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Topic Title: Installation Conditions - Cable Size
Topic Summary: Maximum length that can be ignored
Created On: 09 November 2017 10:14 AM
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 09 November 2017 10:14 AM
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Apostolos1983

Posts: 120
Joined: 03 December 2012

Good morning.
If you have a cable installed in a specific way, e.g on perforated tray.
And for a specific length it has to dip into the ground, in ducts, to cross a road and get to another building where it will continue on a perforated tray.
As you can imagine, this dip into the ground creates conditions more onerous for the cable. Different installation method, different cross section area (possibly).
Is there a maximum cable length for which we could ignore this dip into the ground?
0.5 meters?
1 meter?
5% of the total length?
Is there anywhere as guidance on this?
Thanks!
 09 November 2017 10:38 AM
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AJJewsbury

Posts: 15997
Joined: 13 August 2003

I'd say that in principle it's similar to covering the cable with a short length of thermal insulation - so I'd be looking at regulation 523.9 and table 52.2 for inspiration. The ground won't be anything as thermally insulating as mineral wool so the de-rating factor itself is probably way overboard, but the principle that you can de-rate more gradually between the 'out of insulation' and 'completely in insulation' ratings over a short distance is clear. You can't completely ignore the 'dip' (based on table 52.2 even 50mm produces a definite re-rating) but if it's only a small distance then the de-rating may not be anything like as bad as the full 'in ground' rating. Table 52.2 only works for cables up to 10mm2 so if your cables are thicker you might consider that the lengths could be extended a little.

Other tricks include using a cable with a higher temperature rating than you can use at the terminations (e.g. using 90 degree XPLE but derated to 70-degrees) - many metres away from the termination there's no need to keep to the 70-degree limit so you can use the full 90-degree rating for the 'dips'.

- Andy.
 09 November 2017 11:30 AM
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mapj1

Posts: 9557
Joined: 22 July 2004

The key to how long is a 'neglligable' length is how much temperature rise occurs for heat travelling 'longways' along a piece of copper is when compared to heat escaping sideways through a particular thickness of insulation.The answer is complicated and relates to the ratio of the thermal conductivities of the two cooling mechanisms, and the thickness of surrounding material over which most of the temperature drop occurs.
The answer is jolly variable, but usually ends up being some tens of cable diameters. If you can arrange airflow though the ducts, or even water (!) then the problem is greatly changed.

-------------------------
regards Mike
 09 November 2017 11:41 AM
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broadgage

Posts: 2433
Joined: 07 August 2007

In my view, if the cable crosses a road in a buried duct, then it needs to be correctly sized for installation in a buried duct.
The fact that a much larger length of cable may be installed under more favourable conditions on perforated cable tray is not in my view relevant.

When a very short length of cable passes through less favourable conditions, then less de-rating might reasonably be applied.
IMO, "very short" means passing from one room to another via an insulated wall cavity, and similar conditions. The reasoning being that heat will be partially conducted away along the length of the cable and will escape to the surroundings outside the thermally insulated part.

In the case of a road crossing, even a narrow road, then in my view no significant heat can escape along the buried part of the cable.
Therefore the cable must be rated for the conditions.

It is a design choice as to whether to increase the cable size over the whole length, or whether to joint to a larger size each side of the road crossing.
A larger size throughout has the merit of simplicity and also reduction in losses.
Have you yet done voltage drop calculations ? you may find that the cable needs upsizing significantly beyond current carrying capacity in any case.
 09 November 2017 11:47 AM
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Apostolos1983

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So,
If the cable inside the two buildings needs to be 4c10mm2 and for the part that crosses the road between the two buildings the cable needs to be 4c25mm2, how would we joint these 3 pieces of cables?
400V junction boxes either side of the road??
And when we jump from 10mm2 to 25mm2 and back to 10mm2, do we need to fuse?
 09 November 2017 12:14 PM
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broadgage

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Jointing may be done either by terminals in suitable boxes, or by a cast in resin joint as used by the DNO.
When cables of different sizes are used in a circuit, or indeed the same cables under different conditions are used in different parts of the circuit, then the fuse at the origin of the circuit should be selected for the least favourable conditions. No additional fuses along the run would be needed.

In the example given, of relatively small cables, it might be cheaper to use the larger size throughout, rather than paying the labour and material costs of jointing.
Joints are a potential point of failure, and I would prefer to minimise them.
 09 November 2017 12:59 PM
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mapj1

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No need to fuse the transition - after all both sizes of cable have been chosen for the same circuit load and presumably there is a suitable fuse or breaker at the origin.
The economics of joining or not depends how long it is, but somewhere around 50m to 100m long voltage drop bites on 'normal' sized 230/400V stuff anyway, by which I mean, cables need to be (up) sized to combat voltage drop, not decided by current rating.

As a badly rounded arm waving example - consider 10mm cable - call it 1.6 milliohms per metre per core and, say, carrying 80A max for easy sums, so 128millivolts drop per metre at 80A. (not the book values, I know, but I'm looking out of the window and working from memory/ rules of thumb)
For a voltage drop of say 10V, which is at or near the upper end of happy, depending on what is happening in the rest of the system, that suggest a maximum of 80 metres of round trip, or 40m of actual cable length... Longer than that and its next size cable anyway )
You may need to perform a similar sanity check, with the right numbers for your cable ratings and an acceptable voltage loss for just that leg of the circuit. How wide is the road and how long is the tray?

-------------------------
regards Mike
 09 November 2017 02:51 PM
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OMS

Posts: 22388
Joined: 23 March 2004

Originally posted by: Apostolos1983

So,

If the cable inside the two buildings needs to be 4c10mm2 and for the part that crosses the road between the two buildings the cable needs to be 4c25mm2, how would we joint these 3 pieces of cables?

