Help in re-wiring a long 350m outdoor run

[JohnW2]

for 350m with a 3kw rating clipped direct that would be ....

'Clipped direct' to the soil?

Kind Regards, John

 

[holmslaw]

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[holmslaw]

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[JohnW2]

With VD at 2.8mV/A/m

Yes thats correct for a temperature of 70°C, do you live in a particularily hot part of the uk. A 16mm cable carrying 13A will be as cold as a witches tit and therefore the tabulated vd will be much lower.

I guess that depends upon what you mean by 'much lower'. The tempersture-resistance coefficient of copper wire is around 0.004 per degree C. Hence, as a rough and ready 'in the head' calculation, if one assumes that witches' tit s are about 20 degrees, the 50 degree difference between that and the temperature (70 degrees) for which VDs are tabulated would correspond to a roughly 20% difference in resistance (hence roughly VD, since resistance is the dominant component of impedance in cables up to about 16mm²). So, one might expect that the 2.8mV/A/m might fall to somewhere around 2.2 mV/A/m at 20 degrees C (a proper calculation would obviously be needed to determine exact figures). For 350m of cable, that would allow about 9A total (i.e. about 2kW) before hitting the maximum permitted VD for lighting.

Kind Regards, John.

 

[ban-all-sheds]

Thanks for doing that.

There's a useful calculator for you here holmslaw.

You might like to try it, to check I've used it correctly, because it appears that your "witches tit" temperature needed to get 350m of 16mm² down to a low enough resistance for 13A to drop 6.9V is -147.7°C.

How are you going to arrange for that?

And can we now look forward, from you, to

a) An apology?
b) Ridicule and insults in a doomed attempt to deflect attention from your tw*ttish behaviour?
c) Wriggling and twisting, ditto?
d) None of the above?

 

[JohnW2]

With VD at 2.8mV/A/m, and (presumably) lighting in the workshop?

The lower VD limit for lighting specified in Appendix 12 of the regs is a bit of a pain in situations like this, and may tempt one to light one's distamt sheds and workshops with candles! However, I wonder if there is not a way around this ....

Appendix 12 is only 'informative' but 525.3 indicates that adhereing to that appendix will result in one being deemed to satisfy 525.1 and 525.2. That presumably means that one is free to satisfy 525.1 and 525.2 in other ways is one wishes.

Could one not have ELV lighting powered by stabilised and/or input-voltage-tolerant (e.g.switched mode) power supplies - indeed, in it's crudest form,12V DC lighting run from a car battery that was floating across a charger! The 'lighting' itself would then be receiving the voltage it wanted, regardless of the supply voltage - which I think one could easily argue satisfied 525.1 and 525.2, even if there was more than a 3% drop in the supply. The extra 2% one might therefore 'gain' could save a lot of money, particularly if one is talking about 350m of SWA! What do folk think about that?

Kind Regards, John

 

[Lectrician]

Lets see your calcs then Holms....

If I had done calcs I would not have said "something like 3c16mm swa"

BAS did the incorrect calcs because he has no experience in sizing cables.

So instead of standing on the sidelines sniping, why not advise everyone of a suitable cable size.

Me, sniping? You want to look back over your recent posts....

I placed a smiley at the end of my comment for a reason. I have ommited one on this occasion.

I was jesting at the complexity of the calculation, not you.

 

[holmslaw]

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[holmslaw]

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[ban-all-sheds]

Don't need it.

Actually it appears that you do.

I'm prepared to believe, though, that you don't understand it.

So what.

So your claim

A 16mm cable carrying 13A will be as cold as a witches tit and therefore the tabulated vd will be much lower.

is nonsense.

Care to do the calculation, with or without the use of that website, to show that the voltage drop will be much lower at your chosen wt temperature?

As you're so in awe of the authors of the BRB the calculation you need is this.

That's nonsense holmslaw claim #2, because the book clearly says "This equation applies only where ... the actual ambient temperature is equal to or greater than 30° C.

So as you said "A 16mm cable carrying 13A will be as cold as a witches tit"....

it is nonsense to anyone with an ounce of common sense,

Don't judge everyone by your own standards, holmslaw. It's not your "common sense" which leads you to dismiss that formula, it's your inability to understand it.

Something else to get your head round, IET say max vd for lighting circuit is 3% of 230v = 6.9v, so for a 240v supply it can drop to 232.1volts.

But ESQCR say my supply voltage could be -6% of 230v = 216.2 volts.

So in the former I'm not allowed a lighting circuit at 232, but in the latter my lighting circuit could be at 216.2v - 6.9v = 209.3v and it complies.

Actually at 20°C, assuming a resistive load, your 350m of 16mm² will drop 10V.

