Electrical for a Greenhouse

[imagineer]

I’m only a few (more like 5) years late on my promise to build Mrs. Imagineer a greenhouse, but the project is finally underway.

The Greenhouse will be about 11’ square inside and about 10’ at the roof peak. The “foundation” will be a perimeter of 2x12 that is anchored to 4x4 posts set in concrete. The frame of the greenhouse will be extruded aluminum storefront material (the same stuff Wal-mart entries are made with). The 4 walls and roof trusses will be welded as separate pieces, then bolted together and lagged down to the 2x12s. The glazing will be 8mm twin wall Lexan.

This weekend I’ll be digging to access a ¾” PVC conduit that runs close to the SW corner of the greenhouse. The ¾” conduit was run at the same time as the new electric service to the pole barn. It has Cat5 and Coax cable in it, neither of which are being used, so I’m re-tasking it for the new 30amp service for the greenhouse, which bring me to a few electrical questions.

Is it ok to use a “Main Panel” as a “Sub-Panel”? The sub-panels I’m finding do not have a main switch to de-energize the panel if needed. When I had the new 100amp service run to my pole barn, I used a GE 100amp/24 circuit main panel. It’s fed off a 100amp 2-pole breaker in the electrical in the house. I like that the barn panel has a main switch on it so I can easily kill the power to the barn when needed. I’d like that same feature for the greenhouse.

I plan to pull 2 – 10ga and 1 - 12ga through the existing ¾” PVC conduit for the 30amp electrical service to the greenhouse. This will be fed from a 30amp, 2 pole breaker in the electrical panel in the house. The question, is it ok to use a 12ga for the neutral? For your reference, it will be about 55’ of wire from the house panel to the greenhouse panel.

I plan to drive a ground rod at the greenhouse and bond it to the neutral buss on the electric panel as well as to the extruded aluminum frame of the greenhouse. Any reason not to do this?

I really do not want to have conduit on the outside of the perimeter 2x12, so I’m planning on running the re-directed conduit under the perimeter frame and coming up inside the greenhouse. Any reason I shouldn’t do this either?

 

[TractorJeff]

1. I believe most of the Sparkies will state that the Neutral needs to be the same size as the 10ga wires. Down sizing a Neutral is usually because there are 240v loads in the building.
2. I think the Sparkies will treat this as a Building and want the Neutral and the Ground Bus bars isolated from each other thus requiring 2 Ground Rods not 1.
3. I do not foresee a problem with running the Conduit up inside the Structure.

In conclusion then requiring pulling a 4th Wire (Green) as a Ground Conductor back to the Source. This Wire could probably be 12ga?

As far as "Bonding" the Aluminum Frame to the Ground Rods(2), I believe it would be a good idea!

 

[pattenp]

For a 30A feed all four conductors need to be #10 Cu. You should install two ground rods at the greenhouse at least 6ft apart on the same electrode conductor.

 

[wyliesdiesels]

1. I believe most of the Sparkies will state that the Neutral needs to be the same size as the 10ga wires. Down sizing a Neutral is usually because there are 240v loads in the building.
2. I think the Sparkies will treat this as a Building and want the Neutral and the Ground Bus bars isolated from each other thus requiring 2 Ground Rods not 1.
3. I do not foresee a problem with running the Conduit up inside the Structure.

In conclusion then requiring pulling a 4th Wire (Green) as a Ground Conductor back to the Source. This Wire could probably be 12ga?

As far as "Bonding" the Aluminum Frame to the Ground Rods(2), I believe it would be a good idea!

isolating the neutral bar in the panel does not change the number of ground rods required for a subpanel. Code requires 2 no matter how the panel is setup, unless one can prove 25ohms or less to earth...

ground wire needs to be #10

 

[imagineer]

Thanks for the input. I'll run all 10ga and use two ground rods.

I've not found an outdoor rated panel with a main switch, so I'll be using an 8 space sub-panel. It won't be that inconvenient to have to run down to the basement to de-energize the greenhouse when needed.

 

[Norcal]

Use a 12 space panel with a backfed main breaker it will require a hold down kit on the main breaker to comply with code.

 

[pattenp]

A panel in a detached structure that has more than 6 breakers to kill power to all circuits needs a main disconnect at the detached structure.

 

[imagineer]

A panel in a detached structure that has more than 6 breakers to kill power to all circuits needs a main disconnect at the detached structure.

Does the main disconnect need to be on the inside or outside of the detached structure?

BTW, I got the conduit work done this past weekend, but I'm holding off pulling the wire until there's a roof over the structure. For the record, pounding in 8' ground rods with a small sledgehammer is not easy.

 

[nh_yota]

Why do you need a 30 amp service to such a small greenhouse?

 

[mrVanagon]

Don't need a sledge for driving ground rods, just a bottle of water.

 

[imagineer]

Why do you need a 30 amp service to such a small greenhouse?

For the heater. Based on the minimum temperature Mrs. Imagineer wants to maintain in the greenhouse over winter, and the calculated heat loss, a suitable heater will pull 18-20 amps.

