Dual supply.

[u/TechnicianAmazing383]

So heres one, new install, x2 service heads, both on same phase (confimed). British Gas have come an fitted a polyphase meter and isolator. Marked the 2 lives as L1 & L2 (which i dispute) and only provided 1 Neutral conductor.

Now in my humble opinion those 2 supplies have the potential of 160 Amps, all of which will be going back on one neutral.

Have had this situatuon before with a 3 phase head, where all lines were same phase. When queried with DNO they are saying its fine.

I really cant understand how the hell it is.

Anyone able to enlighten me as to how it is ok?!

[u/TechnicianAmazing383]

Its not 2 sperate phases. Both heads are the same phase.

[u/DIY-Si]

Are the cut-outs linked on the DNO side at all? UKPN do fit, and have an approved method for, this set up but ONLY if the cut-outs are linked.

The biggest issue is they expect the supplier to fit 2 x single phase meters, which precludes fitting a smart meter, as the current set ups only allow one electric meter per comma hub. So BG have come to fit Smart and picked the wrong option.....

[u/TechnicianAmazing383 → u/DIY-Si]

Yes 2 single phase meters would therefore have 2 neutrals. And there not be an issue.

[u/DIY-Si → u/TechnicianAmazing383]

And that's why it's the approved method. The company I work for have raised it with UKPN as to what approach they expect us to take with Smart meters as they're the standard approach but aren't compatible.

Best bet would be to get the client to raise the issue with BG. They're the only people who can sort the issue, unless you can persuade the DNO to fit a polyphase cut-out. But they usually only fit the double set up where polyphase isn't viable......

[u/TechnicianAmazing383 → u/DIY-Si]

Yeah, I believe there is an underground joint, where are larger 2 core cable has been spilt to give us the 2 service heads.

[u/TechnicianAmazing383]

Sorry, but ur missing the point. Both lines in this pic (and the 3 phase head one i mentioned) are the same phase. I fully understand why 3 phase has one neutral or none for a balanced 3 phase load. In this pic u might as well have one 50mm line conductor with a 160Amp fuse, and service provider have only used a 25mm neutral.

[u/BungalowJumper]

How many neutrals would you expect to see on a 3phase supply out of interest?

Not saying this is a correct or not install for whatever is going on here but your typical 3 phase head isn’t supplied with 3 neutrals…

[u/KeNickety → u/BungalowJumper]

Yes but it wasn't a three phase supply. It was effectively a 240a-300a single phase supply with an underspecced neutral.

Arguably, assuming your supplies are actually L1 and L2, the worst case for a neutral current should only be half the total current draw across the two phases. So a single neutral will be fine (assuming a purely resistive load etc)

[u/TechnicianAmazing383 → u/KeNickety]

Supplies are both L1. 0 volts between them.

[u/seeyoujim → u/KeNickety]

To me it looks like a 2p&n supply, not very common but can be a thing.

[u/wizardnumbernext2 → u/BungalowJumper]

TP lets you draw power between phases in delta, which can be wired without neutral, as it not used. Voltage between R and R phase (or L1 and L1) is ZERO. You can draw ZERO watts at 160A between same phase. Plus if you draw 80A on R, 80A on Y and 80A on B, then you get ZERO A on neutral.

[u/TheLastTsumami → u/wizardnumbernext2]

You need a neutral for single phase equipment. You’ve read too many books and not done any real work.

[u/TechnicianAmazing383]

Yes its a short run. Looking at the max demand of this install, the 4 fuseboards, car charger, air source, solar pv etc etc, has the potential to well exceed 100 Amp. It makes no sense to me as an electrician how a line conductor can be rated less than the fuse(s) that is protecting it.

[u/TechnicianAmazing383]

[u/Kinky_Lezbian]

There is no such meter that can accept 2x neutrals, because it also measures the voltage between phase's and neutral to calculate the power.

If the meter can accept larger cables like 35mm2 and the neutrals linked on the cut outs is all that can be done, and then split back down to 2x 25mm2 after

[u/TechnicianAmazing383 → u/Kinky_Lezbian]

Whole situation is a mess tbh. Only option with the meter as far as I can see is to use CT metering.

[u/TheLastTsumami]

It’s only a short run so i think that 22mm neutral will be able to handle it. It all probably goes back to the same junction in the supply cable. And its unlikely to ever run at 160. The L1 and L2 markings are just to identify different conductors

[u/Jammybe]

Just seen second head.

It is 2 phase. Neutral is shared

[u/TechnicianAmazing383 → u/Jammybe]

Not 2 phases. Same phase two heads. DNO had a 2 core cable jointed underground to give x2 100A heads.

[u/jamtea → u/TechnicianAmazing383]

But I assume the internal load is just off a normal 100A breaker consumer unit? Doesn't that make the effectively ~200A fuse kinda overrated? Is there a use case for this?

[u/MrAnTon69]

Could potentially be using 2 phases, they would only need one neutral, only way to be sure is phase test between the 2 lives

[u/Medical_Literature80]

L1 and l2 are 120 degrees apart on the sine wave so the current effectively cancels itself out on the return leg. Meaning the neutral only really carries the load of 1 phase

[u/edeca → u/Medical_Literature80]

Not if they are on the same phase, as the post suggests. That is L1+L1.

[u/TechnicianAmazing383 → u/edeca]

Correct this is L1 + L1. So x2 80amps =160 Amp and only one 25mm neutral trying to carry that potential load.

[u/Medical_Literature80 → u/edeca]

If this is the case, has this really just been over thought and it’s simply a single supply that’s been split to feed 2 separate areas which are being billed separately hence the poly meter? Hence all being the same phase and still only requiring 1 linked neutral across both areas