Cable size to masthead light

[DanTribe]

Help please.
Can some kind soul please check my calculations and reasoning?
I need to re-wire my Aquasignal 40 tri-colour and anchor light.
Bulbs are 35 watts & 10 watts.
Mast length 13m, overal distance to switch panel approx 16m
I make that 2 amps, and from a table I have, reckon I need 2.5mm2 wire.
I'm told I should use tinned cable.
3 core 2.5mm2 tinned cable appears to be very rarely stocked and is expensive, getting on for £90 from the only stockist I can find.
This fact makes me think that it isn't used much, so what do other people recommend?
I have read some of the previous threads on this subject but conflicting advice has confused me a bit.

 

[prv]

1.5mm 3-core tinned seems to be about £2 a metre from a quick google. Would the price difference from 2.5mm pay for LED bulbs for the masthead, reducing the cable size needed and, as a bonus, reducing the draw on the battery? You could probably use even thinner cable if it's available.

Pete

 

[David2452]

I haven't done the proper circular mil calc. but, 2a allowing for a 10% voltage drop you could use 1.5mm2, in which case Fernaux Ridall will do you a 30m reel of tinned thin wall 3 core for under £50 and you will have enough left over for other jobs,

 

[barnaclephill]

This fact makes me think that it isn't used much, so what do other people recommend?

Tinned cable has a the copper strands (for flexibilty) coated in tin/lead/solder stuff to prevent the copper oxidising (corroding), and protection from wicking. For marine use it's recommended. The ordinary stull will corrode at the terminals in a few short years, and the masthead is not fun getting to for impromptu repairs.

Tricolour at 35W divided by 12V is about 3 amps. That seems the maximum to be on at the one time, (i.e. it's either the tricolour or you're anchored).

The 16m run and return is 32m or 105 feet.
From "The 12 Volt Bible for Boats" by Miner Brotherton page 56, for a maximum drop of 3% for 5amps at 70 feet, the AWG is 8. The tables in his book don't go further that that.

The stuff I bought yesterday is called 4mm twin sheath tinned wire, at $2.60/metre and I needed 8m for the mast. I think it's overkill but it's all they used to have. They also had a (new product) packet of lighter wire claiming to be rated at 6 amps over the 10m length of 0.54mm squared each (0.54mm sq each), and only $15, specifically for LED lamps. So I bought that as well.

I'm sorry I can't help you more. I'm sure others will give better advice. My main suggestion will be to change your lamps for LED lamps to save both on power and the thickness and cost of cabling. Sometimes you can change just the bulb for a LED bulb, but light intensity is lost going from a white LED through a green or red lens.

My mast has only one all-round white (4 Watts), used as steaming or anchor, switchable with the (1Watt) stern white. A red & green (1 watt each) are on the cabin sides, and are used in conjunction with a white light. The masthead has been seen at more than 5 nm away.

 

[David2452]

Calculation here:

CM = K x I x L / E
Where:
K = 10.75 resistivity constant for copper
I = Current (amps)
L = Round Trip Wire Distance (feet)
E = Maximum Voltage Drop Allowed

So 10.75 x 3.75 x 105 / 2 = 2116 circular mil or 1.072 mm2
That would give you around 12v at the load with the motor running or about 10.5 without and a well charged battery, but 1.5mm2 will be a quite bit better, you are 12v aren’t you? I ask as you calculated the amps at 2 which would be closer for a 24v system. If you play around with the allowable voltage drop until the circular mil = 5000 that is 1.5mm2

 

[merlin3688]

As you are in Essex just pop and see the boys at Sailspar, they will have the correct wire and at a good price!
Sailspar
Hilliards Road Great Bromley, Colchester, Essex CO7 7US

East Anglia, East of England
p: 01206 251348 f: 01206 255230

 

[Kwik Decision]

While tinned cable is recommended for marine use as it will clearly resist corrosion far better than copper, I believe it is overkill unless you are anticipating keeping the boat for a very long time. Our boat was wired with plain copper cabling from new in 1984, and the cables to the bow and stern lights (which I believe are original) are still fine. On this basis, I'm replacing our cable up the mast with untinned cable, and as I'm expecting the the conections to be kept dry, I'm not anticipating any problems. The reason I'm replacing it BTW is because the insulation at the top has got UVed and cracked!

