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Cable Sizes & Conversions

by Collyn Rivers

All electrical cable resists the flow of current and the larger the diameter of the cable, and the shorter its length, the lower is its resistance. It is essential to minimise this resistance because it reduces the voltage that lights, refrigerators and other electrical bits and pieces need to work correctly.

 In practice, the highest acceptable voltage loss in any 12/24 volt campervan or motorhome wiring is 3% of the battery voltage. That is: 0.36 volts, and 0.72 volts in 12 and 24 volt circuits respectively. Professional installers often work toward 5% losses because it saves on cable costs – but at the expense of their clients whose electric fridges will never work as well as their makers intended.

Given that 3% voltage drop is the maximum acceptable drop (but 1.5%-2% is better) it should be easy enough to select cable accordingly: tables of recommended cable sizes for various current flows are readily available. There’s also a really simple bit of arithmetic that does the job superbly (see below).

 Auto Cable – a Trap!

Unfortunately there’s a huge trap. Automobile cable is commonly sold using a rating system that appears to correspond (but doesn’t) with appliance manufacturers’ cabling recommendations. Nor does it, nor indeed can it, correspond with other cable size standards.

Auto cable is that cable invariably sold by auto parts stores, auto-electricians and many hardware stores. This cable is rated in terms of its outside diameter – insulation and all.  In other words all that measurement tells you is the size hole it can be passed through!

But most appliance manufacturers and virtually all electrical trades specify cable either directly in terms of the cross-sectional area (in of its copper conductor - or by a numbering system (such as AWG) that relates to conductor area.

A typically recommended cable size is 2.5 mm.sq. Generally, the appliance maker will spell this out in full (eg. 2.5 – but not invariably. You can however be certain that if the appliance maker talks about 2.5 mm cable they refer to 2.5 sq mm cross-sectional area.

But when a buyer fronts up to the parts store and asks for the specified 2.50 mm cable what he will almost certainly be sold is not 2.50 mm.sq. cable, but 3.00 mm diameter auto cable (the closest made to 2.5 sq mm).  A 3.0 mm diameter auto cable’s conductor area however is typically a tad over 1.0 sq mm – about one third of  the required size

If such cable is used for example for fridge wiring, the voltage drop across it will be close to three times that specified as the maximum by the appliance maker!

Unless that cable is exceptionally short, that fridge, which may be a top brand in perfect working order, will never work satisfactorily. This is the main reason why so many 12-volt fridges disappoint, and why trailer batteries are often grossly undercharged. Yet countless (in my experience, most) trailers and motorhomes are, at least in part, wired this way!  And that includes professional installations.


Compounding this, automobile cable may also be rated in amps. Thus, ‘3 mm’ cable is highly likely also to be described as ’20 amp’ (the amount of this claim varies from maker to maker).  Most people not unreasonably take this as the recommended current rating. But it’s not.

That ’20 amp’ rating is not a recommendation of the current the cable could/should carry. It is an indication of the highest current that cable can carry before its insulation virtually melts (this rating is often known as ‘ampacity’). It does not take voltage drop in to account. Ampacity is a fire-related thing only.

How to Choose the Right Cable

As noted previously, appliance makers usually specify the minimum-sized cable that may be used. Some supply the actual cable. In all cases, heavier cable must be used if the length of cable is longer than that specified.

But this is of no help to someone wiring a complete vehicle. The rule here is to assess the maximum current that will be carried by each cable run and specify that cable size accordingly.

Tables of cable sizes for various lengths of cable and current carrying capacity are obtainable from cable manufacturers, are also published in books such as my own ‘Motorhome Electrics’. I have included a simplified version in this feature

There is a slight discrepancy in most published conversions (mine are corrected) between the most commonly used AWG gauges and ISO gauges. This discrepancy, due to ‘rounding up’, of the two gauges has resulted in the general use of cables one AWG gauge too small. To be safe, use one AWG gauge size larger (ie. numerically lower than most conversion tables shows. Note the one reproduced here is correct

ISO gauges are easy. The actual gauge number (eg. 2.5, 4.0, 6.0, 10 etc) is the actual cross sectional area, of the conductor, in mm.sq.

