Fuse Sizing

[WotEver]

ISO 10133 states (pretty obviously) that if there is only one fuse in the circuit then it should be sized to protect the smallest conductor in the circuit. I guess this is where the 'mantra' of 'fuse protects the cable' comes from, and it is correct but is also incomplete.  

So in the case of NMEA's pump the 25mm²dedicated supply would not be fused at 70A but at the rating recommended by the pump manufacturer. 

Lighting and power socket circuits are more difficult to calculate as we're not dealing with a dedicated supply to a single lamp but with a cable feeding multiple lamps/sockets. Ideally every lamp or socket should be individually fused; in reality a single fuse of around twice the combined total draw of the lamps (or expected draw from the sockets) would probably be used as a compromise. 

3 hours ago, jddevel said:

So am I correct that the proximity of other cabling and being in a conduit and the distance quoted above means that we`re back to the 2-5amp fuse? 

Only under circumstances where all of the conductors are carrying >30% of their capacity as I stated above. So in a typical narrowboat installation, no. 

Edited June 24, 2017 by WotEver
Missing ]

[WotEver]

1 hour ago, jddevel said:

All very confusing why would the loom supplier quote maximum safe load as 2.5amps on the 1mm square cable they supplied? Any suggestions/advice please.

Because they're taking worst case figures to protect themselves and you. If they've stated it's a 2.5A feed then that's the size of fuse that you should fit. 

Besides, 1mm² is about as small as you should go in a loom. The RCD states that 0.75mm² is the absolute smallest cable size that may be used. 

[cuthound]

1 hour ago, NMEA said:

I really don’t know where this mantra came from, probably domestic situations where each socket is assumed to accept only a fused plug rated for the appliance load. In hardwired situations a fuse is there to protect the downstream circuit which includes but is not limited to cable, the fuse should be rated to protect the weakest component downstream of it. As a for instance, a 25mm2 cable is used to prevent volt drop to a pump using a 150a fuse at source to protect the cable as per mantra could easily cause a winding fire in a failure event or at least destroy an otherwise saveable component unless there is a smaller correctly rated fuse installed closer to the pump.

It comes from good design.

The designer does not usually know what future users may do.

 Hence by fusing to protect the cable you prevent future nuisance outages because the user didn't realise the circuit was "under fused" to protect a long gone load.

 

[jonathanA]

8 hours ago, NMEA said:

I really don’t know where this mantra came from, probably domestic situations where each socket is assumed to accept only a fused plug rated for the appliance load. In hardwired situations a fuse is there to protect the downstream circuit which includes but is not limited to cable, the fuse should be rated to protect the weakest component downstream of it. As a for instance, a 25mm2 cable is used to prevent volt drop to a pump using a 150a fuse at source to protect the cable as per mantra could easily cause a winding fire in a failure event or at least destroy an otherwise saveable component unless there is a smaller correctly rated fuse installed closer to the pump.

Well said that man! 

Some common sense.... 

[WotEver]

7 hours ago, jonathanA said:

Some common sense.... 

But incomplete.

What he stated was correct for the single example he gave but does not cover such things as lighting and power circuits. There's lots of common sense written here but the full story is not always easy to cover in the space of a single forum post. 

[jonathanA]

Seemed pretty complete to me in the context of this thread. I assume your referring to mains power and lighting circuits which as you say is beyond  single forum post to cover ! 

[WotEver]

7 hours ago, jonathanA said:

I assume your referring to mains power and lighting circuits

You assume incorrectly. The context of this thread was about 12V lighting circuits (I added 12V power sockets as another example). NMEA described a specific scenario of a pump feed which had no direct relevance to this thread. 

[jddevel]

Help has been supplied on this subject but one further bit of assistance please. Fuses or contact breakers what is the general consensus of opinion? In fact can you even get fused distribution panels or are contact breakers the norm?

[WotEver]

27 minutes ago, jddevel said:

Fuses or contact breakers what is the general consensus of opinion?

Fuses are cheaper. Breakers are more accurate. 