As for size of cable, the Aquasignal 40 normally has a 25W bulb in the tricolour, not 35W, and will therfore draw ~2A at 12v, (as you have stated). A quick surf to here:

http://www.kilowatts.com.au/calculator-voltage-drop.php

confirms your 2.5 mm2 cable required, to keep the volage drop below 5%.

 

[David2452]

While tinned cable is recommended for marine use as it will clearly resist corrosion far better than copper, I believe it is overkill unless you are anticipating keeping the boat for a very long time. Our boat was wired with plain copper cabling from new in 1984, and the cables to the bow and stern lights (which I believe are original) are still fine. On this basis, I'm replacing our cable up the mast with untinned cable, and as I'm expecting the the conections to be kept dry, I'm not anticipating any problems. The reason I'm replacing it BTW is because the insulation at the top has got UVed and cracked!

As for size of cable, the Aquasignal 40 normally has a 25W bulb in the tricolour, not 35W, and will therfore draw ~2A at 12v, (as you have stated). A quick surf to here:

http://www.kilowatts.com.au/calculator-voltage-drop.php

confirms your 2.5 mm2 cable required, to keep the volage drop below 5%.

You, (and others) may well have been lucky but I have on numerous occasions, one only last week and on not so old a boat chopped back cables at 1m intervals looking for non oxidised conductor only to wind up finding some so close to the switch panel I may have well just gone there in the first place. The cost differential in using tinned cable against good qualityplain copper is nowhere near as great (in common sizes) as people will have you believe. also why would you aim for a 5% drop? that's electronics teritory not incandesant light teratory. 1.5mm2 will be fine. Best, as already suggested is use LEDs and lighter cable as it will be much easier to install.

 

[DanTribe]

Thanks for the advice everybody.
If I can get away with 1.5mm2 cable it will be a big plus.
I've managed to locate some tinned 2.5mm2 3 core at reasonable cost but it's in Australia and the carriage doubles the cost.
I think barnaclephil's suggestion of 8AWG [8mm2] is a bit of an overkill?
I like the suggestion of LED bulbs but the cost of these nullifies any saving in cable and opens up a new can of worms regarding colour rendition.
Also to correct my original post, I meant 25watts [not 35w] and yes, 12V.
Perhaps I'll go back to parafin lamps, but what colour oil to use?

 

[VicS]

You say 35 watt but you then say 2 amps. Do you mean 25 watts?

You do not say what volts ! If its is 12 volts then 2amps is correct for a 25 watt bulb.

Do not forget some tables require you to use the total of positive and ngative cable ie twice the total distance to the light. So that will be 32m or about 100ft

From a table
Assuming a 12 volt system and a 25 watt ( 2amp) load you will require AWG 12 . Awg 12 has a cross sectional area of 3.3mm²


to confirm this by calculation:-

AWG 12 has a resistance of 5.4 ohms per 1000m

32m therefore has a resistance of 5.4x32/1000 = 0.17 ohms

the volts loss at 2 amps will therefore be 2x0.17 = 0.34 volts.

An acceptable volts loss for a filament type nav light is 3%
3% of 12 volts is 0.36v

This confirms AWG 12 to be the correct cable size for your requirements.

1.5 mm² will not be adequate .... nor really will 2.5mm²

I'd not quibble over 3.0 mm² which will give you a volts drop of 0.37 volts.

Note all this is for a 12 volt 25 watt filament bulb!

( If you switched to an LED bulb you will require much lighter cable not only because the current will be much less but also because the LED will probably operate satisfactorily at a lower voltage anyway!)

 

[maby]

VicS' calculations look pretty comprehensive and the results look reasonable. I would just add that physical strength is probably a bigger consideration than current carrying capacity - you don't want those wires breaking at a safety critical moment, so don't skimp on surface area.

 

[Martin_J]

I was in the same predicament a few years back for a 15m mast... When I felt the weight of all that cable (and thought about it being aloft and supporting itself in the mast), I went for the smaller section cable and fitted LED lights...

VicS' calculations look good you see.