It is practicably impossible to give any meaningful conversion between auto cable, AWG or ISO. This is because the insulation thickness of auto cable varies from vendor to vendor. Thus 4 mm auto cable can does have conductor cross-sections from an extraordinary 1.25 sq mm – a more typical 1.8 sq mm. A few makers supply it at 2.0 sq mm.

What can be assumed is that 4 mm auto cable can be used instead of 1.5 sq mm cable; 6 mm auto cable can be used instead of 4 sq mm cable, 8 mm auto cable is almost the same as 8 sq mm. Anything smaller than 3 mm auto cable has so little copper that it’s really only useful for instrument wiring.

Just why automobile cable is sold using this insane and misleading rating is unclear. The sooner it changes the better as it has caused huge problems for countless people for decades.

 A Better Way

As mentioned previously is also very easy to work out the voltage drop for any given length of cable, current flow, and cable size. I always use this rather than published tables (including my own) because once remembered it’s so easy to do – and it gives the right answer every time. The formula is simply:

Voltage drop equals (cable length (in metres) X current (in amps) X 0.017) divided by cable cross-section in mm.sq.

For example: 10 metres X 5 amps X 0.017 = 0.85. Divided by (say) 2.5 (sq mm), the voltage drop is 0.34 volt. This is just acceptable – but (here) 4.0 sq mm would be even better.  That results in a drop of 0.212 volts.  

Cable size tables and the above formula is for the voltage drop across a single conductor path. Where, as is common with trailers, there’s twin conductors (one positive, one negative) the total conductor length must be taken into account. In other words, if there’s a five metre run using twin cable, that’s 10 metres to be taken into account.

The very best cable to use for campervan and motorhome work is so-called ‘tinned’ copper. It’s not actually tinned, but rather copper electroplated with a nickel alloy. It is obtainable from boating electrical equipment suppliers and wholesale electrical suppliers, but is hard to locate otherwise. It’s worth tracking down, not least because it is available in ISO sizes. 

The above is extracted from my previously published articles and books. The theme is greatly expanded in my book ‘Motorhome Electrics – and Caravans Too!’ and this book is being increasingly used as a reference work by auto electricians

Australia-wide. It is also now being used a the basic text in this field by TAFE. It also sells in large numbers overseas – especially in New Zealand and the UK.


Please note that ALL my published writing is fully protected under the Commonwealth Copyright Act. This article and (copyright) Table is reproduced here by express permission from the Copyright Holders – Caravan and Motorhome Books, Broome, WA 6725. It may not be reproduced in any shape or form, or in any media without express permission as above.


Caravan and Motorhome Books

 There is a whole heap of information on various subjects on my website:

Caravan and Motorhome Books by Collyn Rivers

Collyn Rivers

Broome, August 2004.



Cable Conversion Table - ISO
















Auto Cable
















































Note: this table is copyright ‘Solar That Really Works' third edition, 2012.

Caravan and Motorhome Books by Collyn Rivers


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Voltage Drop

by Collyn Rivers


<1 metres

<2 metres


<4 metres

<5 metres

<6 metres

 5 amps

1.0 (16)

1.0 (16)

1.5 (14)

2.5 (12)

2.5 (12)

4.0 (10)

 10 amps

1.0 (16)

2.5 (12)

2.5 (12)

4.0 (10)

6.0 (8)

6.0 (8)

15 amps

1.5 (14)

4.0 (10)

6.0 (8)

6.0 (8)

10 (6)

10 (6)

20 amps

2.5 (12)

4.0 (10)

6.0 (8)

10 (6)

10 (6)

16 (4)

30 amps

4.0 (10)

6.0 (8)

10 (6)

16 (4)

16 (4)

25 (2)

50 amps

6.0 (8)

10 (6)

16 (4)

25 (2)

25 (2)

25 (2)

Cable sizes for 12-volt fridge and other circuits. The sizes, in square millimetres, (and in  brackets AWG) introduce less than 3% voltage drop at the maximum distance in each range. In the above Table, 4 mm   auto cable can replace 1.5 sq mm, and 6.0 mm auto cable can replace any size up to 4.0 sq mm. 

