Power provisions

Greenhouse businesses usually get larger over time by adding more hoophouses or gutter-connect bays. To operate efficiently, an adequate electrical system that can handle the environment control and materials handling equipment must be added. When building a new greenhouse or expanding to an existing one, a determination of how much power is needed, where the service will be located and how power will be distributed has to be made. As electric systems can be complicated with heating, cooling and lighting systems that must be integrated, it is best to hire an engineer or electrician that has design experience with greenhouses.

Demand load

The size of the greenhouse service is usually determined by calculating the electrical demand load. The National Electrical Code (NEC) contains a procedure for determining the electrical load for farm buildings. It includes listing all the electrical equipment for the greenhouse, providing full-load amperes and operating voltage. Next is to identify the equipment that may be operating at the same time. Examples include inflation fans, HAF fans, pumps and lighting, heating or fan cooling equipment. This is known as “without diversity.” The remaining equipment is labeled as “with diversity.”

Once the demand load is established, the service entrance size can be determined and the wire size calculated. A load center, circuit breaker panelboard or fusible safety switch is used as a disconnect. It may also contain individual circuit breakers.

Service drop

A convenient location for the service drop is needed. The best option is inside a headhouse or work area with good accessibility. An alternate is a central distribution pole in the yard with wiring underground to an inside load center. If possible, it should be central to the main electricity use so that the wiring will be as short as possible. From there, load center wiring to the individual equipment can be made, or, in large operations, feeders to supply subpanels. Subpanels with disconnects and distribution circuits are common with multiple hoophouse or gutter-connect installations.

To reduce maintenance or failure, mount electric panels and equipment where it has easy access and away from excessive heat or cold. Watertight boxes should be used in dusty and damp locations.

Wire size

All wiring needs to be of adequate size to carry the load without excessive voltage drop. Excessive voltage drop can result in lights that dim or flicker, lower heater output and motors that lose power or burn out. A common example of this is to have the load panel at one end of a 100-foot to 200-foot greenhouse and fans in the opposite endwall. If the wire size is too small and the voltage drop is more than 5%, the fan output is reduced, and the motor overheats.

Most applications require that the wires be enclosed in conduit. PVC conduit is the most common today, as it is easy to install and provides good mechanical and moisture protection.

Equipment ground

Grounding of the electrical system and all equipment is very important in greenhouses to protect people from electrical shock. It provides a reliable path from each piece of equipment back to the electrical system ground conductor in the main service. For example, if a ground fault occurs that involves the hot wire in a motor that is not properly grounded, a person touching it would receive a shock, as they are providing the path for the electricity to get to the ground. Ground-fault circuit interrupters (GFCI) are required in residences, but in a greenhouse, if crucial growing equipment stops operating due to tripping of a GFCI, the result can be loss of crops. Small amounts of leakage current, especially in circuits longer than 100 feet, can cause nuisance tripping of a GFCI. Circuits with electric discharge types of lighting and permanently installed electric motors should not be connected to a GFCI. These should be connected to a good equipment ground wire run with the circuit wires and connected to all metal equipment.

Demand charge

Most greenhouses operations are on a utility schedule that includes a demand charge. This may be able to be reduced by installing efficient motors, controlling the timing of non-essential loads or installing microprocessor controllers that shed some equipment when the demand reaches a specified level.