Module grounding revisited [RE-wrenches]

[William Miller]

Brad and others:

Again, thanks for the discussion.  Comments and questions below.


At 03:43 AM 10/30/02 +0000, you wrote:
>The trick is get electrical contact between the module frame and the 
>rack. Many modules frames are so well anodized that electrical contact 
>can't be counted on by just bolting them together. There must be a way 
>to break through the anodized coating to get contact.

Is the anodizing resistive by nature?  The only reference in the 2002 NEC
to "anodized" is that some required labels must be made from stock that is
anodized aluminum or other permanent material.

>
>One method used by some is to put a star washer (stainless of course) 
>between the module frame and rack where it is bolted down. Whether this 
>gives adequate conductance or not I'm not sure. I also wonder how it 
>will do over a period of years. It also wouldn't work with a clamp down 
>type rack.

Why not, if the star washer is held firmly between the rack and module frame?

>
>Another corollary issue is how to get a good, long lasting ground on any 
>aluminum rack or module. Somewhere there is likely to be a problem with 
>corrosion between the aluminum frame and the copper grounding wire. 
>Inserting something stainless between might slow it down some. The best 
>I've seen is a copper lug (GBL-4DB from Ilsco) held with a stainless 
>bolt with a stainless washer between it and the aluminum frame (with 
>copper ground wire of course).


Brad is right in that corrosive conditions are to be avoided:

	250.120 Equipment Grounding Conductor Installation.
	An equipment grounding conductor shall be installed in accordance with
	250.120(A), (B), and (C).
	(snip)
	(B) Aluminum and Copper-Clad Aluminum Conductors. Equipment grounding
	conductors of bare or insulated aluminum or copper-clad aluminum shall be
	permitted. Bare conductors shall not come in direct contact with masonry or
	the earth or where subject to corrosive conditions. 

The question is, is this a corrosive condition?  Has anyone observed
corrosion in any installations without these precautions?  Might it be
better to have one connection to a rack where one can see it, with
stainless washers, etc., than having multiple, hidden connections to
individual module frames?

>
>I have a question about the size of the ground wire. I think I've read 
>in the NEC that AWG #6 stranded or #8 solid is required where subject to 
>physical damage. Does ground wiring in an array qualify as subject to 
>physical damage (if the combiner is adjacent)? Or is #10 ground wire 
>adequate? Since #10 or smaller interconnect wire seems to be accepted, 
>maybe that's all that's needed for ground wire. Any thoughts?
>

On ground wire sizing, the 2002 code says this:

	250.120 Equipment Grounding Conductor Installation.
	An equipment grounding conductor shall be installed 
	in accordance with 250.120(A), (B), and (C).

	(C) Equipment Grounding Conductors Smaller Than 6 AWG.
	Equipment grounding conductors smaller than 6 AWG shall be protected from
	physical damage by a raceway or cable armor except where run in hollow
	spaces of walls or partitions, where not subject to physical damage, or
	where protected from physical damage.

One might contend that the grounding wire is protected from physical damage
or not subject to physical damage hidden behind a module that will break if
subject to physical damage and particularly if the modules are on a roof,
or better yet, flat on a roof.

Regarding sizing:

	690.45 Size of Equipment Grounding Conductor.
	Where not protected by the ground-fault protection equipment required by
	690.5, the equipment-grounding conductor for photovoltaic source and
	photovoltaic output circuits shall be sized for 125 percent of the
	photovoltaic-originated short-circuit currents in that circuit. Where
	protected by the ground-fault protection equipment required by 690.5, the
	equipment-grounding conductors for photovoltaic source and photovoltaic
	output circuits shall be sized in accordance with 250.122.

And this:

	250.122 Size of Equipment Grounding Conductors.
	(A) General. Copper, aluminum, or copper-clad aluminum equipment grounding
	conductors of the wire type shall not be smaller than shown in Table
	250.122 but shall not be required to be larger than the circuit conductors
	supplying the equipment.

And the table:

	Table 250.122 Minimum Size Equipment Grounding Conductors for Grounding
	Raceway and Equipment
	
	Rating or Setting of Automatic
	Overcurrent Device in Circuit
	Ahead of Equipment, Conduit, etc., 
	Not Exceeding 
	(Amperes)		Size (AWG or kcmil)	
				Copper		Aluminum or 
						Copper-Clad Aluminum*	
	15			14		12	
	20			12		10	
	30			10		8	
	40			10		8	
	60			10		8	
	100			8		6	
	200			6		4	
	300			4		2	


If your series fuse is 15 amps, then your grounding conductor is 14 AWG.
For multiple series arrays, do we need to increase size of ground wire?  No:

	(C) Multiple Circuits. Where a single equipment grounding conductor is run
with multiple circuits in the same raceway or cable, it shall be sized for
the largest overcurrent device protecting conductors in the raceway or cable.

William


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William Miller
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