Sunday, August 1, 2010

Grounding & Bonding, Part III

This week we finish our Grounding and Bonding installation for the Gym remodel project. In previous blogs we discussed two separated services that enter the building, supplying two separate panels, each with its own set of feeders.

     Our initial grounding choice might be the "Metal Underground Water Pipe" as is specified in 250.52(A)(1). I hesitate to utilize this method however, due to the fact that it can be very difficult to verify the "minimum 10' of direct earth contact" rule. There is simply no easy method to verify that the cold water service doesn't change over to PVC, two feet or so after it goes underground. Therefore, we might bond it regardless, but we would not want to consider it our primary grounding electrode.

     Thus, we're left with a Rod, Pipe, or Plate electrode type. Ground rods are the most commonly installed, and we'll most likely use these in this application. The only draw back to this, is that we MUST then perform a final resistance to ground test, when the installations are completed, in order to comply with 250.56's rule of 25 ohms or less.

     We would size our Grounding Electrode Conductor (GEC) from table 250.66, ignoring the maximum rule of 250.53(E). That rule allows us a maximum of a #6 AWG, but it only applies when the ground rod is a supplementary electrode -- not the sole electrode as is true in this application.

     T250.66 sizes the GEC based on the "Largest Ungrounded Service Conductor." I found 2/0 CU feeders in each panel, thus we would be in the "2/0 to 3/0" row and find a #4 AWG CU would be required. We would encase it to protect against physical damage in accordance with 250.64(B).

     Because there are two separate services, each would receive its own identically sized GEC installation based on the service conductors present in each panel. We would not be allowed to tap the two together. Article 250.28(D)(2) instructs us to supply a main bonding jumper to each service disconnect. We bond each panel per 250.24(A)(1).

     One final note, if we use a metal conduit to encase the #4 GEC, we MUST bond EACH end of the conduit with a grounding type clamp or fitting, attached to the same size bonding jumper as we used for the GEC. This prevents dangerous "choking" potential during high voltage discharge releases, such as found in lightening strikes etc...

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