SUB MAIN QUESTIONS. ADVICE NEEDED

[Nelly72]

Hi all.

I have been asked to take over a job an electrician has started, and has moved abroad.

The house is a big mansion, with a 3PH supply, with 100A fuse on each Phase.

The electrician has ran 2 25mm 3core armours to another 2 DB’s in the mansion, and there has to be a DB fitted where the incoming supply is, bringing it to a total of 3 DBs.

In the plant room, there is also a 3PH heat pump.

However, my question is how would you all terminate the sub mains. Would you bring each phase into a Henley block, and branch off into a 80A switched fuse for the armour sub mains, and take a tail off the Henley blocks for each phase into a separate enclosure with a protective device for the heat pump, or would you fit a TPN board, and bring all armours into it + the heat pump supply?

If it’s a TPN board would you fit MCBs or MCCBs for the 25mm armours? Is there even

I’m a new build house basher, so this is abit more complex to what I’m used to, even though it’s a new build, so please go easy on me I am simply just wanting advice to keep me right.

 

[Rockingit]

Fuses are old technology and cheap by comparison, but they naturally have a "delay" in how the wire heats up, and they offer exceptionally good energy-limiting. So if you really don't expect them to be blown/tripped, they are a very good alternative and will just sit there for decade after decade after decade, and still work if called upon.

Hence my suggestion of a small Ryefield!

 

[pc1966]

Hence my suggestion of a small Ryefield!

If it is feeding sub-mains so 5s disconnect and all 63A or above it makes a lot of sense.

But I think there is at least on 3P load for a heat-pump, that would be better served by some 3P MCB or similar so on fault all phases go down. Still a DIN box and MCB from a set of fuses is still quite a viable option.

 

[Rockingit]

If it is feeding sub-mains so 5s disconnect and all 63A or above it makes a lot of sense.

But I think there is at least on 3P load for a heat-pump, that would be better served by some 3P MCB or similar so on fault all phases go down. Still a DIN box and MCB from a set of fuses is still quite a viable option.

Thing is, it's never just a heat pump, is it. It's some local sockets, a light or two...... so my choice would be to supply a small 4way 3ph board from said Ryefield. If you're building a plant room, build a plant room! Not some abomination of tails spaghetti.

 

[nicebutdim]

Not some abomination of tails spaghetti.

I think this part bothered me most about the proposed installation.

It's the sort of set up one might expect to be faced with in an old and regularly repurposed commercial property.

 

[Mark42]

Not sure if I’ve understood this properly, but it appears to have parallels with my own installation. I highly dislike the idea of feeding SP sub-mains from a TP board, unless for very small local loads - which are not likely to increase in the future - like, maybe, a garden shed. I would never even consider a single phase 80A sub main.
For anything else I run 4 (or 5) core SWA to additional TPN distribution boards. Then you have the ability to properly balance loads between the phases, and to run any future 3P loads.
And it looks a whole lot better.
I buy Hager 3P DBs when they come up on EBay, and populate them with Hager RCBOs from the same source. So it doesn’t even cost much.
OP: are these new 3-core runs long, buried, or otherwise non-negotiable?

Edit: Apologies -I just noticed this is a zombie thread. I came to it through a ‘recent posts’ link.