Greenhouse install

[FREEBALL]

I am wondering what type of conduit for a greenhouse job. Ive seen a lot of opinions so Im steering away from PVC, that and my own witness to PVC in a Greenhouse I am working on. I am thinking IMC a little better than EMT and lighter for the cross braces. But what I see in the PVC boxes is a lot of bending rupturing seals and the like. I was told the GFIS were removed from these boxes because they kept tripping and the heat mats would not operate. The one pvc box is actually melted inward where the pvc enters it. Im thinking extreme heat, but why wouldn't the breaker trip. The entire greenhouse is underserved and is expected to be serviced to new service soon. The existing greenhouse is fed from a 30 amp circuit 250 ft away to a ML 8 cir. panel branching into 2 20 amp breakers, and then into the greenhouse 50 additional feet. The farm mngr said they never tripped a breaker only the GFI's. There has to be considerable voltage drop for the size of load and length of run. Anyhow would like ideas and thoughts on both issues.

As always thankyou


Jeff

 

[Dennis Alwon]

I have always used emt

 

[growler]

I was told the GFIS were removed from these boxes because they kept tripping and the heat mats would not operate.

Wonder why those GFCIs kept tripping? Yes, get rid of those things, nothing but problems and the world is over populated anyway.

On the other hand you may want to check those heat mats.

 

[growler]

The one pvc box is actually melted inward where the pvc enters it. Im thinking extreme heat, but why wouldn't the breaker trip.

Why doesn't a breaker trip when you plug in a space heater ( lots of heat ) but not enough current draw to trip a breaker.

A corroded connection acts like a resistor, heats up but not a high current flow.

 

[Fulthrotl]

I am wondering what type of conduit for a greenhouse job. Ive seen a lot of opinions so Im steering away from PVC, that and my own witness to PVC in a Greenhouse I am working on.

Jeff

every greenhouse i've seen (all of about 3 of them), used pvc or emt.

i'd use schedule 80, pvc. what emt i've seen has decomposed pretty badly
with the humidity. as in disintigrated completely.

i'm not understanding the issue of heat in the boxes. where is the heat
coming from? bad connections, or are the conduits right next to a source of heat?

 

[dfmischler]

My family has had greenhouses all of my life: mostly glass, some double-poly, and a bit of polycarbonate. Some of it is wired in EMT, some of it in RMC, and some in PVC.

Most of the EMT is OK, but a few places have had to be replaced. The RMC advantage is that it doesn't need to be supported as often. PVC is great for wet areas, but supporting it every 3 feet can be a challenge in a greenhouse unless it is in/on the floor, or following structural members. In a few spots we have PVC fastened to glazing bars, and you need to take precautions against expansion problems due to the large temperature swings in the top of the greenhouse.

Think about how hot things must be getting to deform your PVC boxes/seals. I would also assume resistive connections because plants are not going to be happy at the environmental temperatures that would allow PVC to deform (160 degrees F plus).

Heat mats leaking enough current to trip a GFCI is certainly a problem we've heard here before. How many mats are you trying to run on each GFCI? How much leakage current does the manufacturer say you should expect? Of course, if the existing wiring is in bad shape and devices are getting wet then that may increase the leakage current seen by the GFCI.

 

[FREEBALL]

dfmischler

There are heat mat cord connections with 2 doub duplex recpt and 800 watt max w/ 200 watts per mat. They have taken out the gfis to stop the tripping of those due to leakage, but I don't see why, if they are connecting too many, the breakers aren't tripping. There is one pvc fs box in particular that is sunken in at the top from melting. I see the issue of the weakest link in a resistance type load circuit but man. any thoughts.

 

[dfmischler]

We do not have electric heat mats in any of my family's greenhouses. We have seed germination chambers heated by hot water (stainless steel basin with a water heater element controlled by a thermostat), and we have a bench with mist watering and root zone heat (PEX tubing carrying hot water pumped from a gas-fired water heater; pumps and valves are managed by digital controls) for early growth of seedlings and cuttings.

However, I do know that the usual cause of electric heat mats tripping a GFCI is due to current leakage to ground from the wiring or the mat itself. Even a brand new mat in perfect condition is likely to leak some current to ground through capacitance, and I can only imagine that older mats may experience additional leakage through cracks or other imperfections in the mat. This example propagating mat spec sheet unfortunately does not list the expected leakage current. I guess you would have to talk to the manufacturer to try to find out what they know. I'm also not sure of the best way to measure the leakage current, but here is an article from Fluke (trying to sell equipment, but still informative).

Note that 800 watts of power draw is only about 6.7 amps, so nowhere close to popping a 20 amp breaker, but if each mat leaks 2 mA then the total leakage of around 8 mA for 4 mats will trip a typical GFCI because you exceeded the 5 mA of leakage it allows.

I would never compromise on employee safety by eliminating a GFCI simply because it trips: something is wrong and needs to be fixed, or redesigned to present less leakage current to the GFCI. One option might be to use a separate GFCI for each heat mat right at the point of use if that seems appropriate.

On the PVC overheating issue I would say that nothing should be heating up the PVC conduit or boxes if appropriate wiring and devices are used with in-use covers as needed, and I would have no problems recommending PVC conduit for a greenhouse as long as everything is installed correctly.