Table of Amps Vs Cable Length For 3% Volt Drop

[Alan de Enfield]

This may be of interest, as a quick reference guide to what size cable may be needed for your re-wiring job.

 

http://www.boatelectricals.co.uk/marine-cable-selection-guide/

 

The table / matrix will not 'paste' correctly, but here is the heading

Marine Cable Selection Guide

 

12 Volts Cable Thickness Table©Boat Electricals Ltd 3/9/2010
Allowing for a 3% Voltage Drop

Critical Applications: Electronics, Bilge Pumps, Navigation Lights, Fluorescent Deck Lights

 

[Guest]

Useful. Seems to agree with the sizes I calculated. My front 12V socket should have been 6 sq mm really but I used 4 sq mm. Always been OK for the TV and audio stuff, but it is marginal. Perhaps one day...

[NMEA]

3% is a target for marine nav kit and other stuff not really found on inland boats and overkill (not that overkill is a bad thing), 10% is more apropriate for most stuff found on narrowboats.

[WotEver]

3% is a target for marine nav kit and other stuff not really found on inland boats and overkill (not that overkill is a bad thing), 10% is more apropriate for most stuff found on narrowboats.

For all except charging circuits, fridge feeds and Webasto/Ebberspacher I agree.

[Jrtm]

Its very close we worked out 10mm cable for ours and its says between 10-16mm

you need to remember if using cable you need to go there and back so 10 foot is actually 20 foot ect if your using cable there and back.

Usefull info though.

Edited November 25, 2016 by billybobbooth

[Guest]

I prefer 3% mostly. 10% seems an awful lot to lose in my world!

Except tunnel lamps, they shaould have a compulsory 20% drop. Might then stop them burning the retinas out of my eyes in tunnels.

Edited November 26, 2016 by Guest

[Jrtm]

We did ours on 3-5%

[Guest]

We did ours on 3-5%

I messed up a bit on the tV socket, but evrything else works much closer than a 10% drop. That is measured BTW not just calculated.

Just as I would be unhappy if somebody robbed 10% of my pension daily, I don't wish to lose voltage either, for the sake of a slightly thicker electric wire

[WotEver]

Just as I would be unhappy if somebody robbed 10% of my pension daily, I don't wish to lose voltage either, for the sake of a slightly thicker electric wire

That 10% isn't just 'not arriving at the destination' it's been used heating up the wiring and still has to be put back into the battery.

 

You only wire your boat once (hopefully!) so why penny-pinch?

[Alan de Enfield]

For those who do wish to 'penny pinch' the link also gives a "10% Voltage Drop" table.

 

It seems false economy on a £50,000 + boat to worry about saving £100- £200 and use the 'wrong size' wiring.

[WotEver]

For those who do wish to 'penny pinch' the link also gives a "10% Voltage Drop" table.

 

It seems false economy on a £50,000 + boat to worry about saving £100- £200 and use the 'wrong size' wiring.

My thoughts exactly.

[Guest]

That 10% isn't just 'not arriving at the destination' it's been used heating up the wiring and still has to be put back into the battery.

 

You only wire your boat once (hopefully!) so why penny-pinch?

I know, my very point! : "I don't wish to lose voltage either, for the sake of a slightly thicker electric wire ."

For those who do wish to 'penny pinch' the link also gives a "10% Voltage Drop" table.

 

It seems false economy on a £50,000 + boat to worry about saving £100- £200 and use the 'wrong size' wiring.

Yep, in the grnad scheme of things, using the next size up cable makes little difference to the cost of the build. Over specced with our gas piping too should we ever want to add a Morco or similar.

Our lighting wiring is way over spec now with changing to LEDS though! Nothing lost.

Edited November 26, 2016 by Guest

[Chalky]

If you do the calculations properly you factor in the heat rise in the cable bundle at max current before the fuse blows as well as volt drop.

[WotEver]

If you do the calculations properly you factor in the heat rise in the cable bundle at max current before the fuse blows as well as volt drop.

