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What size/amperage wire to use.


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The length of wire from the master switch on my boat which is on the port side with the battery is in excess of twenty feet to reach the starboard side of the cabin.Runs over half the length of the port side and then a similar length back along the starboard side.Installed a diesel heater on the starboard side,but it wouldn't work displaying the low voltage fault code even with a fully charged battery.

Fixed that by wiring a much shorter run to the heater and now it works ok.

Fitted two LED reading lamps port and stbd in the cabin,and the port side one has a cable run of about a metre,and the stbd one is powered from the 20ft +wire.

the port light is noticeably brighter than the stbd one.

Fourteen year old daughter tells me I need to use thicker wire,"cos we just done that in science"

If she is right,what thickness wire should I use.I have not seen it advertised in thickness,but in Amps; 10amp 15amp 20 and 30amp.

Thanks for any advice.

 

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You should NEVER  believe the Amperage in catalogues and on cable drums unless the total run is only a meter or two. You need to do a calculation involving maximum current flow, cable conductor cross sectional area, and total cable length (out & back), and the voltdrop you would find acceptable or use a calculator on the internet

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Wire specification in "Amps" is meaningless, a dumbed down specification that ignores most of the reasons for picking any given size of conductor.

 

To specify your cable first you need to decide or know what voltage drop along it you (or the equipment you will run from it) will tolerate. Then you need to know the current in the conductor. Then you can use Ohm's Law to work out how low the resistance of the cable needs to be, then you can compare this with cable specifications available and select the right size.

 

 

 

 

 

 

Edited by Mike the Boilerman
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I can’t really understand your description but sounds like the 20+ cable is undersized? Usually led reading lamps draw a very small current  1 or 2 amps say.   
 

you should be able to run them  off speaker wire (not recommended) 

 

what size are the cables especially the 20 foot one and what else is it supplying.

 

simple solution is to run a new feed for the lights with suitable fusing etc

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37 minutes ago, Mad Harold said:

The length of wire from the master switch on my boat which is on the port side with the battery is in excess of twenty feet to reach the starboard side of the cabin.Runs over half the length of the port side and then a similar length back along the starboard side.Installed a diesel heater on the starboard side,but it wouldn't work displaying the low voltage fault code even with a fully charged battery.

Fixed that by wiring a much shorter run to the heater and now it works ok.

Fitted two LED reading lamps port and stbd in the cabin,and the port side one has a cable run of about a metre,and the stbd one is powered from the 20ft +wire.

the port light is noticeably brighter than the stbd one.

Fourteen year old daughter tells me I need to use thicker wire,"cos we just done that in science"

If she is right,what thickness wire should I use.I have not seen it advertised in thickness,but in Amps; 10amp 15amp 20 and 30amp.

Thanks for any advice.

 

A lot of cables are specified for 220v (mains power).  So the current rating is to avoid overheating the wire.  On a long run at 220v, if you loose say 10v that is less than 5% so not a big problem, but that same current at 12v will still loose 10v, leaving you with just 2v to work your appliance (eg diesel heater, light etc), which is going to be a failure.  Cables sold for vehicle use, will have an advised current capacity, but it will be based on a cable length of a couple of feet, so may allow for a volt drop of say 0.5v, if you have a run of 20ft, the volt drop will be 10 times what it would be at 2ft, so you will be loosing 5v in the cable leaving about 7.5v for your appliance.  Hence low volt alarm etc.

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There are lots of on-line voltage drop calculators. A typical one here. Worth reading the entire page the calculator is on. Buy from a supplier that tells you the cross sectional area of the wire in mm2, not Amps current capacity.

 

Jen

Edited by Jen-in-Wellies
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1 minute ago, Alan de Enfield said:

Half way down the page on this link

 

https://www.12voltplanet.co.uk/cable-sizing-selection.html

 

Fill in the boxes with the length of run and the current demanded by whatever appliances are connected (worst case scenario - use 2 amps for every light bulb and 1 amp for every LED lamp.

