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Help needed with shed wiring setup running off of kitchen ring main

Postby temple-of-seven » Fri Oct 16, 2015 2:57 pm

I have installed a 15 meter length of 2.5mmsq 3 core SWA to my shed and intend to attach it into the kitchen ring which is the closest ring which is 32 Amp breaker protected. The SWA will terminate in the shed on a plastic 4 way consumer unit with a 100A main switch.

The house consumer unit is RCD protected already as it's recently been upgraded to current specs.

The kitchen ring runs a 2.3KW cooker so 9 Amps, a kettle of 2.2KW so another 9 Amps and a 800W microwave so 3 Amps. 21 in total. 11 to spare according to the breaker in the main consumer unit.

I intend to install a fridge freezer and fluorescent light in the shed so can’t see it drawling more than around 7 amps.

My question is, should I

1.Terminate the SWA in the kitchen onto a 13 Amp 1 gang RCD connected into the ring, replacing the 13 Amp fuse with a 10 Amp.

2.Terminate the SWA on a standard 13 Amp double pole fused spur into the ring replacing the 13 Amp fuse with a 10 Amp?

In both instances I’m unsure if I should
Earth the armour of the cable to the rings earth end only, observing attaching the internal earth core of the SWA to the ring earth and shed consumer unit earth bar?

Earth the armour of the cable to the ring earth end and at the consumer unit earth bar in the shed, observing attaching the internal earth core of the SWA to the ring earth and shed consumer unit earth bar?

The shed is installed onto concrete paving of roughly 6 inches deep and I intend to seal the shed base with several layers of bitumen to prevent rot and provide additional electrical insulation.

Help please Temple!
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Simply Build It

Postby ericmark » Sun Oct 18, 2015 2:54 am

In house you need a fused connection unit (FCU) where connecting to ring.

No real point in a consumer unit in the shed a switched FCU as the light switch with 3 amp fuse is all that is required.

Fridge/freezer in the main have minimum ambient temperature which is higher than found in out buildings and also very strict volt drop limits. Some of the new units are three phase with an inverter drive these still say don't use extension lead but in real terms don't have the same problem with volt drop. It's down to start current which is lower with inverter units.

Chest freezers are often OK in cold sheds as long as volt drop is OK, in fact work better. But clearly a fridge can't maintain 4 degs C if colder than that in the shed. Although my inverter fridge/freezer has solenoids to control fridge and freezer independent that is unusual. Normally the fridge gets a percentage of the sent to freezer and relies on the unit being used between +10 and +32 degrees C (climate class SN) or +16 ~ +32 (climate class N).

One problem is auto defrost or frost free it needs enough heat to stop water freezing in the drain pipes.

Start amps on inverter freezer is less than the 160W used for the defrost heaters. The old fridge freezer was more like 10A on start. So you need a line - neutral impedance of around 1.52 ohms to be within volt drop limits. The ring should be 1.44 ohms at worst so 5 meters is about the max without actually measuring the impedance.

The Martindale EZ150 socket tester has 1.5 ohm as it's lowest range not a clue why since the rules ask for lower than that on a ring final but as far as you are worried if it shows 1.5 ohms then you are good to go with freezer able to run in those temperatures.
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