
structure with no exposed steel and no metallic water system.
What does one use as a grounding electrode and/or to what do you bond X/O ?

 

[Smart $]

A 208Y/120 transformer is added to a 480v distributon system. The transformer is added to a rtail shop on the 2nd floor of a precast concrete
structure with no exposed steel and no metallic water system.
What does one use as a grounding electrode and/or to what do you bond X/O ?

I'm fairly confident the 480 service has a GES. Extend the GES by way of a GEC to the SDS. The GEC can be run with the primary feeders in lieu of EGC.

 

[pete m.]

Although it won't be necessarily easy or convienent, there will need to be a grounding electrode conductor routed from the point on the SDS where the system bonding jumper is installed to any electrode recognized in 250.52(A). Reference: 250.30(A)(7)exc. 1.

XO will be connected to the transformer case or the first means of disconnect via the system bonding jumper. The grounding electrode conductor will be connecetd to the SDS at the point where this bonding takes place. Reference 250.30(A)(1)

Pete

 

[benaround]

Smart$,

Could you point me in the direction of the article that allows using a GEC

in lieu of an EGC. Thank you.

 

[don_resqcapt19]

Frank,
I see no reason why grounding conductor pulled in with the primary can serve the purpose of both the primary EGC and the secondary GEC.
Don

 

[benaround]

Don,

I know your position on this subject, but, 250.52 has a list of grounding

electrodes and the service equipment grounding bus is not on the list. I'm

sure an EI may feel the same way. Your reasoning is on the money, the

' words' have to be followed IMO.

 

[tom baker]

250.30(A)(7) has a list of the acceptable grounding electrodes. The grounding conductor could be pulled in with the primary if it was bonded to metal boxes and each end-250.64.
But, if you think about it, the SDS secondary is bonded to the GES, and via the bonding jumper to the XO and case, which is bonded to the primary EGC, to the service, and via the bonding jumper to the GES.
I am not sure why SDS transformers in a building need a GES. Perhaps I'll submit a code proposal to only require a GES for SDS on the exterior of a building.

 

[don_resqcapt19]

Frank,

I know your position on this subject, but, 250.52 has a list of grounding electrodes and the service equipment grounding bus is not on the list

The conductor can be pulled through the main switchgear and connected directly to the grounding electrode. A tap could be connected in the switchgear from this conductor to the EGC terminal bar.
Don

 

[don_resqcapt19]

Tom,

I am not sure why SDS transformers in a building need a GES. Perhaps I'll submit a code proposal to only require a GES for SDS on the exterior of a building.

I tried that and it was rejected.

5-99 Log #2716 NEC-P05
(250-30(A)(4) Exception No. 2 (New) )
Final Action: Reject
Submitter: Donald A. Ganiere Ottawa, IL
Recommendation:
Renumber existing Exception as Exception No. 1 and add new Exception No. 2 as follows:
"Exception No. 2: Where a transformer is used as a separately derived system and where the transformer primary feeder circuit
originates in the same building or structure, a grounding electrode shall not be required."
Substantiation:
There is no electrical or safety related reason to require a grounding electrode for a transformer used as a separately derived system when
the primary power source for the transformer is located in the same building or structure. The requirements of 250.4(A)(1) are met
without the use of a grounding electrode at the secondary side of the transformer. Lighting is not a problem within the building or
structure. The only possible contact with a higher voltage system is a fault on the primary feeder and this fault will be cleared by the
primary feeder equipment grounding conductor. The bonding required by 250.30(A)(1) will stabilize the voltage to earth under normal
operating conditions. The same bonding in combination with the primary feeder equipment grounding conductor will limit the effect of
line surges. The addition of a direct connection from the secondary side of a separately derived system to a grounding electrode
provides no additional protection over that which is already provided by the primary feeder equipment grounding conductor.
Panel Meeting Action: Reject
Panel Statement:
The present requirements for separately derived systems to be grounded to a reference earth by a dedicated grounding electrode
conductor apply to all systems without regard of installation inside or outside the building or structure served. The primary reason is to
establish an earth ground reference and stabilize the system voltage around this reference. The equipment grounding conductor from the
source to the separately derived system does not meet the requirements for size, not having a choke effect when installed in metal
raceways, multiple terminations, etc. The equipment grounding conductor's primary purpose is to provide a low impedance path for
fault current in the event of a ground-fault on the system up to and including the primary of the transformer, not to act as the low
impedance earth reference conductor.
Number Eligible to Vote: 16
Ballot Results: Affirmative: 15 Negative: 1
Explanation of Negative:
RAPPAPORT: The purpose of the grounding electrode conductor in the proposed Exception is to stabilize voltage. See my Comment
on Affirmative on Proposal 5-91. An equipment grounding conductor, run with the feeder to the separately derived system and sized for
the separately derived system, should be adequate.

 

[Smart $]

Smart$,

Could you point me in the direction of the article that allows using a GEC

in lieu of an EGC. Thank you.

Well, perhaps I wasn't explicit enough. I didn't mean to imply there would be no EGC. I was thinking of the instance where the conduit containing the primary feeder(s) is considered the EGC and instead of a wire EGC within the conduit, run a GEC.

