Planning a 240v installation

[DHutch]

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

Also about the best price i could find this time last year for 50m of 2,5 arctic.

 

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

Yes, more of this please! If not essential, certainly a better job, and easier to install.

 

 

Daniel

[NBMike]

I have landline entry at bow and stern with a changeover switch in engine room, leading to RCD + consumer unit. My BSC examiner pointed out that I had no protection on the cable from either input plug to the consumer unit. He suggested RCBO trips just inside of the boat at both cable entry points. Maybe overkill but better safe than sorry!

Mike

[nicknorman]

I have landline entry at bow and stern with a changeover switch in engine room, leading to RCD + consumer unit. My BSC examiner pointed out that I had no protection on the cable from either input plug to the consumer unit. He suggested RCBO trips just inside of the boat at both cable entry points. Maybe overkill but better safe than sorry!

Mike

Wouldn't hurt but not necessary IMHO. The RCD is generally to protect against failures where personal contact would be an issue - if your cabling from sockets to changeover / rcd are properly installed this should be unlikely. Surely the higher risk is from the trailing cable to the shore bollard, outside, wet, likely to be stood on / trapped, rather than a cable installed inside in trunking etc. Hopefully the marina supply has breaker / rcd protection for this, and that protection would extend to any (much more unlikely) faults in the cabling to your RCD. I can see that in your case the argument is swung slightly in favour due to length of cabling, but for a more normal installation where the run of cable from socket to rcd is short, surely not.

[WotEver]

I agree with Nick - overkill.

 

Tony

[by'eck]

I have landline entry at bow and stern with a changeover switch in engine room, leading to RCD + consumer unit. My BSC examiner pointed out that I had no protection on the cable from either input plug to the consumer unit. He suggested RCBO trips just inside of the boat at both cable entry points. Maybe overkill but better safe than sorry!

Mike

 

I also agree with Nick & Tony on this being overkill. There seems to be an increasing trend on this & other forums for recommendations of extra fusing in the case of DC & extra RCD's in the case of AC when a cable run is of any great length.

 

Is there really significantly greater likelihood of a cable problem in say a short run through two bulkheads & a longer run through the same if the cable is installed in a chafe free way in trunking?

 

ETA: I always understood you fuse to protect a particular device with a margin so start-up currents won't blow/trip the fuse. If the cable gauge has been chosen correctly this will also protect the cable feeding it with a large safety margin.

 

Remember you generally have no control over how trouble-free an electrical device will perform, but you do with the cable feeding it.

Edited December 11, 2011 by richardhula

[Bob18]

Passing through a bulkhead is more likely to be a "weak point" than a long, joint free run in conduit.

[rjasmith]

The RCD ISO 13297 (para 7.2.2) requires a circuit breaker and RCD to be located within 0.5m of the "source of power" (which for a shoreline it defines as the location of the fixed shoreline input connector). If this is impractical it requires the cabling between these locations to be protected in trunking or similar. If so protected, the distance can be increased to 3m. If however this distance is also impractical then it requires an additional circuit breaker (but not also an additional RCD) to be fitted at an intermediate point that is less than 3m from the source of power.

 

I realise that this may not be relevant to a boat that doesn't have to be RCD compliant but it always intrigues me that the BSS guide refers its readers to and suggests they take account of, the appropriate RCD ISOs at the beginning of each chapter.

 

Could this be why NBMike's inspector made his suggestion?

 

Richard

[nicknorman]

The RCD ISO 13297 (para 7.2.2) requires a circuit breaker and RCD to be located within 0.5m of the "source of power" (which for a shoreline it defines as the location of the fixed shoreline input connector). If this is impractical it requires the cabling between these locations to be protected in trunking or similar. If so protected, the distance can be increased to 3m. If however this distance is also impractical then it requires an additional circuit breaker (but not also an additional RCD) to be fitted at an intermediate point that is less than 3m from the source of power.

 

I realise that this may not be relevant to a boat that doesn't have to be RCD compliant but it always intrigues me that the BSS guide refers its readers to and suggests they take account of, the appropriate RCD ISOs at the beginning of each chapter.

 

Could this be why NBMike's inspector made his suggestion?

