Mains cable

[vintagescubaman]

Hi everybody, I am planning to run new mains cable through my boat, I have carried out a search on the forum, but cant find a specific answer to my query. I am wondering what people recommend as the most suitable size cable. 25mm Artic seems a bit on the small side if its running round a 60 foot boat? It will be fed by a 3000 watt inverter.

 

Any advice much appreciated!

[paulcatchpole]

Hi everybody, I am planning to run new mains cable through my boat, I have carried out a search on the forum, but cant find a specific answer to my query. I am wondering what people recommend as the most suitable size cable. 25mm Artic seems a bit on the small side if its running round a 60 foot boat? It will be fed by a 3000 watt inverter.

 

Any advice much appreciated!

 

2.5mmsq, presumably?

 

It'll be fine - is this one or two radials? There's reasons for not using a ring, someone will be along to remind me what they are soon...

 

2.5mm is rated to 20-25A depending on brand, although it's worth remembering that's in free air and gets de-rated for enclosing etc. Even so, more than enough for protection by a 16A MCB.

 

PC

[Artimis]

If your doing a ring main, you potentially divide the load in 2, so dont need that thickness as current only has half the distance to go

 

There ya go.

WHat is the reason for not using a ring Paul?

 

I saw a newly wired boat last week, done by a full time professional boat electrician who used 2.5 in a ring and not in conduit

Edited May 7, 2010 by Artimis

[paulcatchpole]

If your doing a ring main, you potentially divide the load in 2, so dont need that thickness as current only has half the distance to go

 

There ya go.

WHat is the reason for not using a ring Paul?

 

I saw a newly wired boat last week, done by a full time professional boat electrician who used 2.5 in a ring and not in conduit

 

Sorry, I really can't remember - but I suspect a search of the last time this came up would probably reveal all. I'll have a gander.

 

Note, that I wasn't recommending conduit, just reminding that the current carrying ability is derated for enclosure...

 

PC

[Gibbo]

2.5sq mm cable is good for 35 amps in free air.

 

But they're not usually in free air and rather than expecting people to calculate lots of sums involving heat transfer and so on, clever people draw up tables for us. 2.5 is good for 20 amps in a conduit in a wall, however it's only good for about 13 amps if inside an insulated wall with no conduit. The conduit contains an air gap though how this is supposed to help I don't know!

 

Remember that these rating are purely to do with fire safety. Voltage drop has to be calculated (not difficult).

 

Rings weren't such a good idea with modifed sinewave inverters as they tend to radiate lots of noise into the middle of the boat. It doesn't matter with a pure sinewave inverter so a ring is fine.

[paulcatchpole]

2.5sq mm cable is good for 35 amps in free air.

 

But they're not usually in free air and rather than expecting people to calculate lots of sums involving heat transfer and so on, clever people draw up tables for us. 2.5 is good for 20 amps in a conduit in a wall, however it's only good for about 13 amps if inside an insulated wall with no conduit. The conduit contains an air gap though how this is supposed to help I don't know!

 

Remember that these rating are purely to do with fire safety. Voltage drop has to be calculated (not difficult).

 

Rings weren't such a good idea with modifed sinewave inverters as they tend to radiate lots of noise into the middle of the boat. It doesn't matter with a pure sinewave inverter so a ring is fine.

 

Ta for putting me right, Chris...

 

I was thinking, also, about rings and the potential for someone to use cable just sized for the ring current maximum, and then getting a loose connection and breaking the ring... Also, for me, it's a nice thought to have two radials on separate trips, not that we've ever tripped one, mind...

 

PC

[blackrose]

2.5sq mm cable is good for 35 amps in free air.

 

But they're not usually in free air and rather than expecting people to calculate lots of sums involving heat transfer and so on, clever people draw up tables for us. 2.5 is good for 20 amps in a conduit in a wall, however it's only good for about 13 amps if inside an insulated wall with no conduit. The conduit contains an air gap though how this is supposed to help I don't know!

 

Remember that these rating are purely to do with fire safety. .

 

So what would happen in the latter case if one exceeded the 13amp current (say to 15 or 16amps)? Would anything trip or would the cable get hot and potentially lead to a fire?

Edited May 8, 2010 by blackrose

[Robbo]

So what would happen in the latter case if one exceeded the 13amp current (say to 15 or 16amps)? Would anything trip or would the cable get hot and potentially lead to a fire?

 

Your circuit breaker will kick in, but as the OP's inverter is only 3000 watts this would quit before a 16a MCB! On shore power either your MCB or the on shore one will kick in.

Edited May 8, 2010 by Robbo

[Gibbo]

So what would happen in the latter case if one exceeded the 13amp current (say to 15 or 16amps)? Would anything trip or would the cable get hot and potentially lead to a fire?