Yes, both types are available. You pays yer money and makes yer choice.

[cuthound]

9 hours ago, WotEver said:

Fuses are cheaper. Breakers are more accurate. 

Yes, both types are available. You pays yer money and makes yer choice.

I disagree that fuses are less accurate, although it does depend on the quality of the fuse. BS88 fuses rupture at twice rated current. Miniature circuit breakers at 3-5 times rated current depending upon type.

Mcb's are much more convenient to reset, rather than replacing fuses though.

[WotEver]

1 hour ago, cuthound said:

I disagree that fuses are less accurate, although it does depend on the quality of the fuse. BS88 fuses rupture at twice rated current.

But automotive fuses, the type most commonly seen on boats, are not to BS88. BS88 are cartridge fuses which aren't usually fitted in this application. Automotive fuses are both slow in response and low in accuracy. 

[Chewbacka]

I use automotive blade fuses as they were much cheaper than decent breakers, and I figured probably a lot more reliable (dependable) than very cheap breakers.  The main advantage of breakers is they are easy to reset, but having used the boat for 5 years I have only ever had one fuse blow and that was me working on the wires without isolating.  Same as cars, fuses just don't blow on a properly wired boat.  -  Must not temp fate............

Edited June 29, 2017 by Chewbacka

[hounddog]

On 6/24/2017 at 15:51, rusty69 said:

I think it is stated on tb training website and the smartgauge website. 

Well it's wrong. As has been said; the cable will be overspecced for voltage drop and presumably rated higher than its maximum load so you fuse for the maximum load. 

An instance; should you wire a lighting circuit for LEDs when they had incandescent previously the LED bulbs could easily catch fire with the cable and fuse still carrying less than rated current. 

Edited June 29, 2017 by hounddog

[rusty69]

2 minutes ago, hounddog said:

An instance; should you wire a lighting circuit for LEDs when they had incandescent previously the LED bulbs could easily catch fire with the cable and fuse still carrying less than rated current. 

What size fuse would you fit to a circuit with a number of led lights on, each individualy swiitched. 

[hounddog]

4 minutes ago, rusty69 said:

What size fuse would you fit to a circuit with a number of led lights on, each individualy swiitched. 

Depends on how many of what power on a circuit but 5A runs theoretically 20 x 3W LEDs at 12V so a 5A fuse will do 10 x 3W LEDs sensibly. 

Edited June 29, 2017 by hounddog

[rusty69]

Just now, hounddog said:

Depends on how many of what power on a circuit but 5A runs theoretically 20 x 3W LEDs at 12V so 5A will do 10 x 3W LEDs sensibly. 

So what happens when you only have one led light on that develops a fault. Will it be protected by the fuse? 

[hounddog]

I don't know what this guff about 8A fuses is anyway, i expect you can get one but 1,2,5,10,15A etc are the commonly available blade fuses. The most reliable and easy to fit in my opinion.  

 

2 minutes ago, rusty69 said:

So what happens when you only have one led light on that develops a fault. Will it be protected by the fuse? 

Well theoretically i guess it's possible to cause a fire with 4.5A but short of fusing each light fitting that's the nearest you'll get. 

The way things fail is they get hot, melt stuff and cause bigger short circuits, at some point ( before the cable melts - important. ) your 5A fuse will blow. 

[rusty69]

3 minutes ago, hounddog said:

Well theoretically i guess it's possible to cause a fire with 4.5A but short of fusing each light fitting that's the nearest you'll get. 

I suspect that is why some suppliers of led lights are selling products with built in fuses. 

[WotEver]

48 minutes ago, rusty69 said:

I suspect that is why some suppliers of led lights are selling products with built in fuses. 

Absolutely.

It's complete rubbish to state that you should 'fuse for the load' when the load is variable. A single 2W LED with tiny 0.5mm or smaller internal wiring would need a 1A or smaller fuse. 20 x 2W LED lamps would need at least a 5A fuse - now the fuse is too big for a single lamp. 

