10mm cable and 6mm cable

[PrenticeBoyofDerry]

In my opinion it is less than ideal, for circuit end to end identification reason also that documentation will need to state two CSA core sizes.
Plus you will always be calculating on worse case, what if someone decides to now move oven to another location, joins an additional length at the existing oven point via junction box, using 10mm cable?
Gets a little confusing and assumption start to be made, that could lead to dangerous outcomes.
Same CSA for unless discrimination is being offered!
As far as voltage drop that should be addressed at the design stage and I would consider joining two differing core sizes in same circuit, under one protective device not particularly well thought out.
It may well help with Zs reading but if r1 and r2 are sufficient (again design issue) and no external loop problems. Why would you?

 

[JohnW2]

In my opinion it is less than ideal, for circuit end to end identification reason also that documentation will need to state two CSA core sizes.

Fair enough, but documenting the two cable sizes would not be significantly more arduous than documenting one size - and I would have thought that having different CSAs at the two ends of the circuit make it less, rather than more, likely that one will erroneously identify the two ends of the cable.

....what if someone decides to now move oven to another location, joins an additional length at the existing oven point via junction box, using 10mm cable?

That's different. Any increase in cable CSA along the length of a run is obviously always going to be potentially dangerous, particularly if preceded by a decrease in CSA (which may be hidden). We are talking about a decrease in CSA.

As far as voltage drop that should be addressed at the design stage and I would consider joining two differing core sizes in same circuit, under one protective device not particularly well thought out. It may well help with Zs reading but if r1 and r2 are sufficient (again design issue) and no external loop problems.

I am talking about 'design stage' ('ideal, or non-deal design'). You say that two different cable sizes is 'not particularly well thought out', but surely it could be quite the opposite? In the case of very long circuits, one may have 'very carefully thought out' the fact that using two cable sizes was the most efficient way to achieve the desired/required VD and R1+R2?

There's not that much difference between what we're talking about and having, say, a 4mm² radial final with 2.5mm² unfused branches ('spurs', if you wish), is there?

Kind Regards, John.

 

[PrenticeBoyofDerry]

I would have thought that having different CSAs at the two ends of the circuit make it less, rather than more, likely that one will erroneously identify the two ends of the cable.

Why? You have found 10mm core at CU, logically you would be looking for 10mm core at end of circuit "oven!"

That's different. Any increase in cable CSA along the length of a run is obviously always going to be potentially dangerous, particularly if preceded by a decrease in CSA (which may be hidden). We are talking about a decrease in CSA.

So we don't take the potential for alteration in to consideration?

I am talking about 'design stage' ('ideal, or non-deal design'). You say that two different cable sizes is 'not particularly well thought out', but surely it could be quite the opposite? In the case of very long circuits, one may have 'very carefully thought out' the fact that using two cable sizes was the most efficient way to achieve the desired/required VD and R1+R2?

Explain how?
Having continuous length of cable, that offers the suitable values would be much better thought out, rather than having joints in cables where the potential for failure is higher!
Can't see how reducing cable CSA along a route is going improve efficient of Vd and r1+r2.

There's not that much difference between what we're talking about and having, say, a 4mm² radial final with 2.5mm² unfused branches ('spurs', if you wish), is there?

There is, as that is common and expected.

 

[17thman]

During a recent PIR i discovered that the voltage drop was calculated at at 7% for the 2.5mm2 swa serving an outbuilding. I calculated that it would be possible to replace the section of swa from the cu, around the length and width of the house with 6mm2 swa to satisfy voltage drop requirements. The length of 2.5mm2 swa from this connection to the outbuilding remained in situ. I left a circuit description attatched to the cu for this modified circuit.

Everyones happy. No Electrical concerns.

 

[JohnW2]

I would have thought that having different CSAs at the two ends of the circuit make it less, rather than more, likely that one will erroneously identify the two ends of the cable.

Why? You have found 10mm core at CU, logically you would be looking for 10mm core at end of circuit "oven!"

