Online 12V cable thickness chart?

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ChasB

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I've had a 12V toilet rated at 30A wired in by a boatyard with what looks to me like domestic 13A wiring.

Does anyone know where I can access an authoritative on-line chart or table that gives the required cable thicknesses set against the current running through the cable?

Cheers.
 
Cheers again.

I'm going to go down to an electrical wholesalers (a few yards away as luck would have it) on Monday morning.

So far I've got 8 or 10 AWG on the first site, and a cable size of 6 or more on the other. Anyone able to eligthen me on what these figures mean?

And how do they compare to a domestic 240V 13A cable?
 
13A 'mains' cable is typically 2.5mm2.

Assuming that the cable is large enough for the current, you should also consider the voltage drop along the cable, if it is of any significant length.

Voltage drop for 2.5mm2 cable is 18mV/A/m [0.018 volts per amp, per metre].
 
Is that calculated at 240V or 12V?

So at 12V with a 30A load, a cable of 2.5mm2 thickness will drop 2 volts over 4 metres. (0.018 x 30 x 4 ?)

Is that bad? Or reasonable?


I think I'll do the obvious - ring the toilet's manufacturers!
 
The correct figure is...

The correct voltage drop figure for a 2.5 sq mm conductor is about 8mV per amp per metre. Voltage drop is regardless of voltage.

If your toilet takes 30A, and if it's 4 metres away from the battery, you'll need much heavier gauge wire. I'd suggest you use at least 10 sq mm wire, which would result in a voltage drop of around 0.5V. 15 sq mm wire would be even better, giving a voltage drop of around 0.3V.
 
Re: The correct figure is...

Thanks guys. That's very helpful.

I have a follw-up question, but I'll make that a seperate thread.
 
Re: The correct figure is...

Sorry to disagree, but the voltage drop for 2.5mm2 2-core cable IS 18mV/A/m.

This is taken from the IEE 16th edition, Table 4D1B, for 2 DC cables, 'Single-core 70oC thermoplastic (pvc) insulated cables, non-armoured, with or without sheath (COPPER CONDUCTORS).

For reference, the voltage drop for other sizes are:

1mm² - 44mV/A/m
1.5mm² - 29mV/A/m
2.5mm² - 18mV/A/m
4mm² - 11mV/A/m
6mm² - 7.3mV/A/m
10mm² - 4.4mV/A/m
16mm² - 2.8mV/A/m
25mm² - 1.75mV/A/m
 
Re: The correct figure is...

A quick Google yields:
http://www.batt.co.uk/images/pics/43864D1A.pdf

So if I read table 4D1A correctly, the current carrying capacity of a cable is independant of the voltage? So a 30A toilet will need at least the kind of cable you would expect to see in a mains fuse box handling 30A?

If that's so that's an easy rule of thumb to remember.
 
Re: The correct figure is...

Svenglish Tommy made a Volts Drop Calculator, that when the cable length and current load was entered, would give you the AWG and mm2 size cable and the current carrying capacity of each size A or mm2.

For the 30 amp load, and assuming 5mtrs there and back, it gives a size of 10mm2, 8 AWG/77.7 amps.

If you would like a copy of it, PM me with your email address, and I will send it to you.
 
Re: The correct figure is...

I'm not sure you are disagreeing - you're clarifying your previous post by talking now about "2.5mm2 2-core cable", whereas before you were just talking about "2.5mm2 cable". My statement that "The correct voltage drop figure for a 2.5 sq mm conductor is about 8mV per amp per metre" is correct. As you'll know, the convention in boat electrics is to quote the voltage drop in a single cable, rather than a 2-core cable. There's a good reason for this, because some of the most crucial cable-size issues relate to the single wire from the alternator to the batteries. Voltage drop tables commonly used in boating applications use single cable values and the cable length is specified as the round trip length from battery to equipment and back to battery.

I always use this very easy rule of thumb for calculating voltage drop:-

* 1 amp going 1 metre along a 1 sq mm wire gives a 20 mV drop.

That’s the basic figure, and all you need do now is change the 20mV figure in proportion to the other changes. So, for example:-

* an increase in current increases the 20mV proportionally

* an increase in length increases the 20mV proportionally

* but an increase in wire size decreases the 20mV proportionally.

Now before the purists start, I did say that this is only a rule of thumb. It’s about 95% accurate, which I reckon is close enough for leisure boats. But, and this is important, it’s easy to remember, and easy to work out without needing to resort to a calculator (which is why I believe it’s easier to think in millivolts, as there’s less chance of confusion with the decimal point).
 
Re: The correct figure is...

out of interest -surely the figures shd be adjustd for multistrand wire? Does not the current flow predominantly on the outside - hence multiple stand of the same x-section far more efficent than single strand?
 
Interesting question...

I don't think it makes any significant difference. If it did, surely cables would be made with flat conductors to maximise surface area?
 
12V applications are more sensitive...

12V applications are much more sensitive to voltage drop than mains applications. That's why it's important to size cables for an acceptably low voltage drop at the expected current. If you size for, say, a 0.5V drop you won't normally have problems.
 
Re: The correct figure is...

[ QUOTE ]
surely the figures shd be adjustd for multistrand wire?

[/ QUOTE ]

I would have thought so too, but a quick bit of research on the net reveals it to only be the case if the wires are individually insulated. Since in most wire bundles they are in electrical contact and the electrons can migrate across the strands the whole bundle acts (nearly) as if it were a solid lump. I think this may be because the electrons will tend to repel each other and push 'outwards' towards the conducting mass's surface. Maybe you also need to factor in the oxidation likely to occur on a wire's surface and the impact this will have on conductivity...


So, uh, a 30A 12V loo needs cabling of at least the type you would find on a 30A fuse in a 240V fuse box...? /forums/images/graemlins/confused.gif
 
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