conductors between service transformer and service disconnect
- Thread starter mshields
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Got me thinking...
Got me thinking...
HMM got me thinking, if you have a small 45kva xfmr somewhere downstream in the system would it be more acceptable to bond the neutral and ground at the xfmr enclosure or wait until the first disconnect? Would it be a correct statement that when the grounding of a transformer for a sepratly derived system happens within the xfmr. Enclosure the connection from the xfmr. Grounded conductor (XO) to the ground bar in the xfmr case is required to be sized the same as the phase conductors? This would be considered a grounded conductor correct?
Got me thinking...
HMM got me thinking, if you have a small 45kva xfmr somewhere downstream in the system would it be more acceptable to bond the neutral and ground at the xfmr enclosure or wait until the first disconnect? Would it be a correct statement that when the grounding of a transformer for a sepratly derived system happens within the xfmr. Enclosure the connection from the xfmr. Grounded conductor (XO) to the ground bar in the xfmr case is required to be sized the same as the phase conductors? This would be considered a grounded conductor correct?coulter said:
250.6(B)(3) does apply. . It tells you the solution. . The outdoor transformer electrode system should not be directly connected to the building electrode system except thru the service neutral.
The 250.50 requirement to "bond together" "All grounding electrodes" "to form the grounding electrode system" only applies within "each building or structure". . If the transformer is not in the building, 250.50 does require interconnection of electrodes. . And 310.4 requires the electrodes not to be interconnected since there is more than one bonding point.
On the line side of the main disconnect, the service neutral is can be used for bonding [ such as in the meterbase ] and it will keep the to electrode systems at near exact equal potential, as near exact as a normal operating current carrying conductor can.
David
dnem said:
David -
I'm struggling with understanding your intent and interpretation. Let me tell you the ramifications of this statement. First a disclaimer - I don't do houses or commercial - strictly industrial experience.
Your statement implies:
1. A ground mat that underlies the entire area including the utility owned transformer and a process area where the service disconnect is located is a poor design practice. That would suprise most of us with industrial experience.
2. I could not use metal conduit between the xfm and disconnect. Actually this is not a porblem. I don't tend to bury metal conduit if I have a choice. Breaks too easily.
3. I could not use a metal cable tray between the xfm and the disconnect. CT is my perferred for above ground.
Do you propose that I cut the ground mat, install a dielectric coupling, cut the cable tray?
4. I could not use MC-HL cable between the xfm and the disconnect. I don't know how one might cut the armor on this one. This is my preferred for buried.
5. On a 13.8kV service what do you do with with the shield tapes? Are you planning on not bonding both ends? That does not sound like a good idea at all. This one may not count. I did not check to see if there was an exemption for over 600V. The code tends to recognize their poor attempts at design in the <= 600V areas are really a bad idea in the >600V arena.
I'm in agreement your proposed designs are suitable for most small installations - but not necessarily good design practice for all installations
carl
coulter said:
Industrial installations commonly have exceptions that can't be used for commercial or residential applications.
coulter said:
coulter said:
Absolutely not ! . Don't damage any equipment or install it differently than intended.
250.6(B) has other solutions other than (3)
(1) Discontinue one or more but not all .....
Which would mean that your particular installation would require you to "discontinue" or ignore the "additional" that is called for in 250.24(A)(2).
"ground mat that underlies the entire area"
I think 250.6(B)(1) is your best choice
coulter said:
"Are you planning on not bonding both ends?"
I would never tell anyone to skip bonding but I would say to skip making electrode connections to more than one point in the system, in most situations. . All bonding must still be done. . But don't forget that the service neutral can be used for bonding on the line side of the main disconnect. . When you bond using the neutral, you're not skipping bonding, you're still doing it.
Taking the simple example of a house system. . You can bond the meterbase to a ground rod with an electrode conductor. . And then bond the main disconnect to the water main with a different electrode wire. . You don't have single point bonding but you do have a code compliant installation and you do have bonding thru the service neutral.
