Installing a wall light

[pressld2]

I am installing a wall light and it will be much easier to connect it to the ring main than to try and connect it to the lighting circuit. I know I have to use 2.5mm ring main cable on the supply side of the FCU but can I use 6amp lighting cable on the load side?

[plugwash]

you can use 1.0mm on the load side

no sizes smaller than this should be used for normal fixed wiring

[pressld2]

Thanks Plugwash!

Just for my own interest, how does the fcu prevent the full 30 amp load on the ring main from travelling across the light I'm installing? Without blowing the fuse in the fcu?

[plugwash]

hmm if your that ignorant about electricity i wonder wtf you are doing with wiring in the first place

the voltage stays constant

the current will (under normal conditions) be only what the appliances need to draw

if equipment develops a fault and draws too much current or worse shorts live-neutral or live-earth directly we have protective devices like fuses and mcbs to cut off the faulty equipment before any real damage is done

[pressld2]

plugwash wrote:

hmm if your that ignorant about electricity i wonder wtf you are doing with wiring in the first place

I'm starting to wonder that too - but I'm learning (I think)

I'm actually struggling to understand what a friend told me. He said that the reason for installing an FCU and not just tapping off the ring main directly is to prevent the full 30amp load going through the light. If I've understood you correctly, what you're saying is that if everything is working correctly then the only current on the spur will be what the light bulb draws. However there is the potential for the full 30 amp load in the case of a fault or short on the spur - hence the fuse in the fcu to protect it. This makes sense to me but my friend seemed to be implying that the fcu was somehow preventing the full 30amp load travelling over the spur under normal conditions.

Thanks again for your help.

[plugwash]

pressld2 wrote:

said that the reason for installing an FCU and not just tapping off the ring main directly is to prevent the full 30amp load going through the light.

under *normal* conditions 30A will not go through the light

pressld2 wrote:

If I've understood you correctly, what you're saying is that if everything is working correctly then the only current on the spur will be what the light bulb draws. However there is the potential for the full 30 amp load in the case of a fault or short on the spur - hence the fuse in the fcu to protect it.

in the event of a short a *LOT* more than 30A will flow (we are talking from just over a hundred amps up to thousands of amps here) however provided the cuircuit is properly designed this current will not flow for very long before a protective device operates (mcb trips/fuse blows)

the main reason for the fcu is in case there is a fault that doesn't draw enough to rapidly trip the 30A/32A fuse/breaker on the ring (ie is not a dead short) but is still dangerously high for lighting equipment

[ban-all-sheds]

pressld2 wrote:

I'm starting to wonder that too - but I'm learning (I think)

Start learning here.

And here: http://www.kevinboone.com/electricity.html

And here: http://www.kevinboone.com/domesticinstallations.html

And here: http://www.diydick.com/site-main-button-index-pages/electrical-menu-frame.htm

And stop "learning" from your friend. I'm sure he's a fine chap, salt of the earth, would do anything for a mate, good man to have around in a crisis, always gets his round in etc, but either he's useless at explaining things or he's clueless about electricity.

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I mustn't warn people that the "experts" on the plumbing forum can't be trusted to tell the truth.