If a number of devices are all fed from a single feed then ideally they should be individually fused. 

[WotEver]

On 24/06/2017 at 14:51, rusty69 said:

I think it is stated on tb training website and the smartgauge website. 

I don't believe that it is.

I've read both sites regarding fusing and I can't see that written on either site. What they both DO say is that any fuse should be no greater than the current carrying capacity of the cable being protected, and that they exist to protect the circuit. This is why it is a nonsense to suggest that a specific fuse size 'fits all' for, for example, 12V sockets. You might have a single old phone being charged, or you might have 3 phones, 2 tablets and a laptop. The only sensible way to fuse such a circuit is to have a large fuse on the feed and have each socket individually fused with lower value fuses (maybe either 3A, 5A, or 10A, depending on what you plan to plug into them and the quality of the sockets). 

[rusty69]

14 minutes ago, WotEver said:

I don't believe that it is.

I've read both sites regarding fusing and I can't see that written on either site. What they both DO say is that any fuse should be no greater than the current carrying capacity of the cable being protected, and that they exist to protect the circuit. This is why it is a nonsense to suggest that a specific fuse size 'fits all' for, for example, 12V sockets. You might have a single old phone being charged, or you might have 3 phones, 2 tablets and a laptop. The only sensible way to fuse such a circuit is to have a large fuse on the feed and have each socket individually fused with lower value fuses (maybe either 3A, 5A, or 10A, depending on what you plan to plug into them and the quality of the sockets). 

Perhaps i have it wrong, i haven't read both sites, but excerpts below:

http://www.tb-training.co.uk/MarineE03.html

The fuses/circuit breakers are fitted to PROTECT THE CABLES. If a fault in a load or damage to the cables causes excess current flow, it is already too late for the load or that piece of cable, but you do not want the fault setting fire to the cable run.

 

http://www.smartgauge.co.uk/sb_fusing.html

 A fuse will not protect electronic equipment. By the time the fuse blows, the damage to the electronic compenents is already done.

So now we have clarified what a fuse is actually for i.e. to protect the cabling, 

Edited June 29, 2017 by rusty69

[WotEver]

Just now, rusty69 said:

Perhaps i have it wrong, i haven't read both sitws, but excerpts below:

http://www.tb-training.co.uk/MarineE03.html

The fuses/circuit breakers are fitted to PROTECT THE CABLES. If a fault in a load or damage to the cables causes excess current flow, it is already too late for the load or that piece of cable, but you do not want the fault setting fire to the cable run.

 

http://www.smartgauge.co.uk/sb_fusing.html

 A fuse will not protect electronic equipment. By the time the fuse blows, the damage to the electronic compenents is already done.

So now we have clarified what a fuse is actually for i.e. to protect the cabling, 

Yup, and that is correct. Fuses are there to protect the cables, not the equipment. That is ALL of the cables in that circuit, not just the feed from the dis panel. Where that's impossible due to there being 'many' items being fed by the one supply then the individual items should have their own fuses. 

[rusty69]

3 minutes ago, WotEver said:

Yup, and that is correct. Fuses are there to protect the cables, not the equipment. That is ALL of the cables in that circuit, not just the feed from the dis panel. Where that's impossible due to there being 'many' items being fed by the one supply then the individual items should have their own fuses. 

But that is what i stated in post #4 "the fuse is to protect the cable" 

[WotEver]

1 minute ago, rusty69 said:

But that is what i stated in post #4 "the fuse is to protect the cable" 

But that's too simple. That suggests that the fuse is ONLY there to protect the supply cable from the dis board. 'All the cables in the circuit' includes the cables feeding the equipment at the far end. 

Further, for a dedicated feed to say a pump, there is no point (and it would be incorrect) in fusing larger than the pump manufacturer advises. 

[rusty69]

1 minute ago, WotEver said:

But that's too simple. That suggests that the fuse is ONLY there to protect the supply cable from the dis board. 'All the cables in the circuit' includes the cables feeding the equipment at the far end. 

I see how it could be interpreted that way.