 

That depends on the size of the circuit breaker.

 

If the CB was 16 amps, and you pulled 15 amps through the cable, the cable would overheat, possibly melt the insulation and set something on fire.

 

Your circuit breaker will kick in, but as the OP's inverter is only 3000 watts this would quit before a 16a MCB! On shore power either your MCB or the on shore one will kick in.

 

Sorry but this is completely wrong. Don't rely on this advice.

 

Ta for putting me right, Chris...

 

I was thinking, also, about rings and the potential for someone to use cable just sized for the ring current maximum, and then getting a loose connection and breaking the ring... Also, for me, it's a nice thought to have two radials on separate trips, not that we've ever tripped one, mind...

 

PC

 

I've often struggled with the logic behind rings because of this problem. The idea is just mental. To my mind, the two feeds of the ring should each be fed by their own separate circuit breaker.

[Robbo]

Sorry but this is completely wrong. Don't rely on this advice.

 

I misread the question, agree don't follow that advice, I wasn't thinking this morn..

[Radiomariner]

name='Gibbo' date='May 8 2010, 01:03 PM' post='526542'

 

 

I've often struggled with the logic behind rings because of this problem. The idea is just mental. To my mind, the two feeds of the ring should each be fed by their own separate circuit breaker.

 

Surely, as the circuit breaker is rated according to the cable used, safety wise there would be no compromise if the loop circuit was broken it would still trip before wiring heats up. (Downside is that when it does trip you loose all sockets in the circuit). On the Pro. side the ring only requires one cb and with most multisocket layouts significently less cable than a number of radials. I am not convinced that having a cb on each feed of a loop is a good idea as both will have to trip to isolate the loop. If the loop is broken and only one cb has tripped then there is the likelyhood of a hazard remaining where the loop is broken.

However, if expense is no object, go for radials every time!

Edited May 8, 2010 by Radiomariner

[Gibbo]

Surely, as the circuit breaker is rated according to the cable used, safety wise there would be no compromise if the loop circuit was broken it would still trip before wiring heats up.

 

Well that would be correct if that's how they were rated. But they're not. The fuse for a ring is rated assuming that the ring is intact. A UK ring is usually 2.5mm cable with a 32amp CB. If one ring breaks then the fuse is too big for the remaining half of the ring. Put a 25 amp load on the end (with a broken ring) and you've got a fire on your hands. That's just the way it is.

 

(Downside is that when it does trip you loose all sockets in the circuit). On the Pro. side the ring only requires one cb and with most multisocket layouts significently less cable than a number of radials. I am not convinced that having a cb on each feed of a loop is a good idea as both will have to trip to isolate the loop.

 

I'm asolutley certain it's a good idea.

 

A ring of 2.5mm cable would be safer with a 15 amp fuse at each end. How can that cause a problem?

Edited May 8, 2010 by Gibbo

[Radiomariner]

Well that would be correct if that's how they were rated. But they're not. The fuse for a ring is rated assuming that the ring is intact. A UK ring is usually 2.5mm cable with a 32amp CB. If one ring breaks then the fuse is too big for the remaining half of the ring. Put a 25 amp load on the end (with a broken ring) and you've got a fire on your hands. That's just the way it is.

 

 

 

I'm asolutley certain it's a good idea.

 

A ring of 2.5mm cable would be safer with a 15 amp fuse at each end. How can that cause a problem?

 

 

I am surprised. I have never looked into domestic wiring requirements unless I have needed to.

Looking at my own home wiring I note that my lighting rings are of 1.5mm² and fused with 5A mcb's the power rings are of 2.5mm² and protected by a 15A mcb and my cooker and water heater are radial circuits of 6mm² cable and protected by 30 amp mcb's.In each case the trip current of the breakers is considerably less than the safe current of the cables. The house is 32 years old. An inspection some years ago showed the wiring earthing etc to be in good condition, an RCD was added and the old rewirable fuses (an American Radar manual I had to use once kept referring to an "over current protection device" which I had a hell of a job finding until I realised it was simply a effing fuse!), were replaced with plug in mcb's. It has never occurred to me that any updating of British Standards would result in a reduction in safety!

 

By the way the Recreational Craft Directive sub section 2.8.2( b ) requires that the current rating of each circuit breaker or fuse shall not exceed the smallest size of conductor. This is regardless of being a ring circuit.

 

As for your second comment you missed my point. If there is break in the loop and there is cause for overcurrent, with a fuse at each end of the loop only the one fuse will rupture. This means that somewhere there is an apparrently dead power socket with live wires loose somewhere in or close by; a hazard to the unwary electrician who may consider the loop to be isolated. My argument was also based along the lines that the circuit breaker would trip at a current less than that of the maximum permissable current for the wiring, so the mcb would trip the complete ring even if the loop is broken, but you tell me that this is not now the case.

