During the process of designing our new house, I had made arrangements for an outside 50-amp 240-volt line to be made available to my shed and greenhouse area. The builder had ensured this capability by running an empty conduit from an electrical box just outside our house to a location just outside the concrete slab that later became the floor of our shed. Once a decision would be made to install the wiring in the shed and greenhouse, the needed wires would be pulled through the conduit using a vacuum device.
The first step in electrifying the greenhouse was to wire the shed, including a sub-panel that could later be used as a base for extension to the greenhouse, which is adjacent to the shed. Then, conduit was put underground, between the shed and the greenhouse to allow for extending the 240-volt service to the greenhouse.
The design for the electrical service in the greenhouse required knowledge of the different types of equipment I anticipated I would be utilizing in the greenhouse and which ones would be in service at the same time. To do this, I had to do some research regarding the hydroponics and grow lighting systems I would be installing. I will describe the hydroponics system and the grow lighting system separately, but at this point, I have simply listed the electrical requirements for the greenhouse in this table.
The result of my research indicated that I would need two 240-volt receptacles and about ten 120-volt receptacles. The contractor suggested that it would be easier to put in twelve 120-volt receptacles, since they come in gangs of four. However, a strange thing happened while I was away playing tennis. The contractor mistakenly installed 24 120-volt receptacles instead of the agreed upon twelve, as can be seen in the accompanying photo. Each bank of 12 120-volt receptacles is protected by a 20-amp circuit breaker. There are two more 20-amp 240-volt circuit breakers, one to control the receptacle for the grow light system, the other to control the receptacle for the backup electric resistance heater. This adds up to 80-amps, which is in excess of the total amperage available to the circuit serving both the greenhouse and the shed. More about this anomaly below.
All of these loads are protected by connection to my whole-house emergency generator. In the event of a power outage, the generator would automatically start up to keep the lighting and other loads in the greenhouse going. The LP gas heater would be unaffected by a power outage, except for its blower, which is non-essential. But, if the gas heater was unavailable for some reason (e.g., out of fuel, clogged fuel line, faulty thermostat, etc.), continued operation of the electric resistance heater would be essential, so putting it on a line supported by the generator is a good idea. In an emergency, use of the grow lights would not be essential. Therefore, in an emergency, the grow light system could be turned off to allow the electric heater to operate. That kind of tradeoff might have been necessary if I didn’t have as large a line as 50-amp.
Regardless of the number of receptacles provided, there is a limit to how much current can be run through them all at the same time. A minimal amount of 120-volt load is taken up by the lighting in the shed, maybe two or three amps. The total for all the anticipated loads in the greenhouse is just short of 40 amps. (Using a 240-volt heater and 240-volt ballasts for the grow light system halves the amount of current that would have been required if 120-volt versions of this equipment was used instead.) There is just enough capacity to support the intended load. However, if one were to plug in a vacuum cleaner or other appliance with a sizeable motor, it is conceivable that the 50-amp circuit breaker for the entire line could be tripped. Therefore, I will have to be vigilant not to exceed the design load just because there are enough receptacles to plug in more appliances.
One more wrinkle to deal with is the question of what kind of 240-volt receptacles need to be put in place for the electric heater and the grow light system. My research indicated that the electrical resistance heater requires a different style of receptacle than the grow light system. Our strategy is to wait until the equipment is delivered to make sure the correct styles of receptacle are installed.
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