Now as far as those mentioning that a GEC is not among the listed electrodes, how do you guys plan to bond to such electrode without a GEC?

Then, perhaps you guys can tell me if I'm incorrectly interpreting...

250.30(A)(3) Grounding Electrode Conductor, Single Separately Derived System
Exception No. 2: Where a separately derived system originates in listed equipment suitable as service equipment. the grounding electrode conductor from the service or feeder equipment to the grounding electrode shall be permitted as the grounding electrode conductor for the separately derived system, provided the grounding electrode conductor is of sufficient size for the separately derived system. Where the equipment grounding bus internal to the equipment is not smaller than the required grounding electrode conductor for the separately derived system, the grounding electrode connection for the separately derived system shall be permitted to be made to the bus.

 

[automate0042]

I dont have much experience w/ SDS systems, but i am curious. If the 480v supply was a grounded B phase (say), and the GEC was routed to the xfmr and connected to case w/ X0, would that change the xmrs SDS status?

 

[Smart $]

I dont have much experience w/ SDS systems, but i am curious. If the 480v supply was a grounded B phase (say), and the GEC was routed to the xfmr and connected to case w/ X0, would that change the xmrs SDS status?

No. It would still be an SDS. I believe the only time(s) when a transformer would not be an SDS is when there is a direct electrical pathway between the primary and secondary circuits, such as with autotransfomers and buck/boost xfmrs where the primary and secondary windings are isolated but the connections make it into an autotransforming arrangement.

 

[automate0042]

Thanks for the reply. Looking at art 100 definitions, I wondered about " no direct electrical connection, including solidly grounded ckt conductor."

 

[Smart $]

Thanks for the reply. Looking at art 100 definitions, I wondered about " no direct electrical connection, including solidly grounded ckt conductor."

Your point is most noteworthy, to say the least. Yes, the direct electrical connection most definitely exists. The only differentiation I can see is the terminology and intent of the conductor, that being a GEC is not a grounded circuit conductor.

 

[automate0042]

Thanks. I am not good at interpreting the NEC, esp. when it involves anything "green" or grounded. Your use of the word "intent" helps. I am probably loosing site of common sense as to the "direct electrical connection", the way I interpret the GEC in this case as a direct electrical connection between primary and secondary. This was just a situation that I had wondered about, and when I saw the post, I thought I'd ask.

 

[benaround]

Smart$,

250.30(A)(3) is only allowed when the transformer is part of the listed

service equipment, the exception rather than the rule. This could not be

used in the original discussion even if the EGC was the conduit. Do you agree.

 

[crossman]

benaround, thank you for that comment. I was a bit confused on exception 3 to 250.30(A)(3), both here and in a past thread. You cleared the confusion. Now, you did mean to say 250.30(A)(3) Exception 3, right?

It dawned on me that a "seperately derived system" is not the entire transformer, it is only the secondary of the transformer! This is implied by the definition of SDS in Article 100. The primary of a transformer is typically fed by a service, therefore that part of the xfmr cannot be an SDS. In the past, I was thinking of the entire xfmr as being the SDS.

So, IMHSO, you are entirely correct in your assessment of 250.30(A)(3) excp 3.

 

[Smart $]

Smart$,

250.30(A)(3) is only allowed when the transformer is part of the listed

service equipment, the exception rather than the rule. This could not be

used in the original discussion even if the EGC was the conduit. Do you agree.

No, I respectfully do not agree.

Most, if not all of the passages refer to the transformer as the SDS source. The exception here and in other passages differs in that it uses the phrase, "where the separately derived system originates..." IMO (or to be trendy, IMSO) the transfomer's primary OCPD is where the system originates.

However, I'm open to correction, as I do not see any clear definition...

 

[crossman]

2005 and 2008 NEC Art 100 definition of Seperately Derived System:

A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

Based on the definition of Premises Wiring System, the seperately derived system would be the secondary windings and any wiring connected to that secondary extended to the outlets.

So a seperately derived system isn't just the xfmr. It is the entire secondary system including all circuitry extended from the SDS.

The primary of a xfmr does not fit the definition of an SDS. The primary is typically fed from a service and this is a direct contradiction of the definition of Seperately Derived System.

Therefore my conclusion is that 250.30 (A)(3) Excp 2 and 250.30(A)(7) Excp 2 are both referring to listed service equipment that has a xfmr secondary contained within the equipment, just like Benaround has stated.

Note that these exceptions do not relieve one of the requirements in 250.104(D). This concerns bonding of the SDS to the waterpipe and structural steel in the immediate area of the SDS. There are, of course, some exceptions to this section.

 

[Smart $]

Interpretation of the defintion is not limited to the one which you have made. As stated previously, please note the use of the word source and originate (or grammatical forms thereof).

A premises wiring system whose power is derived from a source... other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

Note that these exceptions do not relieve one of the requirements in 250.104(D). This concerns bonding of the SDS to the waterpipe and structural steel in the immediate area of the SDS. There are, of course, some exceptions to this section.

I did not state otherwise. The OP concerns instances where these grounding electrodes are not available in the immediate area.