ASK THE EXPERTS: Grounding DC Systems

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Do I need to ground the negative poles of the batteries and modules in a 24- or 48-volt off-grid solar-electric system? If so, how? I’m thinking about connecting one wire from the positive battery terminal to the load (with breaker), and two from the negative battery terminal—one to the load and the other to ground. Is this the correct way to do this?

James Tohls • via email

Many things can go wrong with electrical systems to make them unsafe, so the National Electrical Code (NEC) has been developed to help mitigate these dangers. One of the most basic guidelines is to connect metal, which can get energized, to earth. This is called “grounding” or “equipment ground.” All electrical systems need to be connected to earth in case the exposed metal becomes energized by some sort of equipment failure. Earth-grounding reduces the danger of shock if the exposed metal is subsequently touched. And, if there is a short circuit, it also allows the fuse or circuit breaker to trip. Equipment grounding also helps protect the electrical system and appliances from lightning surges.

All touchable metal parts of an electrical system need to be kept at the same potential (voltage) as earth. So wires (usually bare or green wires) are routed through the system, and attached to all the metal boxes, metal conduit, module frames, metal equipment cases, and other metal. The other end of these “grounding” wires leads down to an earth connection, often a ground rod. In this way, we keep the metal that can be touched from shocking someone standing on the earth. (See “Get Grounded” in HP118 for a more thorough treatment of this subject.)

In electrical systems in the United States, one of the wires carrying the electricity (typically the “neutral” in AC systems and the negative in DC systems) is connected to the equipment grounding. (This is also what makes an electrical system “grounded.”) This is in addition to the requirement that all metal be connected to earth. However, it is unfortunate that the word “ground” is used to describe these very different concepts.

There’s more than one option for your situation, which can make figuring out what to do difficult. The NEC allows a stand-alone system that has a 24 V (or lower) battery bank to be “ungrounded.” This means that you don’t have to attach one of the wires carrying the electricity to the equipment grounding. But the requirements for ungrounded systems are rigorous, such as requiring overcurrent protection on both the positive and negative wires; it is much easier to configure a grounded system. Here’s what I recommend:

  • Attach equipment-grounding wires to all the exposed metal of your system—PV module frames, PV racking, metal electrical boxes and conduit, and metal equipment boxes, like load cases and the ones that house the charge controller. Make sure to establish a good electrical “bond” (low electrical resistance) that can last the lifetime of the system. Read the equipment instruction manuals to see what is recommended. Run the equipment grounding wires to a grounding terminal block in a DC circuit breaker box (like MidNite Solar’s Big Baby Box, fitted with a grounding terminal block). Attach a single, larger wire (commonly a bare #6 AWG solid copper conductor) to that grounding terminal block with the other end attached to the ground rod.
  • There will only be one place in your system where you will attach the negative wire to the equipment grounding. (If you have SunPower PV modules, you’ll attach the positive wire to the equipment grounding.) If there is ground fault protection (GFP), this connection is normally made through the GFP (follow the GFP instructions.) If there is no GFP, this connection will normally be made on the battery side of the charge controller at the grounding terminal block. Be sure to make that connection (between negative and equipment grounding) in one place only.

Kelly Larson, renewable energy consultant • SolarKelly.com

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