45A mcb too hot?

[plugwash]

well normal width mcbs come in 63A so we've got quite some headroom yet

 

[Skip_2000]

It's just that some instructions are written by monkeys. They're not fit to wipe your @rse on..

Who's shower was it anyway? Give me the details and I'll ring em up & give em some stick...I like that sort of challenge!

To obtain full advantage of the power provided by this unit, use the shortest
possible cable route from the consumer unit to the shower. It is also necessary
to satisfy the disconnection time and thermal constraints which mean that, for any
given combination of current demand, voltage drop and cable size, there is a
maximum permissible length of circuit.
As a guide only, the following maximum permissible lengths are given for
6 mm2 and 10mm2 cable protected by a 45A MCB type B and allowing for a 5
volt drop:
Mira Sport 10.8 kW (240 V AC) at 240 V - 15 metres 6 mm cable
Mira Sport 10.8 kW (240 V AC) at 240 V - 25 metres 10 mm cable

See page 13 of the instructions:

http://www.mirashowers.com/onlinecatalog/pdf/p3279_2a.pdf

Named and shamed...

 

[ban-all-sheds]

How may people have their shower cables clipped to the surface of the walls?

 

[plugwash]

im sure it came up a while ago on here that burried directly in a normal (non insulateing) wall was installation method one (same as clipping direct)

can any sparkysconfirm this?

 

[Mark80]

Guys,

Mira rate the shower as 10.8 kW @ 240v, but only 9.9kw @230v. Does that make a 45A mcb OK?

MarkP

 

[ban-all-sheds]

Just OK.

50A would be better, and as I would seriously advise 10mm cable that would be OK.

 

[dingbat]

im sure it came up a while ago on here that burried directly in a normal (non insulateing) wall was installation method one (same as clipping direct)

can any sparkysconfirm this?

BS7671 Appendix 4, Table 4A1 (page 210 of latest version)

In the section entitled, "Cables embedded direct in building materials" it says that the appropriate reference method for, "Sheathed cables embedded directly in masonry, brickwork, concrete, plaster or the like (other than thermally insulating materials)" is, indeed, Method 1. But it's never quite as clear cut as that because many cable runs will encounter insulation somewhere along the way, either now or at some time in the future, and may need to be derated accordingly - for domestic installations, ref method 6 and table 4D5A is a safer bet.

 

[securespark]

But for capped, conduited or trunked cables it is method 3.

 

[dingbat]

Ah but... table 4D5A was introduced in the last but one revision to cover flat twin and earth cable of the type normally used in domestic installation. Indeed it is the only table to specifically refer to such cable. Appropriate installation methods are 1, 6 and 15. There is no method 3 indicated in that table, because method 3 is primarily used to describe singles in conduit or trunking, although it can be used to describe multicore cable in surface mounted conduit - not a normal domestic method, except in garages, etc.

Ref Method 3 generally de-rates to around 82-85%, whilst Method 6 de-rates to around 72-74% of nominal capacities for the smaller conductor sizes. So Method 6 has a greater margin for safety, partly to take into account the difficulty of assessing a single installation method for domestic wiring systems.

Capping is not specifically referred to in any of the installation method descriptions, nor is it mentioned in Guidance Note 1. Capped and plastered-over cable is the same as embedded direct - Ref Method 1.

 

[securespark]

Hmmmmmmmm.

 

[dingbat]

Hmmmmmmmm.

Seriously! Look it up!

In the brown covered, latest amended version of 7671, go to Table 4A2 'Table of Appropriate Current Rating Tables' (p.216) and in the second row it refers to PVC insulated and sheathed flat cable with protective conductor to Table 8 of BS6004, which is 6241Y surface wiring cable, as used extensively in domestic installations. It tells you to use table 4D5 for this kind of cable.

Now look at table 4A1 'Schedule of Installation Methods'. In row 6 you find in the Appropriate Reference Method column it says, "or Method 6 for cable type covered by table 4D5A" In row 15 it says "or Method 15 for cable type covered in Table 4D5A"

Both Method 6 and Method 15 deal with cable installed, with and without conduit, in a thermally insulating wall. If you feel your capped cable covered in plaster is thermally insulated then by all means use these figures, but this is where you have to use your judgement. I judge that, if anything, a strip of galvanised capping will improve the rate of heat transfer rather than increase the insulation.

But you don't have to believe me. Just read the regs.