In a recent project we were able to value engineer a DC combiner feeder by running an overhead line rather than trenching an underground duct bank. The feeder would have been a 175 feet underground duct bank routed across a high traffic driveway with multiple existing underground its path. The overhead aerial line was installed at a much lower cost (no trenching, conduit, backfill, or asphalt repair) without disrupting the faculties operations with a road closing.
The overhead run was constructed from EMT conduits with service heads installed at the top on both sides being supported to (2) 2” RMC support poles fived to the adjacent buildings. The 3 conductors were pulled from the combiner box through the service head and on the other end from the DC disconnect to the service head with 3 feet of additional conductor to create a drip loop. A 66’ piece of aluminum triplex 4/0-4/0-4/0 was fastened to an Arlington 610 porcelain wire holder on each RMC support pole. The overhead conductors were spliced to the THWN feeder conductors using Ilsco PBT-250 insulated splice connections.
In this installation, an AC panel board was located at ground level which combined the AC circuits from string inverters on the roof. We wanted to avoid entering the top of the enclosure with conduits, so a trough was used so that the feeders could enter through the bottom of the enclosure.
During O&M we often find combiner boxes and disconnect switches with water inside them, despite the box being NEMA 4 and fully gasketed, no top or side penetrations, and weather tight connectors properly installed.
Where is the water coming from?
The water enters the enclosure as moisture in the air, which later condenses into water. During the day, the bus bars heat up and warm the air in the enclosure. When air warms, the amount of moisture it can hold increases. At night, the air in the conduit or enclosure cools, and the water vapor in the air condenses. Each daily cycle only generates a tiny drop of water, but after several months this begins to add up.
Water may only travel downwards due to gravity, but moist warm air can enter and travel through a conduit and enter the enclosure from any direction. This is how water enters an enclosure even when there is no apparent way for water to run down and into the cabinet.
The solution is to seal the conduits where they enter the enclosure to prevent fresh air from getting inside the box. Traditional duct seal is effective and readily available. Expanding foam sealants area also available and very effective, however they must be approved for direct contact with the electrical cables.
Here are two examples of enclosures with only bottom entry conduits, but ground water entered the underground conduit, evaporated up the conduit into the enclosure, then condensed.
Many roofs are designed to support thousands of pounds of HVAC units and large air handlers, so the additional weight of an inverter is feasible in many cases.
If the structural engineer determines the building can handle the load, the next thing to consider is how you will mount the inverter on the roof. One option is to cut out a section of roof membrane and insulation and install a flashed curb that is bolted directly to the roof deck below. Effective, but involves cutting a hole in the roof, which we always try to avoid.
Pure Power has a better solution that is more cost effective, easier to install, and that doesn’t involve cutting a hole in the roof.
You can place the inverter directly on the roof surface, using an outrigger pad to evenly distribute the weight across the roof membrane and insulation. Polyisocyanurate insulation is has a compressive strength of least 20 PSI, which converts to 2,880 PSF. A 100kW inverter weighs around 3,000 across a footprint of 15 square feet, which translates to an average load of 200 PSF, less than one tenth the compression strength of the roof insulation.
In the past, we have used 2” thick white outrigger pads (used to distribute the weight of crane outriggers) with a slip sheet which look great! One tip, locate near a wall so you can provide a lateral support to address the concern of tipping over during a hurricane.
The cost savings of Aluminum conductors is too good to ignore. Both the engineer and the electrician must respect the differences between copper and the less forgiving aluminum. However, if designed and installed properly, Aluminum can reduce the installed cost and perform just as well as copper.
The four steps and the pitfalls of aluminum will be discussed below
Strip insulation off conductors
Use a wire brush on the bare conductor strands
Apply No-Ox compound
Install a lug appropriate for the thermal cycling
Stripping the insulation – Extreme care must be taken when stripping the insulation from an aluminum conductor. Using a utility knife may be acceptable for copper, but it is not for aluminum. If the soft aluminum in nicked, over time those strands will break free and the lose strands will cause small shorts and sparks over the air gap of the nick, eventually leading to a failure.
Wire Brushing – You should wire brush the exposed conductor to remove oxidation before applying the oxide inhibitor and terminating the conductor. This step will remove any excessive oxide from copper or aluminum wire and remove any pieces of insulation or other contaminants that might interfere with your connection.
Oxidation – Oxidation is to aluminum what rust is to iron. Oxidation will increase the resistance of the connection. Anti-oxidation compound (AKA “no-ox”) will mitigate oxidation and promote a low impedance connection to the lug.
Thermal Cycling – Due to the thermal cycling from day to night, the conductors expand and contract. Aluminum expands and contracts more than copper and is much softer, so when mechanical lugs are used the cable can work itself loose. Mechanical lugs should not be retorqued as part of any routine maintenance procedure because repeated tightening of any set crew mechanical lug could result in the eventual biting through of the conductor which will ultimately cause a failure of the connection. This is why Pure Power Engineering strongly recommends the use of compression lugs, which will ensure a long life and secure connection.
Note: A hydraulic crimper for compression lugs runs about $2,000, but once you have the crimper the material and labor cost is equal for mechanical and compression lugs.