12w for an led, you must like bright lights ?

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4 minutes ago, Chewbacka said:

12w for an led, you must like bright lights ?

I do ! 
 

However I was keeping it simple for the OP

 

Which just makes it worse he must be using bell wire....

Edited by jonathanA
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1 minute ago, WotEver said:

I bet there’s at least one dirty and corroded chock block in the circuit too. 

My LEDs are 3.5w - 5w

But I did say worst case and I'd rather overestimate and get the cable too large than the other way.

 

OP has no idea about volt drop (although it appears his daughter does) so better to over estimate.

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4 minutes ago, Alan de Enfield said:

My LEDs are 3.5w - 5w

But I did say worst case and I'd rather overestimate and get the cable too large than the other way.

 

OP has no idea about volt drop (although it appears his daughter does) so better to over estimate.

Best get the daughter specifying cables then, I think mine could with a bit of prompting, but I don’t think she is interested in boats just yet ?

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38 minutes ago, Chewbacka said:

if you loose say 10v that is less than 5% so not a big problem, but that same current at 12v will still loose 10v, leaving you with just 2v to work your appliance

 

C'mon, that's rubbish Shirley!!

 

The same current may drop the same voltage, but for 12v to deliver the same power as 240v, the current has to be 20 times larger so the cable would prolly catch fire 

 

 

 

 

 

 

 

 

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57 minutes ago, Alan de Enfield said:

Half way down the page on this link

 

https://www.12voltplanet.co.uk/cable-sizing-selection.html

 

Fill in the boxes with the length of run and the current demanded by whatever appliances are connected (worst case scenario - use 2 amps for every light bulb and 1 amp for every LED lamp.

Many thanks for the link.

All is much clearer now.

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The 12v planet calculator is very good for steady loads but for fridges it is undersizing due to the starting current required. As a rule of thumb for a 12v fridge never use anything less than 10mm cross section cable and 16mm if more than 25 feet away from the batteries, 25mm if over 50 feet away. That way you can't go wrong.

 

The same applies to diesel fired heaters like Ebersnackers and Webastards, the ignitor takes a lot of current, use big cable for trouble free working.

 

I cheat with my fridge, the solar goes in at the fridge so it uses the same heavy cable back to the batteries.

Edited by Boater Sam
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10 hours ago, Mike the Boilerman said:

 

C'mon, that's rubbish Shirley!!

 

The same current may drop the same voltage, but for 12v to deliver the same power as 240v, the current has to be 20 times larger so the cable would prolly catch fire 

 

 

 

 

 

 

 

 

The point I was failing to make is if the supplier states a cable is ok for 10A assuming 220v use then a resulting 5v drop in the cables is fine, if I use that same cable for my boat where I want it to carry 10A at 12v then it will still loose 5v which is not ok.  I was not interested in what was being powered by the cable as that was not relevant, just the consequences of carrying the same level of current (obviously for different appliances) at different supply voltages.

 

Which explains why supplier ‘advised’ max currents are normally much too high for 12v boat applications.

Edited by Chewbacka
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I still doubt Chewbacka's statement. The amperage stated on the drum is related the the heating effect of the current flow and the ability of the insulation to cope with the heating effect. 10 amps at 12V and 10 amps at 240 will have the same heating effect. The heating effect is proportional to the conductor's resistance and thus the volt drop. Mains or 12V you still   have to rake account of voltdrop although as Chewbacka say any given voltdrop on mains circuits will have less serious effects than the same voltdrop at 12V.

 

It has to be due to the heating effect and insulation otherwise one would not have to de-rate for bundled and loomed cables.

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55 minutes ago, Chewbacka said:

The point I was failing to make is if the supplier states a cable is ok for 10A assuming 220v use then a resulting 5v drop in the cables is fine, if I use that same cable for my boat where I want it to carry 10A at 12v then it will still loose 5v which is not ok.  I was not interested in what was being powered by the cable as that was not relevant, just the consequences of carrying the same level of current (obviously for different appliances) at different supply voltages.

