Extension to outsde shed.

[JohnW2]

Appliances that have a fixed power rating and adjust the current they take to maintain that fixed load could take excessive current (overloading the cable and/or the MCB) if the supply voltage dropped significantly.

I think that barrels are being rather scraped again but, in any event, the regulation about voltage drop says nothing about "overloading the cable and/or the MCB", only requiring that the safe functioning of the equipment is not impaired
Edit: having just re-read the above, I should probably have added (but didn't, because it seemed so obvious) that in the very unlikely the event that under-voltage (somewhat ironically/perversely) resulted in an 'overload' current, that's precisely the sort of thing that the MCB/whatever is there to protect the cable from.

As I often say, I think that, in general, any considerations of 'voltage drop' (from the origin of an installation) are pretty daft, given the very wide range of voltages at the origin which are permissible!

Kind Regards, John

 

[SUNRAY]

I think that barrels are being rather scraped again but, in any event, the regulation about voltage drop says nothing about "overloading the cable and/or the MCB", only requiring that the safe functioning of the equipment is not impaired

As I often say, I think that, in general, any considerations of 'voltage drop' (from the origin of an installation) are pretty daft, given the very wide range of voltages at the origin which are permissible!

Kind Regards, John

As long as the voltage doesn't drop below 216V at it's final point then it's within regs.

 

[JohnW2]

As long as the voltage doesn't drop below 216V at it's final point then it's within regs.

Exactly - which is why, with a voltage-at-origin virtually never below 245V, I am personally not in the slightest concerned about any circuit for which the 'theoretical maximum' VD is appreciably over "5%" (which would take the voltage-to-load down to maybe 230V).

The problem, of course, is that there might be some installations out there whose normal voltage-at-origin is not a lot over 216.2V. In fact, if they started out with that voltage at origin, just a 5% (of 230V) VD would taken the voltage-at-load down to just 204.7V.

Kind Regards, John

 

[SUNRAY]

Exactly - which is why, with a voltage-at-origin virtually never below 245V, I am personally not in the slightest concerned about any circuit for which the 'theoretical maximum' VD is appreciably over "5%" (which would take the voltage-to-load down to maybe 230V).

The problem, of course, is that there might be some installations out there whose normal voltage-at-origin is not a lot over 216.2V. In fact, if they started out with that voltage at origin, just a 5% (of 230V) VD would taken the voltage-at-load down to just 204.7V.

Kind Regards, John

Indeed, and I've known 220V with a fairly high loop Z and an electric drill pretty much directly on the CU was enough to take it down below 216V. A kettle took it below 200V.

On the other hand, on a camp site I ran 175m (50m + 50m + 45m + 15m + 15m) of 4mm² to a tent for an amateur radio station. The feed is ~130m 25mm² (submain to a 3ph DB) + ~13m 16mm². My gut feeling says that's about 2-2.5Ω total from the main DB in the building, which inturn was ~15m from the substantial sub (½MW ish).

I know it's a brilliant low impedance supply and way over 240V but I was gobsmacked that we still had nearly 230V with a 3KW kettle.

 

[bernardgreen]

I was asked by a friend of a friend to cast an eye over his DIY electrical installation as he was planning to install a potter's kiln in the bungalow.

A 1 kW kettle took the supply down by approx 10 volts. Incomer to the bungalow was SWA that ran 70 metres to a shed where the SWA was glanded into a very rusty box. Twin and earth from the box went up a pole to a 80 metre run of open wire. This open wire ended up with a Twin and earth down into another shed where the meters were. From the DNO cutout by the meters to the highway was about 20 metres of overhead.

I strongly advised a new cable from meters to bungalow.

 

[plugwash]

As I often say, I think that, in general, any considerations of 'voltage drop' (from the origin of an installation) are pretty daft, given the very wide range of voltages at the origin which are permissible!

A line has to be drawn somewhere. We can argue about at what point exactly volt drop becomes excessive but there clearly is such a thing as excessive volt drop (though I suspect in most cases you will run into issues with insufficient fault protection first).

The problem with taking local supply conditions into account is that unless you control the supply transformer and if-appropriate it's associated tap changer, you have no guarantee that those supply conditions will remain the same.

The other thing to remember is that we base volt drop calculations on normal load current, but transient behavior is IMO equally if-not more important. If you have a 10% volt drop under normal load and under startup conditions your load draws 5x normal load current (IIRC this is not unreasonable for things like motors) then you have a 50% volt drop under startup conditions. I could easilly see that being the difference between a motor starting successfully and stalling until the breaker trips.

 

[Haydn R]

I will install a 13A FCU next to the originating socket.

Double pole switch at the start of the shed installation to enable shed to be totally isolated quickly. This is a tenanted property. I think it is important to limit what can be done in the shed. Hence, a limited current supply and quick isolation.

Discussion about voltage drop interesting but does not apply in this case.

I will also revert to a 2.5mm2 SWA cable. I only selected to spur from an existing ring main socket as it is near to the back door and access to the shed. To run back to the distribution board to provide the shed with its own circuit would be a major impact in time, expense and difficulty,

 

[Taylortwocities]

I will install a 13A FCU next to the originating socket.

That will provide the DP isolation for the shed, and the SWA. There’s no need for another one in the shed.

 

[JohnW2]

A line has to be drawn somewhere. We can argue about at what point exactly volt drop becomes excessive but there clearly is such a thing as excessive volt drop ....

Sure. However, that doesn't alter the fact that, given that what matters is the voltage at the load, and given the wise range of possible/permissible voltages at the origin of the installation, to think of that 'drawn line' in terms of VD relative to the origin of the installation is really not very helpful - unless, as below, one considers the 'worst-case-scenario' (which can get rather silly, hence also 'not very helpful'.

...(though I suspect in most cases you will run into issues with insufficient fault protection first).

That's what I would have thought, although it's not difficult to come up with scenarios in which, if one took VD very seriously, it would be the length-limiting factor in a circuit for a given size of cable.

The problem with taking local supply conditions into account is that unless you control the supply transformer and if-appropriate it's associated tap changer, you have no guarantee that those supply conditions will remain the same.

Sure - but the only real alternative would be to consider the worst-case scenario (or voltage at origin being 216.2V) which, with currently-available current-using products could easily result in the conclusion that almost any degree of VD (hence any circuit!) would be unacceptable

So, in essence, when we decide on where to 'draw the line' (in terms of VD relative to origin), we are really fooling ourselves, since what we are actually doing (even if only subconsciously) is making some assumption about the likely voltage at the origin.

The other thing to remember is that we base volt drop calculations on normal load current, but transient behavior is IMO equally if-not more important. If you have a 10% volt drop under normal load and under startup conditions your load draws 5x normal load current (IIRC this is not unreasonable for things like motors) then you have a 50% volt drop under startup conditions. I could easilly see that being the difference between a motor starting successfully and stalling until the breaker trips.

I've often wondered about that. I don't know what the truth is in practice, but I've often seen people talking of motor start-up currents of 10 or 20 (sometimes even more) times the running current - and, if that were the case, considerations of VD might make one wonder how such motors ever manage to start satisfactorily

Kind Regards, John