 

[imagineer]

Don't need a sledge for driving ground rods, just a bottle of water.

I tried that, and also using a hammer drill. Neither worked. Our soil is almost all clay. With the water trick, at about 15-20" down, I wasn't able to pull the rod back up without clamping a vice grip to it and levering it using a 2x4.

 

[mcbane]

Don't need a sledge for driving ground rods, just a bottle of water.

Not everyone has marine mud for soil. In my location, the soil is alluvial till. You use a rotohammer with 36” bit to find a location with no boulders near the surface. Then you use a pavement breaker to try to drive a rod. If the rod deflects off a rock below the 36” drill hole the breaker has enough power to continuously bend the rod as it passes that obstruction. Maybe 1 in 5 rods, the rod hits a boulder dead on and stops. In that case the breaker is only able to slowly mushroom the head of the rod. Time to cut the rod flush with the ground surface and try again elsewhere.


Sent from my iPhone using The Garage Journal mobile app

 

[Sumboodie]

For the heater. Based on the minimum temperature Mrs. Imagineer wants to maintain in the greenhouse over winter, and the calculated heat loss, a suitable heater will pull 18-20 amps.

4800 watt heater?

 

[larry4406]

Can't help on the electric but I am hoping to see your build of the greenhouse. I would like one one day...

 

[nadogail]

If a panel does not have an installed entrance breaker, you feed it from a disconnect switch?

A 30 amp fused disconnect, if I remember correctly, was inexpensive.

 

[imagineer]

Can't help on the electric but I hoping to see your build of the greenhouse. I would like one one day...

Here's a rendering of the frame on the base, looking at the south east corner...

 

[imagineer]

4800 watt heater?

Actually we're going with a 3000 watt main heater (which I already have) and a smaller 1500 watt heater as a backup should the temps really plummet.

 

[brianh]

I made a greenhouse a few years ago for growing greens and starting plants for the big garden. For overwinter growing I put incandesent lights over the plants on a thermostat I grow cold tolerant stuff in the winter spinach, mustard greens, arugula, beets, lettuce ect.

I also have a tarp that goes over at night makes a huge difference in heat retention I have since doubled its size it is great to have fresh salad when 2 feet of snow is on the ground.

North wall is insulated with 3" foam south wall has the first 30 inches.

 

[imagineer]

I made a greenhouse a few years ago for growing greens and starting plants

Nice work.

 

[jacric2005]

I'm considering solar for my 12X24 horse trailer cover. However, not that much use compared to a greenhouse. Probably a couple of LED shop light in the peak. It's a ways away from an outlet and don't want to run an extension cord.

 

[imagineer]

Thought I'd share a couple of pics of the project. Despite my limited spare time to work on it, I'm actually about a week ahead of schedule. The conduit is run, but I'm holding off pulling wires until the house is weather proof. The base and floor (brick pavers) are done and frame is assembled.

The aluminum extrusions used for the frame are the same storefront extrusions you would find in Walmart entries. The 4 walls and roof trusses are all TIG welded. the 4 walls a re though bolted together at the corners and the whole thing is lag bolted to the 2x12 base. The roof supports between the trusses are held in place with brackets and rivets.

I added cross bracing and last weekend, got the polycarbonate paneling installed on the south wall. My goal is to have it done and ready for use by the end of September.

 

[imagineer]

Ok, the green house is done and in use. Mrs. Imagineer has it full up of plants and as winter is approaching, is concerned about the possibility of a power outage, and not being able to run the greenhouse heater.

My plan . . . I install an extra 30amp/2 pole breaker in the greenhouse electric panel and wire it to a weatherproof 2 pole socket mounted on the outside wall of the greenhouse. This new breaker would remain off while there is power coming from the main house panel. In the event of a power failure, I would throw the switch on the greenhouse electric panel (isolating it from the main house), turn on the extra breaker, then plug a cord between a generator and the outside socket on the greenhouse, essentially back feeding generator power to the greenhouse electric panel. After main power is restored, I would disconnect the generator, turn off the breaker to the outside socket, then throw the main switch on the greenhouse electric panel.

Is this a stupid idea? (and yes, I assume its against electrical code)

Some extra info . . . In the main house, have a generator panel with the few necessary circuits (furnace, well, microwave, misc lights) run through it. I.e., if/when the power goes out, I wheel the generator outside and fire it up. We have a large 220v cord that plugs into the generator and into a weatherproof socket on the outside of the house. After sufficient generator warmup (3-5 minutes), I throw the switch on the generator panel, isolating the necessary house circuits from the main power and connecting them to the generator. It’s not the most convenient set up, but it has worked for us for over 12 years.

Note, the circuit from the main house feeding the greenhouse is not on the generator panel. Also, there is no more spaces available in the generator panel. Even if there were space, I’d be opposed to adding the greenhouse to the generator panel as our portable generator is only 5500 watts and isn’t large enough to run the few house circuits AND the greenhouse heater.

Yes, we’ve looked into getting a larger generator and are considering taking the plunge and having an automatic 12K watt, NG fuel Generac installed, however we probably won’t be able to get it installed before winter hits.