Garage Electrical supply

Is the second terminal marked with a white or yellow dot?

If yellow, that looks like three phase, and you could presumably take a tap from the other phases, subject to regs about not mixing single phases in the same premises (which you probably won't fall foul of). You would presumably need the permission of whoever owns this garage.

If you want to piggyback on the existing single-phase connection, it's a question for the DNO whether it's thick enough to support another 100A (or whatever) on the same feed.

(Am I correct in understanding that it's wired in star form and the black is the neutral from the transformer? That suggests an additional tap would be yellow+black or red+black. Would that give the right 230V live-neutral?)

Theo

Surely 40A would do for a garage.

NT

Thanks, the second terminal is indeed yellow, so I was hoping that a yellow+black or red+black for another feed would be acceptable, sounds like it should be. The owners of the garage are very happy for a connection to be made from this, and fed through the wall, but I guess there'll have to be some legal sign off. I hadn't thought about piggybacking on the existing supply but I guess it might be possible, perhaps more so if a 40A supply is an option. 100A is overkill for a garage, I just need to run a chest freezer and the occasional power tool, but the options I was given were 100A single phase or three phase.

William

Agree 40A would be more than sufficient, but it wasn't given as an option when I enquired. I was asked about projected usage, but it seems that was just to determine single/three phase requirement, after I pointed out I really didn't need a three phase supply all the discussion was about a 100A single phase supply. I'm guessing it'd be cabled up for

William

In message , at 07:20:00 on Tue, 19 Sep

The connection work (and your own meter) would have to be done by National Grid, and therefore you need to understand what their policy was about sharing a three-phase box like that between two properties.

They'll want to make it 100amp so that in a few years time someone can charge their EV.

Everything else being equal, one question will be whether either of the two "spare" phases has enough extra capacity in your street (from the substation). If the phase with that spare capacity is Blue, it'll be a non-starter (because that cable will be potentially feeding 200A on just one phase).

And of course, I'd expect them to want to get a formal wayleave rather than just the informal agreement of whoever happens to live there this week.

Not the DNO?

Chris

What is the problem with multiple phases in the same premises?

John

the possibility of 415v shocks.

If its on private property though, you would need written permission for your agent, ie the leccymob to enter and work on it, and of course it could mean a short interruption in supply. It also needs to be said that are you sure its up to the job of the extra load and do you know who owns the property its in? Brian

Indeed. SWMBO worked in a grammar school (quite an upmarket one) and had a sixth form 'computer lab' (she put in a Linux server and several clients).

She discovered that two adjacent power points on the PC desks were on different phases. Because I'd educated her well (!) she complained that at the very least they should be labelled. Nothing was done and *she* was labelled a troublemaker. She left not long afterwards.

However, 3-phase power is sometimes installed in UK homes. I know of several. In many other countries it is commonplace. How much risk is there of simultaneously grabbing hold of two live wires at the same time? It is very hard to think of any way this could happen accidentally except perhaps if multiple phases are present in the same switch or connection box and alterations are being made on a live system.

John

I sympathise. BTDTGTTS.

It's not just shocks, it's equipment damage.

Suppose you have a laptop and a powered USB hub plugged into adjacent sockets which happen to be on different phases. They're double insulated, hence no earth. There's a tiny bit of leakage to local ground on each one, not enough to trip the RCD. Suddenly there's 415v across the USB cable. If you touch both ends, you might get a belt. When you plug it in there may be a spark and it may zap something inside. While connections like Ethernet are magnetically isolated, USB is a direct electrical connection and is not expecting a 415v difference between signals.

Theo

For a voltage of 415V to be present between the USB connectors just before they make contact it would be necessary for all the leakage to come from one pole of each of the power supplies, which seems very unlikely.

Even if this did happen, the equipment should not care. USB ports have to cope with situations like a double-insulated USB hub being connected to an earthed computer. In this case, making the same assumption that all the leakage comes from one side of the mains they would still have to deal with 240V from the double insulated device to the earthed device. In CE-marked equipment the USB ports will have been tested with electrostatic discharges to the connector body of many kV. It is the connector body that makes first contact when USB connections are mated.

So whether the source voltage is 240V or 415V is really not going to make much difference when the devices are already protected against much higher voltages.

If you are holding both devices, you may get a small electric shock. However, the maximum shock current from double-insulated devices powered from different phases will be LESS than that from a single-phase powered device to an earthed device. This is because the voltage will be about 1.7x higher, but the leakage impedance will be doubled as there will be two sets of double-insulation in series with each other. So the worst-case 3-phase shock current will be about

So why worry?

John

In message , at 08:18:01 on Tue, 19 Sep 2017, Chris J Dixon remarked:

Possibly; the vans I see have "UK Power Networks" on the side, which maybe I'm mistakenly conflating with National Grid. If you'd asked me who my DNO was I'd have replied "Eastern Electricity".

None.

The only requirement is that where 400V (two or three phases) is present and might not be expected, it must be labelled as 400V.

That's the 'Euro harmonised' phase to phase voltage. As lethal as makes no difference to the actual pre-harmonisation values of 415 (UK) and 380 (European) voltages within a couple of volts of the actual voltages in use.

No more lethal in real terms than the new "EU 230V" that most properties have if you follow the simple rules of proving dead before working on a circuit.