I am taking a supply from a 3 phase board to a single phase CU (with a 100A main breaker) and want to make sure it is done right (and before you ask I am planning on getting a qualified install as I am not an electrician but I want to know what I am talking about when I try to find that qualified help!) 

 

I have a 36m run of 3 core 16mm cable going from CU to a Lewden 3 phase board. There is one existing breaker in the 3 phase board that is a 3 pole 32A with the neutral going to a shared bar. Can one get a 3 pole MCB and just connect up the one pole for the 240V supply to the CU? Or are there specific breakers that allow you to go from 400V to 240V? Or can one just put in a single pole MCB and leave the other two blank?

 

I was looking at something like this

https://www.cef.co.uk/catalogue/products/4553693-63a-triple-pole-10ka-type-c-mcb

 

Is it an issue that it is a 63A 3 pole MCB if it is going to a 240V CU with a 100A main breaker (given that I am only going off one phase) - I will not be using more than 63A so I am assuming it should be ok.

 

I hope this is enough info and thanks for any help.

@ProDave I think will probably have best ideas but you can definitely get 3 phase isolators which I would put in the line at the terminal end for the two phases you won't be using and take your 240 from there. Then, if at a later date, you, or a future occupier, want to use the other phases - perhaps to increase your PV return to supply you have a way of getting to the connection in safety.

If taking a SINGLE phase supply out of a 3 phase board, you use a single pole MCB. I would not advise using a 3 pole mcb and only using 1 pole.

 

Maximum of 80A for your 16mm cable. Have fun finding the right mcb for it.  CEF have a 63A single pole https://www.cef.co.uk/catalogue/products/4553645-63a-single-pole-10ka-type-b-mcb?q=Lewden+single+pole That might be the largest you can get for that board, which is fine as long as your maximum demand does not exceed 63A

thanks for the replies - a single pole MCB seems the way to go