 

I do agree that especially on a small boat where expense is not a major issue individual radial circuits are preferable. (As a "fault finding man" it woud be ideal.) However to some, a couple of hundred quid on extra breakers and wiring just might be too much. There are a lot of people out there on tight budgets. For them, if they need power sockets all over, consider a ring main but be sure that the trip setting of the mcb is less than the maximum capacity of the cable used.

 

Edited because a smiley came up instead of ( b )

Edited May 9, 2010 by Radiomariner

[bottle]

the power rings are of 2.5mm2 and protected by a 15A mcb

 

If they were/are rings then this seems unusual as back in the late 60's early 70's we wired ring circuits (13 amp square pin sockets) in 2.5mm2 and fitted 30 amp fuses.

[Arnot]

There is one advantage of ring mains that has not been mentioned here…

 

In buildings where people work it is conventional to have the installation fully tested regularly. With a ring main, it is possible to disconnect the tails at the distribution board and test the continuity of live neutral and protective earth rings individually as well as testing for any insulation breakdown.

 

On most new builds, there tend to be socket outlets on both sides of the boat so there has to be wiring up each side. The addition of an extra few metres of cable makes the loop allowing proper testing, better fault tolerance and lower cable losses.

 

Whilst conduit is a good idea when the cable is located where mechanical or other damage is likely or possible, with most boats it would be impractical to conduit every inch and the use of sheathed artic cable essentially achieves the same end.

 

The reasoning behind using 2.5mm flexible cable on boats stems from the best practice for mobile electrical installations in the wiring regulations. This is to minimise the danger of cable damage through the effects of vibration rather than the effects of over current.

 

The BSS carefully avoids any detail on mains electrical systems, so testing is not mandatory but none the less it is good practice.

 

FWIW I do this sort of thing professionally and unless there are very good reasons for deviation, I wire up all the socket outlets on a boat with 2.5mm artic in a ring main and protect them with a single B16 MCB.

 

Regards

 

Arnot

[Gibbo]

Looking at my own home wiring I note that my lighting rings are of 1.5mm² and fused with 5A mcb's the power rings are of 2.5mm² and protected by a 15A mcb and...............

 

So what's the point in using a ring?

 

The ring circuit was developed in order to reduce cable size so that a (nominally) 30 amp circuit could be produced using 15 amp cable by the mere addition of an extra few feet connecting the ends together. That's the whole point of it. That's why it was developed.

 

If you fuse the ring at 15 amps then nothing has been achieved (apart from wasting that extra few feet of cable).

Edited May 9, 2010 by Gibbo

[Radiomariner]

So what's the point in using a ring?

 

The ring circuit was developed in order to reduce cable size so that a (nominally) 30 amp circuit could be produced using 15 amp cable by the mere addition of an extra few feet connecting the ends together. That's the whole point of it. That's why it was developed.

 

If you fuse the ring at 15 amps then nothing has been achieved (apart from wasting that extra few feet of cable).

 

The builders have decided to introduce a wider safety margin for just the same reasons you have already mentioned! The extra few feet of cable to complete the loop provides a better safety margin at less cost than using heavier cable throughout!

 

This is a pointless argument, as what goes around seems to come around.

[Gibbo]

The builders have decided to introduce a wider safety margin for just the same reasons you have already mentioned! The extra few feet of cable to complete the loop provides a better safety margin at less cost than using heavier cable throughout!

 

This is a pointless argument, as what goes around seems to come around.

 

Me thinks someone has been spinning you a yarn.

 

If a 15 amp CB is in there, then there is no point whatsoever in using a ring. Absolutely none. The margin between 2.5mm cable and a 15 amp circuit breaker is already so huge that adding even more is pointless.

[GUMPY]

Your circuit breaker will kick in, but as the OP's inverter is only 3000 watts this would quit before a 16a MCB! On shore power either your MCB or the on shore one will kick in.

 

Just be warned I came back to the mooring yesterday and after a day connected everything up, batteries were down to about 75% and the wife wanted the washing machine on.

Watched as the 240v AC amp meter climbed past 20 amps and stayed there for well over half hour. As expected none of the 16amp MCBs tripped even the pony B-type one in the shed.

So its real easy to draw much more current than an MCB is rated for, use less than 2.5 at your peril.

Edited May 9, 2010 by idleness

[Radiomariner]

Me thinks someone has been spinning you a yarn.

 

If a 15 amp CB is in there, then there is no point whatsoever in using a ring. Absolutely none. The margin between 2.5mm cable and a 15 amp circuit breaker is already so huge that adding even more is pointless.

