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Topic Title: Cable Sizing ERA vs BS7671
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Created On: 02 May 2013 09:33 AM
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 02 May 2013 09:33 AM
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mrh105

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A project that is being built in the UK (with a French client) is using cable current carrying values provided by ERA Technology. This was a contract stipulation. The values differ compared to BS7671 especially with larger cables buried in the ground.
For example, for a 185mm XLPE/SWA 3-phase cable buried directly in the ground, BS7671 table 4E4A recommends the maximum current carrying capacity is 281 amps. The equivalent ERA Technology value for the same size cable is 458 amps.
Can anyone advise of the implications (legal etc) of using ERA values instead of the values in BS7671. Obviously, I am concerned that cables may be undersized.
 02 May 2013 11:55 AM
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AJJewsbury

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Probably one for OMS, but my first thought would be to compare the conditions for each set of ratings. The laws of physics don't differ, so I suspect the ERA ratings are for a lower ambient temperature, higher conductor temperature or lower thermal soil resistance.

Some of those differences might have knock-on effect that BS 7671 would be interested in - e.g. conductor temperature at terminations, or would require correction factors to bring them into line with your site's characteristics.

- Andy.
 02 May 2013 02:46 PM
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OMS

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You need to compare apples with apples - as Andy says, the laws of physics don't change.

The BS 7671 tabulated values are at worst case of depth of lay, ground temperature, spacing etc.

Having had exactly the same set of design criteria imposed on me EDF project), I evaluated the data using the relevant corrections to BS 7671 to match those in the ERA report, took acount of cyclic loading etc and the numbers were pretty well identical.

I would start with a table of the criteria that influence the cable thermal capability and re evaluate the BS 7671 data with corrections against that stated for the ERA cable data.

You can then adjust either set of data for your actual installd conditions for BS 7671 compliance

Regards

OMS

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 02 May 2013 06:27 PM
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John Peckham

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Yes I have noted that the ERA cable ratings are higher than BS7671. If you have a look at the Draka cable calculation AP on your phone or the on line version you can choose the ERA method or the BS7671 method for the calculation. The ERA method always comes out with a smaller CSA. I assume the designer and the inspector who have to sign the electrical installation certificate for compliance with BS7671 have to declare the departure from BS7671 on the front page of the certificate if the ERA method has been used?

OMS

I don't see how you can adjust the data for BS7671 compliance?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 02 May 2013 06:42 PM
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OMS

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I don't see how you can adjust the data for BS7671 compliance?


Grouping, actual temperatures, depth, etc etc John ? - as I said, for the installed conditions.

Both ERA data and BS 7671 data are stated for refernce conditions - each set of data, when baselined against the same reference conditions is about the same - like any circuit, you then adjust for actual installed conditions.

In simple terms, ERA cable data (ERA 69-30) is for an Air ambient of 25C, ground temp of 15C, soil thermal resistivity of 1.2k.W/m and 0.8m depth of lay.

Compare that with BS 7671 and you'll see why the cable current ratings differ.

If you now correct the BS 7671 data with the factors in Tables 4B1, 4B2, 4B3 and 4B4 you should see reasonable correlation with ERA 69-30)

From there, you can then correct either set for the actual installed conditions for BS 7671 compliance - as you would for any circuit

regards

OMS

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 02 May 2013 07:32 PM
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weirdbeard

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Hi,

Sorry to the OP for being no help whatsoever, but having never heard of the ERA before, a quick google of OMSs ERA 69-30 brought up drakas site as mentioned by JP and this quote from there:

"There are two optional criteria for sizing buried cables. BS7671 uses a reference ground temperature of 20 degC and thermal resistivity of 2.5 K.m/W. At Draka we feel that this does not represent the soil conditions in Britain which are closer to a ground temperature of 15 degC and thermal resistivity of 1.2 K.m/W. Using ERA criteria (69-30 parts III & V) will give a result more representative of British conditions."

Isn't it a bit odd that BS7671 doesn't represent the conditions found in Britain?

Is it just me or is electrical installation is the only industry in this country where over-engineering is encouraged by the regs?
 02 May 2013 07:48 PM
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OMS

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Is it just me or is electrical installation is the only industry in this country where over-engineering is encouraged by the regs?


How so - if you know the ground conditions then just adjust the BS 7671 numbers to suit (I gave the factors tables above)

If you happen to have ground conditions more similar to the ERA refrence criteria, use that.

As I said, just compare apples with apples.

A good engineer will know how to deal with this and eliminate the "over engineering" in favour of adequate engineering.

That said, cable is cheap compared to excavation and ducting and given a design life of perhaps 40 years and knowing how load "creep" occurs, then there is no point in taking on undue risk and undersizing.

If the client suspects you are being a bit profligate with his money then good old I2R can show the savings overall in using bigger cables up front - it also saves a few polar bears

Regards

OMS

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 02 May 2013 08:26 PM
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weirdbeard

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Hi OMS, dumbing down a bit re the overengineeing, taking an example of domestic smokes detectors, they are a power circuit, powering only a very few watts, but they must be wired in 1.5mm, over a large estate of houses thats a fair waste of copper?
 02 May 2013 08:39 PM
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Legh

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A good engineer will know how to deal with this and eliminate the "over engineering" in favour of adequate engineering.


Sorry to be APITA,

By 'Good Engineer', Do you mean one that has a moral obligation for a solid workable solution or one that works to a 'quality' manual that might specify a workable solution within a specific time, usually called a 'Manufacturer's Warranty' ?

