two conductors into main isolator?

edexlab · Jul 10, 2012

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.

telectrix · Jul 10, 2012

personally, i'd insist on henleys as you suggest. 2 sets of differently sized tails in isolator terminals is asking for poor connections leading to arcing and overheating. not sure if there's a reg. against it though.

steveberry11 · Jul 10, 2012

Don't have a problem as long as the containment is suitable and the conductors don't pass through the switch, and protective device will protect the smallest cable. It would be better if the terminations were on stud and crimp. If you use Henley blocs not a lot of difference in wiring method.
30kw is around 130A single phase are the cables suitable for this load or is this spread over the 3phases.
As you are subbing to a firm I guess you have drawings. Nothing wrong in clarifying any queries you have.

This is a industrial install so there are different methods used. As long as you follow the basics you should be Ok

edexlab · Jul 10, 2012

Thanks for the replies
re steve berry11 it is 3 phase and I do agree with the idea that the installation method is similar to using henley blocks but I think I will stick to the original plan as the terminal has only the one screw clamping the two conductors so I suspect there is a good chance of arcing as suggested by telectrix ,whereas if it was stud and crimp thats no different to connecting to common busbars.
As for the drawings ,I wish ! most jobs I've been on have usually been of the get there and design/sort it out yourself onsite which is a good way of learning but makes it hard as you have to check things like this as they arise.