Another cable size question...

[Reptile Smile]

Hi all,

I'm really sorry - I haven't much of a clue about this sort of thing.

Someone at work has a bunch of cheap cable that I could temporarily use for my boat. I want to wire in a 0.8W LED nav light. The cable run is 6m, so 12m in total. The cable is 1mm squared. Am I pushing my luck? Is it safe?

 

[nrcoyle]

I am sure someone more knowledgible than me will be along a moment, but if you look at the link below:

http://www.the12voltshop.co.uk/Shop/about-cable-sizes.html

it would suggest that 0.5mm² is the minimum for your run so what you are proposing should be fine

 

[VicS]

Hi all,

I'm really sorry - I haven't much of a clue about this sort of thing.

Someone at work has a bunch of cheap cable that I could temporarily use for my boat. I want to wire in a 0.8W LED nav light. The cable run is 6m, so 12m in total. The cable is 1mm squared. Am I pushing my luck? Is it safe?

No problem .

0.8 watts will draw 0.8/12 = approx 0.07 amps

1.0 mm² cable has a resistance of approx 17 ohms per 1000m, so the resistance of 12m is 12 x 17 / 1000 = 0.2 ohms

volts drop will therefore be 0.2 x 0.07 = 0.014 volts ........... ie virtually nothing!



I take it though that the 0.8 watt nav light does give you the required visible range ?

 

[nigelmercier]

... Is it safe?

Yes, if it has a fuse below about 15A.

 

[VicS]

Yes, if it has a fuse below about 15A.

but since we don't know anything about the nav light fitting itself a more sensible fuse rating would probably much lower 5A 3A or even 1 A

 

[nigelmercier]

but since we don't know anything about the nav light fitting itself a more sensible fuse rating would probably much lower 5A 3A or even 1 A

Indeed.

 

[deep denial]

I would however advise against cheap cable, particularly up a mast. It will corrode quickly. you really should go for tinned marine cable

 

[VicS]

I would however advise against cheap cable, particularly up a mast. It will corrode quickly. you really should go for tinned marine cable

how long would you expect ordinary untinned domestic cable to last?

 

[peter2407]

On another thread i asked a question about size of cable to support an 80 W solar panel where the recommendation was 6mm, so went off to Maplins to see if they had any... Their recommendation was 1mm!

 

[AndyJ76]

On another thread i asked a question about size of cable to support an 80 W solar panel where the recommendation was 6mm, so went off to Maplins to see if they had any... Their recommendation was 1mm!

Did Maplin ask about the length of the cable run? The 6mm recommendation was for a cable run of 40ft (which is quite a long way for such a small panel)

Apologies for thread drift, although the question is suitably answered by VicS et al

 

[Daydream believer]

I would however advise against cheap cable, particularly up a mast. It will corrode quickly. you really should go for tinned marine cable

Surely it is only the end inch that deteriorates so if you were to leave a 6 inch loop just before the fitting you could cut the black wire off every 2-3 years & have 8years of life
or was it a fluke that my wiring lasted that long up the mast

 

[peter2407]

Did Maplin ask about the length of the cable run? The 6mm recommendation was for a cable run of 40ft (which is quite a long way for such a small panel)

Apologies for thread drift, although the question is suitably answered by VicS et al

Yes. Let me turn thar around - using the calculator, 2 metres equates to 1mm and 3 metres to 1.5mm. There was a prebuilt cable in a packet that was around that. So, what the Maplins guys seem to have been told to say/are actually saying is "regardless of length" it is always 1 to 1.5 mm, which is disappointing because I have generally found them to be good/enthusiastic etc. you are right though - my measurement was based on worse case, as the optimum position may be the bow on a 36 foot boat.

 

[Coaster]

how long would you expect ordinary untinned domestic cable to last?

Clearly the answer depends on location.

Our Westerly is now 30 years old. There is still a fair bit of original untinned cable in situ, mostly in lighting circuits. The cables running internally are fine. I expect they will last for many more years.

However the cables led to external light fittings had copper strands that were blackened. This extended from the ends of the cables, back beneath the sheathing, for half a metre or more in places. This copper seemed to be brittle. I have now replaced most of these cables. All of the new cable on the boat is tinned.