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Posted By Topic: House mains subbed from neighbours house!

Oct 16 2017 09:35

I have a client that is wanting to put in a heat pump for a new pool he is going to install. 40A max load.I said i would check his loading.When I asked to see his meter box he showed me the front houses meter box.His 16mm main comes from their house!(15m)They only have a 16mm main! (15m) He has seperate meter and a 40A sub fuse.I told him to speak to Unison as I thought this was very strange.Now he is asking me to connect up a 20A heat pump .He has a gas hob and HW but I\'m sure the load will be to much for the front house.Not sure best course of action.??

Oct 16 2017 11:56

I think the first thing you need to do is sort out what the actual load of this pool is going to be .
You say max 40A,I suspect that would be if it used its internal element to heat the pool and not the heat pump \"option\".
Also you say the heat pump is 20A , if that is correct then its a pretty big heat pump, more likely that\'s the recommended breaker size , you will probably find the heat pump running current is around 6 to 8 amps.
What type of pool is it ? a Spa ? or a Swim spa or something like that.


Oct 16 2017 12:14

Two possibilities:
Either this is one installation comprised of 2 structures; with point of supply (as usual) at boundary).
Or it\'s two separate installations and the network has decided the PoS for each is in the meterbox. Not likely, but possible.

The 40 A \"sub fuse\" suggests strongly that the set-up is a single installation; because most networks don\'t permit supply fuse less than 63A. The metering configuration and commercial arrangements with energy supplier(s) are not relevant to whether it is one or two installations. Nor are ICPs definitive. What matters is where the PoS is.

The presence of the 40A fuse almost certainly makes the meterbox a \"switchboard\", in which case it would have to be the MSB; containing not only the \"main switch (controlling entire installation) but also the MEN point.

Regardless of compliance (or otherwise\" of existing set-up 9which may pre-date today\'s Wiring rules) ; the issue is whether the extra load can be added in a compliant manner.
Start with a full max demand calculation for both the existing houses. Then a volt drop calculation using estimated those loads + 20 A for the submains and mains route distances. If you apply max demand by limitation, it likely won\'t fit. So you need to do it by either calculation (counting points per circuit, as per Table C 1) or measurement over time. Plenty of houses manage on 32A load-limiter (reduced capacity tariff) even with electric cooking and heating; but adding another 20A may require upgrading of both submains and mains.
In which case, getting a separate supply may be a preferable option, even if an easement is required to get mains from new POS to rear house past the other one.


Oct 16 2017 13:02

He says its a 16A heat pump now.
I\'ll run it through table C1 and see what I come up with.I guess I\'m going to have to calculate the load on both house really.

Oct 16 2017 14:28

Yes; you can\'t get volt drop for either house without having a max demand figure for each house.

To illustrate; assuming actual MD for each is 25 A; there will be 50 A in the \"mains\"
x (minimum) 2.54 mV x 15 m = 952.5 mV up to meterbox of the 11.5 V total allowed.

Then calculate for submain; 25 A over cable route distance.

Then for final subcircuits. Not just the proposed new one (where any excess VD can usually be dealt with by increasing cable size); but all existing subcircuits, because the added load in mains / submains will increase VD in those sections. In turn, potentially making one or more existing circuits non-compliant.

So without MD figures for both, you can\'t know whether it\'s possible to add the extra load - whatever it turns out to actually be.

Most of us seldom have to do this sort of thing, unless for extra-long mains or similar. For standard situations, we can usually just say \"MD is by limitation\", use the supply fuse as limit, and be confident we\'re on the safe side. and we can use the \"rule-of-thumb\" of 2.5 % each for mains and for everything else.
But adding even 20 A of new load when there are already 2 houses means it\'s a significant addition; so you\'ll want to be sure.

This is professional advice & design work; so you should be paid for working it out. You won\'t want to do all that work, & develop provide a specification; and then either someone else gets the job or there is no job \'cos it\'s too expensive.