I have a greenhouse that I built and wired myself. Four weeks ago, on a beautiful sunny day when no one was home, the circuit breaker ‘popped’, shutting down my ventilation system and heating the GH to about 130F or so. I lost almost half of my orchid collection, including many plants I have grown for 10 years. This was the second time that happened, and it will be the last. I am taking 2 directions to prevent the problem. First, I have re-designed my ventilation control system to provide some redundancy. Second, I am tying in my monitor system to my house security system. I am looking for some general advice based on experience.
I have a two stage ventilation system. The lower temperature system is passive, opening 2 power louvers (~ 1/2 amp ea continous when open) and a 22 ft ridge vent. When the temp gets too warm, the ridge vent closes and a pair of 1/6 hp exhaust fans turn on. There is a large inside fan (1/3 hp, heavy duty industrial totally sealed) that provides air circulation 24/7 except when the exhaust fans are running. The ridge vent does require power to open or to close. The ridge vent, the louvers, the exhaust fans and the thermostats are all made by companies that make greenhouse ventilation equipment. The drive motor for the ridge vent is a Baldor 1/6 hp reversing gear motor that I supplied. Rotation direction is controlled by a DPDT relay (20 amp rating).
On two occassions my ventilation has shutdown while it was warm. On both occassions I found the ridge vent closed, which means that either the ventilation system failed trying to open the first stage or the exhaust fans were running and something tripped the breaker. The ridge vent requires 5-10 seconds of power to close after the exhaust fans turn on. Originally I had a GFCI breaker on this circuit. However, an electrician told me that wasn’t required and I had just replaced it with a standard breaker a few days before the last diasaster. The breaker is a 15 amp breaker and all wiring is 12 ga. outdoor wiring. I ran a conduit through the ground and have conduit throughout the GH. The control system is a collection of thermostats, relays and limit switches.
The curcuit breaker has plenty of capacity (15 amp) to handle the load, including startup. It is a dedicated circuit. My wiring is clean (no shorts, etc.). This system works 99% of the time. I think that the problem is related to the exhaust fans, but I can’t prove it. I have tried disconnecting one fan at a time to determine which one might be bad but it hasn’t failed. Maybe a single fan isn’t enough to cause the failure. I have purchased a power meter that I will install on different devices to record peak power to try to determine what sometimes pulls too much power.
Can anyone make any further comments, suggestions, ideas, critism or share further experiences or insight?
I and my plants thank you!
Replies
The first thing that comes to mind is that the motors all tried to start at the same time; thereby causing a surge on the inrush which tripped the breaker. Motors start-up draws are usually about twice the running draw. You might want to try putting the smaller motors on a time delay relay so they can stagger the start-ups.
Just an idea
locolobo; Edmonton, AB
Maybe seems too obvious, but if you have 12 gauge wire, why not just put in a 20 amp breaker?
It is just one of my fetishes. I always use 12 ga wire. I always install bigger boxes than I think that I need. I always try to include extra capacity. The reason that I don't want to try a 20 amp breaker is that, by all reviews, the 15 is sufficient and I really do have a problem somewhere. However, I have a saying, if you look properly and long enough and you don't find a problem, maybe there isn't one. I may just do that with the watt meter to moniter power draw and continue looking.
surge on the inrush which tripped the breaker. Motors start-up draws are usually about twice the running draw
That is undoubtedly what it is.
Inrush can be a lot higher than twice. like 10X. Dont have my NEMA standard handy, you should be able to find the NEMA peak starting current (locked rotor amps, LRA) on the web, most motor nameplates list LRA or a letter (you gotta go to the NEMA spec to see the LRA values that correspond to the letter)
Put a 20A breaker on the line and run a second line for the 2nd fan.
Edit PS - many fans have notoriously high inertia and long spinup times which exacerbates the peak starting amps - e.g the current stays above 15A with both motors starting for longer than the breaker trip curve allows for.
Edited 10/17/2005 3:14 pm ET by junkhound
SCSI computer drives have an option called "delay start" that is ID*some amount of time, specifically to avoid this type of problem. I wonder if something similar is available for fans? Not that I'm suggesting you convert a bunch of SCSI drives to fans or anything, though that might be interesting...
Well, overbuilding is one of the best characteristics of DIY in general, so, to follow that thought, why not put each motor or significant load on its own circuit. Those orchids are expensive.
I do not understand "The ridge vent does require power to open or to close" if it has a motor. It also seems that if the temp is too hot the ridge vent would stay open and not close.
But I am not an electrician or greenhouse person. If my system was wired with 12ga and otherwise protected I would use a 20 amp with that many items with startup loads.