 

Which explains why supplier ‘advised’ max currents are normally much too high for 12v boat applications.

 

Whilst agreeing with you I think Tony has put his finger on it and highlighted why I felt the need to query your post.

 

When the manu claims a cable is good for 10A (say), they are making no claims about how many or how few volts will be dropped. The cable 'spec' is all about the insulation performance and nothing more. 

 

My point is to work out a suitable size for a cable one needs to use Ohm's Law, not rely the label on the reel. 

 

 

And to use Ohm's Law, first one needs to decide what volt drop is acceptable for the equipment being powered.

 

 

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12 minutes ago, Mike the Boilerman said:

And to use Ohm's Law, first one needs to decide what volt drop is acceptable for the equipment being powered.

Has any party committed to repeal Ohm's Law in their manifesto? We could use much thinner cable then. Should this be on the general election thread? Is it a law based on an EU requirement? Should it be in the Brexit thread?

 

Jen

Edited by Jen-in-Wellies
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8 minutes ago, Mike the Boilerman said:

 

Whilst agreeing with you I think Tony has put his finger on it and highlighted why I felt the need to query your post.

 

When the manu claims a cable is good for 10A (say), they are making no claims about how many or how few volts will be dropped. The cable 'spec' is all about the insulation performance and nothing more. 

 

My point is to work out a suitable size for a cable one needs to use Ohm's Law, not rely the label on the reel. 

 

 

And to use Ohm's Law, first one needs to decide what volt drop is acceptable for the equipment being powered.

 

 

 

Totally agree except I have doubts about asking people to use Ohm's law. I know they will be doing that when using a voltdrop calculation or cable size chart but on explicitly. I have been in front of too many classes who must have been taught Ohm's law at school and when introducing the topic seen half the classes eyes shut off and some complaining "its too hard". In fact we have enough instances on here of people saying electrics are hard. Personally I would far rather avoid phrases like Ohm's law, left hand rule, right hand rule and so on in case it turns people off.

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4 minutes ago, Tony Brooks said:

 

Totally agree except I have doubts about asking people to use Ohm's law. I know they will be doing that when using a voltdrop calculation or cable size chart but on explicitly. I have been in front of too many classes who must have been taught Ohm's law at school and when introducing the topic seen half the classes eyes shut off and some complaining "its too hard". In fact we have enough instances on here of people saying electrics are hard. Personally I would far rather avoid phrases like Ohm's law, left hand rule, right hand rule and so on in case it turns people off.

 

Yes you are right about the way people shut off when they hear mention of Ohm's Law. Weird given how simple it is. One cannot get mathematics more basic, it is equations at their simplest. 

 

However, reviewing the OP he didn't ask how to work out the right cable size, he asked what cable size to use. So wants us to work it out for him.

 

In order to do this we need some concrete info from the OP please:

 

1) Actual length the cable will be

2) What exactly it will be powering so we can look up the manufacturer's allowable volt drop

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1 minute ago, Mike the Boilerman said:

 

Yes you are right about the way people shut off when they hear mention of Ohm's Law. Weird given how simple it is. One cannot get mathematics more basic, it is equations at their simplest. 

 

However, reviewing the OP he didn't ask how to work out the right cable size, he asked what cable size to use. So wants us to work it out for him.

 

In order to do this we need some concrete info from the OP please:

 

1) Actual length the cable will be

2) What exactly it will be powering so we can look up the manufacturer's allowable volt drop

Which is more or less what I told him in post 2.

 

Actually I don't think I have seen a maximum voltdrop stated for 12V equipment and can't recall seeing a minimum supply voltage which is why I work on a maximum of 0.5 volts for general equipment, 0.3 volts for charging and starting circuits and "use the size the manufacturer says" for fridges.

 

I recall that when  delivering an electrics course for the engineers at a major hire fleet I saw the dawn of realisation about succession water pump failures when we discussed how excess voltdrop makes most 12V mores draw more current and thus can burn out.

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