So no - it does not comply. Try to get your head round the fact that supply voltage variations don't mean that you can ignore voltage drop.

So I'm quite happy with the 350 mts of 3c16mm swa for a 3kw load.

I'm sure you are.


Anyway - I reckon that was c) Wriggling and twisting in a doomed attempt to deflect attention from your tw*ttish behaviour.

Nothing else to say,

Thank heavens for that.

bye, bye.

There's no need for you to be in any hurry to return.

 

[Paul_C]

Would the regs allow the use of a step up transformer to send power over the 350 metre at say 460 volts to reduce the current by half and then step down at the far end ?.

My thought immediately upon considering a potential 3kW or so load over that sort of distance (over one-fifth of a mile). Convert up to something like 600V and you're looking at a much more manageable 5 amps at full load.

 

[riveralt]

We need to rewire a long outdoor run from a house to a workshop approx 230m away through many trees, and to a further shed another 120m on from that (total run 350m).

We're looking at replacing it with a single run of proper SWA cable. Is 3-core 1.5mm OK or should it be thicker because of the voltage drop? Any thing else we should be doing (like extra RCDs in the workshop/shed?)

All advice gratefully received,
many thanks

Just to get back to the OP - Bob you can see that from the 'debate' there is some significant variance in the size of the cable needed to accommodate your desire to provide power to the shed.

Clearly your 3 core 1.5mm SWA is not okay and something in the range of 16/32mm cable would be required.

The price range for 16mm SWA cable hovers around £8 per meter and the price of 32mm SWA is around £12 per meter. Even 25mm SWA cable is around £10 per meter.
So just to provide the cable - never mind the labour - digging the trench or as JohnW2 suggests 'clipping it direct to the soil' is going to average around £3500.
It may be worth you while investing in a
http://www.justgenerators.co.uk/pages/portable_generators.htm
and getting a spark in to wire up you shed and outbuilding to accommodate this.

 

[JohnW2]

That equation is not really applicable in the context of the discussion above, for a couple of reasons:

• 1...It only applies when ambient temperatures is 30 degrees C or above. Given that, it actually seems strange that they have included it in the UK Wiring Regulations!
  
  2...this equation is not really applicable when, as in this discussion, the actual (on load) 'operating temperature' of the cable is known, assumed or asserted (in this case, "as cold as a witch's' tit") - as you can see, one cannot 'plug in' an operating temperature into the equation to get a correction factor for that temperature. What this equation is doing is providing a correction factor for VD in situations in which the operating temperature is less than the maximum permitted (tp) because design current (Ib) is less than the tabulated maximum CCC of the cable (It) at the maximum operating temperature of (70 degrees C in the case we're discussing). If one knows or assumes the operating temperature, a far simpler calculation such as I outlined last night is probably adequate.

Having said all that, if one forgets the silliness about witch's whatsits, one can apply that equation to the situation of interest (if one assumes ambient temp of 30 degrees C), For 16mm² 2-core SWA, without grouping and with an ambient temp of 30 degrees C, buried in ground (Method D) the correction factor becomes about 0.87, so the tabulated VD of 2.8 mv/A/m would reduce to about 2.436 mv/A/m if the design current was 13A - i.e. a VD of about 11.08V for 350m and 13A (unacceptable for lighting, just OK for power).

A little bit of maths enables one to derive a useful answer in terms of the maximum possible decrease of VD one can get by using a cable sized very generously in relation to the design current (hence low heating and lower operating temperature). With no grouping, 30 degrees C ambient temp and a maximum operating temp of 70 degrees C, the above equation reduces to:

• Ct = 0.866 + 0.133(Ib²/It²)

The most extreme case possible is when the design current, Ib, is zero, in which case it can be seen that the correction factor simply becomes 0.866. In other words, no matter how generous one is in sizing cable relative to design current, the lowest VD one can get (using that equation) is 0.866 times the value tabulated for 70 degrees C.

Kind Regards, John.

 

[OwainDIYer]

Would it be cheaper to run as uninsulated overhead on poles?

Depending on the loads in the workshop/shed would batteries and an inverter be feasible, trickle-charging the batteries over the long wire, which would then be a lot smaller?

 

[JohnW2]

Something else to get your head round, IET say max vd for lighting circuit is 3% of 230v = 6.9v, so for a 240v supply it can drop to 233.1volts.
But ESQCR say my supply voltage could be -6% of 230v = 216.2 volts.
So in the former I'm not allowed a lighting circuit at 232, but in the latter my lighting circuit could be at 216.2v - 6.9v = 209.3v and it complies.

If you look at the first sentence in Appendix 12, you will see that the 3% permitted drop for lighting is 3% of the nominal voltage of the installation (i.e. 230V), regardless of what supply voltage you are actually getting.

Kind Regards, John.