Confused about fuses

[pophops]

Finally got round to looking at the fuse board today. I thought it was about time I labelled all the fuses. Falcon has a fairly uncomplicated electrical system. No inverter or charger. Everything runs off 12v.

The fuses are "continental" ceramic type fuses, rated at a mixture of 8amp and 16 amp and the fuses holders are not marked. I got to thinking "How do I know that the correct fuse is fitted for any particular circuit?" I can well imagine that a few of them may have been wrongly substituted over the years. I understand that the fuse is there to protect the cable but I have no idea what size cables are in place or how this relates to the load they are carrying. Perhaps I am asking one of those 'how long is a piece of string?' type questions. I haven't had any issues with fuses blowing but I'm concerned in case some of them are too high a rating.

 

[MtB]

Finally got round to looking at the fuse board today. I thought it was about time I labelled all the fuses. Falcon has a fairly uncomplicated electrical system. No inverter or charger. Everything runs off 12v.

The fuses are "continental" ceramic type fuses, rated at a mixture of 8amp and 16 amp and the fuses holders are not marked. I got to thinking "How do I know that the correct fuse is fitted for any particular circuit?" I can well imagine that a few of them may have been wrongly substituted over the years. I understand that the fuse is there to protect the cable but I have no idea what size cables are in place or how this relates to the load they are carrying. Perhaps I am asking one of those 'how long is a piece of string?' type questions. I haven't had any issues with fuses blowing but I'm concerned in case some of them are too high a rating.

 

 

You are correct about fuses being rated to protect the fixed wiring.

 

the only way to establish the correct fuse rating for each circuit is to inspect the wiring and establish the size, and to a lesser degree the length. Then you can calculate (or look up) a suitable fuse rating.

 

MtB

P.S. The environment makes a difference too IIRC. A cable in an insulated space is down-rated as it cannot dissipate heat so well. Bear in mind I'm a plumber not an electrician though

[NMEA]

Unfortunately even measuring the cable dia will not help a lot as it could be standard or thin wall of the same rating, though you could measure it and assume worst case. It may be simpler to trace the load from each fuse by turning everything on and removing the fuses one by one, that will identify what they supply. You can then work out the amperage of the loads at least.

[alan_fincher]

Here's an answer that many may put down as "slightly dodgy", but it's a starter!

 

Assuming you can establish the boat is wired with something like good quality automotive cable, then as a general rule, all but the very thinnest is good for at least 16 amps.

 

Or at least it is good for at least 16 amps in as much as it will not actually overheat, and melt the outer insulation - it may not be truly "good for 16 amps" to actually supply something at long range, without excessive volts drop - but that's just a performance issue, not a safety one.

 

Unless the cable is insubstantial, it is probably good for 16 amps, even if the cable is covered, and can't ventilate much.

 

So I would say that if none of your fuses exceed 16 amps, then the wiring is only at risk in the event of a fault, if it is particularly undersized. (Clearly if bits f the boat were wired in bell wire, then it is not good for 16 amps!).

Have a look here Only the very thinnest (1mm squared) cable is rated at under 16 amps, and, in all honesty, will probably withstand 16 amps passing through it without failure - albeit that volts drop would normally make it unsuitable for supplying more than its rated current.

If you had circuits fused at more than 16 amps, then I think an assumption that any cable to it was capable of passing higher currents would start to become a lot dodgier, but if you think all cable is at least 2mm squared or more, then in my view the fuses are adequately protecting it from catastrophic failure.

Edited January 7, 2014 by alan_fincher

[steelaway]

Hi

 

With only 6A & 8A fuses I should think every thing will be OK. They are very small fuses for a 12V system and if they are not blowing all should be OK.

 

See http://www.cable-ratings.co.uk/

 

Boats are typically wired with 4mm single flex cable nominally rated at 25A.

 

Simple formula is Amps = Watts / Volts . 60W bulb at 12V with be 5A. You need to read the wattage of your pumps etc to work out the Amps.

 

Alex

[pophops]

Thanks for all replies so far. The cabling looks competently done, if a little dated. The boat was built by Stowe Hill, which I understand had a reputation for good quality workmanship so I assume the cables are all up to spec. How do you measure the cable? Is it simply a case of measuring the diameter of the conductor?

[Alan de Enfield]

No - the cross sectional area refers to the amount of copper and not the overall (including the insulation)

 

If you can access the ends of the cables, and can count the strands you can work out the CSA (cross sectional area) of the wires :

 

Typical 'domestic' single core flexibles, classification "2491X"

 

16 strands each strand 0.2mm is 0.5mm squared

24 strands each strand 0.2mm is 0.75mm squared (6 amp rating)

32 strands each strand 0.2mm is 1,0mm squared (10 amp rating)

30 strands each strand 0.25mm is 1.5mm squared (15 amp rating)

50 strands each strand 0.25mm is 2.5mm squared

56 strands each strand 0.3mm is 4 mm squared.