Back to the transformer example. . When you're disconnecting an electrode wire between an outside transformer electrode system and a building electrode system, both systems must still have full bonding.
David
David -
You sound dangerously close to agreeeing with me.
Kind of interesting the rules change depending on who owns the xfm.
carl
You sound dangerously close to agreeeing with me.
I think that is a great idea.dnem said:
Now lets take a look at what the NEC says if the customer owns the transformer, the service point is on the xfm primary. Consider an outside xfm, a ground mat under the entire area, and let's use MC-HL cable for the secondary feeders to the OCPD, 500" away. Exactly the same installation as if the utility owned the xfm, except this one is an SDS. I say we make the N-G bond either at he xfm or at the frist disconnect, but not both. 250.30.A.1.dnem said:
Kind of interesting the rules change depending on who owns the xfm.
carl
coulter said:David -
You sound dangerously close to agreeeing with me.
Now lets take a look at what the NEC says if the customer owns the transformer, the service point is on the xfm primary. Consider an outside xfm, a ground mat under the entire area, and let's use MC-HL cable for the secondary feeders to the OCPD, 500" away. Exactly the same installation as if the utility owned the xfm, except this one is an SDS. I say we make the N-G bond either at he xfm or at the frist disconnect, but not both. 250.30.A.1.
Kind of interesting the rules change depending on who owns the xfm.
carl
We are dangerously close but I think the point of disagreement isn't the bonding requirements or the paralleling conductor requirements. . I think we are viewing SDS differently.
In your paragraph above you state that "the service point is on the xfm primary". . I don't believe the service point matters in determining an SDS. . The service point matters in determining NEC jurisdiction but not SDS.
Service Point. The point of connection between the facilities of the serving utility and the premises wiring.
Separately Derived System. A premises wiring system whose power is derived from a source of electrical energy or equipment other than a service.
[ note that it says service not service point ]
Service. The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.
No matter who owns the transformer or where the service point is, the question is where is the service main disconnect ? . Is it on the load side of the transformer or on the line side ? . If it's on the line side, then you have an SDS coming out of the secondary of that transformer.
If you have a transformer on the load side of the service point, then you have NEC jurisdiction and 250.24(A)(2) applies.
If you have a transformer on the line side of the service point, then you have no NEC jurisdiction and 250.24(A)(2) doesn't apply.
If the service main disconnect is on the line / primary side of the transformer, then you have a SDS and 250.30(A)(1) applies.
If the service main disconnect is on the load / secondary side of the transformer, then you don't have a SDS and 250.30(A)(1) doesn't apply.
But even if 250.30(A)(1) applies, when you're dealing with a transformer outside of the building, you're dealing with a separate structure because the transformer is "built or constructed" [ 100 defs ] in a factory. . So you have the option of connecting the 2 structures according to 250.32(B)(1) or 250.32(B)(2). . If you use (1), you have to let the neutral "float" unconnected from the grounding on one end or the other or you'll get a 310.4 violation. . If you use (2), you have to bond neutral to grounding at both ends.
David
ramsy
NoFixNoPay Electric
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
mshields said:
If your colleague anticipates the customer's need for isolated grounding, when they call back for the iso-ground, he just lifts the extra service ECG to an insulated bus bar, performs a nice dog & pony show then presents a big fat bill.
coulter said:
I have something on my mind about this 13.8kv shielded cable.
I have never seen a voltage level that high that was not used for either:
supplying a transformer
supplying high power demand 3 phase equipment
In either case I can't imagine anybody supplying these 13.8kv cables from a wye secondary transformer. . There's significant disadvantages in distributing high wattage thru a wye. . Any lower voltage needed for controls in the 13.8kv machinery would be supplied from a transformer in the equipment itself.
End result; no neutral. . So neither 250.24(A)(2) nor shield bonding brings up the possibility of a 310.4 violation.
David
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