Planning a 240v installation

[MtB]

Currently I have a couple of small 240v convector heaters I connect directly to the shoreline for frost protection in extreme weather. The shoreline trails up the inside of the boat and is pretty untidy so I'm planning to fit a 16A weatherproof shoreline socket outside and some fixed wiring inside the boat to two 13A twin sockets, one by each heater.

 

From a BSS point of view, can I connect the 16A socket directly to the two 13A twin sockets or do I need a circuit breaker (or anything else) to protect the wiring and/or users?

 

And in particular, what sort of cable should I use for the 240v wiring? 2.5 T&E, or flex of some sort?

 

And what are the cable-clipping requirements? And is there anything else I should know before starting?

 

Many thanks in anticipation,

Mike

[the grinch]

you must use multi strand cable, single strand cables are not allowed. personally i would use a CU and wire every socket seperately but that is probably overkill, at least keep different ampage circuits seperated.

[Chris Pink]

You need a garage consumer unit one or two breakers plus RCD, I would run 2.5 blur 'artic' cable down 20mm conduit clipped 4 saddles per 3 m length. You will need a hull earth bond (6mm machine screw to clean steel) in the vicinity of the consumer unit, which would be sensibly mounted just inside the cabin from the 16A inlet. Run the two sockets as one 'spur' ie 1 run of cable to the two daisy chained sockets. Keep the 240v conduit logically and physically separate from the 12v. Except that your 12v and 240v earth bonds are best close together if possible.

 

The BSS says very little about 240v installation.

Edited December 8, 2011 by Chris Pink

[pmms]

you must use multi strand cable, single strand cables are not allowed. personally i would use a CU and wire every socket seperately but that is probably overkill, at least keep different ampage circuits seperated.

Note that there is no requirement to use multi strand cable, the use of twin and earth is not a failure point (private boats), BUT best practice says use multi strand.

 

There is no BSS requirement for there to be fuses/circuit breakers or Residual current circuit breaker (AKA RCD). BUT best practice says use both. A lot of installations use a "garage" consumer unit from Screwfix (or similar) this comes with an rccb and a 6amp (lights) CB and a 10amp CB for sockets.

 

Mike, most installations use 2.5mm 3 core flex "Arctic" cable.

 

I think that the lack of a BSS requirement for fuses/CB's/RCCB reflects the diverse opinions within the marine industry on the subject of 'lectricity.

 

Cable should be supported clear of anything that could cause damage, clear of fuel/gas pipes and protected from chafe when passing thru bulkheads, note there is no designated supporting distance, nor a distance from gas/fuel lines just clear of them. Note also that double insulated cable is regarded as being adequately protected from chafe where it is in good condition.

 

Ref BSS Pt 3.2/3.3/3.4

 

Paul M

[Bat & Frog]

I was also advised to put bootlace ferrules on any cable end that will be fixed by a screw.

[nicknorman]

Note that there is no requirement to use multi strand cable, the use of twin and earth is not a failure point (private boats), BUT best practice says use multi strand.

 

There is no BSS requirement for there to be fuses/circuit breakers or Residual current circuit breaker (AKA RCD). BUT best practice says use both. A lot of installations use a "garage" consumer unit from Screwfix (or similar) this comes with an rccb and a 6amp (lights) CB and a 10amp CB for sockets.

 

Mike, most installations use 2.5mm 3 core flex "Arctic" cable.

 

I think that the lack of a BSS requirement for fuses/CB's/RCCB reflects the diverse opinions within the marine industry on the subject of 'lectricity.

 

Cable should be supported clear of anything that could cause damage, clear of fuel/gas pipes and protected from chafe when passing thru bulkheads, note there is no designated supporting distance, nor a distance from gas/fuel lines just clear of them. Note also that double insulated cable is regarded as being adequately protected from chafe where it is in good condition.

 

Ref BSS Pt 3.2/3.3/3.4

 

Paul M

I would say you want a 16a breaker for the sockets - that is the rating of the external connectors and normally the rating of the marina socket. There is normally no need for an additional breaker unless you want to have a 240v lighting circuit, which is unusual unless a residential boat. If you only have a 10a breaker you are limited to around 2.4kw total appliance drain from all the sockets. Definitely an RCD unless you don't value your's and your friends/family's lives.

