Why does 690.47(B) exist?

[tortuga]

690.47(B) basically says you can have an auxiliary grounding electrode and size it per 250.66.
Article 250 already allows this in general, is there something I am missing here?
The entire paragraph is just saying you can do stuff 250 already says you can do.

 

[wwhitney]

Because it used to say it was required, and administratively it's easier to make it optional than to delete the section?

Cheers, Wayne

 

[tortuga]

I am making a PI to delete it. Per 90.3 all parts of article 250 already apply to article 690, this subsection should be deleted as it is not modifying the requirements in article 250, nor is it adding additional requirements.

 

[shockdudude]

If you want to delete something I would delete having a neutral derived system over all....all creating neutral by grounding,Should be straight 240 single or 3 phase with ground , would cure all neutral issues... ha ha

 

[tortuga]

If you want to delete something I would delete having a neutral derived system over all....all creating neutral by grounding,Should be straight 240 single or 3 phase with ground , would cure all neutral issues... ha ha

Is that in 690?

 

[jaggedben]

The GEC size requirement in 690.47(B) does not exist in 250.54 and therefore might be worth keeping. It is arguably unsafe in lightning country for anyone to be connecting an auxiliary electrode to a rooftop array with a 14awg GEC. But it is not clear to me where 250 disallows that.

 

[pv_n00b]

The history behind this is pretty epic. If you can get an old timer from CMP 4 to talk about it it's fascinating. The one thing I learned from it was that someone can make what seems like an innocuous public input to the NEC and some cowboy on the CMP can take that PI and twist it and make it into something completely different and damaging to the industry. So you have to evaluate all PIs with an eye towards that possible outcome and accept that some things are best left untouched.

 

[tortuga]

The GEC size requirement in 690.47(B) does not exist in 250.54 and therefore might be worth keeping.

Good point

It is arguably unsafe in lightning country for anyone to be connecting an auxiliary electrode to a rooftop array with a 14awg GEC.

I don't personally think its in the scope of the NEC to deal with lightning protection.
NFPA 780 has a chapter (12) devoted to roof top solar systems.

To me 690.47(B) sizing the bonding wire smells like lighting protection and unless I am missing something, could potently conflict with NFPA 780.
For example take the following scenario:
A PV system is installed on a building where 250.66 requires a 3/0 GEC,
A lightning protection system is then installed per NFPA 780,
Per 780 table 4.1.1.1.1 the bonding wire from the roof array to the auxiliary electrode is effectively a #6 AWG (26,240 cir. mil flat braided)
690.47(B) would step in and require this to be a 3/0 bonding wire.

But it is not clear to me where 250 disallows that.

Anyone can run a #14 off any other large electrical equipment on a roof to a 250.54 ground rod if it makes them feel better, so why a special rule for solar ?

 

[jaggedben]

...


Anyone can run a #14 off any other large electrical equipment on a roof to a 250.54 ground rod if it makes them feel better, so why a special rule for solar ?

Fair enough, and a good point.