Caravan and Motorhome Books

Caravan and Motorhome Books by Collyn Rivers


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Lead Acid Batteries

by Collyn Rivers

Batteries are like teen-agers. They come in different shapes and sizes, some are prone to sulking, others behave like (flawed?) angels, but by and large their behavior reflects how they are treated.

Starter batteries, and deep cycle batteries work in a basically similar way. Energy is stored within them as a result of an electro-chemical reaction between lead plates and a water/acid mix (called the electrolyte).

Charging is effected by imposing a voltage across the battery that is greater than the voltage ‘within’ that battery. The greater that voltage difference the quicker and deeper the battery will charge. When battery voltage reaches the charging voltage, charging ceases.

A starter battery delivers a very heavy current (up to 400 amps). It does this (hopefully) for only a few seconds. This discharges it by a mere 2%-3% and that small amount of energy is replaced within a minute or two of the engine starting. In practice, starter batteries spend most of their life somewhere between 65% and 70% of full charge. Lead/calcium batteries may be charged a bit higher.

Providing such heavy current requires the lead plates to present a large surface area to the electrolyte. To enable this, starter batteries have a large number of thin plates, but whilst this form of construction allows heavy currents to flow for a few seconds, it will not withstand more than a few extended discharges. Flatten most starter batteries half a dozen times and they are dead

 Deep Cycle Batteries

A deep cycle battery differs from a starter battery in that it is intended to operate consistently over a much large part of its capacity. It cannot however supply remotely as heavy a current, in fact it will be damaged if discharged regularly at greater than 25% or so of its amp/hour capacity.

Despite their name, deep cycle batteries cannot consistently provide remotely the energy that the label on the side may lead you to believe. Without adequate solar or ‘smart’ battery charging a deep cycle battery is unlikely to exceed 70% of nominal charge. So there’s 30% lost up front. Worse, battery makers advise not to discharge below 50%.  As that leaves only 20% for actual use almost everyone ignores the battery makers’ advise and many people routinely run them down until the lights go dim (about 80% discharged).

If treated as above, the best deep cycle battery yet made is good for 80-100 charge/discharge cycles. If run down to about 30% remaining charge they are good for 150-200 cycles. Doing as the makers advise gives you 500-1000 cycles.

Sadly, magazine article after another (and advertisers who should know better) routinely claim a fridge that (for example) draws 5 amps will routinely run for 20 hours on a 100 amp/hour battery. You’ll see pigs flying in formation (whilst singing arias from Tosca) before a 100 amp/hour battery can do that!  The reality is that ‘deep cycle’ batteries would better be described as ‘less-shallow cycle’ batteries.

 How Low Should I Discharge?

Because the amount of air shifted by a cooling fan is proportional to the cube of its rotation, low voltage doesn’t do a great deal for fan-cooled motors, but otherwise running the battery way down is unlikely to do any harm. Most fridges have a voltage-sensing cutout that disconnects the incoming power if it drops (typically) below 11.4 or so volts. This is usually promoted as for protecting the battery, but its main job is to protect the fridge motor from overheating.

Discharge level is essentially a trade-off between convenience and your bank account, but unless you keep the discharge to 65% (i.e. 35% left) you are better off buying the cheaper traction batteries.  These, and true deep cycle batteries are likely to last about as long. But please do not take the above as a recommendation to discharge batteries below 50%. I’m simply explaining what happens if you do.

A far better way is to have a charging setup (which will generally require solar) such that the batteries are routinely charged close to 100% and discharged overnight by a probable 15-20%. This way the batteries stay much of the time around 85%-90% fully charged – and last forever. That’s how my OKA works and its last set of deep cycle batteries lasted over seven years. (Just how to do this and why, is explained in my book ‘Solar That Really Works!’  I have published it in separate ‘Caravan’ and ‘Motorhome’ editions.

 Microwave Ovens - A Trap

Be aware that an ‘800-watt’ microwave oven may draw, via a 12-volt inverter, close to 150 amps. Such heavy current damage will damage any deep cycle battery bank of less than 350-400 amp/hours.