 

[ffiill]

2.5mm is same size as standard 25 amp fused 15 amp plugged domestic cable-just sounds a lot for a single masthead bulb.
A good place to look for the right ratings is one of the firms selling 12v automotive cable such as Vehicle Wiring Products also good for anything 12v electrical from switches to heavy duty relays;battery switches etc.-well worth a look http://www.vehicle-wiring-products.eu

 

[VicS]

2.5mm is same size as standard 25 amp fused 15 amp plugged domestic cable-just sounds a lot for a single masthead bulb.
A good place to look for the right ratings is one of the firms selling 12v automotive cable such as Vehicle Wiring Products also good for anything 12v electrical from switches to heavy duty relays;battery switches etc.-well worth a look http://www.vehicle-wiring-products.eu

It is a lot and it is a lot because for an application like a navigation light the voltage drop along a pretty long cable has to be taken into account.

There are some circuits where the voltage drop may not be so important but it is relatively important for a navigation light or its brightness and visible range will be adversely affected.

You see my calculation above!

Firstly I used a table to look up the advisable cable size. Then I confirmed that was in fact correct by calculation.
Please check my arithmetic if you feel I have made an error.


As a very rough rule you can say that for cable runs of a couple of metres or less the important factor to take into account is the max safe current that the cable can carry.
Once the cable run length exceeds a couple of metres you have to start considering the voltage drop and making allowances for it.

 

[aitchem]

I read here, disbelieved.
Asked the engineers at work, didn't want to believe it.
Learnt OHMS Law.
Downoaded various software calculators and put all the numbers in, they all said 2.5mm.

So, My mastlights have 2.5mm cores, panel breaker to head, there's no way round it.

 

[VicS]

I read here, disbelieved.
Asked the engineers at work, didn't want to believe it.
Learnt OHMS Law.
Downoaded various software calculators and put all the numbers in, they all said 2.5mm.

So, My mastlights have 2.5mm cores, panel breaker to head, there's no way round it.

Well there is now. You can use an LED light!

.

 

[PetiteFleur]

I went through the same exercise last winter. Ended up with 20m x 2.5mm² x 2 core for the tricolour, I had a 'screwed on' anchor light on top which I removed and converted to a LED lamp to be hung in the foretriangle. For the steaming & deck lamp I used 20m x 1.5mm² x 3 core.
1.5mm² bought from Furneaux Riddall and the 2.5mm² a seller off ebay. And I've some left for all the odd little wiring jobs around the boat.

 

[DanTribe]

Thanks, once again, for all your advice especially VicS.
I didn't like your conclusion, but am now convinced it's absolutely correct.
I can only find two suppliers of 2.5mm 3 core tinned cable; Aquafax and Power Store. lowest price is still over £100 to wire 1 light, gulp.
There was lots of discussion previously about using LED in tricolour lenses, has this been sorted out now?

 

[neil1967]

Take a look at seascrew.com - 10m of 2.5 mm tinned 3 core is £33. Of course only one pair of wires has to be 2.5mm - the anchor light could be 1.5mm, so you could run a twin core 2.5 mm cable (pos & neg) for the Nav light, and a single 1.5 mm pos for the anchor light, using the neg return of the Nav light. Not tidy, but would work, and cheaper.

Neil

 

[VicS]

Thanks, once again, for all your advice especially VicS.
I didn't like your conclusion, but am now convinced it's absolutely correct.
I can only find two suppliers of 2.5mm 3 core tinned cable; Aquafax and Power Store. lowest price is still over £100 to wire 1 light, gulp.
There was lots of discussion previously about using LED in tricolour lenses, has this been sorted out now?

Ideally the proper red LEDs in red lights, green in green and coolwhite in white lights.

I have fitted an LED light in my Aquasignal tricolour light that has three coloured sectors to match the lenses. Many chandlers sell the ECSmith one http://www.seamarknunn.com/acatalog/info_ES100167.html

Mine came with a special replacement holder so that the LED could be properly orientated with the coloured sectors of the tricolour.



Apparently an acceptable compromise is a warm white LED assembly. It gives reasonable colors in the red and green sectors and although not as bright as a cool white is acceptable for the white light