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Fuse Sizing


jddevel

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Fuse to the cable specification, after taking into consideration temperature, i.e. how the cable is mounted, whether it is a single cable or in a bunch, buried in insulation etc.

All derating information is in BS7671 IET Wiring Regs.

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15 minutes ago, stegra said:

Ok. So you have a redundant 50mm² cable and choose to wire an LED warning light from it rather than run a new cable?

If it was already in place, presumably it would also be properly fused. 

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Assuming that you are connecting a DC load, then fit a DC fuse (automotive type) at the beginning of the supply cable, to match the connected device current. This must be lower than the theoretical maximum cable current which will depend on the size, type and length and location of the feed and return cable. The best cable to use is Tri rated as it has the best insulation and temperature rating.

If you are connecting 240 volt AC then consult the 17th wiring regulations but do not use solid drawn conductors ( twin and earth type cable), either use tri-rated or flexible cables.

Edited by adrianh
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Further information. I am at this time assessing the fuse requirements for my distribution panels. Some might and indeed have commented in the past that instead of having a loom designed by a company it would have been worth dealing with it myself. However total ignorance of 12 volt electricity (for me last discussed over 50 years ago in my Physics classes at school) led me to decide on a loom. The loom is enclosed in corrugated conduits running either under the gunnels (power) or in again corrugated conduit in an enclosure both port and starboard (lights domestic, navigation tunnel and horn) at roof/ceiling height. It is not believed that either location will be directly affected by heat. So for example the bathroom lights are shown on the specification sheet as being supplied by a 1mm squared size cable with a total cable run of 27.22 metres that`s there and back not one way with a maximum safe load of 2.5 amps. For that reason I`m assuming that a 2.5 amp rated fuse will be required is that the guide? 

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9 hours ago, jddevel said:

For that reason I`m assuming that a 2.5 amp rated fuse will be required is that the guide? 

No, No, No.

As has been said above, 'fuse to the cable' - what would happen if you changed lights, increased the number of lights, took a spur of for something else.

1mm2 cable is rated at 8 amps so use an 8 amp fuse.

 

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9 hours ago, jddevel said:

Further information. I am at this time assessing the fuse requirements for my distribution panels. Some might and indeed have commented in the past that instead of having a loom designed by a company it would have been worth dealing with it myself. However total ignorance of 12 volt electricity (for me last discussed over 50 years ago in my Physics classes at school) led me to decide on a loom. The loom is enclosed in corrugated conduits running either under the gunnels (power) or in again corrugated conduit in an enclosure both port and starboard (lights domestic, navigation tunnel and horn) at roof/ceiling height. It is not believed that either location will be directly affected by heat. So for example the bathroom lights are shown on the specification sheet as being supplied by a 1mm squared size cable with a total cable run of 27.22 metres that`s there and back not one way with a maximum safe load of 2.5 amps. For that reason I`m assuming that a 2.5 amp rated fuse will be required is that the guide? 

If the supplier has quoted a max safe load as 2.5amps then you should not use a fuse bigger than 2.5A  You could use a fuse that is smaller if you wished, but remember how a fuse works.  It works by getting hot and melting, so if a current is just below the fuse rating the fuse will be warm/hot which will oxidise and eventually fail, even though there are no faults, which is not good.  To reduce the chance of nuisance fails the fuse needs to be rated about 25% or so bigger than the load so long as you do not exceed the max safe spec.  Also consider the start up surge, so a fridge may only take an amp or so when running - and that is usually what will be on the rating plate - but on start-up may take many times more amps, so to avoid nuisance fuse 'blows' the fuse must be able to withstand the repeated start up surge over many years.  The same applies to other devices with a start-up surge, such as pumps and some high wattage tungsten filament bulbs.  So without knowing what load and type of load is present on each circuit it is difficult to suggest a fuse rated lower than the safe load for the cable, assuming the cable has been correctly sized for the application.  Note that if this were 220vAC (domestic mains) generally speaking the cable size would be based only on load, however with 12V systems, cables are often over sized for the  specified load to minimise volt drop to an acceptable level.

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10 hours ago, adrianh said:

Assuming that you are connecting a DC load, then fit a DC fuse (automotive type) at the beginning of the supply cable, to match the connected device current. This must be lower than the theoretical maximum cable current which will depend on the size, type and length and location of the feed and return cable. The best cable to use is Tri rated as it has the best insulation and temperature rating.

