Power Supply


Registered User
Hi I have run a cable underground 2.5 armour plated to my shed,the cable is about 50 metres in length,in the shed I have a light and a power point for a freezer and on two occasions my freezer seems to partly defrost,my question is am Ioosing power due to the length of cable.

50th VFR

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Could simply be that shed has become too cold for freezer to work. Check minimum operating temperature on freezer.
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Registered User
this was just a second hand freezer I got for nothing I didn't once think about it being too cold to operate properly.


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50th VFR is bang on the money with his thoughts here ;),

I know it sounds stupid but some of the modern upright fridge freezers (especially the big twin door american style type) will go into cut-out mode if the surrounding temperature becomes too cold, they then have to be re-booted to get back to normal working, and this cannot be done until the surrounding outside temperature raises itself to something like +5*C, (guessing a bit at this figure as I cant remember the actual setting right now)
At a push some of these fridge/freezers can be fooled by putting some form of fan heater near to the air intake, but this is not as simple as it sounds, as a lot of them take their air from all around the four sides of the base,
And yes I do know, as I've been there, done that, Had to get ride of the nice posh big freezer in the end :shakehead:, Got around the problem by buying a second hand older chest freezer (possibly about 15yr old) These older chest freezers don't seem to be effected in the same way, (Well the one I got isn't anyway) and that's been working well (in the same position as where the newer upright one wouldn't) for at least the last 8yrs or so :sing:,


Registered User
We had a chest freezer there before but I got fed up digging through finding what I wanted. I took the fridge freezer over the skip earlier today so cannot check any operating temperatures but probably think now that it has been too cold for it to operate properly,thanks for pointing me in the right direction.
The drop you are allowed is 4% of the supply voltage taken at 230v =9.2 volts.

2.5mm cable has a voltdrop of 18mV /per amp/per metre.


9.2/(0.018x50)=10.2 amps as a maximum supply current.

But that doesn’t account for your earth loop impedance, Zs which will need to be under a certain value to prevent you getting frazzled when you touch your freezer


r1+r2=0.014 ohms/metre x 50metres = 0.7 ohms.
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The maximum Zs for a 10amp type 'B' breaker for a 0.4 second disconnection time, using the 80% method is 0.87 ohms.

So question, is the external impedance of your supply less than 0.17 ohms?
All I can say is, at a guess, it’s more likely it is than isn’t but only just.
You are topped out at that. So don’t run it in anything bigger than a 10 amp breaker.

Is it on an rcd ?

For anyone else reading it’s hardly worthwhile running this length in anything but a minimum of 6mm or maybe 4mm at a push if you really don’t want to spend the money. It’s as much about complying with the regs under fault conditions as it is for handling the load. Cables generally will handle loads when maxed out, but what you really need to think about is the protection of people when a fault occurs. A bigger cable always offers a bit more scope when it comes compliance or changes of use over time.
It’s not good to squeak in over the line by the skin of your teeth.

Oh and I think Pete is probably right...
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Another reason for wanting to comply is Part P of the building regs.
An external supply like this is now noticeable works.
(Well so what, there’s no one to enforce it I hear you say)
Technically it’s a criminal act. But if the work is ok they won’t bother coming after you.
If it’s not ok and to the point of being dangerous then they might well have you and fines of £4K have been heard of.


Registered User
Thank you all very much for replies,this underground cable was in when I bought the house in 1995 and in view of Rawhides replies I think I will get it checked out as I am no electrician.


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I went through something similar when buying a freezer for our detached garage which is around 25m away from the house. You must ensure that the freezer you buy is capable of cooling when in low surrounding temperatures. Most shut off and cause problems as you describe.

Fortunately they do make freezers for exactly this situation that operate down to -15 and a partial list can be found here: https://markselectrical.co.uk/fridg...MIitbLveDd3wIVb7vtCh34KwOfEAAYASAAEgLIs_D_BwE


Registered User
yes it is on an rcd and the cables armour is connected either end I am not sure if there is an earth cable inside as I have never looked ,we had our rcd changed last November so I will ask our electrician to look at this link to the shed to see if it is safe meanwhile the fridge freezer is over the skip and probably not going to be replaced Thank you very much for your imput. Mike.