Battery cable size - confused by calculators

maej

maej

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I'm getting confused :confused: I'm trying to decide on the correct battery cable size for replacing my aged and corroded cables. The existing cable has no markings on it and the conductor thickness is only 7mm (9mm including insulation) which seems a little on the small side.

I've tried a few online calculators and calculations but I'm getting some confusing answers :( The 'Length' the calculators want, is it 'one-way' (battery to load) or 'round trip' (battery to load and back to battery)? The results I get from these calculators below are double what I get when using the calculation below which makes me think that the calculator wants one-way distance. If that is wrong and I use a cable half the required size it could be disasterous :eek:

Online Calculator 1
Online Calculator 2
Volt Drop Calculation

For 12v DC with a 'round trip' distance of 3.8m and a load of 108 amps, what cable size do I need to keep volt drop below 3%? Is it 35mm2?

Are there any calculators that work in mm2 rather than AWG?
 
maej said:
For 12v DC with a 'round trip' distance of 3.8m and a load of 108 amps, what cable size do I need to keep volt drop below 3%? Is it 35mm2?
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35sq.mm would certainly fit with Calder's reckoning for that 'round trip'.

'Round trip' is my understanding of the normal convention but some calculators seem to work on round trip, some on one-way: you're right it would be helpful if they all specified which.
 
Because of the confusion caused by some online calculators not stating if the length to be used is for the round trip or not I now usually calculate voltage drop from a table of cable resistance vs crossectional area
 
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There are 2 factors you need to consider when selecting wire size.

Voltage drop
Maximium current.

Normally the former is the problem, but in short runs maximium current carrying capacity can be the factor that dictates a larger wire.

25 mm squared will give a voltage drop of 0.45v or 3.7%. It's current carring capacity is listed in my tables as only 105A.
Current carring capacity is complicated to get a proper rating you need to know the type of insulation. If the cable is bundled or passes through a conduate and finally if it is the engine room. The 105A is a simplified figure.

Given that it fails both tests voltage drop is a touch over 3% and the current carring capacity is too low I would to up to 35mm squared
 
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noelex said:
25 mm squared will give a voltage drop of 0.45v or 3.7%.
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I don't know how you work that out. 25mm2 gives a voltage drop of 0.28v, less than 3%.

The calculation usually quoted for voltage drop is:-

Voltage drop = (total cable length (in metres) x current (in amps) x 0.017) / (divided by) cable mm2
 
pvb said:
I don't know how you work that out. 25mm2 gives a voltage drop of 0.28v, less than 3%.

The calculation usually quoted for voltage drop is:-

Voltage drop = (total cable length (in metres) x current (in amps) x 0.017) / (divided by) cable mm2
Click to expand...

Likewise .28 volt, approx 2.3 % of 12 volts
 
The voltage drop will all give slightly different results depending on the assumptions.
My figures were based on O'connells book " The marine electrics book" which lists 25 mm squared as having a DC voltage drop of 1.09mV per amp per metre.
I looked up some tables of resistance of 25mm squared wire and they varied between 0.832 ohms per Km and 0.78 Ohms per km. This would give a voltage drop of between 0.34v and 0.26v which is a voltage drop of between 2.8-2.6%

This fits in with your figures so my apologies.

However, the current carring capacity of wire is insufficient, or at least very marginal, so I would still recommend a larger 35mm squared wire to the OP.
 
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noelex said:
However, the current carring capacity of wire is insufficient, or at least very marginal, so I would still recommend a larger 35mm squared wire to the OP.
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I refer usually to an Ammerican table which gives two figures for current carrying capacity.
One for what they call "chassis wiring " were the cable is in free air and not bundled with other cables. The other for what they call "power transmission" where the cables may be enclosed or bundled.

For AWG 3, the nearest AWG size to 25mm² the max rating is 158 amps for chassis wiring but only 75 amps for "power transmission"

For short periods or where the cables are not enclosed 25mm² will be adequate but for long periods where the cables are enclosed or bundled with other cables it is not. AWG 1 would be required in the latter case.

AWG 2 or 35mm² might be a good compromise
 
VicS said:
For AWG 3, the nearest AWG size to 25mm² the max rating is 158 amps for chassis wiring but only 75 amps for "power transmission"
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Vyv Cox suggested the Merlin Equipment website. Their chart (http://www.merlinequipment.com/page/document.asp?id=298) doesn't show AWG3, but it does show AWG4 (19 mm2) and says it has an ampacity of 136 inside the engine compartment, or 160 outside the engine space. These are rather more generous than the ratings you quoted.
 
pvb said:
Vyv Cox suggested the Merlin Equipment website. Their chart (http://www.merlinequipment.com/page/document.asp?id=298) doesn't show AWG3, but it does show AWG4 (19 mm2) and says it has an ampacity of 136 inside the engine compartment, or 160 outside the engine space. These are rather more generous than the ratings you quoted.
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The table I am looking at gives the two figures for AWG 4 ( 21.15 mm²) as 135 amps and 60 amps So yes AWG 4 would be Ok here the cables are in free air. Not if enclosed or bundled and subjected to a continuous load.
 