 

The two main automotive cables (BS6862) are :

 

14 strands each strand 0.3mm is 1mm squared (8.75 amp rating

28 strands each strand 0.3mm is 2mm squared (17.4 amp rating)

 

Current rating depends on installation conditions, number of cables etc etc.

Different guides will quote diffrent ratings for the same cables - "check the small print"

Edited January 7, 2014 by Alan de Enfield

[MtB]

Good post from Alan De E.

 

Measure the diameter of the strands with a vernier like this: http://www.ebay.co.uk/itm/6-Digital-VERNIER-CALIPER-Gauge-Electronic-Micrometer-Measurement-150mm-Depth-/171097224139?pt=UK_Measuring_Tools_Levels&hash=item27d63073cb

 

Another way to count the strands is to use a Stanley knife to make a longitudinal cut in the insulation to get access to the gizzards. Once you've counted the strands, reassemble the cable and wrap it in insulation tape.

 

MtB

[alan_fincher]

Or just go and buy a very short length of 2.5 mm2 cable, which we know from its spec will carry 16 amps with no issue, and use it for comparison purposes.

 

If the number and size of the strands in the least substantial wiring you can find looks to at least match or exceed that, then without measuring anything you can be fairly confident that your wiring is adequately protected if no fuse is rated greater than 16 amps.

 

I'd argue you only need to start detailed measurement if you have less substantial wiring - an even half well fitted out boat should not have, unless someone has made DIY modifications in something less suitable.

[Alan de Enfield]

Or just go and buy a very short length of 2.5 mm2 cable, which we know from its spec will carry 16 amps with no issue, and use it for comparison purposes.

 

 

 

But which single core cable have you bought - they are all to different specs and have different insulation thicknesses so there is a big difference in O/D of cable to BS6500 (2491x), BS6231 and BS6862.

 

If you want to "guess" - then use 2491x as it has the thinnest insulation (0..6mm wall thickness on 1mm squared)

[alan_fincher]

 

But which single core cable have you bought - they are all to different specs and have different insulation thicknesses so there is a big difference in O/D of cable to BS6500 (2491x), BS6231 and BS6862.

 

If you want to "guess" - then use 2491x as it has the thinnest insulation (0..6mm wall thickness on 1mm squared)

 

I'm not suggesting you compare the outers, over the insulation thickness. I am suggesting you compare what the conductors look like.

 

Copper is copper, and if there are at least the same number of strands, of at least the same thickness each as a sample you compare too, I think it is going to be good enough.

 

I have already said if any of it looks marginal for carrying 16 amps, then you can resort to more scientific methods, but I think at the end of the day, all OP is worried about is "if I get a fault condition on what the wiring is supplying, are the fuses adequately protecting it?". You don't need masses of copper to carry 16 amps safely, without the wiring being at risk.

[nicknorman]

Although it is true that the fusing is there to protect the cable, this only determines the maximum fuse size, not the minimum. Boats wiring is often selected to be "oversized" regarding considerations of max current / heating, so that the voltage drop on a long run doesn't become excessive - after all we are only starting out with 12v. Therefore it may be appropriate to select fuse sizes that relate to the size of the load rather than the size of the cable, with the caveat that the fuse size must not be greater than the current rating of the cable.

 

If I have cable capable of taking 30A, feeding some LED lights that consume 2A, I would select a 5A fuse, not a 30A one.

[alan_fincher]

Although it is true that the fusing is there to protect the cable, this only determines the maximum fuse size, not the minimum. Boats wiring is often selected to be "oversized" regarding considerations of max current / heating, so that the voltage drop on a long run doesn't become excessive - after all we are only starting out with 12v. Therefore it may be appropriate to select fuse sizes that relate to the size of the load rather than the size of the cable, with the caveat that the fuse size must not be greater than the current rating of the cable.

 

If I have cable capable of taking 30A, feeding some LED lights that consume 2A, I would select a 5A fuse, not a 30A one.

 

Yes, I agree with all of that, and ideally each fuse would not be rated massively higher than the sum of all loads on that circuit if they were all switched on together, (with a bit of margin for any start up surges, of course).

 

It would perhaps be useful, but not IMO essential, for the OP to actually work out what each fuse is supplying in total, (don't need to trace cables - withdraw one at a time, and see what stops working!), and to reduce any that are much higher than they need to be. This could be the case if, for example, LED lights had been used to replace incandescents, without changing anything else. However, even then the advantages of changing don't seem that great to me, as the cabling is already well protected - I suppose it could slightly reduce the chance of a small fire in a light fitting if an LED got a high current fault, but not a short circuit fault.

