Grounding and bonding

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joeyww12000

joeyww12000

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Chatsworth GA
I terminated a 120/208 transformer yesterday. The xo/ground bond was made at the transformer. Heres how I did my grounding and bonding. From XO, GEC to building steel, bonding jumper to case frame. From case frame, Incoming EGC from 277/480v panel with primary conductors for transformer feed, EGC with secondary conductors feeding 120/208v panel. OK, specs called for 1/0 cu for GEC, #6 cu for EGC's on Primary and Secondary sides of transformer. I also used a #6 cu for my bonding jumper from xo to case frame, was that correct? My question stems from 250.102 (c) and (d). Is the primary side of a transformer considered "supply side of service" and secondary side considered "Load side of service"?
 
The EGC in the primary feeder is sized according to 250.122 and is based on the size of the OCPD protecting that feeder. The GEC, the system bonding jumper (between the XO and the case) and the equipment bonding jumper (run with the secondary conductors) are all sized according to 250.66 and based on the size of the secondary (derived) conductors. If your GEC is required to be #1/0, your system and equipment bonding jumpers must be #1/0 also.
 
So bonding jumper needs to be sized same as GEC to steel which is 1/0 by specs? If I were to figure GEC own my own for a seperately derived system, would GEC be sized according to Secondary conductors? Im just thrown by Engineers numbers in specs. He has 1/0 for GEC which isnt correct for table 250.66 or 250.122. Primary conductors are #1, secondary are 4/0. He's good on table 250.66 just oversized I suppose. Which table would I use for GEC and bonding jumper? I say 250.66, which is based on service conductors, which is line side or primary. So on a transformer how do you figure supply and load? Would transformer be load side of service, if so Im wrong....bonding jumper would be based on 250.122, based on 250.102 (d). Im actually getting a headache!
 
joeyww12000 said:
Thanks Infinity, Im clear now.
Click to expand...

You're welcome.

Take a close look at 250.66 and you see note "a" at the bottom. This tells you that the table is also for separately derived systems.

Your #1/0 cu is over sized for #4/0 secondary conductors. You could have used #2 cu or #1/0 al.
 
Supply Side of Service

Supply Side of Service

Okay, on the question regarding whether the primary of the transformer is considered to be the supply side of service. If the transformer is connected downstream from the service disconnect then the primary is actually on the load side of the service. Section 250.102(C) is referring to actual service conductors being used as reference for sizing the Equipment Bonding Jumper.
But if it's a utility transformer then the secondary would be the supply side of the service because it is connected upstream from the service entrance.
 
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