 

Richard

You are of course right, it does say that. But perhaps worth thinking about why it says that. If you have a long run of cable from the inlet to the breaker, the risk is not really one of personal shock since a reasonably well installed cable would be away from direct personal contact. That is why it does not require an intermediate RCD. However there remains the possibility (if a remote one in a well installed system) of a cable fault eg short circuit overloading the cable that could ultimately cause a fire in hidden cable. Nasty! However, if the boat is connected to a shore installation that is fused or breakered at an appropriate rating eg 16amps, which it of course should be, the only outcome is that the shore supply trips.

 

Therefore it boils down to whether you think there is ever a chance of connecting to a shore supply that is unprotected. For an international standard such as ISO 13297 it would be brave to assume that you could sail to anywhere in the world and never encounter an unprotected shore supply. In UK inland waterways / marinas, perhaps a less bold assumption?

[GUMPY]

ETA: I always understood you fuse to protect a particular device with a margin so start-up currents won't blow/trip the fuse. If the cable gauge has been chosen correctly this will also protect the cable feeding it with a large safety margin.

 

Actually the fuse is there to protect the cable, not the device, the device should have its own internal fuse to protect it.

Also cables over 25m length should be protected by an RCD, as should anything that is used outside.

 

The real wonder is that somewhere earlier in this thread someone debunked the heating effect of a cable in a coil. I would love them to explain just how a coiled cable carrying its rated current will catch fire after a while.

[nicknorman]

The real wonder is that somewhere earlier in this thread someone debunked the heating effect of a cable in a coil. I would love them to explain just how a coiled cable carrying its rated current will catch fire after a while.

that would be me then! There is no additional heat generated because a cable is in a coil vs straight, what there is is a reduced capability for the cable to shed heat, since instead of being surrounded by air that can cool the cable by both conduction and convection, it is surrounded by more cable that is producing heat. Thus whilst the overall heat produced remains the same, the temperature will get much hotter because the same heat is concentrated in a smaller volume with reduced ability to shed heat.

 

Once the coiled cable gets hot, it's resistance will increase slightly so at that point a bit more heat may be generated, but that small effect is the result of the cable getting hot, not any magic properties of a coil!

 

As an aside, a coil of AC powered single conductor wiring on a ferrous core may generate increased heat due to resistive losses of eddy currents in the ferrous core. However mains flex contains both live and neutral, so the overall current flow is null, so no magnetic field worth talking about, so no additional heating even on an iron core.

 

Actually the fuse is there to protect the cable, not the device, the device should have its own internal fuse to protect it.

That is true, however it really determines the max fuse size. There is nothing wrong with cabling to a single service being protected by a smaller fuse that protects both the service and the cable.

Edited December 11, 2011 by nicknorman

[system 4-50]

Actually the fuse is there to protect the cable, not the device, the device should have its own internal fuse to protect it.

Also cables over 25m length should be protected by an RCD, as should anything that is used outside.

 

The real wonder is that somewhere earlier in this thread someone debunked the heating effect of a cable in a coil. I would love them to explain just how a coiled cable carrying its rated current will catch fire after a while.

My sister-in-Law visited my parents-in-Law to find them in their sitting room heated by an portable electric fire run through a cable extension sat on a chair with only about 8 ft extracted from the drum. The drum AND the cushion it was on were gently smouldering! The old folk were a bit bothered about the smell but unable to locate it, possibly already affected by the fumes.

[the grinch]

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.

1.5 cable on an 13amp 2KW immersion would overheat even 2.5 gets warm, butyl cable should be 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!

Arctic as its designed for colder working temperatures as far as i know

 

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. no reason to use unless the cable comes into contact with anything that will harm it

 

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?

good question that i have never understood why when the elecs are not attached to the boat do we then attach the two together? a battery charger or inverter would be the only reason i can think of but i'm sure someone knows the answer.

 

Many thanks,

Mike

[nicknorman]

Possibly some confusion here between heat and temperature. They are fundamentally different things of course.

[nicknorman]

good question that i have never understood why when the elecs are not attached to the boat do we then attach the two together? a battery charger or inverter would be the only reason i can think of but i'm sure someone knows the answer.

 

As I mentioned before, the SG website has a good if long explanation Here. If you bother to read it you will understand and never again have to say "never understood why..."

