MV Service, Grounding and Disconnect

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nhee2

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I do not have experience with primary metered systems, where the transformer is customer owned and the service disconnect is on the primary side of the transformer.

  • For purposes of meeting the service disconnect and OCPD requirements of 230.205 and 230.208 ? do the optional load break switches and fusing available from the xfmr mfr provide this function (are they service rated) or is a separate switchgear assembly required? IF separate, does this normally take the form of a padmount switch? And if a separate padmount switch is installed, is there any need for a load break switch on the transformer?
  • For supply to a delta primary, from a MGN utility feed, my understanding is 250.24(C) does not apply (>1000V). I see nothing in 250 requiring me to bring the grounded conductor to the service equipment. If there is no grounded conductor, it is not clear to me what the requirements for bonding are at the service disconnect ? I am not understanding the requirements of 250.186(B) ? is this saying a conductor must be run with the service conductors?
 
nhee2 said:
I do not have experience with primary metered systems, where the transformer is customer owned and the service disconnect is on the primary side of the transformer.
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There is very little difference between a utility service at medium voltage and one at 600V. Both have utility service points, utility metering, service entrance equipment, and customer owned step-down transformers. While there are different voltage related pertinent NEC references, the concepts are still pretty much the same.
 
nhee2 said:
I do not have experience with primary metered systems, where the transformer is customer owned and the service disconnect is on the primary side of the transformer.

  • For purposes of meeting the service disconnect and OCPD requirements of 230.205 and 230.208 ? do the optional load break switches and fusing available from the xfmr mfr provide this function (are they service rated) or is a separate switchgear assembly required? IF separate, does this normally take the form of a padmount switch? And if a separate padmount switch is installed, is there any need for a load break switch on the transformer?
  • For supply to a delta primary, from a MGN utility feed, my understanding is 250.24(C) does not apply (>1000V). I see nothing in 250 requiring me to bring the grounded conductor to the service equipment. If there is no grounded conductor, it is not clear to me what the requirements for bonding are at the service disconnect ? I am not understanding the requirements of 250.186(B) ? is this saying a conductor must be run with the service conductors?
Click to expand...

As you may have noted, 250.186 is new to the 2014 NEC. The point of it is to make certain that there is a fault return path as has been always required in less than 1000 volt services (previously 600 volt). 250.186(B) is telling you that even if there is no grounded conductor provided by the utility and the utility supply is grounded at any point, you must run a bonding jumper back to the utility grounding point along with the ungrounded conductors.
 
Thanks for the responses. I assume the utility grounding point is generally at the pole (for an OH distribution?)
 
I am re-asking one of my original questions on this post - is the integral load break switch / fusing available from most pad mount transformers suitable to be used as the service disconnecting means/service OCPD for a facility fed from a primary metered service?
 
nhee2 said:
I am re-asking one of my original questions on this post - is the integral load break switch / fusing available from most pad mount transformers suitable to be used as the service disconnecting means/service OCPD for a facility fed from a primary metered service?
Click to expand...

I have never seen this stuff in a transformer primary compartment. In my area (PECO/Exelon), the customer owns a pad-mounted load-break MV disconnect (could be a fusible switch or circuit breaker.) The utility's metering transformers go in a compartment of nearly the same size as the main disconnect, and fed by the switch so the CT's and PT's can be de-energized for service. There is no way to fit all that into a padmounted transformer primary compartment.
 
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