I know its a delicate balance but as we know with various industries at present there appears to be quite a few recalls.....

Legh

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 02 May 2013 08:56 PM
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OMS

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Originally posted by: weirdbeard

Hi OMS, dumbing down a bit re the overengineeing, taking an example of domestic smokes detectors, they are a power circuit, powering only a very few watts, but they must be wired in 1.5mm, over a large estate of houses thats a fair waste of copper?


But the reason is "robustness" in a life safety system - not the load supplied - it's a bit like bonding sizes - in many cases a 6.0mm2 minimum is still far too big - but we use that minimum as the basis of it won't easily get disconnected by a bit of rough and tumble

Regards

OMS

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Failure is always an option
 02 May 2013 09:01 PM
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OMS

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I don't have any morals Legh - I'm a consultant.

"Good Engineer" is just the phrase I picked to describe someone who knows how to read the data and deploy it in a risk averse contractual strategy.

That may be client risk or my risk or a combination of both - ideally I'd want to shift all the risk to a contractor - but sometimes we have to work for our money - and we are quite good at stating assumptions and limiting the parameters over which we might fail - so we tell clients what the design is based on - and more importantly where the limits of failure are - if they then operate outside those limits, I'm away scott free other than my legal "duty of care" which is objective

regards

OMS

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 02 May 2013 10:02 PM
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leckie

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Originally posted by: weirdbeard

Hi OMS, dumbing down a bit re the overengineeing, taking an example of domestic smokes detectors, they are a power circuit, powering only a very few watts, but they must be wired in 1.5mm, over a large estate of houses thats a fair waste of copper?


I didn't know you had to wire domestic smoke alarms in a minimum cable size of 1.5sq.mm. Is this in BS5839 pt6?
 02 May 2013 10:27 PM
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AJJewsbury

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I didn't know you had to wire domestic smoke alarms in a minimum cable size of 1.5sq.mm. Is this in BS5839 pt6?

BS 7671, table 52.3 - on the basis that it's not a lighting circuit.

I wonder if you could class it as signalling or control circuit though.

- Andy.
 03 May 2013 06:07 AM
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Jaymack

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Originally posted by: AJJewsbury
I didn't know you had to wire domestic smoke alarms in a minimum cable size of 1.5sq.mm. Is this in BS5839 pt6?

BS 7671, table 52.3 - on the basis that it's not a lighting circuit.

I don't follow that logic, can you elaborate?

Regards
 03 May 2013 09:36 AM
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AJJewsbury

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I don't follow that logic, can you elaborate?

BS 7671 has minimum limits on the size of conductors - traditionally, for most things, it was 1mm2 for copper and 16mm2 for aluminium. The 17th Ed revised the 1mm2 limit upwards to 1.5mm2 - with an exception for lighting circuits.

I'm presuming that the smoke alarm wiring is still within the scope of BS 7671 (unless BS 5839 or whatever has usurped that).

Like I said though, I think there's an argument that even though the wiring 'powers' the detectors, it could be regarded overall as signal/control wiring, which would make 0.5mm2 or above acceptable.

Or have I missed something?

- Andy.
 03 May 2013 11:09 AM
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OMS

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BS 5839 Part 1 dictaes a minimum conductor size of 1.00mm2 for fire alarm circuits and power supply wiring is also a fire alarm circuit by definition.

BS 5839 Part 6 (for domestic) is silent on the matter and refers to BS 7671.

So we have a power supply requirement that's also a BS 5839 fire circuit.

By inference, BS 5839 requires a minuimum of 1.0mm2 and compliance with BS 7671 - which tells us in table 52.3 that we need a minimum of 1.5mm2 copper for any cicuit that isn't a lighting circuit.

So the accepted deduction (and possibly incorrect one) is that mains operated fire detctors in a domestic setting need a minimum of 1.5mm2 - in any event it's a widely held belief (presumably as installers love simple rules)

regards

OMS

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 03 May 2013 02:31 PM
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mrh105

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Thanks to all that have replied.
After posting I went away and compared the values again and it appears that they are down to the different soil thermal resistivity values. As mentioned in one of the posts above BS7671 uses a thermal resistivity of 2.5 K.m/W and ERA use 1.2 K.m/W. On further investigation it does seem that 1.2 is a more sensible figure. So why does BS7671 use 2.5? Is this just a worst-case figure?
 03 May 2013 02:47 PM
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AJJewsbury

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Is this just a worst-case figure?

Presumably so - but BS 7671 does provide table 4B3 in appendix 4 for correction for other soils.
- Andy.
 03 May 2013 02:56 PM
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OMS

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Originally posted by: mrh105

Thanks to all that have replied.

After posting I went away and compared the values again and it appears that they are down to the different soil thermal resistivity values. As mentioned in one of the posts above BS7671 uses a thermal resistivity of 2.5 K.m/W and ERA use 1.2 K.m/W. On further investigation it does seem that 1.2 is a more sensible figure. So why does BS7671 use 2.5? Is this just a worst-case figure?


As well as different ambient temps, ground temps and depth of lay.

BS 7671 uses what it uses - it' not a "worst case figure" - just the figure at which the data is referenced - BS 7671 provides the adjust factors

Regards

OMS

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 03 May 2013 03:28 PM
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SherlockOhms

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Oh, this could be a good one for the EICR types.......

I'll pose the question "what code for smokes wired in 1,0"?

S.
IET » Wiring and the regulations » Cable Sizing ERA vs BS7671

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