Grounding and fmc

Status
Not open for further replies.
neonjoe

neonjoe

Member
Location
Kentucky
I guess I'm going to have to start bringing my 2005NEC home with me. One of these days I'm going to quote something out of my '02 and it will have changed. I don't think that's so in this case.

Art. 600.7(A) states that listed fmc that encloses the secondary circuit conductor from a neon transformer shall be permitted as a bonding means if the total accumulative length of the conduit in the secondary circuit does not exceed 100ft. What I call the 100' rule. Before this was code, the 6' rule as stated in Art. 250.118(6) applied.

BTW, I must have read Art. 600 fifty times between 2002 and 2005 before the 100' rule struck me, and I must've done dozens of jobs using the 6' rule. I sure wish you would've said something, Gaylord.

What is it that makes fmc a better bond in 600.7(A) than in 250.118(6)?

Another BTW. Due to the limitations on the length of neon even the largest transformers can control, I can't think of any circumstance where there would be 100' of secondary conductor for neon in a single ground path, and still comply with the NEC and transformer manufacturers' guidelines. 60' at the most and that would be extreme.
 
Joe,
What is the output current of the sign transformer? I expect that it is very low compared to a branch circuit. I think that may be the reason.
Don
 
The last neon transformer I got shoced by, I mean that I worked on, was rated for 30kv at 15ma.

Believe me, it was quite an experience.


By the way, the secondary was center tapped; did that make it two phase?
 
The code allows up to 300mA max output current, very rare. I've never seen one higher than 120mA, which is used for what we call cold cathode lighting. In my area, 30mA is the most popular. In colder climates, 60mA is used frequently, but 30mA is probably the most popular. I never use 60ma unless specified.

"By the way, the secondary was center tapped; did that make it two phase?"

I'm afraid my knowledge of phases is limited (actually nil), but I believe it is still single phase. The center tap is what we call mid-point ground. A 15kv transformer is actually two 7.5kv's sharing the mid-point. We use the mid-point whenever possible to keep our secondary leads as short as possible. When properly loaded using this method, for instance in border tubing, the voltage at the end of each run is between 1 and 20v, making that long run back to the transformer (or between the two end electrodes) virtually insusceptible to insulation degradation and eventual short to ground.

I believe that it was the '96 NEC that required that all transformers 7.5kv and above be secondary ground fault protected and the 100' rule was in effect for the '02 NEC. Could this be the reason for the change from 6' to 100'? Though I don't see the logic in that.
 
The last neon transformer I got shoced by, I mean that I worked on, was rated for 30kv at 15ma.

Believe me, it was quite an experience.
Click to expand...

Gee Larry, it looks like it even affected your spelling!:p ;)
 
Status
Not open for further replies.
Top