 

If you are going to leave it on shore power (sounds like you are) and you bond the earth to hull (which you should do unless you don't value your life etc) you should consider a galvanic isolator to prevent shore earth currents from causing galvanic corrosion of the hull. There will now follow an argument about whether it should be a GI or an isolation transformer, but IMHO a GI is adequate if no perfect...

 

One last point which I have never quite understood, there is a requirement to separate 12vdc and 240v wiring, eg not running them in the same trunking etc. But I have never been clear as to whether this is met by having double-insulated mains flex - ie each conductor insulated, then overall insulation of the flex. Any BSS expert views on that?

Edited December 9, 2011 by nicknorman

[MtB]

Ok thanks everyone. I think pmms's answer is the closest to what I wanted to hear

 

A few points strike me:

 

1) I'm surprised that 2.5 flex is considered necessary with a 10amp circuit breaker. 1.5 is considered generous for 13 amp immersion heaters in household installations, and I often see 1.0 flex used.

 

2) I have no specific 12v earth bond, other than through the alternator casing/brackets. Why is one necessary? The system works fine.

 

3) Is it 'arctic' or 'artic' cable? I see both words used!

 

4) Is arctic cable considered 'double insulated' for chafing protection?

 

5) Conduit seems a bit of a pain to use. Is there any reason not to use P clips to fix it if the cable is not exposed to damage? The cable runs are short and rather convoluted.

 

6) I hadn't considered a 240v earth bond. I think I can see why this will be necessary if I install a CU with RCD, but is it necessary without? If so, why?

 

Many thanks,

Mike

[nicknorman]

Ok thanks everyone. I think pmms's answer is the closest to what I wanted to hear

 

A few points strike me:

 

1) I'm surprised that 2.5 flex is considered necessary with a 10amp circuit breaker. 1.5 is considered generous for 13 amp immersion heaters in household installations, and I often see 1.0 flex used.

 

2) I have no specific 12v earth bond, other than through the alternator casing/brackets. Why is one necessary? The system works fine.

 

3) Is it 'arctic' or 'artic' cable? I see both words used!

 

4) Is arctic cable considered 'double insulated' for chafing protection?

 

5) Conduit seems a bit of a pain to use. Is there any reason not to use P clips to fix it if the cable is not exposed to damage? The cable runs are short and rather convoluted.

 

6) I hadn't considered a 240v earth bond. I think I can see why this will be necessary if I install a CU with RCD, but is it necessary without? If so, why?

 

Many thanks,

Mike

 

1) On the subject of cable size, it can be to do with the thermal insulation of the cable's installation - eg if in a conduit it can get hotter than if in fresh air, such as the trailing cable to a domestic heater from the cable outlet a couple of feet away might be.

 

2) Explained on the Smartguage website but basically its a protection against corrosion should a service short circuit the +ve to hull. With the bond, that will blow the fuse, without it, you do not notice and galvanic corrosion can ensue. If the alternator/engine is earthed, it will depend on the engine mounts but if rubber, probably no continuity to hull.

 

3) It's arctic (as in cold) rather than artic(ulated lorry).

 

6) There is well-explained discussion about mains earth bonding on the Smartgauge website, I recommend you read that rather than us trying to replicate it here, ditto for RCD IIRC.

Edited December 9, 2011 by nicknorman

[Chris Pink]

Although my feeling is that an RCD is not required with trailing cables ( and praise the freedom to say it without being flamed) in an installation it is necessary as the potential for live hull faults are much higher.

 

The 12v earth is a different matter. Generally it is to avoid the fault condition of large currents inadvertently running down inappropriate cables, an example being a roof mounted aerial.

 

On the subject of cable size, it can be to do with the thermal insulation of the cable's installation - eg if in a conduit it can get hotter than if in fresh air, such as the trailing cable to a domestic heater from the cable outlet a couple of feet away might be.

 

 

The length of the run is the factor. And the total theoretical load of the system.

Edited December 9, 2011 by Chris Pink

[nicknorman]

The length of the run is the factor. And the total theoretical load of the system.

For 12v systems where voltage drop is critical - yes, but for mains circuits where voltage drop is much less of an issue - no, not really unless you are doing something like running 240v down to the bottom of the garden to run high power in the garage. Certainly in terms of safety, voltage drop is not an issue whereas protecting the cable from overheating is. Have you ever failed to unroll an extension cable reel fully, only to find it has melted itself together (I have!)?

