GES for separately derived system

R

rlqdot

Member
Location
St. Louis, MO - USA
Occupation
Professional Engineer (multiple states) - building design
I am designing a building that has 480Y277 volt service entrance equipment and several 480 - 208Y120 volt step-down transformers. I would normally call for the transformer XO terminal to be bonded to the structural steel frame of the building in order to establish a bond to the service entrance GES, but this building is precast concrete (I had a similar problem in a wood-framed building previously) and the precast manufacturer says we cannot rely on the rebar as an electrically-continuous path through the structure.

Can I increase the size of the wire-type EGC in the primary (480 volt) feeder so that it is equal to the size of the GEC required by the step-down transformer and use one conductor for both the 480 volt EGC and the transformer GES bonding conductor??? If the 480 volt feed for a given transformer originates in a sub-panel, is it suitable to use the EGC from the service equipment all the way to the transformer as the GEC for the transformer as long as the size of the EGC for its entire length from service equipment to transformer is equal to or larger than the transformer GEC size requirement???

Looking at 250.30(A)(5) it seems like what I am proposing is allowed by code.

Seeking opinions on either side of this suggestion.
 
If you meet all of the rules for both an EGC and a GEC you can do that. The no splice and the bonding to ferrous raceways often create problems trying to use a common conductor. You have to have an unspliced conductor back to the grounding electrode system. That would be difficult where the transformer primary circuit is originating anywhere other than the service equipment.
 
don_resqcapt19 said:
That would be difficult where the transformer primary circuit is originating anywhere other than the service equipment.
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Just playing devils advocate here, would that really be that hard? As long as one has the appropriate crimping equipment, couldnt one just crimp on jumpers around any ground bar connections and to a bonding bushing? seems like throwing on say half a dozen H-taps would often be cheaper and easier than running an entire GEC.
 
electrofelon said:
Just playing devils advocate here, would that really be that hard? As long as one has the appropriate crimping equipment, couldnt one just crimp on jumpers around any ground bar connections and to a bonding bushing? seems like throwing on say half a dozen H-taps would often be cheaper and easier than running an entire GEC.
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Maybe.
 
thank you both for your input. now i have to see if the contractor has installed conduits and, if so, whether the larger EGC / GEC conductors will fit. I appreciate your quick replies!!
 
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