Fw: Grounding Information: [RE-wrenches]

Mon Mar 21 10:02:27 PST 2005

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Grounding Information:        
----- Original Message ----- 
From: John Wiles 
Sent: Monday, March 21, 2005 10:38 AM

[Allan: Would you please post the following on the wrenches list:]

Dear Wrenches who are getting grounded by inspectors on grounding.

The 2005 NEC in Article 690 discusses grounding electrode conductors for ac and dc systems in Section  690.47 in terms similar to these.

690.47(C) Systems with Alternating Current and Direct Current Grounding Requirements

Photovoltaic power systems with both alternating current and direct-current (dc) grounding requirements shall be permitted to be grounded as described in (1) or (2).

       (1) A grounding-electrode conductor shall be connected between the identified dc grounding point to a separate dc grounding electrode.  The dc grounding-electrode conductor shall be sized according to 250.166.  The dc grounding electrode shall be bonded to the ac-grounding electrode to make a grounding electrode system according to 250.52 and 250.53.  The bonding conductor shall be no smaller than the largest grounding electrode conductor, either ac or dc.

        (2) The dc grounding electrode conductor and ac grounding electrode conductor shall be connected to a single grounding electrode. The separate grounding electrode conductors shall be sized as required by 250.66 (ac) and 250.166 (dc).
**************************************************************************************************

HERE IS SOME BACK GROUND

Grounding SMA and Other Inverters

Most PV inverters have transformers that isolate the dc grounded conductor (usually the negative) from the ac grounded conductor (usually the neutral). Think of it as a dc separately derived circuit.  Shortly after the Sunny Boy 2500 was listed, UL and SMA America decided that the National Electrical Code (NEC) required that the dc system must have an appropriate grounding system.

The typical 14 AWG conductor frequently used as the ac equipment-grounding conductor, although duplicating the electrical path described below, does not meet the code requirements for a dc grounding electrode conductor.

An astute observer will note that the path between the PE terminal to the chassis ground, and then to the dc grounding electrode terminal, through the dc grounding electrode conductor (GEC) to the dc grounding electrode, and then to the ac grounding electrode is electrically similar to the path from the PE terminal through the ac equipment-grounding conductor to the ac load center to the ac grounding electrode.   However, the NEC allows the ac equipment-grounding conductor to be as small as 14 AWG and to be spliced any number of times by numerous different methods.  The code requirements for the dc grounding electrode conductor are entirely different.  It is usually 6 AWG or larger and if it is spliced, the splices must be made by irreversible means or welded.  The significant differences in these two conductor requirements dictate that both the ac equipment-grounding conductor and the dc grounding electrode conductor be used.  See the International Association of Electrical Inspectors (IAEI) Soares Book on Grounding for more details.

Current production SMA inverters have a 4 AWG terminal installed to the right of the ac output terminal strip and it is connected to the chassis, the PE terminal, and the chassis end of the ground-fault protective device (GFPD) fuse/circuit.  It is marked "grounding electrode".   SMA will furnish a lug kit for any of their products without this lug.    Utility-interactive inverters by other manufacturers should have a similar lug marked with a grounding symbol in addition to the terminal(s) required to accept the ac and dc equipment-grounding conductors.

An 8, 6, or 4 AWG (to a Ufer-concrete encased electrode) grounding electrode conductor is attached to this lug, run out the center knockout through a cord grip and to 1) a separate dc grounding electrode which must be bonded to the ac grounding electrode/system or 2) directly to the ac grounding electrode.  The usual NEC Article 250 sizing, splicing, and other requirements apply to this dc grounding electrode conductor.  Article 690 in the 2005 NEC will be revised along these lines.

The dc equipment-grounding conductor from the dc disconnect should be spliced to the ac equipment-grounding conductor from the ac disconnect and then the spliced conductor should be connected to the PE terminal.   If additional equipment-grounding terminals are available, they may be used instead of splicing the equipment-grounding conductors together.  Normally only one conductor is allowed per terminal.   The 1/4" "Fast-on", quick-disconnect terminals located at various points inside the inverter case are not to be used and SMA will be removing them in later production runs.

UL 1741 is being rewritten to require the dc grounding electrode conductor terminal.

Other inverter manufacturers will also have to meet this grounding attachment point requirement as the standard is published.

*************************************************************************************
AND HERE IS A WAY TO MEET THE INTENT OF THE 2005 CODE FOR THE SMALLER "STRING" INVERTERS:

Some installers are finding it difficult to run a 6 AWG DC grounding electrode conductor to a new dc ground rod or back to the ac grounding electrode that is required by the 2005 NEC.  Here is an optional connection that might be acceptable.

Connect a bare (best) or green unspliced 6 AWG conductor from a grounding terminal in the inverter all the way to the ac grounding electrode.  Route this conductor in the conduit with the ac output circuits.  Do not connect or splice it anywhere except at the ends.  An 6 AWG conductor run in this manner and connected to these two locations only should meet most inspectors' and the NEC requirements for both an ac equipment-grounding conductor and the dc grounding electrode conductor.  Some inspectors may accept an 8 AWG conductor-it does not hurt to ask.  If you have to splice it, the splices must be irreversible crimp splices or welded splices.   If the ac grounding electrode is a concrete-encased electrode (UFER) and has a 4 AWG ac grounding electrode conductor, then you may be out of luck as the inspector may require a 4 AWG conductor all the way back to the inverter.   If the ac grounding electrode is not accessible, then most inspectors will allow splicing this 6 or 8 AWG conductor with an irreversible crimp splice to the exposed ac grounding electrode conductor where it is accessible.   If the ac grounding electrode conductor is in conduit, then the installer and the inspector will have to agree on some other method of meeting the requirements.

I will try to get this word out to the inspectors, but they are as hard to reach as the typical PV installer.

See the new 2005 PV/NEC Suggested Practices Manual in PDF Draft at the link below and visit our web site to read the very detailed articles that I am writing for the inspectors.  The the IAEI does not impose the samer length restrictions on my articles that Home Power Magazine does.
-- 
You may download the Draft 2005 version of the "PV Power Systems and the National Electrical Code: Suggested Practices" Manual from this web site:  http://www.nmsu.edu/~tdi/roswell-8opt.pdf 
This manual will not be finalized or printed for several months.  Our web site (URL below) has additional information. To see the "Perspectives on PV" and the "Code Corner" articles, go to "Photovoltaics" and then "Codes and Standards".

If I can provide further information, please do not hesitate to call, e-mail or fax me.

John

John C. Wiles,  Program Manager
Southwest Technology Development Institute, New Mexico State University
505-646-6105      505-646-3841 (FAX)   http://www.NMSU.Edu/~tdi
SWTDI/NMSU
Corner Research Drive and Sam Steel Way
Box 30001/MSC 3 SOLAR
Las Cruces, NM 88003-8001
[Non-text portions of this message have been removed]

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