Calculation required

Hello All

I was looking to find out what size (Kw/watts) greenhouse heater I could safely plug into the supply I currently have in my greenhouse or not .The greenhouse (8ftx6) is 25m from the house and is connected by a 3 core 1.5mm SWA cable.

In the greenhouse the cable comes in at ground level into a double socket using a gland,the socket is a rcd protected outdoor type and looks like a BG storm. The socket is mounted on a concrete post set into the ground.

The cable is cleated to the concrete fence posts about a foot off the ground all the way to the house then into a wiska box, again with a gland then through the wall to a fused switched spur.

The spur  is connected to a sockets ring in the garage the ring is on a 32amp mcb.

I think the greenhouse socket was originally used to plug the mover in to reach further down the garden.

I plan to have the cable  run to the Consumer unit in due course as it will reach.

Until then any thoughts or calculations on what could be safely used as it is and then when its run directly to the consumer unit would be much appreciated. 

When you say its connected as a spur from the ring ,    are you saying there is actually a spur unit  with a 13A fuse in it  , at the start of the circuit ?  

If thats the case  then you are restricted to 13A    which protects the 1.5 cable  from overload.   You could  certainly go with a couple of 500W  tubular heaters   .  

Evans Electric said:

When you say its connected as a spur from the ring ,    are you saying there is actually a spur unit  with a 13A fuse in it  , at the start of the circuit ?  

If thats the case  then you are restricted to 13A    which protects the 1.5 cable  from overload.   You could  certainly go with a couple of 500W  tubular heaters   .  

Click to expand...

Yes there's a cable from the double socket to a fused spur  and the swa is connected to the load side.

Two 500w tubular heaters would be fine.

Out of interest when the cable is taken to the consumer unit directly would that increase what could be plugged in? and if so to what limit?

If you move it to the Consumer unit , fit a 16A   MCB   /RCBO   .      I can't imagine you needing more than those heaters TBH   Other than a light perhaps. 

You could push it to 3KW   but why would you ?  

No I agree just interested in what a small cable like that could power as i found researching it very confusing.

Will keep to a couple of 500w heaters, only looking to keep it frost free to over winter some plants.

Having a garden room put in next year so will have the electrician look at it when he does the work for that and take his advice as to taking it into the consumer unit.

Thank you very much for your advice.

Cheers Matt

Use a thermostat too, or you will waste a lot of power. 

My greenhouse is a similar size to yours, lined in winter with bubble-wrap insulation. It really doesn't seem to take much to stay above freezing, though I admit I live in a relatively mild area.

I use a 2kw greenhouse fan heater with a built in thermostat, and optimise the setting using a max/min thermometer. Adjust the 'stat so the temperature never goes below +2 or 3 degrees as frost protection. 

Yes a thermostat control built in or otherwise is a must really thanks

I fitted one of these:

https://www.keengardener.co.uk/bio-green-thermostat-thermo-2.html?utm_source=google&utm_medium=cpc&utm_campaign=BIOG-TER2/GB&gclid=Cj0KCQjw2or8BRCNARIsAC_ppyYJ40SNzpyqD-SK5No9WuSsJftAsKoYaSewSYr5nIGIU49YYr5Qnl4aAtQ-EALw_wcB

Into a greenhouse a year or so back - neat and clever. Customer very happy

Thanks for the recommendation 

matt 68 said:

Yes there's a cable from the double socket to a fused spur  and the swa is connected to the load side.

Two 500w tubular heaters would be fine.

Out of interest when the cable is taken to the consumer unit directly would that increase what could be plugged in? and if so to what limit?

Click to expand...

You need to consider the maximum current carrying capacity of the cable so assuming it is a 2 core x 1.5mm cable clipped direct, this is 27A, so your protection device in the Consumer Unit would need to be below this to protect the cable hence why @Evans Electric said to use a 16A MCB or RCBO. If you wanted more, you could look at a 20A device or replace the cable to 2.5mm.