why 6' apart

[jayrush13]

why is it that the 2nd ground rod has to be spaced six feet from the first?

 

[iwire]

why is it that the 2nd ground rod has to be spaced six feet from the first?

Lets be clear, it does not have to be spaced 6' from the first, it must be at least 6' from the first, they could be 20' apart.

The reason they have to be at least 6' apart is because the closer they are together the less effective the second rod is.

If you placed them side by side the second rod would do almost nothing.

I believe this is due to the resistance of the soil.

 

[M. D.]

I agree with Bob 6' is a minimun . Two times the length of the rod would be better.
From;

Bringing Grounding Down to Earth

Don't let the rules and requirements intimidate you. Grounding isn't as mysterious
as you might think.

By Tim Janof, P.E.

(published in EC&M, February 2005)

​

....The Code requires you to space rods
at least 6 feet apart [250.53(B)]. However,
this spacing is a minimum?and far from
ideal. When using the typical 8-foot or 10-
foot ground rod, you get the best results by
spacing the rods at least 16 or 20 feet apart,
respectively. This is much greater than the
Code minimum 6-foot spacing.
Ground rods spaced less than two
rod-lengths apart will interfere with each
other because their effective resistance areas
will overlap (Figure 2A). For reference, see
IEEE-142 and Soares Book on Grounding.
The overlap increases the net resistance of
each rod, making the grounding electrode
system less effective than if the rods were
spaced farther apart (Figure 2b)......

​

 

[bphgravity]

Per A.4.13.2.4 of the NFPA 780, "No benefit is gained from the second ground rod if placed closer than the length of the longer rod. No additional benefit is gained if the second rod is placed over four times the length of the longer rod."

 

[Pierre C Belarge]

When current flows through the rod, it creates "spheres of influence" in the soil. If the spheres of influence touch each other, it effectively makes both rods as one. There are other variables, but that is generally the reason why the rods actually should be apart from each other at least the length of the longest rod as Bryan pointed out.

 

[brian john]

Because the Code making panel likes even numbers otherwise it would have been 7 feet.

 

[tom baker]

6 ft is a number that is used a lot in the NEC. distance from a sink, maximum length of flex w/o EGC, maximum length of bonding jumper.
At some time, long ago, someone figured this all out.

 

[brian john]

Tom

Are you saying the CMP actually figured out 6' was a minimum effective distance or that 6' was a nice sounding number that works in a variety of instances.

 

[Pierre C Belarge]

6 ft is a number that is used a lot in the NEC. distance from a sink, maximum length of flex w/o EGC, maximum length of bonding jumper.
At some time, long ago, someone figured this all out.

...in a land far, far away!!! :grin:

 

[iwire]

They where sitting in a meeting and one of them stretched their hands as far apart as possible and said "How 'bout this far"?

 

[brian john]

Bob, best laugh of the week (maybe the month) almost dropped my laptop.

 

[walkerj]

Yea, I'm great at applying the 6' rule to anything. Just stretch my arms out and there ya have it

 

[Pierre C Belarge]

That may be a good laugh, but I would not put anything like that past the way things are sorted out...