Shore power -ring main? Question

[nigelmercier]

why?

[1] Split at socket: the "inner" cable takes twice the load of the "outer"
[2] 10A RCB: why limit the current when you could have 16A?

 

[Simondjuk]

[1] Split at socket: the "inner" cable takes twice the load of the "outer"
[2] 10A RCB: why limit the current when you could have 16A?

1. Provided the wiring is rated above the breaker, so what?

2. He said milliamps, RCD and RCBO. What have any of those got to do with the rating of the MCB?

 

[nigelmercier]

1. Provided the wiring is rated above the breaker, so what?

2. He said milliamps, RCD and RCBO. What have any of those got to do with the rating of the MCB?

[1] Because it is simply bad practice.
[2] My mistake, I read it as 10A MCB. A 10mA RCD (rather than 30mA) is a good idea, never seen one that will fit in a CU though.

 

[Jean]

Take your pick, providing the conductors are of a size that the protective device adequatelly protects the cable, then you can spur it off another socket or direct from the RCB (see BS7671 Appendix 15 and 433.1, 433.4.1 and 434.4 if in doubt). Cables should though be multistranded for vibration protection as metioned by someone early (hope the one you bought is?).

 

[prv]

A 10mA RCD (rather than 30mA) is a good idea, never seen one that will fit in a CU though.

I believe they're around, the suggested use being for a dedicated circuit supplying outdoor sockets (or sockets indoors but likely to be used for powering outdoor equipment).

Pete

 

[nigelmercier]

... you can spur it off another socket or direct from the RCB (see BS7671 Appendix 15 and 433.1, 433.4.1 and 434.4 if in doubt)...

I believe that this refers to the practice of taking a spur from a ring main, which is a totally different concept. Taking a spur from a radial circuit (which itself is technically a fused spur) is asking for trouble.

 

[Simondjuk]

It won't be a spur off a radial though. He's only got one socket at present, so he'll simply be extending the one outlet radial which is already in place.

 

[PeterGibbs]

Out of interest, what are Marina individual shorepower loads generally limited to?

In practice it depends how old the installation is. Modern pontooning will support 13 amp just fine - mostly the case in the UK - but numerous sites on the continent with older equipment could be as low as 1 amp.

As more marinas convert to power charging at the berth the ratings have been lifted. You sail on the continent these days with a stack of 50 C coins - just watch how they fly through the power pylons!

PWG

 

[Jean]

Nigelmercier, if you look at the radial circuit in appendix 15 (the second diagram), it states “An unfused spur may be connected to the origin of the circuit in the distribution board.” Perhaps you would like to tell us just what “trouble” you are referring to?

 

[nigelmercier]

Nigelmercier, if you look at the radial circuit in appendix 15 (the second diagram), it states “An unfused spur may be connected to the origin of the circuit in the distribution board.” ...

See my underlining.

It won't be a spur off a radial though. He's only got one socket at present, so he'll simply be extending the one outlet radial which is already in place.

Err... that is a spur off a radial. See above.

 

[Jean]

nigelmercier: but the origin of the circuit is the MCB in the distribution board is it not? I'd also appreciate a comment on what "trouble" you are refering to?

 

[Simondjuk]

See my underlining.



Err... that is a spur off a radial. See above.

No, it's not.

A spur off a radial will have three cables in the pattress at which the spur originates - the incoming radial, the outgoing radial and the cable feeding the spur.

In the scenario described there will only be two cables in the first (existing) and one in the last (new) pattress. There is no spur as there is no fitting containing three cables. The last fitting with just one cable is simply the end of the radial, not a spur.

 

[nigelmercier]

A spur off a radial will have three cables in the pattress at which the spur originates...

Why so? A spur is any circuit that does not originate from the CU. It is obvious that this is not good practice in the case of a radial circuit, as the original cable will be under increased load. Whether this causes any problems is a function of the MCB and the existing cable, but why take chances?

 

[VicS]

Why so?

Simple numeracy I would imagine.

One. The incoming supply from the mcb or the previous socket on the circuit.

Two. The connection going on to the next one in the circuit

Three. The connection to the spur

 

[nigelmercier]

Simple numeracy I would imagine.