If you are connecting 240 volt AC then consult the 17th wiring regulations but do not use solid drawn conductors ( twin and earth type cable), either use tri-rated or flexible cables.

The wiring regulations apply to both ac and dc cables.

The IET also produce  code of practice booklet  covering dc.

"Code of Practice for Low and Extra Low Direct Current for Power Distribution in Buildings"

Although not specifically designed for boats, it covers the relevant areas of separation, protection and volt drop.

After all, the cable doesn't know that it has been installed in a boat does it? 

Edited by cuthound
To unmangle the effects of autocorrect.
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Why not avoid all of the good intentioned advice (both right & wrong) and just install your wiring properly and in accordance with the necessary standards :

BS EN ISO 10133-2001 "Small Craft Electrical Systems. Extra Low voltage DC Installations"

 

There could then be no suggestion of culpability (or insurance payout refused) in the event of a 'problem'

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1 hour ago, Alan de Enfield said:

Why not avoid all of the good intentioned advice (both right & wrong) and just install your wiring properly and in accordance with the necessary standards :

BS EN ISO 10133-2001 "Small Craft Electrical Systems. Extra Low voltage DC Installations"

 

There could then be no suggestion of culpability (or insurance payout refused) in the event of a 'problem'

This might help:

http://www.siranah.de/manuals/ISO10133.pdf

Note that the specific example given of 1mm cable is specified as 8A @ 60C 

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9 minutes ago, WotEver said:

This might help:

http://www.siranah.de/manuals/ISO10133.pdf

Note that the specific example given of 1mm cable is specified as 8A @ 60C 

This is the 2000 version and has been superseded by the 2012 version, so it would be a wise move to have a look on the Manchester Library web site and check if the bits you are interested in have changed. 

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2 minutes ago, Chewbacka said:

This is the 2000 version and has been superseded by the 2012 version, so it would be a wise move to have a look on the Manchester Library web site and check if the bits you are interested in have changed. 

That's why I said 'it should help' as opposed to 'here it is' ;)   I guess I should have pointed it out though.

At least you can print this one out then annotate any changes when referring to the latest version. 

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5 hours ago, Alan de Enfield said:

As has been said above, 'fuse to the cable' - what would happen if you changed lights, increased the number of lights, took a spur of for something else.

1mm2 cable is rated at 8 amps so use an 8 amp fuse.

 

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.

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10 minutes 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.

Cabling is usually rated in open air, this is not it's in a loom which will reduce the rating.

Edited by Robbo
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Just now, Robbo said:

Cabling is rated in open air, this is not it's in a loom

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? 

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Just now, 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? 

As cuthound said in post 2 

16 hours ago, cuthound said:

Fuse to the cable specification, after taking into consideration temperature, i.e. how the cable is mounted, whether it is a single cable or in a bunch, buried in insulation etc.

All derating information is in BS7671 IET Wiring Regs.

I fuse at what is usually specified by the item it's attached to, the fuse is to protect the circuit and the item the cable is attached to is part of that circuit.

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2 minutes ago, Robbo said:

I fuse at what is usually specified by the item it's attached to, the fuse is to protect the circuit and the item the cable is attached to is part of that circuit.

Which is why in the case I quoted namely bathroom lights (two outlets in number) the loom supplier appears to have specified a maximum load of 2.5amps. but used a 1mm cable which could in some circumstances ( separate cabling over a short length etc.) capable of carrying 8amps .Is that correct?

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17 hours ago, cuthound said:

Fuse to the cable specification, after taking into consideration temperature, i.e. how the cable is mounted, whether it is a single cable or in a bunch, buried in insulation etc.

All derating information is in BS7671 IET Wiring Regs.

Bear in mind however that unless I've misunderstood the rules, cable derating assumes that all of the circuits in the bundle are carrying their maximum load. In the case of a typical narrowboat loom the frequency of that happening are... never. Under which circumstances the derating tables can be ignored. 

"The grouping factors are based on the assumption that all cables in a group are carrying rated current. If a cable is expected to carry no more than 30% of its grouped rated current, it can be ignored when calculating the group rating factor. For example, if there are four circuits in a group but one will be carrying less than 30% of its grouped rating, the group may be calculated on the basis of having only three circuits."

With just about all ultra low voltage cables on a narrowboat being selected to minimise voltage drop they are unlikely to exceed 25% of their rating unless there is a fault. 

So... 1mm2 can be fused at 8A. 

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17 hours ago, rusty69 said:

The fuse is to protect the cable

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.

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17 minutes 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.

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

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