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Topic Title: Busbar chamber and cable size
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Created On: 18 January 2013 08:50 AM
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 18 January 2013 08:50 AM
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ianr

Posts: 6
Joined: 28 May 2010

I have to put in a temporary supply onboard a vessel for a upcoming job.

They have a busbar chamber with a 400Amp breaker vacant for me to wire into, feed from a busbar.

The distribution board I am fitting has a main switch of 250Amp.

What size cable would be required from the 400amp switch to my DB ?

Would 4 core 95mm be ok to use and is there a max distance as i need roughly 10 Metres ?
 18 January 2013 09:07 AM
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GeoffBlackwell

Posts: 3580
Joined: 18 January 2003

You will have to supply more information - you have mentioned that this is on a vessel - that may well change everything.

So is it a vessel, if so is it in commission, size, classification, etc.

Regards

Geoff Blackwell
 18 January 2013 09:24 AM
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ianr

Posts: 6
Joined: 28 May 2010

Thanks for a reply.

Yes it is in commission, details:

DWT: 3,426 tonnes
Type: ROV/PSV
Deck: 600.00m2
Overall Length: 80.50 metres
Breadth: 18.00 metres
Built: 2005 (Wuhu, China)
Bollard Pull: n/a
DP: 2 Accommodation: 60 (beds)
BHP: 6,434 SAT Diving: n/a
Crane: 35 tonnes @ 10m radius
GP Crane: n/a
Offshore Crane: n/a
FO Capacity: 1,000 m3
Drill Water: 900 m3
Liquid Mud: 399 m3
 18 January 2013 09:48 AM
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GeoffBlackwell

Posts: 3580
Joined: 18 January 2003

That puts it outside the scope of BS 7671 and into the realms of marine standards. You could start with:
BS 8450:2006 Code of Practice for Installation of Electrical and Electronic Equipment in Ships - yours for £55.25 from BSI

It also means you should probably be using marine cables and current rating data may have to be obtained from the manufacturer.

Basically you can have short circuit protection at the source and overload protection at the load end so you need to select a cable which can cope with the prospective fault current at the source for whatever the permitted time is for the particular cable.

Note that BS 8450 is just a code of practice and that it refers you to IEC 60092-352 Electrical installations in ships - Part 352: Choice and installation of electrical cables
if you want cable current rating tables. That is probably expensive.

Regards

Geoff Blackwell
 18 January 2013 10:46 AM
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ianr

Posts: 6
Joined: 28 May 2010

Yes we will use marine cables and the spec of the cable i have at present is 287Amps.

What else will i need to work out if the cable will be ok to use from the 400Amp breaker to my 250amp main switch ?

Thanks
 18 January 2013 11:32 AM
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GeoffBlackwell

Posts: 3580
Joined: 18 January 2003

The 'cable spec' giving a current rating of 287A is just the starting point. That rating would have to be adjusted to take account of installation method, grouping of cables, ambient temperature and so on. All this needs data - either from a cable manufacturer or a standard.

If we assume that the load end 250A device will provide adequate overload protection (and please note that a notional rating of 287A does not prove this) - the next step is to determine if the cable can handle the prospective fault current when protected by the source end device. So you need to know the prospective fault current and the characteristics of the 400A device.

None of this is easy and getting it wrong could have very serious consequences. If the vessel is in the UK - it will have to operate within the requirements of the Marine and Coast Guard Agency.

This is an extract from the standard concerning fault handling.

3.8 Short circuit capacity (withstand capability).
Cables and their insulated conductors shall be capable of withstanding the mechanical and
thermal effects of the maximum short circuit current which can flow in any part of the circuit in
which they are installed, taking into consideration not only the time/current characteristics of
the circuit protective device, but also the peak value of the prospective short circuit current
during the first half cycle. Further information is given in IEC 60724 and IEC 60986.


Unless you are familiar with the calculations required you are going to find it hard going. The cable manufacturer may help provided you can supply then all of the data they will need.

You ask what else you would have to work out. I suspect you are referring to voltage drop. Once again the standards or cable manufacturer will have the data - but if the run is only 10 metres it may not be an issue

If this is not you normal line of work I would recommend that you check that you have adequate insurance cover - most shore side policies exclude it.

Regards

Geoff Blackwell
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