Edited December 9, 2011 by nicknorman

[Chris Pink]

For 12v systems where voltage drop is critical - yes, but for mains circuits where voltage drop is much less of an issue - no, not really unless you are doing something like running 240v down to the bottom of the garden to run high power in the garage. Certainly in terms of safety, voltage drop is not an issue whereas protecting the cable from overheating is. Have you ever failed to unroll an extension cable reel fully, only to find it has melted itself together (I have!)?

 

That is the heating effect of AC in a coil rather than the enclosure of the cable.

[nicknorman]

That is the heating effect of AC in a coil rather than the enclosure of the cable.

What heating effect of AC in a coil? (there is none). No, it is to do with the packing of the cable close together so that cable not adjacent to fresh air overheats as it can't shed heat.

[WotEver]

What heating effect of AC in a coil? (there is none). No, it is to do with the packing of the cable close together so that cable not adjacent to fresh air overheats as it can't shed heat.

Exactly this.

 

Tony

[Chris Pink]

What heating effect of AC in a coil? (there is none). No, it is to do with the packing of the cable close together so that cable not adjacent to fresh air overheats as it can't shed heat.

 

I see you're right and i perpetuated a myth - learn something everyday.

[nicknorman]

I see you're right and i perpetuated a myth - learn something everyday.

Jolly good, anyway back to the point - I dug out my wiring regs handbook and amongst other stuff there is a table showing the de-rating of cable according to how it is installed and how many conductors are bunched together. The max de-rating is more than 50% ie less than half the nominal current allowed, though for a more typical installation of one cable in trunking, it is nearer 90% of the nominal current allowed .

[WotEver]

There are some derating tables taken from the 16th on the TLC site here.

 

Tony

[nicknorman]

There are some derating tables taken from the 16th on the TLC site here.

 

Tony

That is really useful. Maybe you should post the link on the pinned "cable size selector" thread?

 

I was also advised to put bootlace ferrules on any cable end that will be fixed by a screw.

That is a good idea, though rarely seen unfortunately. Certainly the standard failure mode is for the copper strands to be partially cut through by the screw, finally failing some time later due to vibration/movement. Do you have a source for such ferrules in modest quantities?

Edited December 9, 2011 by nicknorman

[WotEver]

Do you have a source for such ferrules in modest quantities?

Farnell. I'll find a link later.

 

Tony

 

Here you go.

 

 

Tony

[nicknorman]

Farnell. I'll find a link later.

 

Tony

 

Here you go.

 

 

Tony

Great thanks, didn't realise Farnell did them.

[Bob18]

Picking up on "the boiler man's" question about conduit.

Several reasons:

It makes for what is considered a "tidier" installation by keeping the cables in neat bundles (probably more important where you have more than one cable along a particular route)

It allows easy segregation of the "AC" and "DC" cables, which is always a good thing.

To protect the cable insulation from being attacked by the boat insulation - some mixes don't like each other, particularly PVC cable against expanded polystyrene.

It makes it easier to add an extra cable in the future - you don't have to rip out the lining to run it in (provided you've not jammed the conduit too full - aim for 30% filling and you'll be OK, aim for 90% filling and it will be next to impossible)

 

 

If you do use conduit then leave a draw string in so you can pull in that extra wire that inevitably gets forgotten...

 

Picking up on "the boiler man's" question about conduit.

Several reasons:

It makes for what is considered a "tidier" installation by keeping the cables in neat bundles (probably more important where you have more than one cable along a particular route)

It allows easy segregation of the "AC" and "DC" cables, which is always a good thing.

To protect the cable insulation from being attacked by the boat insulation - some mixes don't like each other, particularly PVC cable against expanded polystyrene.

It makes it easier to add an extra cable in the future - you don't have to rip out the lining to run it in (provided you've not jammed the conduit too full - aim for 30% filling and you'll be OK, aim for 90% filling and it will be next to impossible)

 

 

If you do use conduit then leave a draw string in so you can pull in that extra wire that inevitably gets forgotten...

[by'eck]

Ok thanks everyone. I think pmms's answer is the closest to what I wanted to hear

 

A few points strike me:

 

1) I'm surprised that 2.5 flex is considered necessary with a 10amp circuit breaker. 1.5 is considered generous for 13 amp immersion heaters in household installations, and I often see 1.0 flex used.