 

No "yarn" spun. I just replied with a possibility for my home system being as it is. As I mentioned these are "plug in" mcb's (replacing re-wireable fuses) and mcb's of different values can be plugged in. Perhaps it was originally considered for a higher fuse rating.

 

Any how this conversation is pointless

[alan_fincher]

If a 15 amp CB is in there, then there is no point whatsoever in using a ring. Absolutely none. The margin between 2.5mm cable and a 15 amp circuit breaker is already so huge that adding even more is pointless.

 

2.5sq mm cable is good for 35 amps in free air.

 

But they're not usually in free air and rather than expecting people to calculate lots of sums involving heat transfer and so on, clever people draw up tables for us. 2.5 is good for 20 amps in a conduit in a wall, however it's only good for about 13 amps if inside an insulated wall with no conduit. The conduit contains an air gap though how this is supposed to help I don't know!

Pointless though the conversation may now have been declared, can you elaborate, please ?

 

Are you agreeing that the cable can be good for as little as 13 amps if fully insulated around ?

 

If so, then if you are going to draw 15 amps, might it not be useful if that is reaching a socket by two different routes, neither of them (hopefully!) carrying as much as the 15 amps, as some will have (hopefully again!) come around the other way ?

 

Clearly various factors will determine how much is supplied by each route, (length of run mainly, I'd have thought), but surely in most cases the ring would reduce the overall current carried at any point.

 

Also the volts drop will surely be less because of the overall combined resistance between breaker and socket ?

 

I didn't bother though, as I can't conceive needing to, (or being able to) work with anything like these kind of currents!

[Gibbo]

Pointless though the conversation may now have been declared, can you elaborate, please ?

 

I'll try

 

Are you agreeing that the cable can be good for as little as 13 amps if fully insulated around ?

 

Yes, that's why you're not allowed to use it for a 15 amp circuit unless it is in a conduit (IEE regs).

 

If so, then if you are going to draw 15 amps, might it not be useful if that is reaching a socket by two different routes, neither of them (hopefully!) carrying as much as the 15 amps, as some will have (hopefully again!) come around the other way ?

 

But you're not allowed to use it for a 15 amp circuit unless it's in a conduit, in which case it's good for much more.

 

Clearly various factors will determine how much is supplied by each route, (length of run mainly, I'd have thought), but surely in most cases the ring would reduce the overall current carried at any point.

 

Also the volts drop will surely be less because of the overall combined resistance between breaker and socket ?

 

Volt drop isn't a problem on 230 volts with 2.5mm cable until you get to silly lengths. For instance at 15 amps it will drop less than 5% on a 165 foot cable.

 

I didn't bother though, as I can't conceive needing to, (or being able to) work with anything like these kind of currents!

 

Especially on a boat.

[alan_fincher]

Thanks for reply, and largely seems fair enough - I hadn't considered what the volts drop might be, and of course you are right.

 

My only reply might relate to.....

 

Yes, that's why you're not allowed to use it for a 15 amp circuit unless it is in a conduit (IEE regs).

Surely you are actually allowed to install a 240 volt circuit on a leisure narrow boat without complying to IEE regs, aren't you ?

 

True, false or grey area ?

[Gibbo]

Surely you are actually allowed to install a 240 volt circuit on a leisure narrow boat without complying to IEE regs, aren't you ?

 

True, false or grey area ?

 

It's greyer than grey.

 

There's nothing written down saying it has to comply with IEE regs. There are ISO standards for boat AC wiring but they are at variance with IEE in certain parts.

 

If your boat electrocuted someone, or set on fire and the insurance company were looking for a get out, then the courts could possibly argue that it should have been wired in accordance with "good practice". Trying to argue what is "good practice" might prove a bit difficult because the ISO standards and the IEE regs disagree in several places and the ISOs miss out parts that the IEE does cover.

 

The logical solution to this would be to wire it in accordance with the relevant ISOs but revert to IEE for the parts that the ISO misses and they don't conflict. But then soeone else could argue that you followed IEE for part of it then deviated. It also means it's open to personal interpretation.

 

On the other hand, if the boat is classed as residential, then it has to be wired in accordance with IEE regs and carried out by a part p certified sparky. A sparky who, almost by definition, knows nothing about boats and will make an almightly cock up of it. Expecially when it comes to a galvanic isolator which IEE regs state, by implication, are against the regs.

 

IEE doesn't really suit boat systems yet residential boats should, by law, comply. This results in a worse system than following the ISOs and/or good marine practice.

 

It's all a bit of a mess really.

[Guest]

I'm retired from't leccy game (thankfully), but there used to be a limit as to how many sockets could be installed in a radial circuit. Does this still stand?