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..

 

[telectrix]

25A . simple as that.

 

[Sintra]

Use a 25A MCB

 

[Paul.M]

Just use a 25A mcb.

- - - Updated - - -

E3 of us posted the same thing at the same time lol.

 

[spark 68]

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...

Why ?, it should be to your design current Ib also taking diversity into account.

Obviously it is better to use a 32A MCB at the supply end if possible, as this is twice the In of the 16A in the shed giving partial discrimination.
But I see no reason not to use a 20A at the supply end depending on your anticipated load at the shed (power tools etc.), but you know your requirements rather better than us on a forum

Voltage drop needs taking into account too, depending on your Ib.

 

[JUD]

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..

Base your calculations on design current NOT the size of your MCBs i.e. total current = 16A for the sockets and 1.43A for the lights (330 ÷ 230) for a total of 17.43A so would be fine on a 20A breaker.

Voltage drop on 20m of 4mm² is 3.8V at 17.43A. How far is it to the CU after you enter the house?

 

[HappyHippyDad]

Thanks all,

Obviously I feel a bit silly now as I have only seen 6, 16 and 32A MCB and didn't realise there are sizes in between. We have done very little work on MCB's so far on the course, but thats still no excuse.

I'd be really interested to know more about voltage drop and how that affects your decision on cable/MCB's? If you have voltage drop does that actually mean there is more current running through the cable?

I would think I'll be using another 8m's 4mm2 cable in the house.

 

[telectrix]

volt drop..... any cable has a finite resistance. therefore , according to ohms law. V=IR, (V being the volt drop over the length of cable), the volt drop is dependant on both the resistance ( an therefore the length) and the current. tables in BS7671 and OSG give us the VD in mV/A/m for different cable CSA, so you can calculate by mA/V/m x.I x L.

 

[Mr.Ian]

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.

 

[JUD]

So 28m for your distribution circuit gives you a voltage drop of 5.37V which will be included in your total voltage drop for the final circuits.

The maximum voltage drop for your lighting circuit is 6.9V (3% of the supply voltage). This leaves you with around 1.5V to play with. Do you know what the lengths of the final circuits in the shed are going to be?

 

[JUD]

As others have said V = I x R (Voltage Drop = Current x Resistance)

Resistance decreases as csa increases and increases as csa decreases.

Resistance increases as length increases.

Therefore the more resistance there is in a circuit the higher the voltage drop will be which is why you may need larger cables for long runs.

 

[HappyHippyDad]

There is half a meter (1mm) to a 2 gang light switch, then 2m's (from switch) to 2 lights and 2 metres (from switch) to the other light. Also 2m's from shed CU to PIR light on shed front.

As for sockets, around 5m's of 2.5mm cable inside shed.

Thanks Jud for using figures and a real life example to understand voltage drop, I'm finding it easier to understand.

 

[JUD]

There is half a meter (1mm) to a 2 gang light switch, then 2m's (from switch) to 2 lights and 2 metres (from switch) to the other light. Also 2m's from shed CU to PIR light on shed front.

As for sockets, around 5m's of 2.5mm cable inside shed.

Thanks Jud for using figures and a real life example to understand voltage drop, I'm finding it easier to understand.

So to work out the voltage drop for the lights...

mV/A/m for 1mm² is 44.

Let's say we work it out on 100W per light fitting. 2m to 2 lights (200W or 0.87A) would be (44 x 2 x 0.87) ÷ 1000 = 0.08V (hardly worth worrying about). Add this to the 5.37V on the distribution circuit and you get 5.45V which is fine as it's less than the allowed 6.9V.

The maximum volt drop for sockets is 11.5V(5% of the supply voltage) so...

mV/A/m for 2.5mm² is 18.

(18 x 5 x 16) ÷ 1000 = 1.44V. Added to your 5.37V gives you 6.81V so well under the maximum.

 

[HappyHippyDad]

I'm almost there, I understand the maths but the equation is confusing. Telectrix has said that mV/A/m is how VD is expressed for a given CSA in the OSG, but you cant express VD just with CSA you need length and amps. V = I x R so perhaps this mV/A/m is resistance as VD = I x mV/A/m x L which looks very similar?

What is the figure given for mV/A/m (i.e 18 for 2.5mm[SUP]2[/SUP]) expressed in. 18 what? ohms? Or is it just an arbitory figure for a given CSA which is then used in the equation mV/A/m x I x L to find out VD (given length and ampage)?

I reckon this might be my last question!

 

[JUD]

Yes the mV/A/m is a simplified way to work out voltage drop and is expressed in mΩ. It's worked out by using the resistance of 1m of a given size of cable carrying 1amp at 70°C.

So the formula for working out the mV/A/m values is ((R1+Rn) x 1.2) x I. The 1.2 is the correction factor for the operating temperature of the cable. This is because the temperature of a conductor when testing resistance is usually given as 20°C. Resistance increases with temperature by 0.4% per °C. Therefore with the 50°C difference between 20°C and 70°C the resistance increases by 20% (50 x 0.4% = 20%).

 

[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

 

[Guitarist]

Isn't it "resistance is futile" ?

 

[Jimmy Boy]

COMPLETELY off topic..but this dude is unbelievable and if your in to shooting stuff Sky Net is here.

Prototype Quadrotor with Machine Gun! - YouTube