For the majority of applications (a few metres or more of run) if you're aiming for 3% voltage drop you'll never get anywhere near the capacity of the cable or fuse.

[Guest]

For the majority of applications (a few metres or more of run) if you're aiming for 3% voltage drop you'll never get anywhere near the capacity of the cable or fuse.

That is how I see it. We have mostly either 4 mm sq or 6mm sq cables depending on run/load. Most of these have a 6A mcb behind them. Somewhere in the region of one quarter of the current rating. The smallest/shortest cables are 2.5 mm sq, still way more than adequate for the current concerned.

Edited November 26, 2016 by Guest

[NMEA]

You only wire your boat once (hopefully!) so why penny-pinch?

So I assume you would use tined cable also, and F type open barrel tinned terminations, it's not just about how it works now but later on when corrosion sets up voltage reducing resistances. It's no good using oversized cables for a lasting job if they corrode, or rubbish Blue Red Yellow pre insulated terminals that you can't even inspect for full contact. To quote you anything else is penny pinching.

[ditchcrawler]

 

Except tunnel lamps, they shaould have a compulsory 20% drop. Might then stop them burning the retinas out of my eyes in tunnels.

And then they started using LEDs

[Guest]

And then they started using LEDs

Aye. Can't remember which tunnel, but a boat came out of one earlier this year with three LED spotlights on the bow. Looked like one of those trucks on the motorway. I am glad it was a one way tunnel!

[WotEver]

So I assume you would use tined cable also, and F type open barrel tinned terminations, it's not just about how it works now but later on when corrosion sets up voltage reducing resistances.

No I wouldn't. Because these are inland waterways not salty boats.

[NMEA]

No I wouldn't. Because these are inland waterways not salty boats.

I spend quite a lot of time chasing back corroded terminations on inland waterways boats, not nearly so much as on coastal craft but still quite a lot. For instance it is a major factor in poor starting of evaporator heaters.

[WotEver]

I spend quite a lot of time chasing back corroded terminations on inland waterways boats, not nearly so much as on coastal craft but still quite a lot. For instance it is a major factor in poor starting of evaporator heaters.

In that case, for such critical applications then yes it makes sense to use tinned cable and quality crimps, fuse holders, terminal blocks etc.

 

For a bunch of LEDs in the saloon, not so much.

[Alan de Enfield]

So I assume you would use tined cable also, and F type open barrel tinned terminations, it's not just about how it works now but later on when corrosion sets up voltage reducing resistances. It's no good using oversized cables for a lasting job if they corrode, or rubbish Blue Red Yellow pre insulated terminals that you can't even inspect for full contact. To quote you anything else is penny pinching.

 

As an example :

 

If you were wiring a boat requiring 500 metres of 2mm² plain copper conductor single core cable the cost (from ASAP Supplies) would be £191.30

If you were, instead, using cable 3x larger CSA (ie 6mm²) and, with Tinned copper conductors the cost (from ASAP Supplies) would be £620.00

 

Adding just over £400 to the cost of a (?) £50,000 boat is less than 1%

 

I have learnt from past mistakes that buying 'cheap' normally means 'buying twice' (and that includes everything from car tyres to 'tools' to household appliances).

[NMEA]

 

As an example :

 

If you were wiring a boat requiring 500 metres of 2mm² plain copper conductor single core cable the cost (from ASAP Supplies) would be £191.30

If you were, instead, using cable 3x larger CSA (ie 6mm²) and, with Tinned copper conductors the cost (from ASAP Supplies) would be £620.00

 

Adding just over £400 to the cost of a (?) £50,000 boat is less than 1%

 

I have learnt from past mistakes that buying 'cheap' normally means 'buying twice' (and that includes everything from car tyres to 'tools' to household appliances).

Indeed, and there is also the satisfaction that you have done the best possible job you could, using the best materials you could, and the knowlege that it should last the life of the boat and reduce maintainance time, inconvenience and costs. Something that seems to be sadly lacking nowadays.