Transformer secondary conductor ground

[cppoly]

Just a recap because I always keep forgetting.

Transformer primary ground is sized on OCPD
Transformer grounding electric conductor is sized on the largest conductors

How is the transformer secondary conductor ground sized? Based off of the OCPD it terminates in ?

 

[infinity]

Let's start with some "ground" terminology to make it easier.

Primary = EGC (equipment grounding conductor) Based on OCPD ahead of the primary conductors according to T250.122.
Secondary = SSBJ (supply side bonding jumper) Based on the size of the secondary conductors and T250.102(C)(1).
SBJ = System Bonding Jumper. Same as SSBJ
GEC = Grounding Electrode Conductor. Based on secondary conductors and T250.66.

 

[charlie b]

If it's a utility service transformer, there will not be a ground wire run with the secondary (i.e., service) conductors.

If it's a step down transformer inside the building, then the ground wire is actually a Supply Side Bonding Jumper. It is sized on the basis of the secondary conductor size. Reference Table 250.102(C). Please note that that table is virtually identical to table 250.66. The difference comes in to play for large secondary conductors. 250.66 gives you an upper limit on the ground wire size. 250.102(C) does not.

 

[cppoly]

Thanks guys

 

[Ejones77]

Secondary conductor grounding

Secondary conductor grounding

This is a comment made by an owners commissioning agent "The Transformer Schedule on sheet E400 calls for the ground wire size of the Secondary Side to be sized per NEC 250.122; however, NEC 250.30 requires the secondary side ground to be sized per 250.102(C) as a supply side bonding jumper for a separately derived system. Should the ground wire from T1NLB1 to the panel be sized as a #2? Also, this rule appears to impact the 150kVA transformers scheduled for the project. Please advise." Is he correct?

 

[infinity]

This is a comment made by an owners commissioning agent "The Transformer Schedule on sheet E400 calls for the ground wire size of the Secondary Side to be sized per NEC 250.122; however, NEC 250.30 requires the secondary side ground to be sized per 250.102(C) as a supply side bonding jumper for a separately derived system. Should the ground wire from T1NLB1 to the panel be sized as a #2? Also, this rule appears to impact the 150kVA transformers scheduled for the project. Please advise." Is he correct?

The bold is correct, a SSBJ is sized according to T250.102(C)(1).

Not sure what the part is red means. :?

 

[david luchini]

Should the ground wire from T1NLB1 to the panel be sized as a #2? Also, this rule appears to impact the 150kVA transformers scheduled for the project. Please advise." Is he correct?

Would need to know the circuit conductor size to know if #2 is the proper SSBJ size.

 

[Ejones77]

250 amp panel board.

 

[infinity]

View attachment 22662
250 amp panel board.

The #4 SSBJ is too small.