400V junction boxes either side of the road??

And when we jump from 10mm2 to 25mm2 and back to 10mm2, do we need to fuse?


Seriously ?

what are you burying it in - hot compost ?

First go and check the calculation - it's not credible to go from cable on ladder to cable in ground and reach a more that two fold CSA increase unless you are exiting a cooled building and burying underground somewhere very hot and dusty (and even then you would be foolish to make a claim on the cooling system giving you a lower ambient to allow a smaller cable unless you know how resilient it was)

Joints could be in line resin encapsulated with crimped connections, via adaptable boxes with fixed terminals, etc etc - there are a variety of methods depending on application - but see above.

Why would you need to fuse anything other than origin - circuit amps haven't changed - all you have done is install more copper to avoid exceeding the permitted temperature of the insulation due to the installation methods

On small cables such as you describe (and I don't think 10mm2 to 25mm2 is credible), you would tend just to accept the largest cable size driven by the worst case of the installation route for the whole run - a bigger cable is pretty cheap compared to the labour, space etc required for jointing

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 09 November 2017 03:24 PM
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mapj1

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I like the idea of compost ! Worm proof insulation.
I think we should assume it may not be a simple buried line under a road. Or the 'soil' is made of styropore insulation. But most likely it is grouped in a pipe, and a pessimistic poorly cooled case is assumed. Like the domestic immersion heater that can be on 4mm or 1.5mm T and E depending on cable route, but usually isn't either.

-------------------------
regards Mike
 09 November 2017 04:15 PM
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OMS

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Joined: 23 March 2004

I think we should assume that the OP has read the regs book and is using ground thermal resistance values of 2.5 K.m/W for Ref method D and should perhaps look at Table 4B3 and select a more credible ground condition for UK soil types (eg more like 1.2 K.m/W)

From there a browse through Table 4B2 for a ground ambient of around 15C and Table 4B4 for depth and 4C2 for installation spacing of the ducts

To avoid the pain of all that - and presuming Amtech is available, then selecting XLPE insulated armoured cable, run to 70C and installed to ERA data for buried cables will hopefully be illuminating

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 13 November 2017 11:45 AM
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Apostolos1983

Posts: 120
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Thanks everyone. I think the sensible thing would be to go to the bigger CSA through the whole length. I do not think the client would like additional points of failure (junction boxes).
So, to return to the original question, there is not any definite answer in the regs about the max length we could ignore the change in the installation conditions.
Thanks!
 13 November 2017 11:54 AM
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OMS

Posts: 22388
Joined: 23 March 2004

That would be normal for small cables - you might think again for bigger cables (and joints are not that unreliable if made correctly)

I would still query the increase from 10mm2 to 25mm2 just by burying the cable however

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 13 November 2017 12:08 PM
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Apostolos1983

Posts: 120
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Thanks OMS.
Even if instead of 25 is 16mm2 the outcome of the discussion is the same. I am going to use the worst case CSA.
Thanks a lot.
Regards
 13 November 2017 12:40 PM
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OMS

Posts: 22388
Joined: 23 March 2004

Sure - I was just pointing out that to use a lot more 25mm2 rather than 10mm2 in areas where you don't need to de-rate is expensive and wasteful of resources

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 13 November 2017 12:55 PM
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Apostolos1983

Posts: 120
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OMS this was an example to be honest.
If I remember correctly the real scenario was at a Water Treatment Works where the cables used were 4 single core cables 240mm2 each and for the road crossing between the two buildings it would have to be 4 single core 300mm2 (one size up).
But I do not remember all the specifics that is why I created an example to express my question.
Cheers
 13 November 2017 02:02 PM
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alancapon

Posts: 6914
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Originally posted by: Apostolos1983
. . . So, to return to the original question, there is not any definite answer in the regs about the max length we could ignore the change in the installation conditions. . .

No, that is where you have to use your professional judgement / experience.

Regards,

Alan.
 13 November 2017 02:13 PM
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Apostolos1983

Posts: 120
Joined: 03 December 2012

Yes....if only I had one (kidding...)!!!!
:-) :-) :-)
 13 November 2017 04:49 PM
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OMS

Posts: 22388
Joined: 23 March 2004

Originally posted by: Apostolos1983

OMS this was an example to be honest.

If I remember correctly the real scenario was at a Water Treatment Works where the cables used were 4 single core cables 240mm2 each and for the road crossing between the two buildings it would have to be 4 single core 300mm2 (one size up).

But I do not remember all the specifics that is why I created an example to express my question.

Cheers


OK - my point was that you seemed to have more than one cable size increase due to dropping to trenches

On larger cables, depending on the lengths involved not in trenches, it may be more economic to joint them and use the cable required for the differing installation conditions

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 13 November 2017 05:41 PM
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leckie

Posts: 4390
Joined: 21 November 2008

Just to go off on a tangent very slightly, I have just realised that I have never had to do an underground resin joint for different size conductors. I can only remember jointing the same sizes together.

How do you crimp a 70mm to a 125mm? I didn't think you could, I thought you had to use a screwed through connector.

I presume cable jointers still solder service cables when they are teeing into a cable, or is there some specific T or Y connector available?
 13 November 2017 06:14 PM
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alancapon

Posts: 6914
Joined: 27 December 2005

Our standard is screw connectors, albeit with sheer-bolts to get the tightening torque correct every time. The connectors are available for a service tee, without having to cut the main core. We then use a joint shell which is filled with a cold pour resin. Hot jointing techniques fit well with the old lead cables, but are not really correct for XLPE cables.

Regards,

Alan.
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