Shore power -ring main? Question

[Rafiki]

I have the fairly usual DIY shore power installations based on a typical 2 circuit breaker consumer unit. I currently run one socket from this dedicated to the battery charger (Sterling 20 Amp), via the 6 amp circuit breaker and one socket for a fan heater or kettle direct from the 16amp circuit breaker. I installed all this about 4 years ago.

I now want to install a second socket in a more convenient place at the galley. My question is - In order to install two sockets from one circuit breaker do I have to install a ring-main? I appreciate that if I was installing 4 or 5 sockets all over the boat then a ring main would ultimately save cabling and time and effort. BUT in my case it won't because the existing socket is 30cms from the consumer unit and the new socket will involves about 4 metres of fiddly wiring crossing over the boat and above the engine.

I can't see what the benefit would be of, effectively, running two cables across the boat (there and back), over simply having one cable for the new socket direct to the consumer unit, along with the existing socket similiarly direct to the consumer unit.

Can anyone enlighten me? if a ring main is neccessessary then what exactly is the benefit?

Regards
Andrew

 

[sailorman]

I have the fairly usual DIY shore power installations based on a typical 2 circuit breaker consumer unit. I currently run one socket from this dedicated to the battery charger (Sterling 20 Amp), via the 6 amp circuit breaker and one socket for a fan heater or kettle direct from the 16amp circuit breaker. I installed all this about 4 years ago.

I now want to install a second socket in a more convenient place at the galley. My question is - In order to install two sockets from one circuit breaker do I have to install a ring-main? I appreciate that if I was installing 4 or 5 sockets all over the boat then a ring main would ultimately save cabling and time and effort. BUT in my case it won't because the existing socket is 30cms from the consumer unit and the new socket will involves about 4 metres of fiddly wiring crossing over the boat and above the engine.

I can't see what the benefit would be of, effectively, running two cables across the boat (there and back), over simply having one cable for the new socket direct to the consumer unit, along with the existing socket similiarly direct to the consumer unit.

Can anyone enlighten me? if a ring main is neccessessary then what exactly is the benefit?

Regards
Andrew

i would just extend from the 16Amp circuit

 

[ProDave]

A ring main is just a cunning way to wire a 32A circuit with cable only capable of about 27A at most, and to save cable in the process.

As your circuit breaker is only 16A there's no point. Just wire it as you suggest which is called a radial circuit.

 

[VicS]

Pro Dave is an electrician..... no more to be said!

 

[nigelmercier]

if a ring main is neccessessary then what exactly is the benefit?

Ring circuits were invented after WW2 to save copper, they are a kludge at best. Just run a radial circuit using appropriate sized wire: 1.75mm for a short run, 2.5mm for more than a couple of metres.

 

[PeterGibbs]

i would just extend from the 16Amp circuit

Exactly - run a spur from the most convenient source. That's what I do, making sure the total loading does not exceed 13 amp. Make sure all your wiring is suitable for 13 amp as a safety feature.

As you appreciate, overloading is not so easy to do on boats when alongside since the shoreside supply is almost always limited and will snap out if exceeded. Best not to rely on this.........

PWG

 

[Colvic Watson]

Exactly - run a spur from the most convenient source. That's what I do, making sure the total loading does not exceed 13 amp. Make sure all your wiring is suitable for 13 amp as a safety feature.

As you appreciate, overloading is not so easy to do on boats when alongside since the shoreside supply is almost always limited and will snap out if exceeded. Best not to rely on this.........

PWG

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

 

[alan17]

Marina outlet loads

In UK I believe most are 16a but there are some that are 30a.

At Calais the one that I was plugged in to was only 6a. Had to keep resetting if I turned kettle on. A real PIA when it was raining.

 

[st599]

Ring circuits were invented after WW2 to save copper, they are a kludge at best. Just run a radial circuit using appropriate sized wire: 1.75mm for a short run, 2.5mm for more than a couple of metres.

You should upgrade the entire radial circuit to the larger cable size, not just the new bit.

 

[stuhaynes]

In UK I believe most are 16a but there are some that are 30a.

At Calais the one that I was plugged in to was only 6a. Had to keep resetting if I turned kettle on. A real PIA when it was raining.

Maximum we get here is 10 amps which does mean that we have to 'juggle' things around.

Don't know if it's helpful but don't use single core copper wire, it must be multicore (flex) because of possible failure and associated fire risk through vibration.

 

[sailorman]

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

Look in the pontoon dis. Box . You have 10amps I think. Elsewhere in the marina
it drops to 5 amps.

 

[Boathook]

A lot seem to be 16 amp nowdays but if everyone takes that it trips the master unit.

 

[westernman]

10A in my marina.

 

[emnick]

Ring main

Other advantage of using a ring main is that the earth comes from both ways so if one earth wire was somehow disconnected you still have an earth from the other side of the ring. IIRC the earth should be separated in the consumer unit and into different earth screws. Might be stating the obvious though!

 

[CreakyDecks]

Exactly - run a spur from the most convenient source. That's what I do, making sure the total loading does not exceed 13 amp. Make sure all your wiring is suitable for 13 amp as a safety feature.

As you appreciate, overloading is not so easy to do on boats when alongside since the shoreside supply is almost always limited and will snap out if exceeded. Best not to rely on this.........

PWG

If your breaker is 16A your cable should be capable of carrying significantly more, otherwise there is no point having the breaker because the cable will catch fire first.

 

[Blueboatman]

Spurring out from the 16a side of the consumer unit is fine and I did that for a year with two outlets and the 1KW immersion heater.
Having identified that I actually want no extension leads visible, and the ability to plug in the kettle for a cup of green tea by the bed up for'd, as you do.. ..and a discrete socket for a oil filled hester or dehumidifier without traipsing over wires, etc..in the end the two spurs up each side of the boat were separated by so little-accessible- space that I did a ring main, 6 sockets, one immersion element.
Sterling charger on the 6a side from the consumer box.
As said it does mean you 'double up' on the earths and can use slightly thinner cable which is a LOT easier to feed through the various conduits. ( with fairy liquid lubricant).
Each to their own, I guess. I am not a qualified sparks.

 

[christhefish]

spur from existing socket but I would check original cable is 2;5mm and use the same for spur.it should be protected by a 10 ma rcd or rcbo

 

[Rafiki]

Ring main Q

Thanks guys, some helpful advice indeed. It's going to betwo spurs off the 16amp breaker. Now I hav ethreaded one cable through I'm gladon't have to do the return bit. Only wish I had bought 10m rather than 25metres now - anyone in Ipswich want the remaining 15metres of 2.5mm section, bargain at £10 ??

 

[nigelmercier]

spur from existing socket ... protected by a 10 ma rcd or rcbo

No, and no. It should go direct to the MCB, which should be 16A.

 

[alteredoutlook]

No, and no. It should go direct to the MCB, which should be 16A.

why?


Does it only matter if you have 2 items plugged in? i.e. if you only had one item plugged in and that was plugged in to a socket spurred off of another socket surely it would not make a jot of difference?