Thanks

Sounds like anything from AWG 4 (21.15mm2) and up will be safe :)

Considering the volt drop, and that cold starting my elderly bukh takes a lot of cranking, I think I'll go for the 35mm2 to get the best out of the battery and ensure I don't overheat the cable where it is bundled with others in the conduit.

Thanks for all the advice, now I feel more confident.

Now I just need to decide 'tinned' or 'un-tinned', but that's a decision my wallet is going to have to make. It's only £25 more, but it's £25 I can't really spare yet.

Kojaycat is the cheapest I've found so far for tinned marine battery cable, crimp lugs, battery terminals etc. Are there any cheaper places out there that are still good quality?
 
I installed my windlass well over ten years ago, using untinned welding cable running bows to stern. Welding cable is made up of very fine strands that might be supposed to have more of a corrosion risk. Despite that they remain bright and uncorroded, although it must be said that the boat is invariably dry.

My conventional starter cable is also untinned, I replaced the original at about 20 years old, when it was not particularly corroded.
 
The ABYC guidelines are here:
http://bluesea.com/files/resources/reference/2010_ABYCexcerpts.pdf

As you can see they vary considerably depending on the condititions. For wire like this I would always put in a conduit if possible. Note these ratings only apply up to an air temperature of 30C.

The ratings for 25mm with 75C insulation are:
Single wire in air 141A
Single wire in air going through engine bay 106A
In conduit (or 2 wires together) 99A
In conduit (or 2 wires together) in the engine bay 74A
 
maej said:
Sounds like anything from AWG 4 (21.15mm2) and up will be safe :)

Considering the volt drop, and that cold starting my elderly bukh takes a lot of cranking, I think I'll go for the 35mm2 to get the best out of the battery and ensure I don't overheat the cable where it is bundled with others in the conduit.

Thanks for all the advice, now I feel more confident.

Now I just need to decide 'tinned' or 'un-tinned', but that's a decision my wallet is going to have to make. It's only £25 more, but it's £25 I can't really spare yet.

Kojaycat is the cheapest I've found so far for tinned marine battery cable, crimp lugs, battery terminals etc. Are there any cheaper places out there that are still good quality?
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I've bought a fair bit of tinned wire from AES a while ago. Service was good and I think that they were the cheapest I could find at the time, might be worth a look. http://www.autoelectricsupplies.co.uk/product/885/category/124

I think AES quoted prices with VAT and had low P&P so worked out cheaper than Kojaycat by time I'd added VAT + P&P to their prices. No problem with quality either with any of my orders so far.
 
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I'm confused, £25 more? so are you buying 4m of untinned 35mm2 for £2.50 a metre?, 4 metres of 35mm² tinned is only £36 at retail inc VAT, and that's 455/0.30 high flex.
 
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David2452 said:
I'm confused, £25 more? so are you buying 4m of untinned 35mm2 for £2.50 a metre?, 4 metres of 35mm² tinned is only £36 at retail inc VAT, and that's 455/0.30 high flex.
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Actually I need 10m. 3.8m is just the round trip distance for the engine starter circuit, I'm also doing the domestic bank circuit at the same time so that needs more cable. 6m of red and 4m of black is what I'm after in total. I'm keeping the domestic bank cable the same grade as the engine bank as the domestic bank is sometimes needed to help (boost) the engine battery on particularly cold mornings and I like to keep things balanced. It all needs replacing because it's corroded and brittle with some broken strands around the terminals.

It's £7.44 inc vat per metre for tinned at Kojaycat, and £5.20 inc vat per meter for untinnned at battery megastore. Hence the ~£25 difference for the 10m.
 
maej said:
Actually I need 10m. 3.8m is just the round trip distance for the engine starter circuit, I'm also doing the domestic bank circuit at the same time so that needs more cable. 6m of red and 4m of black is what I'm after in total. I'm keeping the domestic bank cable the same grade as the engine bank as the domestic bank is sometimes needed to help (boost) the engine battery on particularly cold mornings and I like to keep things balanced. It all needs replacing because it's corroded and brittle with some broken strands around the terminals.

It's £7.44 inc vat per metre for tinned at Kojaycat, and £5.20 inc vat per meter for untinnned at battery megastore. Hence the ~£25 difference for the 10m.
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When it comes to a starter motor, there are a few things to consider:
1) The initial inrush current to a virtually stalled motor is a lot more than the nominal current.
2) Once the inrush is over, the difference between 0.2 and 0.4V wiring drop is probably not what decides if your engine starts well or not. It pales into insignificance, compared to the effect of how well charged or healthy the battery is.
3) During the inrush, or while trying to start with a low battery, cranking at low RPM, you are giving the starter motor a hard life.
4) keeping the starter circuit as short as possible, including the return helps more than fatter cable.
If you are routinely needing to boost the engine battery with the house batteries, I suspect the charging is not right, or the battery is not up to it.
 
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