Such simple numeracy ignores the fact that a spur does not involve a third wire, viz:

The wrong way:

[CU] ---- [SOCKET] ---- [SPUR SOCKET]

The right way:

____ /---- [SOCKET]
[CU]{
____ \---- [SOCKET]

 

[VicS]

Such simple numeracy ignores the fact that a spur does not involve a third wire, viz:

The wrong way:

[CU] ---- [SOCKET] ---- [SPUR SOCKET]

The right way:

____ /---- [SOCKET]
[CU]{
____ \---- [SOCKET]

???? Subject to correct cable size and fuse/mcb rating either is acceptable

Unless there are any changes in the 17th:-

In a ring circuit the cable leaves the consumer unit and travels to each socket in turn. When it reaches the last socket it returns to the same fuse/mcb in the CU, thus creating a ring.
A ring circuit can serve an area up to 100 square metres in domestic situations. 2.5mm² cable is used to wire the circuit and the circuit has a 30amp fuse or 32amp MCB in the CU.
Ring circuits can have extra sockets added to them by adding a 'spur'. A spur is a branch off the ring circuit, usually from an existing circuit, although a junction box could also be used. Theoretically as many spurs as sockets could be added, but the maximum load of the circuit (30/32amp) still exists).

With radial circuits the cable comes from the consumer unit and travels to each socket, simlar to the ring circuit. However when the circuit reaches the last socket the cable ends.
Radial circuits can therefore only serve a smaller area. Using 2.5mm² cable combined with a 20amp fuse/MCB an area of 20m² is permissible. For 4mm² cable combined with a 32amp MCB or a 30amp cartridge fuse an area of 50m² is permissible.
The number of sockets that can supplied is not specified.
In a similar way to ring circuits spurs can be added at points along the radial circuit if required but high powered appliances, eg cookers and showers, must have their own radial circuit.

At any point in either system a spur will create 3 cables. The existing two plus the cable to the spur.

 

[Simondjuk]

Such simple numeracy ignores the fact that a spur does not involve a third wire, viz:

The wrong way:

[CU] ---- [SOCKET] ---- [SPUR SOCKET]

The right way:

____ /---- [SOCKET]
[CU]{
____ \---- [SOCKET]

By that token, the lights in more or less every home in the UK are wired 'the wrong way', since they're radial circuits. You'd also need a rather large consumer unit to accommodate a cable for each light fitting in the property. Imagine it, perhaps 15 or 20 cables all needing to be crammed into and terminated in there, and that'd be before wiring for sockets, cooker, shower, immersion heater and any other circuits.

As for the terminology, I suppose you can call a radial a spur if it pleases you, but I don't imagine the books will be re-written to suit your whim. A radial is a radial, and a spur is a spur, just as described by Vic S above, whether or not I want to call them flangesproggles and brocklegogglers, or you want to call them what you do.

 

[nigelmercier]

By that token, the lights in more or less every home in the UK are wired 'the wrong way', since they're radial circuits...

Indeed they are. But the difference is that they are fused at less than 50% of the rating of the conductor. This discussion is about using a 16A MCB to feed a pair of 13A sockets, using flexible cable.

 

[VicS]

Indeed they are. But the difference is that they are fused at less than 50% of the rating of the conductor. This discussion is about using a 16A MCB to feed a pair of 13A sockets, using flexible cable.

Yes and the maximum total loading on the radial circuit would be 16 amps. Even if the sockets were wired in parallel to the 16 amp mcb the total load is still restricted to 16 amps

In a domestic situation the circuit should be wired to a 20 amp fuse or mcb using 2.5mm² cable giving a total capacity of 20 amps or using 4mm² cable to a 32 amp mcb or 30 amp cartridge ( not rewireable) fuse giving 32 or 30 amp total capacity.

Its not wrong... its what the IEE regs permit!

 

[nigelmercier]

Yes and the maximum total loading on the radial circuit would be 16 amps. ...

Meaning that the wire from the MCB to the original socket would need to be uprated to 2.5mm to give a sensible margin, or just do it properly in the first place.