 

[FadeToScarlet]

All the cables I've used have, printed on or indented in the insulation, their size and cross sectional area. Might be worth having a look to see if any of them have writing printed on or indented into the insulation somewhere.

[John Williamson 1955]

If you pull a fuse, it is possible to replace it temporarily with an ammeter so that you known how much current is being drawn. I would then replace the fuse with the nearest common value higher than this. So, irrespective of cable size, if the load was 12A, I'd use a 16A fuse, if the load was only 5A, then an 8A fuse, and so on. Assuming the cable was big enough, of course.

[by'eck]

Although it is true that the fusing is there to protect the cable, this only determines the maximum fuse size, not the minimum. Boats wiring is often selected to be "oversized" regarding considerations of max current / heating, so that the voltage drop on a long run doesn't become excessive - after all we are only starting out with 12v. Therefore it may be appropriate to select fuse sizes that relate to the size of the load rather than the size of the cable, with the caveat that the fuse size must not be greater than the current rating of the cable.

 

If I have cable capable of taking 30A, feeding some LED lights that consume 2A, I would select a 5A fuse, not a 30A one.

I agree with the above but would add that its not just appropriate but essential to fuse according to equipment consumption, which should be well within the cable current handling capabilities and providing no more than acceptable volt drop when feeding such equipment.

 

I wish some of the other posters above would stop perpetuating the myth that fuses and breakers are only there to protect the cable. Protecting the equipment they feed can be equally if not more important, and not just for longevity but for fire hazard reasons as well.

[MtB]

All the cables I've used have, printed on or indented in the insulation, their size and cross sectional area. Might be worth having a look to see if any of them have writing printed on or indented into the insulation somewhere.

 

 

Good point. my experience too!

 

MtB

[nicknorman]

I agree with the above but would add that its not just appropriate but essential to fuse according to equipment consumption, which should be well within the cable current handling capabilities and providing no more than acceptable volt drop when feeding such equipment.I wish some of the other posters above would stop perpetuating the myth that fuses and breakers are only there to protect the cable. Protecting the equipment they feed can be equally if not more important, and not just for longevity but for fire hazard reasons as well.

Depends. Quite a lot of equipment is protected by an integral fuse, hopefully of an appropriate rating. This is clearly the best way when one supply fuse is used to feed multiple devices - you can't select a supply fuse that is appropriate for each load switched on separately, and expect it to still work when you turn them all on at once. A 12v circuit feeding sockets into which eg chargers, TV, are plugged into is a good example - all these things will be individually fused.

 

But obviously some services -lights spring to mind - are not individually fused so a fuse relating to the max load rather than the cable is definitely a good idea, especially for LEDs. But with only 1 light on, the fuse will inevitably be larger than optimal for that condition.

 

Then there is the case of a supply fuse feeding a single consumer - water pump circuit, fridge circuit etc. here, selecting a supply fuse that is appropriate to the service, not the cable, is obviously the best thing to do.

[Alan Saunders]

Although it is true that the fusing is there to protect the cable, this only determines the maximum fuse size, not the minimum. Boats wiring is often selected to be "oversized" regarding considerations of max current / heating, so that the voltage drop on a long run doesn't become excessive - after all we are only starting out with 12v. Therefore it may be appropriate to select fuse sizes that relate to the size of the load rather than the size of the cable, with the caveat that the fuse size must not be greater than the current rating of the cable.

 

If I have cable capable of taking 30A, feeding some LED lights that consume 2A, I would select a 5A fuse, not a 30A one.

Yes, I agree.

 

To determine the current draw for each fuse I removed the fuse, connected an ammeter across the fuse-holder and switched on all devices served by that fuse - e.g. tunnel light and horn. Installed the next higher +50% available fuse and marked the fuse-holder with this rating.

 

I just put the meter probes into the vacant fuse socket (minor risk) but you could make up a meter lead terminated by an o/c fuse and an inline fuse.

 

Alan

[alan_fincher]

I wish some of the other posters above would stop perpetuating the myth that fuses and breakers are only there to protect the cable. Protecting the equipment they feed can be equally if not more important, and not just for longevity but for fire hazard reasons as well.

 

I'm certainly not trying to start an argument, but I think I would agree only up to a point.....

 

Quite often equipment will be protected by its own fuse of an appropriate value anyway, whether built in to it, or into a plug, such as a "cigar style" one.

 

I can't really see the longevity bit of the argument applying very often - usually by the time anything has managed to blow an appropriate value fuse, some part of it is already "fried" anyway. From a purely "survival of the equipment" viewpoint, I can't think of too many cases where you could say "that was made far worse by the fuse being 16 amps - had it been 5 amps that bit of kit would still be fine". To draw a parallel, virtually every pre-wired mains plug you will find on any bit of equiment you buy will contain either a 3 amp, possibly a 5 amp, or a 13 amp fuse - if the equipment only needs 0.5 amps, it will still have a 3 amp fuse, and if it will never draw more than 6 amps, the fuse will still be 13 amps, usually.

