Sep Derived System - Secondary Grounding

[sceepe]

Heres the deal, regular old dry type 480V pri, 208 secondary. System bond jumper between Xo and xfmr egc terminals. Assume that you have a proper electrode system and electrode condutor from the xfmr's egc terminal.

Questions:
1. Do you size the ground wire to the secondary side panel based on 250.66?
2. Is this a equipment grounding conductor or an equipment bonding jumper?
3. Say you have parallel feeds to the secondary side panel, do you still size the ground to the panel based on 250.66 in which case the grn in each raceway is based on the total circular mills of all conductors per phase. -OR- do you size the ground using 250.66 based on the size of the conductor in one raceway ignoring note 1????

Maybe this is would be more clear: 112.5 KVA 480 to 208 dry type feeds a 208V panel with a 400 amp main breaker. The system bonding jumper is installed at the transformer. The panel is supplied by 2 sets of 3/0 conductors and a ground. What size is the ground (#4 or #2).


I think #4 is correct based on 250.102(C), but I can't explain why 250.102(C) tells you to size it differently than any other time you use 250.66.

[ June 08, 2005, 04:45 PM: Message edited by: sceepe ]

 

[dereckbc]

Re: Sep Derived System - Secondary Grounding

1. Do you size the ground wire to the secondary side panel based on 250.66?

A. YES

2. Is this a equipment grounding conductor or an equipment bonding jumper?

A Neither, it is a GEC

3. Say you have parallel feeds to the secondary side xfmr, do you still size the ground to the secondary based on 250.66 in which case the grn in each raceway is based on the total circular mills of all conductors per phase. -OR- do you size the ground using 250.66 based on the size of the conductor in one raceway ignoring note 1????

A. No, you need a full size EGC per table 250.122 in each raceway

 

[sceepe]

Re: Sep Derived System - Secondary Grounding

Dereck your answer is confusing. You say the wire between the xfmr secondary and the panel is a GEC (grounding electrode conductor) and should be sized based on 250.66. But if the same wire is part of a parrallel feed to the panel you call it a EGC (equipment grounding conductor) and size it as per 250.122. I am missing something?

 

[fc]

Re: Sep Derived System - Secondary Grounding

Your EGC to the panel is based on 250.122 and wold be # 3 copper or #1 aluminum.
Your GEC is based on 250.66

 

[pierre]

Re: Sep Derived System - Secondary Grounding

Sceepe
If you terminate the GEC from the structural steel to X/O at the transformer, then the conductor that you install to the panel from the transformer is the Equipment Grounding Conductor, sized to table 250.122.

 

[benaround]

Re: Sep Derived System - Secondary Grounding

sceepe,

Base it on the total cm of each phase.The effetive ground fault path can't be divided like the phase conductors.Maybe this is the part yuor missing.Make sense?

frank

 

[kiloamp7]

Re: Sep Derived System - Secondary Grounding

sceepe - Let's be sure all of us are straight on this. Is it correct that the sec. of this 75KVA xfmr is 208Y/120V. 3-phase 4-wire with X0 (the "neutral") brought out & X0 is used to serve loads?

Please use NEC terminoligies when possible.
You say that panelboard is supplied by (2) sets of 3/0's & "a ground" .

 

[sceepe]

Re: Sep Derived System - Secondary Grounding

Is it correct that the sec. of this 75KVA xfmr is 208Y/120V. 3-phase 4-wire with X0 (the "neutral") brought out & X0 is used to serve loads?

YES THATS CORRECT.

Please use NEC terminoligies when possible. You say that panelboard is supplied by (2) sets of 3/0's & "a ground" .

The purpose of the question is to determine the proper terminology for the "green wire" that is run in each of the two conduits between the transformer and the 120/208 panel. Sound like it may be an EGC sized per 250.122. I could swear I read somewhere that it must be sized the same as a GEC and was called a equipment bonding jumper.

Thanks to all for the clarification

[ June 09, 2005, 06:18 AM: Message edited by: sceepe ]

 

[dereckbc]

Re: Sep Derived System - Secondary Grounding

sceepe, sorry for the delay, sounds like you got things cleared up. As mentioned it helps to use the proper terms when dealing with grounding issues. With a transformer you have a:

MBJ sized to 250.66
GEC sized to 250.66
EGC sized to 250.122