Should I use 4mm or 6mm cable? Or which size MCB?

[HappyHippyDad]

Hello all,

I have a 2 way CU in my shed with a 16A MCB for the radial (3 double sockets) and 6A MCB for the lights (four of them, max usage 330w). I have 20 metres of SWA 4mm[SUP]2 [/SUP]which I will take to the house from the shed and then use 4mm[SUP]2 [/SUP]in trunking to the CU.

I then presumed I would have to use a 32A MCB (in the house CU) as it would have to be greater or equal to the total of the 2 way CU in the shed, but...

The OSG states that 4mm[SUP]2[/SUP] (in trunking) can take up to 30A which is absolutely fine for my shed but the MCB obviously has to trip at less than the current carrying capacity of the cable so it should be between 22A (max of the 2 way consumer unit in shed) and 30A.

It seems crazy to use 6mm[SUP]2[/SUP] when I dont need that many amps and already have 20metres of 4mm[SUP]2[/SUP] SWA.

Am I missing something very simple? What MCB should I use in main CU?

Thanks..

 

[Richard Burns]

Volt drop is expressed as millivolts per amp per meter, which is mV/A/m. This is saying for a one amp current in one metre of cable at that csa the volt drop would be that many millivolts.

Therefore you need to multiply this figure by the number of amps (I) you have as the design current and the length of the circuit in metres (l). Because the result would then be in millivolts you need to divide by 1000 to get your result in volts.

 

[JUD]

As Richard said it can also be expressed as milli-volts per amp per meter.

I said it was expressed as mΩ because in the formula used to calculate the values (I) will always equal 1 so can be excluded from the formula leaving (R1+Rn) x 1.2 which gives you a value of resistance only i.e. mΩ. This value multiplied by the length of the cable gives you a total resistance (R) for the circuit which is then multiplied by the design current (I) to give you voltage drop (V) (V = I x R)

 

[ipf]

Suitably qualified, registered and PP approved I hope.

 

[HappyHippyDad]

Thankyou all for responding. I've been at work for 14hrs and it would have been a pretty boring shift without this thread.

I actually understand VD much, much more now. Thanks alot Jud, you've taken alot of time to explain it to me and put up with all my questions. :smiley2:

Nope.. I'm not qualified, registered or PP.... but I will be in one month

 

[Guitarist]

Nope.. I'm not qualified, registered or PP.... but I will be in one month

You can get qualified AND part P registered in 1 month? I very much doubt that. Your lack of knowledge regarding MCB ratings and cable sizing just makes me worry.
Why the hell did I bother wasting my time with a 4 year apprenticeship?

 

[Strima]

You can get qualified AND part P registered in 1 month? I very much doubt that. Your lack of knowledge regarding MCB ratings and cable sizing just makes me worry.
Why the hell did I bother wasting my time with a 4 year apprenticeship?

Only took me five weeks (apart from the registration bit)... :lol:

Which way does the screwdriver go again?

 

[mickys86]

Thankyou all for responding. I've been at work for 14hrs and it would have been a pretty boring shift without this thread.

I actually understand VD much, much more now. Thanks alot Jud, you've taken alot of time to explain it to me and put up with all my questions. :smiley2:

Nope.. I'm not qualified, registered or PP.... but I will be in one month

I'm guessing your on a 5week course. I'm also guessing that your using this job in your own house to become
Registered. (please correct me
If your wrong) I have no problem with This what so ever. My only concern is what your plans are after one months time.
Are you expecting to go out and start working in people's houses? I'm very frightened for you if this is the case.
I don't care about 5 week courses, each to their own, I have just completed my three years at college and I wouldn't change the way I did it. But I am concerned at all the "qualified" electricians being let out without the most vital part of a qualification, EXPERIENCE. I'm glad for my apprenticeship.

 

[HappyHippyDad]

I do find that however the thread starts (i.e talking about cable size etc) it will somehow get on to... Are you qualified, Part P.. was it a 5 week easy peasy course etc, etc..

I dont really mind answering this question each time I start a thread as its good to put peoples minds at rest and I get a lot of support from this forum....but it probably will get a bit boring in a few years!

Here we go.. I only have one month left, but I have already studied for 5 months (doing 1 evening a week - 3.5 hrs each time). I have to do a lot of theory in my own time, which I do.

Now for the important bit.. When I qualify (in one month), I will get registered and then be legally allowed to work in peoples homes.. WHICH IS CRAZY, so I wont do that. I will spend probably up to a year practising in my own home and also working for free alongside an experienced electrician just to get experience.

I wont do anything in other's homes until I have some proper experience and feel capable and confident.

I'm going to copy this entry for future use :smiley2:

 

[ezzzekiel]

Personally I no longer could care less, government isn't interested, part p schemes providers are not interested, even daring to judge on the forum now can get you into trouble, I no longer take a very active role on the forum and no longer get into the debate of Electrical Trainee or partp as I'm not interested, it's all becoming a bore so feel free to ask and good luck with your career choice, it's not your fault your training could not come from a recognised apprentiship.

 

[mickys86]

I do find that however the thread starts (i.e talking about cable size etc) it will somehow get on to... Are you qualified, Part P.. was it a 5 week easy peasy course etc, etc..


Now for the important bit.. When I qualify (in one month), I will get registered and then be legally allowed to work in peoples homes.. WHICH IS CRAZY, so I wont do that. I will spend probably up to a year practising in my own home and also working for free alongside an experienced electrician just to get experience.:

I did say I'm not bothered how you become qualified, it's what you do with that counts. I'm glad you have the opportunity to go out with someone. I know a window fitter who went out and did a 5 week course and labelled himself 'an electrician' and it was really frustrating listening to him saying
' yea I'm an electrician, I did a course I'm qualified to test etc etc...." then
He went on to say ' god knows how I passed though' and when I asked some basic questions about testing he didnt have a clue what I was on about!

 

[Guitarist]

happyhippydad, I'm sure that your intentions are honourable, but I would doubt that after a few weeks of being "qualified" you will want to work for free.
I bet that politicians go into Government promising themselves that they will never claim excessive expenses....

 

[Paul.M]

Because hiphappydad has conducted himself in an adult manor from the first post, worked out VD formula with the help from others and then thanked them for doing so, this man wants learn and become a good sparky so hats off to you.

The point you make about not knowing that a 20A and 25A mcb's exist worries me a bit. Here's a tip for you. Go to all your local wholesalers and catalog sheds and pick up loads of brochures and look through them all. Loads of different types/models of c/u's, rcds, mcbs, rcbos, fittings & fixtures etc etc etc. If you know what is in the market you can design your install better. Good luck.

 

[ipf]

Because hiphappydad has conducted himself in an adult manor from the first post, worked out VD formula with the help from others and then thanked them for doing so, this man wants learn and become a good sparky so hats off to you.

The point you make about not knowing that a 20A and 25A mcb's exist worries me a bit. Here's a tip for you. Go to all your local wholesalers and catalog sheds and pick up loads of brochures and look through them all. Loads of different types/models of c/u's, rcds, mcbs, rcbos, fittings & fixtures etc etc etc. If you know what is in the market you can design your install better. Good luck.

Probably end up with his head spinning. I would.lol!

 

[Thomas123]

1.5 singles stapled to the fence will do the job.. 240mm for the lighting in the shed then.

 

[Jimmy Boy]

Voltage drop is due to the resistance in the cable over a length, smaller cable higher resistance hence greater drop, it's all in the regs.

And of course external influences, a circuit running through a Boiler room might have a completely different design to one running through a cold storage room, embedded in my mind is the saying.'resistance is proportional to length and inversely proportional to CSA ' if all other factors are equal