Wow. Grounding a solar array continues to be a huge issue. In my 5 year IBEW apprenticeship training, 21 years as an electrician, 7 code revisions, 8 years specializing in solar, and cult-like reading of (not always agreeing with) John Wiles Code Corner in HomePower Magazine; I’ve encountered a broad range of grounding know-how. But I’m just getting started in my understanding so please don’t take anything I say here as having one bit of authority. I’m just joe wrench-on-the-roof. I do solar and small wind installations in 2 states, 12 counties, 6 electrical inspection jurisdictions, 6 incentive agency oversight zones, and 11 electrical utilities. On pleasant summer nights I dream sweet dreams of a German Solar Dictatorship overturning our current system of bureaucratic decoupage and implementing a one world solar permitting agency. The dream always digresses to a point where the new regime implants their tracking device into my skull so that they can further streamline the permitting process… Obviously I have issues. But let’s just stick to the facts. All these jurisdictions lead to lots of code confusion and grounding is at the top of the list. To understand solar grounding, understanding these four basic definitions is a good start:
- Equipment grounding
- Grounding Electrode
- Grounding Electrode Conductor
- Grounded Conductor
Equipment Grounding has to do with the green wire that is, under normal operation, not supposed to carry any current. It’s job is to connect all non current carrying metal parts in the system back to the grounding electrode. A Grounding Electrode is a metal thing that IS connected to the earth. The list of possible grounding electrodes and how they are and can be used to ground your system is pretty extensive. Article 250.52 in the code book deals with this. They can be things like concrete encased rebar, metal water piping, building steel, or ground rods brutally pounded 8’ into the earth (these seem to be the favorite). A Grounding Electrode Conductor is the wire that connects the Grounding Electrode(s) to a main bonding point. Bonding is tying the Grounding Electrode to the Grounded Conductor (we’re not there yet) and the Equipment Ground. The Grounding Conductor is the white wire. It does carry current to complete electrical circuits back to ground. The grounded conductor is only supposed to be bonded to ground at one point in your system (white wires and green wires don’t get hooked together except at the main service. Except when there are exceptions). These terms are used interchangeably only by people who don’t speak NEC speak. Article 690.43 of the NEC is about Equipment Grounding for solar installations. It talks about the green wire that goes along with the PV circuit conductors and bonds all non current carrying metal parts – module frames, rails, racks, j-boxes, combiner boxes and etc. The 2008 code also allows for the use of listed devices that facilitate the bonding of the metal parts together such as the WEEB connectors: http://www.we-llc.com/WEEB.html. There’s lots about sizing of conductors in this section that swing you back and forth from 690 to 250. I’m not going to go into that here. Article 690.47 is where it really starts to get fun. GROUNDING ELECTRODE SYSTEM. I’m going to focus on section (C) that applies to systems with both AC and DC grounding requirements. Here’s the basics:
- The DC grounding system shall be bonded to the AC grounding system.
- Size per the largest requirement.
- A single conductor shall be permitted to be used to perform the multiple functions of dc grounding, ac grounding, and bonding between ac and dc systems.
- Sized based on sum of inverters
- A common ground bus shall be permitted to be used for both systems.
- A common grounding electrode shall be permitted to be used for both systems, in which case the grounding electrode conductor shall be connected to the ac ground system bonding point (the ground bus in the main service panel).
- Grounding Electrode Conductor sized per largest requirement.
- For systems with utility-interactive inverters, the premises grounding system serves as the ac grounding system (back to # 6 & the ground bus in the main service panel).
Section (D) of 690.47 is where several installers and inspectors seem to get tripped up. It’s new to the code in 2008 and serves an important purpose but should not be thrown about to rashly. The idea behind installing a grounding electrode is to give the electrical system a reference to ground. If the wiring from a particular system gets too far away from that reference the potential between the real ground and the grounding conductor can be too great. So the code makes us install ground rods at outbuildings and tie the equipment grounding conductor to the grounding electrode conductor. This mainly has to do with the ability of a short to trip the circuit breaker but I won’t go into that. The same concept of installing additional Electrodes (ground rods) is applied here to a solar array. If the solar array is more than 6’ away by ground from the premises wiring electrode (690.47 (D) Exception No. 2) then additional electrodes are required. The verbiage about “as close as practicable to the location of roof mounted photovoltaic arrays” is what gets some inspectors excited about seeing another ground wire run down the lee side of the house with sweating electricians pounding more and more ground rods all around the premises. Let’s go back up to 690.47(C)6 & 8. Unless we are talking about a roof that is over a building that is 6’ away from the building with the main service, or a pole mounted system that is that far away, then we don’t have to talk about additional ground rods. Safe and simple is my motto for a good solar installation. If it takes additional ground wires and electrodes to make a system safe then we must do it. But if we make our systems more complex and ugly while achieving no greater compliance with the code or safety then we are loosing on several levels. My rant has run its course. More to follow.
- Jonathan Lewis