 

2) I have no specific 12v earth bond, other than through the alternator casing/brackets. Why is one necessary? The system works fine.

 

3) Is it 'arctic' or 'artic' cable? I see both words used!

 

4) Is arctic cable considered 'double insulated' for chafing protection?

 

5) Conduit seems a bit of a pain to use. Is there any reason not to use P clips to fix it if the cable is not exposed to damage? The cable runs are short and rather convoluted.

 

6) I hadn't considered a 240v earth bond. I think I can see why this will be necessary if I install a CU with RCD, but is it necessary without? If so, why?

 

Many thanks,

Mike

 

1) 2.5mm² arctic cable is the norm for AC distribution on small boats. Why penny pinch. A lesser gauge might be adequate but why not allow for future use if you are going to install a permanent system.

 

2) You have answered it yourself, you already have a negative battery earth bond to the hull via the alternator & maybe also the starter motor. It can reduce AC hum on some audio devices & again its a standard. It avoids stray currents caused by devices that have their hull mounted casing as a negative feed. Don't rely on such as a current path to feed DC devices though, always run a negative as well as positive cable.

 

3) Its Arctic cable.

 

4) Double insulated yes, but this gives little extra protection from chaffing.

 

5) It makes for a neater installation & with 240v AC on a steel boat, I would want to reduce risk of chaffing to the absolute minimum, despite the double insulation.

 

6) Its absolutely necessary if you want the RCD to work. Incidentally if going for the recommended small consumer unit, use an RCBO (combined breaker & RCD) to save space.

Edited December 9, 2011 by richardhula

[WotEver]

It can reduce hum on some audio devices...

DC don't hum. DC don't have no wiggly up and down waveform.

 

T

[steelaway]

Currently I have a couple of small 240v convector heaters I connect directly to the shoreline for frost protection in extreme weather. The shoreline trails up the inside of the boat and is pretty untidy so I'm planning to fit a 16A weatherproof shoreline socket outside and some fixed wiring inside the boat to two 13A twin sockets, one by each heater.

 

From a BSS point of view, can I connect the 16A socket directly to the two 13A twin sockets or do I need a circuit breaker (or anything else) to protect the wiring and/or users?

 

And in particular, what sort of cable should I use for the 240v wiring? 2.5 T&E, or flex of some sort?

 

And what are the cable-clipping requirements? And is there anything else I should know before starting?

 

Many thanks in anticipation,

Mike

 

 

 

Hi

 

Start with one of these---------------GS controls

Use 2.5 Arctic blue as a single radial circuit (not a ring), protected by the 16A RCD

Land lines are rated at 16a so you are maxing out your requirements for safety..

 

Alex

[WotEver]

That is really useful. Maybe you should post the link on the pinned "cable size selector" thread?

Done

[pmms]

Ok thanks everyone. I think pmms's answer is the closest to what I wanted to hear

 

A few points strike me:

 

1) I'm surprised that 2.5 flex is considered necessary with a 10amp circuit breaker. 1.5 is considered generous for 13 amp immersion heaters in household installations, and I often see 1.0 flex used.

 

2) I have no specific 12v earth bond, other than through the alternator casing/brackets. Why is one necessary? The system works fine.

 

3) Is it 'arctic' or 'artic' cable? I see both words used!

 

4) Is arctic cable considered 'double insulated' for chafing protection?

 

5) Conduit seems a bit of a pain to use. Is there any reason not to use P clips to fix it if the cable is not exposed to damage? The cable runs are short and rather convoluted.

 

6) I hadn't considered a 240v earth bond. I think I can see why this will be necessary if I install a CU with RCD, but is it necessary without? If so, why?

 

Many thanks,

Mike

Mike, I agree 1.5 would be sufficient for a 10 amp @240v load, I only said 2.5 'cause that seems to be the norm.

 

12v earth bond? No BSS requirement but is the subject of a lot of disagreement between the 'lecky experts (Ex-a has been- SPURT - a drip under pressure)

 

Arctic 3 core cable is indeed double insulated in BSS terms, but I would add additional protection.

 

Conduit not required just a requirement to support clear of hazards

 

240v bond? see answer to 12v bond.