Campervans and small motorhomes commonly have microwave ovens, yet may have a battery of only 150 amp/hr. If you use that microwave away from mains power, a marine battery is a better bet. This is an exceptional recommendation however as a marine battery’s capability, of delivering light starter current yet retaining some deep cycle characteristic, is of no benefit in normal use.

Knowing What’s Left

In a deep cycle battery, especially when cold, the ‘charge’ held on the lead plates massively leads what’s happening in the electrolyte. And vice versa. This is because the electro-chemical reactions are very slow. Because of this, any measurement of voltage taken within 12 hours of charging or discharging is all but meaningless. All you are measuring is the surface voltage of the plates.

Hence, a substantially ‘flat’ battery will present as close to fully charged (>12.6 volts) for a long time after being fast charged for minute or two. And an almost fully charged battery may present as near enough to ‘flat’ (<11.8 volts) after driving a microwave oven for a few minutes.

Despite this any camping site will have people ‘checking’ batteries during the day and making hugely incorrect assumptions on the basis of what they find.  Even doing this first thing in the morning, with absolutely everything having been turned off overnight, may still result in 30% or more error. Further one needs a very good meter to have the accuracy and resolution to measure the critical 0.2 –0.3 volt in 12.7 or so volts.

A hydrometer reading is better – but the battery still needs to have rested. Take note only of the hydrometer’s specific gravity reading, NOT of the coloured bands and indications thereon. These markings usually relate to starter batteries and indicate CHARGED at 1.280 SG or more. A deep cycle battery is close to fully charged at an SG of 1.250 and is likely to be high as it will go by 1.260. Also, as ambient temperatures rise, batteries charge at lower specific gravities and voltages.

Measuring What’s Left

As a very rough guide, a well-rested deep-cycle battery that shows 12.25 volts off-load is about 50% charged. Full charge is likely to be 12.6-12.8 volts but the slightest load may pull that down to 12.55 or lower. Table 1 gives a rough guide, but only after batteries have rested for at least 12-hours.

The only really effective way of knowing remaining charge is to measure what goes in and what comes out and deduct charging losses. What’s left is more or less what you’ve got. It’s actually a bit more complex than that as the rate of charge/discharge affects the result. This was thoroughly explained, via Peukert’s Equation, back in 1897 (email me if you’d like a copy).

Almost every good solar regulator costing over $300 or so shows what comes in and what goes out, but do not necessarily include the Peukert correction (although the Mastervolt unit does).  Stand-alone energy monitors are also available but if you are going to use solar it’s far cheaper to use the functions provided by the more up-market solar regulators.

Sealed Batteries

Traditionally, the term ‘sealed battery’ described gel cells and, in recent years, AGM (Absorbed Glass Mat) batteries.  These batteries are heavier, bulkier and costlier than conventional lead acid batteries. They charge close to 100% via standard alternators and may be discharged deeper with far less self-damage. They thus provide much closer to 100% of their nominal capacity and this compensates substantially for their greater weight, bulk and cost.

In recent times the term ‘sealed’ battery has also become a synonym for ‘low maintenance battery’. These batteries use a small proportion of calcium, and other things, in their plates to reduce gassing and water consumption. That, plus a larger reservoir above the plates, enables them to be permanently sealed. This seems a good ideas for starter batteries (which have an intended life of only two/three years) but I am not yet convinced of their suitability in deep cycle form – where a well-maintained and correctly used conventional battery can last five to seven years – unless you prefer to swap battery longevity for freedom from checking the electrolyte level every eight-twelve weeks.

Charging Batteries

A vehicle charging system is deliberately designed to drastically cut back charging at 70% of full charge. The only really safe ways to approach 100% charge (90% is a realistic target) is via adequate capacity solar modules and solar regulator; or via a ‘smart’ charger – also increasing known as a three-stage charger.

These latter chargers are not cheap – they start at $300 or so – but so far most of their vendors’ seem to have overlooked that a 10-amp smart charger will charge a lead acid battery as fast as, and deeper, than any chain-store 20-amp charger. Smart chargers may be left permanently ‘on’. If this is done the batteries will last many times longer. Thus, despite their cost, smart chargers saves you money in the medium-long term.

