Supply to Greenhouse

[rick]

Would appreciate any advice taking power to a greenhouse. Greenhouse about 10 metres from house. Intended loads are lighting and sockets for 3 kw heater and propagator. Supply is TN-S. Run 3 core SWA from FCU off downstairs ring main (on RCD board) inside house with armour tied to earth at house end only. Inside GH terminate SWA in double socket and from there take rubber cable to light switch and on to luminaire mounted at apex of roof. All fittings inside will be IP65 / IP66.

Does that sound about right, and if not any corrections happily received.

Also, does the GH need to be bonded, and if so, bond to armour, CPC or TT spike? I have read many conflicting opinions on this...

 

[Strima]

I would bond the whole lot to the house earth, reason being you have RCD protection on the board, armour and CPC. Dependant upon EFLI readings I would also contemplate sticking a couple of rods in to make sure.

- - - Updated - - -

Just noticed, Biggleswade, not far from me!!!

 

[rick]

Hi Strima

Thanks for your reply. Zs to GH end is fine so unsure about the spike. Also unsure about bonding the armour at both ends as surely that can induce parallel paths or have I misunderstood what you are saying, i.e. bond the GH to the CPC?

And I am still unsure on that - (a) there are many screwed joints in the GH frame and (b) it is powder coated anyway...?

 

[somersetsparks]

Hi Strima

Thanks for your reply. Zs to GH end is fine so unsure about the spike. Also unsure about bonding the armour at both ends as surely that can induce parallel paths or have I misunderstood what you are saying, i.e. bond the GH to the CPC?

And I am still unsure on that - (a) there are many screwed joints in the GH frame and (b) it is powder coated anyway...?

Theres nothing wrong with paralell paths! I would just use 2-core SWA, armour as earth, connected both ends, stake for good measure and bond the greenhouse frame at one point.
I would not seperate the greenhouse earth from the house.

 

[spark 68]

Anything over 2kW is recommended to be on it's own circuit, so maybe a bit much to pull off the RFC from the house.

Edit: The FCU @ 13A will just about supply your 3kW heater, what about your propagator and lighting ?

I would think about supplying the GH from it's own MCB (20A ?) and using a suitable CSA for your armoured cable, taking into account voltage drop, and then a mini CU in the GH.

 

[somersetsparks]

Although thinking about it Rick, it may be best to bond the metal frame of the greenhouse to the MET in the house unless you can prove it is not and exterraneous conductive part, this is probably best achieved by using the 3-core, using the 3rd core as earth, and using the armour to provide the bonding. You would have check this is feasable with the tables somewhere on this forum, i can't be bothered!

Otherwise you could TT it, either way as as been pointed out, a seperate circuit would be nice although maybe not absolutely necessary.

 

[rick]

Thanks for the post chaps.

spark68 - not sure about a CU in the GH as would have to be IP rated? And in any case no way to run a separate circuit from CU on this one although I like the idea and would prefer that if it were possible.

somersetsparks - don't know if the GH would be considered an exterraneous conductive part or not. What you are suggesting is to bond the GH to the armour not the CPC?

I am getting really confused now!

 

[spark 68]

Hi rick,

The CU would have to be IP rated for the environment.

It is getting late now, I will have a look at the regs tomorrow wrt horticultural locations for you if nobody else has given you the correct info. in the mean time.

This is also notifiable work under part P.

You need to also consider your design current at the GH to calculate the size of the armoured cable, the supply OCPD and numerous other things such as Voltage drop, additional loadings on any circuit you plan to modify and Bonding arrangements etc..

It is not a straightforward install, but I think you already know this.

Cheers,
S 68

 

[rick]

Hi Spark68

Yep not simple hence my post! Have looked at the big book myself and it was not clear to me. So interested to hear your input. Ref. Part P, I'm with Elecsa, but hey I think the whole scheme thing is a massive waste of time... but perhaps that discussion is for another thread!!

Look forward to hearing from you.

 

[somersetsparks]

Rick the size of the armouring and the 3rd core together may well be large enough to act as a combined cpc and bonding conductor, depending on which size armoured you used. I think you are going to end up having to bond it and this bond will have to go back to the MET, In my book you can't really spur that amount of load from one point on the ring anyway, a 3kw heater alone is nearly half the ring final circuits capacity.

To prove whether the greenhouse needs bonding or not, you would have to run a wandering lead from the MET to the greenhouse frame and if the reading is above 22.5 kOHms, it does not need to be bonded.

 

[HandySparks]

Just a thought, but it would have to be a gert big greenhouse to need 3kW of heating, especially if a layer of insulation, eg bubble wrap, is fitted inside the glass for winter use.

 

[telectrix]

Just a thought, but it would have to be a gert big greenhouse to need 3kW of heating, especially if a layer of insulation, eg bubble wrap, is fitted inside the glass for winter use.

3kW heating is just about right for growing weed.

 

[rick]

Thanks for all the pointers - I think the answer is to bond back to the MET to be on the safe side.

Ref. separate circuit that would be ideal but not at all easy in this case - I think the answer is to scale down the heater (3kW is way more than needed but I was trying to be future proof in case a larger greenhouse is purchased later on...) and the RFC in question only serves two bedrooms!

 

[spark 68]

Hi Rick,

I had a good look through GN7 today, The main points being the equipment installed have to be suitable for the environment, IP rated etc. (which we already knew), everything requires 30mA RCD protection (which we also knew).
There are several requirements to do with PME which is not applicable to you.

The metalwork of the building needs to be bonded back to the MET if it becomes extraneous or an exposed conductive part, this will need to be min 10mm copper, or equivalent if using the armour (depending on size of armoured cable).
If it is neither of these then bonding need not be applied, you may be better here glanding the armour in to either a plastic gland plate or a plastic adaptable box etc

There is another possibility here and that is to TT it, and earth the armourings at the supply end and insulate the gland at the GH end, and rod it also at the GH end as a caravan pitch is done.
Please note by TT ing it this is not to get around PME regs as is usually the case, but to simplify any bonding at the GH side as this then would not need to go back to the MET in the house, just the local rod earth bar/EMET at the GH.
The problem doing it this way is that you are already planning to have it on the house RCD, which then would mean two 30mA RCD's in series.

This brings us to the next problem your design current as it stands is too large to run off of a FCU and off the RFC in the house, as your heating is 3kW by itself.
You need to know your design current to work out the size of the armoured cable, the supply OCPD (if not spurred of the ring), and also take into account the voltage drop.

You would also need some IP rated equipment in the GH whichever way you go to split the single feed between lighting and sockets, this could be as simple as a weather proof SFCU fused down to 5A, this could double up as your light switch.

All electrical fixtures, fittings and accessories as well as being IP rated (min IP44, may need to be higher) and be class II fittings.

I hope this helps.

 

[HandySparks]

There is another possibility here and that is to TT it, and earth the armourings at the supply end and insulate the gland at the GH end, and rod it also at the GH end as a caravan pitch is done.
Please note by TT ing it this is not to get around PME regs as is usually the case, but to simplify any bonding at the GH side as this then would not need to go back to the MET in the house, just the local rod earth bar/EMET at the GH.
The problem doing it this way is that you are already planning to have it on the house RCD, which then would mean two 30mA RCD's in series.

I'm puzzled, S68. Why would TTing the greenhouse mean that an additional RCD is required? Just trying to understand your reasoning. Thanks.