MCB's


Postby billobach181 » Tue Jan 26, 2010 1:52 pm

I have a Europa Shower Consumer Unit 63 Amp RCCB. But I cannot get a 45 Amp MCB from them [They only do 40 A or 50 A MCB] I am told by my electric cable supplier that 50 amp MCB if fine.
I have calculated my Volt Drop to be 2.7V and the shower Current Rating is 9.8 Kw at 240 V [41 amps current] or 9.0Kw at 230 V [39 amps current] .
The istallation is Method 1 [Clipped to Loft Rafters].
So is it OK to use the 50A MCB for cable protection.
Regards
Bill
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Postby stoneyboy » Tue Jan 26, 2010 10:50 pm

billobach181,
Unless you specify what the CSA of the cables are you are unlikely to get an answer.
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Postby ericmark » Wed Jan 27, 2010 3:34 am

There is no method 1. No 1 is method A which seems an odd method for a house. 16mm cable with installation method 101# and 102# is above 50A. However you don't say what type of cable or what size of cable so sorry can't really answer.

A 6mm cable installation method 103# is rated just 23.5A

I think there is a good reason why Part P requires the LABC inspection on electrical work in bathrooms.
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Postby billobach181 » Wed Jan 27, 2010 9:51 am

THe Cable CSA is 10mm. Method 1 is taken from BS7671. Cable capacity taken from Table 4D2A
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Bill
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Postby kbrownie » Thu Jan 28, 2010 12:48 pm

[quote="billobach181"]THe Cable CSA is 10mm. Method 1 is taken from BS7671. Cable capacity taken from Table 4D2A
Regards
Bill[/quote]
50A should be fine, but because you are changing a protective device, it will be noitifiable work!
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Postby billobach181 » Thu Jan 28, 2010 8:48 pm

K. Brownie
many thanks. The system will be connected and tested by a qualified electrician. I am only trying to get the feel of what I need to allow for in the bathroom [ Cable Holes, Pipe Holes in the Tiles before I get him to fits the Shower etc.
Regards
Bill
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Postby ericmark » Sat Jan 30, 2010 10:02 am

Table 4D2A in BS7671:2008 lists reference methods A, B, C and E. Table 4D5 covers flat twin and earth. There are 4 main reference methods plus C and A and for 10mm cable

Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) 45A

Reference Method 101# (above a plasterboard ceiling covered by thermal insulation exceeding 100 mm in thickness) 36A

Reference Method 102# (in a stud wall with thermal insulation with cable touching the inner wall surface) 47A

Reference Method 103# (in a stud wall with thermal insulation with cable not touching the inner wall surface) 32A

I will guess you have been reading some obsolete copy of BS7671? Although likely you can only use a 45A MCB showers are used for such a short time likely you can draw 50A without it tripping.

Cable rated at 90ºC will have higher current ratting but one has to consider where it is routed and can that also take 90ºC. Ali-tube cable to BS 8436 Guardian, only to 6mm² Earthshield, only to 2.5mm² Flexishield, 10sq.mm and 16sq.mm available on request but your not likely to be using enough. Afumex only to 6mm² and so not suitable there are other XLPE (Cross-Linked Polyethylene) or LSZH (Low Smoke Zero Halogen) cables. Because it is hard to terminate 16mm² cable using 10mm² XLPE may be an option but this is something for your electrician to decide.
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Postby billobach181 » Sun Jan 31, 2010 1:59 pm

erickmark
Thanks for your response. My BS7671 is 2001. My cable does not sit in any insulation but is clipped to the lof rafters and runs up the wall [4 metres] in trunking.
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Postby sparx » Tue Feb 02, 2010 11:48 pm

Hi, since the declared voltage of 230 is the basis for bs7671:2008 and as you state 9kw equates to 39.1A then what's wrong with a 40A mcb?
It's the load that determins the protection level required, so as long as the cable in use and its installation method don't indicate any overload / over temperature likely then no problem that I can see.
I am sure your leckie will confirm this for you,
regards Sparx
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Postby billobach181 » Wed Feb 03, 2010 9:36 am

Hi Sparx
Thanks for your input. As a matter of interst and my education isn't the 40A mcb too close to the 39A operating current or does it not work that way?
Regards
Bill
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Postby sparx » Wed Feb 03, 2010 11:38 pm

hi Bill
yes and no!
yes its close, thats a good thing! the closer the better as it means any overload will cut the supply quickly.

40A rating is the normal design current for the circuit so it will never trip with a 39A load.

FYI a 40A mcb will run at around 50A for many minutes as the trip mechanism is a bi-metal strip, with short circuit protection by a magnetic trip coil.

these little £3/4 devices are a work of art inside, if you havent done so I would recommend drilling out the rivits on an old spare one and carefully opening it up, try to follow the path from in to out terms.....magic,

sad old git aint I.

regards Sparx
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Postby billobach181 » Thu Feb 04, 2010 6:16 pm

Hi sparx
Thanks very much for that. As a retired Mechanical Engineer I like to know how things work and it keeps the old brain ticking over so I will drill one of these units out to see the mechanism.
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