Like most things in life , with batteries one tends to get back less than you put in – about 15% of the charge/discharge cycle is lost in heat. This needs to be remembered, but not always is, when installing solar.


Keep the terminals clean externally (a tablespoon of bicarbonate of soda in a bucket of water works like a charm). Once a year disconnect the terminals, clean them until shiny on their contacting surfaces. After reconnecting, coat with Vaseline or battery protection goo.

Check water levels at least every eight–ten weeks. A correctly charging battery should use some water. About one centimetre every ten weeks is normal in temperate climates. If less the batteries are probably being undercharged. If much more, and unless you are in a very hot area, they are possibly being overcharged.

Avoid Christmas trees of cables hung off battery terminals. Instead, install one or more common power posts, and take a single heavy cable from there to the battery terminal.

There’s a huge amount more one can write about batteries (I know this because I already have in Motorhome Electrics!) but the above gives a run down on the more important aspects of their choice, care and feeding.


This article is published by express permission of the copyright holder.

Please note that all of Collyn Rivers’ published writing is protected under the Commonwealth Copyright Act.  No part may be produced in any form without the written permission of the copyright holder (Caravan and Motorhome Books, Broome WA).

 See also my website


Percentage Charge 


SG  (<25 degrees) 

 SG (>25 degrees) 













































          TABLE 1           

Approximate voltages and specific gravities of deep-cycle batteries (rested for at least 12 hours).

Copyright 2003 Caravan and Motorhome Books, Broome WA.


Caravan and Motorhome Books

Caravan and Motorhome Books by Collyn Rivers


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Solar modules can and will only produce the output claimed by their manufacturers if they are used in specific circumstances. Typical trailer/motorhome usage is not one of them.

Outputs vary from type to type, but in typical installations most solar modules produce a bit over 70% of their apparently claimed output. Many modules have a small panel on their rear face that shows what they actually produce. For an '80-watt' module this is usually about 58 watts.

Whilst this may seem misleading it is legally defendable. (The explanation is included later in this piece).

Two Main Types of Module

There are two main types of solar module: for the purpose of the first part of this explanation they may be seen as Uni-Solar and the rest. One type is not inherently better than the other. They simply have different characteristics that cause them to be more effective in some conditions than in others.

Most solar modules produce less power as they heat up - as much as 20% in seriously hot places like the north of Australia, and they start losing this power from only a few degrees above freezing. Uni-Solar uses a different technology that enables the modules to produce slightly more power as temperature rises.

Because the power output of most solar modules falls as they get hot, from around 35 degrees C (95 F) onward, a 64-watt Uni-Solar module produces much the same output as an 80-watt most anything else.  As they are about 15% cheaper in most countries this can sometimes be a considerable benefit

Uni-Solar modules are also less affected by partial shadowing. If most panels are shadowed by an area even as small as a human hand, they lose virtually all of their output. Uni-Solar modules lose only that area shaded.

No solar module will work in complete shade. I need to spell that out because after I stated (elsewhere) that solar modules produce a small output under high-intensity full-spectrum light, this rapidly turned into 'Collyn says they even work under street lights'. They don't.  

The above would appear to be an overwhelming argument for Uni-Solar. This is not necessarily so. Their heat advantage is only really worthwhile above 25 degrees C (77 F) or so. And a big downside for many is that, because they are less efficient, they are much larger (close to twice the area) of most large later. That's why I use Solarex 80-watt modules on my OKA truck, and 28 by 64-watt Uni-Solar modules on my 5 hectares at home.

In practice it is safe to assume that you will get 58 watts from an 80-watt most-anything module, and about 55 watts from a 64-watt Uni-Solar module.

The amount of energy you will capture each day can be readily worked out by taking the true output of the modules (or about 72% of what it says on the marketing brochure) and multiplying that by so-called the Peak Sun Hours typical for where and when you are going. Maps showing this can obtained from meteorological offices, but their's need translating, and (for Australia) from all of my books. Peak Sun Hours are explained below.

For example, a single ‘120-watt’ module is likely to produce about 92 watts in most places. Broome has a typical six peak sun hours a day (in summer) so the output there would be about 552 watt/hrs/day (or about 42.5 amp/hrs/day).