[by'eck]

Actually the fuse is there to protect the cable, not the device, the device should have its own internal fuse to protect it.

 

 

If it has then fine, most don't. I say again a cable run to a specific (un-fused) device) should be there first to protect that device & fused accordingly. It will also protect the cable, which I don't deny is important. Having fuses at both ends I consider overkill.

 

The only time a breaker has tripped on my boat is when a motor has overloaded. It was usually the Sealand vacuum pump one which stalled due to..... you don't want to know

 

Correct breaker size saved the motor from burning out/breaking the gearbox. If the breaker or fuse had been chosen to protect only the cable it would have allowed significantly higher current to the motor with almost certainly expensive or maybe even dangerous results.

[GUMPY]

that would be me then! There is no additional heat generated because a cable is in a coil vs straight, what there is is a reduced capability for the cable to shed heat, since instead of being surrounded by air that can cool the cable by both conduction and convection, it is surrounded by more cable that is producing heat. Thus whilst the overall heat produced remains the same, the temperature will get much hotter because the same heat is concentrated in a smaller volume with reduced ability to shed heat.

 

Well explained, what was concerning me is that someone could have taken from your original post that it was not a problem to leave cable in a coil, with disastrous consequences

 

If it has then fine, most don't. I say again a cable run to a specific (un-fused) device) should be there first to protect that device & fused accordingly. It will also protect the cable, which I don't deny is important. Having fuses at both ends I consider overkill.

 

The only time a breaker has tripped on my boat is when a motor has overloaded. It was usually the Sealand vacuum pump one which stalled due to..... you don't want to know

 

Correct breaker size saved the motor from burning out/breaking the gearbox. If the breaker or fuse had been chosen to protect only the cable it would have allowed significantly higher current to the motor with almost certainly expensive or maybe even dangerous results.

 

As this thread is about 230v AC that is what I was referring to.

There is negligible voltage drop on 230v systems in boats, so my comment on the fuse being sized for the cable still stands, its actually part of the regulations.

You are talking about 12/24v system where the voltage drop is a concern then you do fuse to cover the appliance as the cable is well overrsized for the appliance.

[Guest]

Actually the fuse is there to protect the cable, not the device, the device should have its own internal fuse to protect it.

Also cables over 25m length should be protected by an RCD, as should anything that is used outside.

 

The real wonder is that somewhere earlier in this thread someone debunked the heating effect of a cable in a coil. I would love them to explain just how a coiled cable carrying its rated current will catch fire after a while.

i agree with your first paragraph re fusing.

 

I have seen numerous extension cables melt when coiled an loaded to near maximum current. There is simply no air to cool the cable and as such they should be downrated when coiled. The old boys used to blame "induction" but I think that is nonsense.

[nicknorman]

Well explained, what was concerning me is that someone could have taken from your original post that it was not a problem to leave cable in a coil, with disastrous consequences

 

No probs- but it's amazing how these threads can go off topic! I used the coiled cable as an example of why the current rating of cable depends on how it installed, then it all went round in a big circle (excuse pun!) but hopefully now we all agree!

[WotEver]

I have seen numerous extension cables melt when coiled an loaded to near maximum current. There is simply no air to cool the cable and as such they should be downrated when coiled.

I used to have a cable reel that actually stated on a large label "Rated 13A when unwound fully. 5A when coiled"

 

Tony

[Guest]

I used to have a cable reel that actually stated on a large label "Rated 13A when unwound fully. 5A when coiled"

 

Tony

Interesting. I have never seen this but it doesn't surprise me.

[by'eck]

As this thread is about 230v AC that is what I was referring to.

There is negligible voltage drop on 230v systems in boats, so my comment on the fuse being sized for the cable still stands, its actually part of the regulations.

You are talking about 12/24v system where the voltage drop is a concern then you do fuse to cover the appliance as the cable is well overrsized for the appliance.

 

Apologies for going off topic, but your mention of fusing (then & now) of cable feeds led me to think you were referring to DC systems.

 

As you know AC systems for many decades now have used circuit breakers to protect the cabling with fuses only in the appliance plugs as you don't know exactly what load will be applied. If however an AC device such as an immersion heater is hard wired to a dedicated breaker, such breaker should cover the current draw from that device only.

Edited December 13, 2011 by richardhula