transformer bonding

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olly

Senior Member
Location
Berthoud, Colorado
Occupation
Master Electrician
I have a 480v meter pedistal feeding a 480 single phase tranny stepping down to 120/240. Its been a while since Ive wired one like this. I have a few questions. I plan on taking the two phases and a EGC. (Equipment grounding conductor) only to feed it, no neutral. Then from my secondary side taking a neutral and a EGC and phases to a 100a panel mounted real close. Putting the bond in that 100a panel. Therefore only two EGC's in the lug mounted to the case on the tranny (no bond at the tranny). The grounding electrode conductor (GEC) is landed in the 480 meter pedestal by the way. This whole set up is right next to each other. Anyone see anything wrong with any of this. ps I'm putting the bond in the 100a panel because the tranny is tiny.

Thank you for your time!!!
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
You will also need to connect the secondary to a grounding electrode. The grounding electrode conductor should connect at the same point your system bond is connected. See 250.30.
 

kwired

Electron manager
Location
NE Nebraska
I have a 480v meter pedistal feeding a 480 single phase tranny stepping down to 120/240. Its been a while since Ive wired one like this. I have a few questions. I plan on taking the two phases and a EGC. (Equipment grounding conductor) only to feed it, no neutral. Then from my secondary side taking a neutral and a EGC and phases to a 100a panel mounted real close. Putting the bond in that 100a panel. Therefore only two EGC's in the lug mounted to the case on the tranny (no bond at the tranny). The grounding electrode conductor (GEC) is landed in the 480 meter pedestal by the way. This whole set up is right next to each other. Anyone see anything wrong with any of this. ps I'm putting the bond in the 100a panel because the tranny is tiny.

Thank you for your time!!!
Assuming this meter pedestal is the service, you still need a service disconnect and overcurrent protection on the primary side of this transformer. This 480 volt service requires a grounding electrode system. The secondary can be bonded at the transformer or at the first disconnecting means, or both if there is no conductive paths between the two. The secondary must also be bonded to the grounding electrode system with an individual grounding electrode conductor, not the EGC of the primary circuit.
 
You will also need to connect the secondary to a grounding electrode. The grounding electrode conductor should connect at the same point your system bond is connected. See 250.30.

BTW I prefer the bond in the panel because it is more accessible than in the transformer, the panel is also more frequently opened and any connection issues maybe easier to discover than in the transformer. On the other hand, since it is more frequently accessed, it is more likely that it gets disturbed. You can derive a rule from this: whenever you get a chance, check your Neutrals ground bond.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
... Then from my secondary side taking a neutral and a EGC and phases to a 100a panel mounted real close. ...
You need a supply side bonding jumper and not an EGC for this part of the circuit. The only real difference is that you use Table 250.66 to size the supply side bonding jumper, not 250.122 like you use for an EGC.
 
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