Ideally solar modules should face the sun, but flat roof mounting is surprisingly effective. Whilst modules can be carried loose, they are readily stolen. In my experience it is not worth arranging for tilting or tracking systems in latitudes less than about 27 degrees.  Adding about 20% more module capacity will make up for any loss.

Heavy cloud and rain cuts output by 50% or more. The highest output is typically on bright days with scattered low cloud. Then, sun shines down, is reflected from earth and bounced down again from those clouds.

Because most modules are heat-sensitive it pays to mount them so there's an air space beneath.  Less important with Uni-Solar but air space provides useful heat insulation in the vehicle.

A solar regulator is essential for all but systems where a very small solar module (<20 watts) float charges a battery bank of at least 250 amp/hr. Even then I’d use one myself.

Technical Stuff

Solar modules are tested using 'Standard Operating Conditions.'

The output of solar modules is obtained by plotting curves of voltage and current and from these using whatever combination of those two parameters gives the highest 'number'. In practice this tends to between 17.0 - 18 volts. Thus a module that produces 4.7 amps at 17 volts is rated at 80 watts.

Power being P = IV, that module will produce 56.4 watts at 12 volts - and 65.8 watts at 14 volts.

In other words module output is a function of the voltage developed across the load.

The industry's 'Standard Operating Conditions' (SOC) measures output at a cell (not ambient) temperature of 25 degrees C at an irradiance of 1 kW sq.m. As this equates to an ambient temperature close to 5 degrees C, the SOC may better be regarded as 'Standard Test Conditions'.

Manufacturers do however also quote a separate NOCT (Nominal Operating Condition Temperature).

This gives an indication of the actual cell temperature at 20 degrees C ambient, but at 80% of the irradiance of SOC operating conditions, a wind speed of 1 m.s, and the back of the module enclosure open to atmosphere. Under these conditions the NOCT is typically 47-49 degrees C. temperature - or looking at it another way, the cell is likely to be 25-30 degrees C hotter than ambient temperature.

Under NOCT  a typical 80-watt module produces about 59 watts.

Mono- and poly-crystalline modules lose output at a rate of approximately 0.4%-0.5% per degree C above about 5 degrees C (voltage drops considerably, current rises slightly). Thus at 30 degrees C ambient, output of modules using that technology is likely to be 15% down (ie. over and above the loss due to working at 12-14 volts).

Amorphous technology modules tend to be increase output very slightly with rising temperature.

The current output of modules is usually shown in the technical data. It may be shown as ISC (short circuit current), or as operating current. The latter is the figure to use. True output wattage is the operating current times the operating voltage. It must then be corrected for temperature.


 Standard Operating Conditions


 Pmax (maximum power)

 81.4 W.

 Voc (voltage open circuit)

212 V.

 Isc (current short circuit)

5.25 A.

 VPmax (voltage max power)

16.60 V.

 Ipmax (current max power)

4.9 A.

 Normal Operating Conditions


 Pmax (maximum power)

59.0 W.

 Ipmax (current max power)

4.0 A.

 This is actual data from the rear panel of an 80-watt module.

 Whilst it is rare for modules to produce their full rated output (Pmax) it is possible to achieve it (or very close to it) when driving a load (such as a water pump) that will operate at the voltage at which peak output was tested. It can also be achieved by using multi-point tracking systems that, in effect act as 'electrical torque converters'. They 'swap volts for amps' (actually a dc/dc convertion).  So whilst people tend to be confused by the rating system, it's both technically credible and legal.

The term Peak Sun Hours (PSH) is not my invention! It's used extensively in the photo-voltaic industry but does not seem widely known by engineers in other disciplines. In effect it's the number of hours of midday sun on a clear day equivalent to the irradiation for that day. One Peak Sun Hour equals 1 kilowatt/hour/square metre.

Full details of what can be run from solar, required module and battery capacity, installation (probably rather more than you wanted to know!) are in my books - and in brief on my website,


This article is copyright Caravan and Motorhome Books, Broome 2004.


It is reproduced by express permission.

 Caravan and Motorhome Books


All information has been reproduced with the kind permission of Collyn Rivers