Exactly. So, if there were no 10mm² cable appearing at the oven end, there would be no risk that you would accidentally 'identify' it as the oven circuit because of its CSA.

That's different. Any increase in cable CSA along the length of a run is obviously always going to be potentially dangerous, particularly if preceded by a decrease in CSA (which may be hidden). We are talking about a decrease in CSA.

So we don't take the potential for alteration in to consideration?

What you were postulating would be as worrying and potentially dangerous in the 'normal' situation as in the one we're contemplating. If you found a 6mm² cable feeding an oven and decided to run something from it in 10mm², the worries/potential hazards are exactly the same whether that 6mm² runs all the way back to the CU or whether it changes into a higher CSA cable before it gets to the CU, aren't they?

In the case of very long circuits, one may have 'very carefully thought out' the fact that using two cable sizes was the most efficient way to achieve the desired/required VD and R1+R2?

Explain how?

Less copper and less money required to achieve the required circuit parameters? In more down-to-earth terms (albeit not initial circuit design) 17thman has also just posted another example.

Can't see how reducing cable CSA along a route is going improve efficient of Vd and r1+r2.

As above, if you can achieve the required VD and R1+R2 without running the whole circuit in the higher CSA cable (the difference between the 10mm² and 6mm² we're talking about is quite a lot), then isn't that 'more efficient'?

Kind Regards, John.

 

[JohnW2]

Everyones happy. No Electrical concerns.

Indeed so - a very sensible pragmatic solution, IMO!

Kind Regards, John.

 

[PrenticeBoyofDerry]

We will take op situation as an example.
So typical costs
10mm @ 2.72 per m
6mm @ 1.50 per m

Total run 10.5m
4m in 10mm = £10.88
5.5m in 6mm = £9.00
plus 60A junction box £5.20
(Plus screw fixing and PVC sleeving)
£25.08

If completed in 10mm
10.5m in 10m = £27.20

There is difference of £2.12 in cost.
Then lets add to the two cable run, the extra labour for making off cables, terminating them and securely fitting accessory and time taken to inspect and test the accessory.
Don't see any efficiency there!

 

[JohnW2]

There is difference of £2.12 in cost. Then lets add to the two cable run, the extra labour for making off cables, terminating them and securely fitting accessory and time taken to inspect and test the accessory. Don't see any efficiency there!

I agree that there will be little, or no, gain in 'efficiency' with a total cable run of only 10.5m, but remember that what I said was:

In the case of very long circuits, one may have 'very carefully thought out' the fact that using two cable sizes was the most efficient way to achieve the desired/required VD and R1+R2?

If you repeated your calculations for a total run of, say, 40m or 50m, your answers would obviously be a bit different.

Having said all that, I agree that, in practice, it will rarely make a lot of difference in terms of 'primary design'. As 17thman illustrated, it's more likely to be a useful approach (and probably pretty efficient, particularly in terms of labour time/costs and 'possibly disruption of the property) as a 'secondary' step, as an alternatively to re-wiring a complete circuit in fatter cable.

Kind Regards, John.

 

[PrenticeBoyofDerry]

As 17thman illustrated, it's more likely to be a useful approach (and probably pretty efficient, particularly in terms of labour time/costs and 'possibly disruption of the property) as a 'secondary' step, as an alternatively to re-wiring a complete circuit in fatter cable.
Kind Regards, John.

It's definitely a solution to a problem but the problem should have been sorted out at design stage! Not as a reactive measure.

 

[JohnW2]

It's definitely a solution to a problem but the problem should have been sorted out at design stage! Not as a reactive measure.

Agreed - but history is history, and (just like so many things), electrical installations (and requirements of electrical installations) evolve. Going right back to square one is, I suppose, always going to be the true 'ideal', but it's often not going to be a very realistic real-world approach.

The electrical trade is (together with a few others) different from many other walks of life, in that the same person is often responsible for initial design, initial 'production' (installation) and subsequent maintenance and modification etc. When, as in some other fields, those activities are undertaken by three or more different people, or groups of people, one often sees that the viewpoints of those three groups differ apreciably.

Kind Regards, John.