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Topic Title: two conductors into main switch
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Created On: 10 July 2012 07:13 PM
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 10 July 2012 07:13 PM
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Joined: 04 September 2010

I've got a question regarding whether its acceptable or not to connect two different sized conductors (or even two equal sized) into the terminals of a main isolator.

this is the set up
TNC-S 3 phase supply incomer 100A main fuses > 25mm tails into 100A (switchfuse) Main isolator > generator switchover panel > Distribution board

Normally we would connect direct into the DB but this is a PV install (30kw) and therefore it needs to be connected before the generator
I'm subbing to a firm on this job and the main contractors electrician reckons its ok to tap off the main isolator (outgoing side of course) by connecting our tails direct into the terminals feeding the gen changeover switch and DB in other words we'll then have the main installation 25mm tails and 16mm tails (feeding pv db) in one terminal designed to take upto 70mm csa.

Although I have seen other installs done this way I've never done this myself and to be honest I'm not convinced its the right thing to do and will probably split the tails via henleys and feed my DB separately ,but as I'm subbing I'll have to justify the extra cost and be sure I have got it right as if I did it the way the firms electrician says it will be slightly cheaper.also as its a new job the main DB etc has been installed by another separate firm and i don't want to cause any issues for them to bring up.
 10 July 2012 09:03 PM
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Doubling up in the main switch is rough as, not sure off hand if theres anything you could find specific in regs that say you cant. (im sure you could interpret something)

I would imagine it goes against manufactures instruction though, i don think they cater for two different sized cables in the connections and or of 42mm

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 10 July 2012 09:47 PM
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Electrically I don't see a problem (we routinely stick multiple conductors into the same terminal behind sockets, MCB outgoing terminals and so on). I'd say it's down to whether the terminal can physically accommodate those conductors adequately - I suspect most would be OK. Clearly if you get into snipping strands to get it to fit there's a problem but 41mm2 in a 70mm2 terminal doesn't sound obviously problematic.
- Andy.

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