Well, if you check the actual installed conditions (or at least those assumed for the BICC data) then you should see good correlation with BS 7671


Regards


OMS

Oms 


thanks for your help. I can see lots of the data match's. I’m just unsure on underground cables. I would have thought bs7671 has been correction factors for uk use. The cable in question is as follows 


250a main switch to DB 400v 

xlpe swa 

0.03ze 

30m: ducted 15m ladder 15m 


the cable installed is 95m 5core swa 

Anthony

You need rather more information than that. What is the value of the fuse or breaker protecting the cable? What is the maximum possible load current? What is the circuit feeding? What surrounds the duct? What environment is around the ladder? What terminates this cable at each end?


All of these affect the cable rating and possible working temperature, and therefore current rating.


Regards

David

OK - you need:


1 - Depth of lay,


2 - Spacing,


3 - In ducts or buried direct,


4 - Ground Temperature,


5 - Soil Thermal Resistivity,


6 - Air temperature,


If you apply the factors in BS 7671 to each of those based on the assumed data in BICC then you should find an exact match (except for the Voltage drop correction factor used in BS 7671 which will further derate the cable)


For the section on ladder, except where the air temperature is higher than normal, the cable size will be dominated by the buried section


Regards


OMS

Thanks guys. 


Im looking at this from a testing point of view so I don’t have all the answers. I’m just trying to understand the numbers 


1 Depth of lay - 1m


2 - Spacing - 4” ducked? 


3 - In ducts or buried direct - duck 


4 - Ground Temperature - 20C A’s guess 


5 - Soil Thermal Resistivity. Not too sure. I understand the area has a very high water table 


6 - air temperature - normal uk of 30C 


250a 60947-2 prob max load of 220a lugged connection  to 250 60947-3 


regard 


tony

Ok - a quick check on that cable at 1m depth, in ducts, not grouped or spaced, at ground temperature of 15C and soil thermal resistivity of 1.2K.m/W in air ambient of 30C suggest a 1 x 4C x 95mm2 (copper) with SWA and XLPE insulation (ie 90C) can carry about 260A - so greater than the device rating of 250A without further setting.


You will need to make a demand on the 90C operation however, unless you can get that MCCB down to around 220A if you operate at 70C - which seems to be the case anyway, by the nature of the load


As a quick check the cable is around 65C at 220A


Personally speaking, it looks OK at the first pass


Regards


OMS

Thank you oms


i completed my 2396 design recently and find myself checking everything. I now have a copy of the bicc and intend on spending time learn more on the cable selection. 


The reason I was doubting it so much was because the engineer response to my question was “amtech selected the cable”


thanks again for for your help

Anthony Sullivan:

Thank you oms



The reason I was doubting it so much was because the engineer response to my question was “amtech selected the cable”


 

OK - I'd like to think that the injuneer selected the cable and then used Amtech to verify it   - but such is life


If you have any queries on cable selection/sizing etc, stick them on here - there is usually a cast of thousands willing to help


Regards


OMS

Wise to be wary of software, it is a good helper and a bad master. Be aware though that the tables at the back of the regs are not the last word on cable installation methods, rather a set of safe figures for designing from with minimal research. There are a great many assumptions that are hidden in the assumptions that certain routing methods are the same (cables don't really cool the same horizontally as they do vertically, assuming you are not in a space station, and convection is the main cooling mechanism, not all plaster walls are equal,  etc.) The figures cannot sensibly cover all cases, and sometimes there will be some installation condition where there is nothing for it but to measure temperatures in situ, or more usefully for inspections, to fit temperature disclosing stickers, and see the 'high tide' markers between one inspection and the next.

Hello,


This is a great thread but I would just like to highlight a point raised in the initial post that I am still not clear about, ie does BICC comply with current regs? I would also like to know what is the purpose of BICC and why not use BS7671 as the main reference for UK work? Also, OMS mentioned there is "good correlation" between the two but it sounds like its not perfect so when the results are different I assume you go with the BS7671 result, which brings me back to why use BICC at all?