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Topic Title: Submain Sizing/CSA Selection
Topic Summary: CSA Selection
Created On: 09 July 2013 02:03 PM
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 09 July 2013 02:03 PM
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Patchy

Posts: 26
Joined: 29 September 2008

Hi all. Could someone please have a look to see if my submain selection is correct or give some advice if they would do things differently?

Distance between the DBs = circa 10m
Maximum load on the final DB = 120 Amps per phase
Final DB protective device = 250 A MCB
Cable type = 4-core CU/XLPE/SWA/LSF
CPC - 1-core CU/LSF (requirement to run a seperate cpc in this case).
Installation - Reference Method E (on perforated cable tray)

I have selected the following conductor csa - 25mm² per core and also 25mm² for the seperate cpc, based on the following:

Current-carrying capacity:
Table 4E4A (BS7671) - 131 Amp (i.e. greater than 120 Amp)

Volt Drop
Table 4E4B (BS7671) - 0.00165 V x 120 A x 10 m = 1.98 (i.e. <1% 230V)

Thanks in advance!
Patchy
 09 July 2013 02:09 PM
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mawry

Posts: 219
Joined: 26 April 2004

can you get a 250A MCB?

Is overload protection not required?

Iz>In>Ib
 09 July 2013 03:09 PM
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Patchy

Posts: 26
Joined: 29 September 2008

So I need to select a cable with a current-carrying capacity (Iz) greater than 250A (In) right? Or reduce the size of the protective device, providing it doesn't fall below the design current Ib?
 09 July 2013 03:25 PM
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AJJewsbury

Posts: 11254
Joined: 13 August 2003

So I need to select a cable with a current-carrying capacity (Iz) greater than 250A (In) right? Or reduce the size of the protective device, providing it doesn't fall below the design current Ib?

That's would be usual - but by no means the only possible way of providing overload protection. Nature of load (e.g. in situations where there's no diversity at the DB the sum total of the outgoing devices might protect the submain from overload), or upstream device(s), or in some (rare) situations it's not required to have overload protection at all.
- Andy.
 09 July 2013 03:39 PM
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Parsley

Posts: 960
Joined: 04 November 2004

433.1.1 unless 433.3.1 applies.

Also is the load equally balanced over the three phases or are there any harmonics to worry about.

Regards
 09 July 2013 03:42 PM
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perspicacious

Posts: 7128
Joined: 18 April 2006

"Table 4E4A (BS7671) - 131 Amp (i.e. greater than 120 Amp)"

I'd love to have sight of the confirmation you got from the equipment/accessory manufacturer after you read Note 1 to the above table Patchy

Regards

BOD
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