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sizing problem? Voltage drop?
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Could you have a wiring/cabling problem, well here in hopefully reasonably simple terms that most of us can understand, I try and explain maybe why! We won't get bogged down in technical explanations here, just some crucial things that you really need to know, so just a few sort minutes reading here, could save you from making some big mistakes and long-term annoyances. With wiring and cable it is critically important that your electrical devices receive all the electrical current that they need, and most importantly, at the correct, designed and needed voltage, for each appliance/device. Now herein lies what I believe is the biggest problem that we have in the automotive electrical world -- voltage drop!! Specially when it come to recreational vehicles and their electrical systems. This voltage drop is so often misunderstood, and is often the main cause of poor or faulty operation of 12V electrical devices. Wire/cable, I'll just call it 'cable' for the rest of this article OK. In my opinion without doubt the biggest misunderstanding is cable size and it's current (amps) carrying capacity, and this not helped by the very misleading and just plain wrong amp rating printed on most cable packaging. It really frustrates me so much when I see that people have been misled over cable amperage ratings. The 'current' rating listed on the packaging of most cable is only a 'fire safety rating', and relates only to the acceptable temperature rise at the specified current that the cable can carry before the insulation becomes unsafe, as in that it may melt, catch fire, or allow a dead short! It has absolutely nothing to do with how much current the cable can carry efficiently, and at the desired voltage, nothing at all! Some very important simplified facts.... Copper is expensive, therefore of course good sized cable is expensive, however under sizing the cable can be life threatening, both physically and mentally! So it is best to get it right the first time so you only pay once, and then have trouble free experiences. Big tip when it comes to the size of cable, batteries, and solar, bigger is always better! In every case, to carry the required current at the 'correct' voltage over a distance, you need to carefully calculate the size of the cable needed, this means the size of the actual copper core that carries all those busy little electrons, and you will also need to know the total length of the cable, and please do not forget to include in the calculation the negative return run back to the battery (rooky mistake). There is a link further down below to my favourite cable sizing calculator sheet, and to a voltage drop calculator. Also a big trap for the unknowing when buying cable is it's real size, the most common terminology is in millimetres, however this fraught with danger, because the most useful measurement to us is the cables true copper core size, expressed in square millimetres, it is the ONLY true measure of cable size that we need to or should use. For instance in Australia, a common rating is like say 4mm Auto cable, and it's often claimed to be 15 amp cable, this is total BS, this cable is really only 1.84mm² (square millimetres), and while it may carry 15A over short distances with no problems, if you were to try it over say 10 meters, now it will not, and can not even run a 5A draw without a large voltage drop, and this will often see the likes of portable fridges just give up and shut down with too low a voltage! By the way, to have 15A run at an acceptable voltage over 10 meters you really need cable with a copper core of 8.4mm², not 1.84mm²! Now when testing with your multimeter for voltage drop on a wire/cable, checking the voltage at one end of the cable, and then the other end is often misleading, and a huge mistake that way too many people make, you see the cable must be under the full load (current draw) that is required, to get an accurate voltage drop measurement! Testing without this full load should, and will, show the same voltage at both ends unless there is some other problem, like a crook join or crimp, or a broken wire/cable. This electrical resistance over distance is the major problem, a very basic explanation would be, that the larger the current draw, the larger the cable needs to be, and the further the distance the cable needs to run, the increasingly larger again the cable needs to be, hence resistance over distance is the problem we need to take into account and make allowances for. Please note, that no matter what size cable that you do use, while under load you can expect that there will always be some voltage drop, that is unavoidable, however we must size the cable so that under maximum load the voltage does not drop below an acceptable level for the particular appliance in use, usually up to 0.5V is acceptable, sometimes a little more, and sometimes a little less, but try and keep it to a minimum. When calculating what size cable you need, you will need to know what will be the maximum amps the cable needs to carry, and the total length of the cable. Now don't forget to include the return negative cable run too in your calculations (rookie mistake), because it is the whole circuit that has to carry the same current, all the way from the battery positive to the device and back to the battery negative, so don't get caught out in this rookie trap here. Some common Australian cable/wire sizes, these are approximate and can vary slightly from brand to brand “Auto Cable” (smaller wire) 3mm (really 1.13mm²), 4mm (really 1.84mm²), 5mm (really 2.90mm²), 6mm (really 4.59mm²) Of the heavier cable, the American's use AWG (American Wire Gauge), however we here in Australia use an identical gauge but call it B&S (stands for Brown & Sharp Wire Gauge) although some think it stands for Battery & Starter! Anyway as I said this is much heaver cable, and in this case the smaller the number, the larger the cable size! 8 B&S = 8.4mm², 6 B&S = 13.3mm², 5 B&S = 16.0mm², 4 B&S = 21.2mm², 2 B&S = 33.6mm², 1B&S = 42.4mm², 0 B&S = 53.5mm², 00 B&S = 67.5mm², 000 B&S = 84.9mm². I hope this article has helped folks from making expensive and frustrating mistakes, the old adages are very true here, if do it right the first time, you only have to pay a bit extra just the once! You can find a Cable Sizing Selection Chart >>here<< to look at or download And you can find a Voltage Drop Calculator >>here<< Here is a very very useful free ebook from our friends at Victron. Welcome to ‘Wiring unlimited’, a book about electrical wiring of systems containing batteries, inverters, chargers and inverter/chargers. Download >>here<< Article by Val Rigoli ©, free advice freely given, my personal thoughts and advice gathered from in-excess of 45 years of practical hands-on experience, learned skills, and industry knowledge. Please share this page
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