Transformer Secondary Grounding

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Keri_WW

Keri_WW

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The ground from the secondary side of a transformer to the piece of equipment it is feeding would be considered a grounding electrode conductor as opposed to an equipment grounding conductor, correct? Is this due to the fact that a transformer is considered as a separately derived system?

I think I have been doing this backwards!!!

Thanks! :grin::grin:
Keri
 
A grounding electrode conductor is a connection to planet Earth. So if you run from the transformer secondary to a panel, for example, and if you bond the neutral and ground within that panel, then you need a GEC from that point to the planet. I believe that the wire that runs from the transformer to the N-G bond point in the panel is a "bonding jumper" of some description or other. There are several things that include "bonding jumper" in their names, and I am not clear which is called what.
 
I am of the belief that 250.30(A)(1) states that the conductor that runs between the transformer (where it is connected to X0 and the separatly derived system grounded conductor) and the panel (where it is connected to the ground bar and the equipment grounding conductor) is the System Bonding Jumber and thusly must be sized based on 250.102(C), which in turn refers you to size this conductors based on 250.66.

So to answer your question, yes. The size of this grounding conductor is based on 250.66 and not on 250.122.

I too had been sizing this wrong for YEARS.
 
Note that if you have an overcurrent device on the secondary side of the transformer and then feed the equipment, the grounding conductor downstream of the OCP is an equipment grounding conductor.
It's pretty much like a service, most things on the line side of your service disconnect (or in this case transformer secondary overcurrent device) are sized 250.66, load side 250.122.
 
All,

Thanks for the input. It's amazing that I've been doing it wrong for the past five years and no one has ever caught it! I'm definitely going to look deeper into this to make sure I know what I am doing from here on out!

Keri
 
A proposed change to Section 250.30(A) of the 2011 NEC (5-69 Log #1836 NEC-P05), directly addresses this issue. It introduces into 250.30(A) a new term: "(2) Supply-Side Bonding Jumper. If the source of a separately derived system and the first disconnecting means are located in separate enclosures, a supply-side bonding jumper shall be installed with the circuit conductors from the source enclosure to the first disconnecting means...
(a) A supply-side bonding jumper of the wire type (shall be sized by) 250.102(C)."

The bottom line is that Table 250.66 is to be used to size the conductor under discussion.
 
If it helps, back when I was an apprentice (Tables were 250.94 and 250.95), I had an instructor tell us in class to write "before fusing/breaker" above Table250.66, and "after fusing/breaker" above Table 250.122. It helped me to keep it in my pea sized brain.
 
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