I have completed the electrical work. This includes wiring the differential controller and pumps with 115 VAC, and running the wires for the temperature sensors on the collectors and the solar storage tank. Although there is no specific NEC section that deals with solar hot water installations (690 deals with photovoltiacs, 720 deals with low voltage circuits), I decided to treat it as a major appliance and run a separate circuit from the breaker panel.
The load calculation shows that all of the equipment requires 237 watts to power when the system is on (TACO 009F pump draws 168 watts, TACO 003B pump draws 54 watts and the controller draws 15 watts). This means that the entire setup draws 2.06 amps when running. A 15 amp circuit breaker with 14/2 NM (aka “Romex”) will do nicely.
Since the controller is running two pumps, and the NEC states that only one conductor is allowed under any screw terminal (NEC 110.14), I installed a junction box to splice the two pump circuits together.
The temperature sensors are connected to low voltage wiring. These sensors are located on the solar hot water storage tank and on the collector. I used 18 gauge thermostat wiring for both of these runs. For outside wiring to the collector on the roof, I used UV resistant wire. I spliced the wire to the pig tails on the sensor, then soldered and covered with heat shrink tubing.
Grounding is a prime concern. The solar collectors are now the highest point on the house. They are electrically connected to the rest of the plumbing system by the copper pipes. In order to reduce the shock hazard they must be well grounded to the rest of the house’s single point ground system. Running a ground wire from the panels down the side of the house and connecting it to a separate ground rod is not recommended. In fact, any use of separate ground rods (other than those attached to the house electrical system) is asking for trouble.
Lightning Grounds
Effective lightning grounds require a bit of planning. Lightning can behave in unexpected ways. I do not expect that my solar panels will take a direct hit from lightning, there are too many tall trees around for that. However, even close lightning strikes can induce an electrical current to flow by means of an electromagnetic pulse (EMP).
Lightning itself is direct current (DC) however, it behaves like alternating current (AC) because of its fast rise time. Therefore, lightning grounds need be designed for moderate frequency AC, around 10 kHz or so. This means an effective lightning ground needs to have minimum inductance to allow the current to flow to the ground rod (electrode). The ground conductor should have no sharp bends, connections or splices should be soldered, brazed, or better yet exothermically (CAD) welded. The ideal lightning ground conductor should be solid copper wire, #2 or larger.
The ground electrode installation is also important. The NEC (NEC 250) calls for two 8 foot ground rods to be driven into the earth and bonded together. These should reach into moist soil. In areas where the water table is deep, longer ground rods should be used. The more ground electrode surface area in contact with the soil, the better the ground will be. This means you can use multiple ground rods, as long as every ground rod is bonded together to present one single point ground system. This is important.
When lightning strikes, the ground all around the lightning strike becomes saturated with electrons or protons depending on the polarity of the strike. The ground cannot bled off the electrical charge fast enough and the entire area is at an increased electrical potential. When different ground paths are presented, the ground potential between the two paths may be different allowing current to flow between the two paths. This will damage any equipment (and or people) that are connected between the to paths. If everything (and everyone) is at the same potential, no matter how high it is, no current will flow and no damage will occur. Sort of like a bird sitting on a high tension power line. That is the basic principle behind lightning grounding.
Still working away at it, not promising to have it done this weekend, but I will try.
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