Power cable to workshop/garage

[EFLImpudence]

Yes, I am saying the same as you.

Why, then, when a 30A BS3036 (20A/0.725 = 27.6A) fuse is replaced by an MCB is it not allowed to use a 40A MCB or -

Why, for new ring finals, do we not use 1.5mm² conductors?



As the 0.625 ( 5/8 ) is only just over half what advantage (today) is there in installing ring finals at all?
A 30A BS3036 ring could have been replaced by two 25A radials.

 

[jmac80]

so...
If i ran a 10mm tc+e, that should have me covered for supplying a garage cu ?

 

[JohnW2]

so... If i ran a 10mm tc+e, that should have me covered for supplying a garage cu ?

That certainly ought to 'have you covered', but it's very probably more than you need! You really have to approach this systematically - firstly decide what maximum load you envisage ever needing on the circuit, then select the MCB/fuse size which is at least as great as that and then determine was size of cable would be required with such a fuse/MCB. Finally, as has been mentioned, you really should check that both voltage drop and loop impedance are acceptable with that cable - although, in your case, that is most unlikely to be an issue.

Kind Regards, John

 

[JohnW2]

Yes, I am saying the same as you. Why, then, when a 30A BS3036 (20A/0.725 = 27.6A) fuse is replaced by an MCB is it not allowed to use a 40A MCB or - ...

I assume you are talking about ring finals. For the reason you imply, it has never made any sense at all to me that 433.1.103 allows a ring final protected by a 30A BS3036, when it restricts one to 32A with an MCB - as you say, these are not consistent. ... you therefore would have to ask the authors of BS7671, not me!

Why, for new ring finals, do we not use 1.5mm² conductors?

The short answer is, of course, that the regs say that you mustn't, unless it's MICC. As you imply, provided one uses Table 4D5 (and not 4D2A!), or singles (Table 4D1A), the CCC of Method C 1.5mm² is 20A - so why 2.5mm² cable with a CCC of 20A is allowed, but 1.5mm² cable with a CCC of 20A is not allowed is, again, something you would have to ask 'them' I suppose they might be sceptical about whether people always pay proper attention to installation method and de-rating factors, and therefore are being a little cautious/'pragmatic' - with 1.5mm², anything short of pristine Method C throughout the cable run would bring the CCC below 20A, whereas with 2.5mm² it's far more likely that the CCC really will be at least 20A

As the 0.625 ( 5/8 ) is only just over half what advantage (today) is there in installing ring finals at all?

Well, that's obviously a whole separate debate, and I need not tell you that some people see no point in, and/or don't like the idea of, ring finals. I'm personally pretty neutral, although I am not all that keen on the idea of 20A multi-socket circuits, since that theoretically can't even support two 13A loads. However, I'm perfectly happy with 32A radials as an alternative to rings.

Kind Regards, John

 

[jmac80]

so... If i ran a 10mm tc+e, that should have me covered for supplying a garage cu ?

That certainly ought to 'have you covered', but it's very probably more than you need! You really have to approach this systematically - firstly decide what maximum load you envisage ever needing on the circuit, then select the MCB/fuse size which is at least as great as that and then determine was size of cable would be required with such a fuse/MCB. Finally, as has been mentioned, you really should check that both voltage drop and loop impedance are acceptable with that cable - although, in your case, that is most unlikely to be an issue.

Kind Regards, John

Thanks John.

I'll go with a 10mm SWA cable i think but will confirm with a sparky first.
SWA is not much more expensive for a relatively small distance like mine (12m).
One think i was thinking though, How much can 10mm swa be 'bent', can you do a 90° turn? Never held the stuff in my hands but looking at the pictures i'd imagine that is a no?
Cheers

 

[JohnW2]

Thanks John. I'll go with a 10mm SWA cable i think but will confirm with a sparky first. SWA is not much more expensive for a relatively small distance like mine (12m).

Fair enough - although, as I implied, over that distance you'd probably only need 10mm² if your power requirements in the future are going to be pretty high. You might also discuss with your electrician whether it really needs to be SWA - given that it's an attached garage, that might not really be necessary.

One think i was thinking though, How much can 10mm swa be 'bent', can you do a 90° turn? Never held the stuff in my hands but looking at the pictures i'd imagine that is a no?

Virtually any cable can be bent through 90°, the question being how large a radius of bend would be required - you obviously cannot have a 'square corner' in any cable! I'm not very used to handling 10mm² SWA, so others here can probably advise what sort of radius you could realistically expect to achieve with it (quite apart from 'rules'!).

Kind Regards, John

 

[ban-all-sheds]

http://www.batt.co.uk/products/view/383/BS5467-SWA/PVC-Cable-IEC-60502-600/1000V

http://www.bs6724.co.uk/

So about 380mm.

 

[ban-all-sheds]

you obviously cannot have a 'square corner' in any cable!

Is a waveguide a cable?

 

[JohnW2]

you obviously cannot have a 'square corner' in any cable!

Is a waveguide a cable?

No - at least, not in my book! (and I'd really love to see a 50Hz waveguide ).

Kind Regards, John

 

[ban-all-sheds]

A waveguide carries electrical energy from A to B. Mentioning 50Hz merely reminds us that frequency is a factor to consider, and that at some frequencies cables might need to become hollow.

 

[ban-all-sheds]

you obviously cannot have a 'square corner' in any cable!

Is a PCB track a cable?

 

[JohnW2]

A waveguide carries electrical energy from A to B. Mentioning 50Hz merely reminds us that frequency is a factor to consider, and that at some frequencies cables might need to become hollow.

... and, in some hypothetical cases, unbelievably large!

Are you having a boring day, by any chance?

Kind Regards, John

 

[JohnW2]

you obviously cannot have a 'square corner' in any cable!

Is a PCB track a cable?

Nope.

You clearly are having a boring day.

Kind Regards, John

 

[bernardgreen]

A waveguide carries electrical energy from A to B. Mentioning 50Hz merely reminds us that frequency is a factor to consider, and that at some frequencies cables might need to become hollow.

The skin effect only starts to have an effect at very high frequencies.

Being pedantic there is no such thing as a square corner. Nearest is a right angle corner. But a square has four corners.

Some high power ICs have miniscule magnetic pellets to pull the electrons round sharp bends in power tracks. Apparently without the pellets the electrons were hitting the atoms at the edge of the track and moving them, thus distorting the bend, weakening the track and / or shorting to adjacent tracks.

 

[ban-all-sheds]

you obviously cannot have a 'square corner' in any cable!

Is a PCB track a cable?

Nope.

Is a copper conductor with a rectangular cross section a cable?

You clearly are having a boring day.

Quite the opposite.