greenhouse: electrical instalation question Ask questions and find answers to many subjects relating to electrics and electrical work

[georgegreco]

Great site :)

I live in finland and plan to grow tomatoes and other veg in a greenhouse and plan to use 6 600 watt HPS lights as light levels are pour here ,
I'm running a cable from my house which will supply electricity to all these lights but would like to hear how I can overcome any cutoffs due to overload;
the max power needed is around 3800watts for 12 hours every day,
please advise on any hardware i need and possible correct configuration
thank you very much
george greco

[kbrownie]

georgegreco,
I'd like to help but as i'm not sure of the regs or your supply voltage and frequency in Finland. I'm unable to do so.
In the Uk domestic Voltage is supplied at 230v and at a frequency of 50Hz,
We would need to make a few calculations and circuit would also need to be protected by RCD or RCBO.
Regards
KB

[ericmark]

British rules and regulations are likely to be different to yours and most of what is needed when doing a project like you talk about is the following of regulations. We may need a TT supply that is one with an earth rod because there is so much metal used in building a green house. A 16 amp supply only needs a 1.5mm² cable but because of the earthing we may need to use 6mm² cable in order that the steel wire armouring would be the equivalent of 10mm² depending on earth system used. Because it is lighting we have maximum of 3% volt drop that’s 14 meters of 1.5mm² cable but 59 meters of 6mm² cable.
Not sure how you intend to switch lamps I would suggest a switch on each one as if all switched on together you may get an inrush problem. 16 amp is out limit for lighting so I would think a 16C or 16D MCB would be used. The whole system would need protecting with 30ma RCD.
Not sure on how you intend to control but you may want to use light cells through a relay so using a larger consumer unit so relays will fit inside consumer unit would make neater job. I would use two light cells and relays or a delay on relay with normal one so there is a delay in switching on last lamp to stop inrush problems. I seem to remember you can get a relay where some contacts are delayed and some switch straight away this would be ideal only need few seconds delay so star / delta delay unit may fit the bill.
With availability and regulations being different we can only give rough guide it I was doing job much of the design would depend what my supplier had in stock and prices.
Not sure on lamp types for growing plants you would need to find someone who has some experience and in this country that would mean drugs so they would not be likely to admit what they knew in case they were caught.
All best your electricity must be cheaper than here would be cheaper to buy the tomatoes! Eric

[georgegreco]

Thankyou Kbrownie and ericmark for your detailed responses,
I should've added that these lights are connected to ballasts of the appropriate wattage. I believe these work as buffers and to maintain a consistent flow of current to the lights. As regards the finish system, there are no differnces with the british system.
The issue remains, how can I draw 3800 watts of power from the same plug without causing the house fuses to blow.
I have learned that my electricity is light industrial ie three phase as opposed to domestic power supply.
Your help is very much appreciated,thank you

[ericmark]

At about 5 amp per phase this is different story. But a ballast takes a fair inrush and I would treat like a three phase motor. Moeller do a PKZM0 overload which also allow the use of aux contact blocks so you can connect an alarm should it trip. These are three phase units and will stand the inrush I think Telemecanique do a very similar unit which in both cases acts as both overload and switch and can with some versions be locked off for maintenance. Some have turn switch and some have push buttons seem to remember push button version slightly more expensive but lockable in off position. These are designed to go with contactors so if you want automatic light sensed operation they will match up to contactor but unlike normal contactor overload do not have to be used with contactor. Because these units have a variable current setting in this country they have to have that part covered if to be used by ordinary person to stop people altering the settings.
Just looked in RS components and Allen Bradley 140M Range, and Telemecanique GV2-Pxx range where xx is max current rating.
These units are not cheap and if you want cheap then a 10D three phase MCB will do the job as with most electrical items you get what you pay for and if used as switch and overload the standard MCB will not last that long and you would need a contactor and control circuit and by time you add all together most likely same price.

[georgegreco]

Thank you eric, this was very useful as I understand the configuartion.
Pity i cant pm you or give thanks as in other forums,
anyways cheers and keep up the good work
regards
george greco