 

I have already conceded that there is maybe a slightly reduced chance of a small fire breaking out in something like an LED light fitting unit, if fused at a higher current than it needs to be, but failures of low voltage electronics like that are usually swift, maybe with a bit of smoke or a "pop" of a component of track on the PCB, but seldom any fire that will spread in a dangerous way, (all in my experience, of course!).

 

On the other hand cable insulation burning away behind wood panelling because the currents through it are far too high seems to me to be a much bigger concern. When plastic insulation really starts to burn, it can keep burning.

 

It doesn't seem correct to me to extend the argument to the point you say protecting the equipment could be more important than the wiring.

Edited January 7, 2014 by alan_fincher

[by'eck]

I'm certainly not trying to start an argument, but I think I would agree only up to a point.....

 

Quite often equipment will be protected by its own fuse of an appropriate value anyway, whether built in to it, or into a plug, such as a "cigar style" one.

 

I can't really see the longevity bit of the argument applying very often - usually by the time anything has managed to blow an appropriate value fuse, some part of it is already "fried" anyway. From a purely "survival of the equipment" viewpoint, I can't think of too many cases where you could say "that was made far worse by the fuse being 16 amps - had it been 5 amps that bit of kit would still be fine". To draw a parallel, virtually every pre-wired mains plug you will find on any bit of equiment you buy will contain either a 3 amp, possibly a 5 amp, or a 13 amp fuse - if the equipment only needs 0.5 amps, it will still have a 3 amp fuse, and if it will never draw more than 6 amps, the fuse will still be 13 amps, usually.

 

I have already conceded that there is maybe a slightly reduced chance of a small fire breaking out in something like an LED light fitting unit, if fused at a higher current than it needs to be, but failures of low voltage electronics like that are usually swift, maybe with a bit of smoke or a "pop" of a component of track on the PCB, but seldom any fire that will spread in a dangerous way, (all in my experience, of course!).

 

On the other hand cable insulation burning away behind wood panelling because the currents through it are far too high seems to me to be a much bigger concern. When plastic insulation really starts to burn, it can keep burning.

 

It doesn't seem correct to me to extend the argument to the point you say protecting the equipment could be more important than the wiring.

It may not seem so to you but consider the following:

 

Many, even most 12 volt equipment doesn't have its own fuse, indeed manufacturers often recommend a suitable external one. Such equipment may otherwise develop a fault, draw excessive current and consequently overheat or worse. Yes it may be beyond redemption then, but what of the case of an overloaded motor. A suitable fuse will then blow indicating a problem that can often be solved mechanically and the equipment saved to use another day.

 

Clearly consideration has to be given to fusing of cables that feed multiple devices although I recommend keeping such to a minimum and certainly using dedicated cabling for high consumption devices such as fridges.

 

Most of the scenarios I describe have happened to me over the years and correct fusing or breaker has saved the equipment, with the correctly chosen cable never being an issue. This is not to say that your argument regarding overheated cabling is not of great importance, but an equipment fault within which are almost certainly thinner gauge wires or conducting surfaces may result in significantly quicker heat build up, and so deserves equal or greater consideration and by its very nature will protect the cabling to a greater degree.

Edited January 8, 2014 by by'eck

[NMEA]

I wish some of the other posters above would stop perpetuating the myth that fuses and breakers are only there to protect the cable. Protecting the equipment they feed can be equally if not more important, and not just for longevity but for fire hazard reasons as well.

+ 1, over-current devices are there to protect the entire circuit not just the cable (though the circuit includes the cable) over-current devices should be of the value to protect the entire circuit and should be the value to do that. For instance a long run dedicated to a single device using heavy cable to prevent voltage loss to a 5a device may well have a 6a fuse even though the cable is rated at say 50a, unless of course there are over-current devices of lower value fitted downstream, either inline or at a distribution panel.

[jonathanA]

for me those continental fuses are just awful - if I'm thinking of the right ones where they are sort of bullet shaped and snap into place between metal prongs.

 

I would look at replacing them with a modern blade type fuse and preferably upgrading to circuit breakers where it is not possible to 'shove a bigger fuse in' and you don't end up scrabbling around in the dark for a replacement

[ditchcrawler]

I bet he wishes he never asked now.

[pophops]

I bet he wishes he never asked now.

Certainly too much information can be confusing for someone who only has a basic electrical knowledge. However, I am really grateful for all the advice given and hope to be able to absorb some of it given a little time!

Thank you all.