Battery cabling

Cupie · Guru

Battery cabling

Aus-Kiwi · Guru

Battery cabling as in battery to inverter where there's a lot of load need to be very large . The lower the voltage the larger the cables need to be ., there's a reason transmission mains into cities are 330,000 volts . DC is often used for low load devices like speakers etc . Voltage drop is critical between charger and regulator due to false reading due to voltage drop . Why SOME. Regulators have seperate wire back from battery . So it reads exactly what it is ! It also depends where the cable is being used ? Being text we can't read minds ., look at your car battery . The cable to starter motor is large , same on larger systems with two batteries with cable between them . Notice cable is generally large of say 25mm and larger while wire etc is 1mm and up . 240v AC should be double insulated . The outer cover is mechanical protection to be extra safe . DC light wire in generally Multy strand to be flexable . Don't go by insulation size . It's the core / size of wire which is important . Amps are VERY high when volts are low as in DC . ( Why big trucks use 24v systems . ) Always directly proportional. The maths doesn't change . If you have to choose between wire size ? "Always" pick the larger !! Btw. 240v or AC should only be carried out by qualified tech . Why there's such a discussion on these posts . Plus there's so many different situations ., AC wiring 1.5 to 2.5mm is too light for fridges etc on 12v DC for example on 3 way fridge . Depending on size ? Ok on compressor fridge . See where things get out of whack . Plus the **** fight if fridge or appliance is upgraded ? Wire size measurement is different too .

-- Edited by Aus-Kiwi on Friday 1st of July 2016 12:06:29 PM

Peter_n_Margaret · Guru

Voltage drop in electrical cables is a function of the amps carried, the cross sectional area of the copper and the length.
The length is the total length of cable and includes both +ve and -ve.

VOTLAGE DROP = [cable length (in metres) X current (in amps) X 0.0164] divided by cable cross-section in mm.sq.

Cheers,
Peter

oldtrack123 · Guru

Cupie wrote:
I often come across the very regular discussions on the correct size of cabling for caravan battery feeds.
I usually wonder why no one mentions the use of 240v power cables. Perhaps this may be a cost effective option.
When one of my mates was wiring his set up I offered a length of very heavy power cabling that I picked up on a mine site that I was was working at. It was probably twice the size of household powerpoint cabling.
He was delighted & has been using it for many years.
Not sure if he stripped off the outer grey insulation. The conductors were multi stranded & somewhat flexible.

HI

Electric cables used in vehicles including vans should flexible multi strand to cope with vibration & movement That is why 12V auto cable has so many relatively fine strands

Normal 240V cable used in fixed wiring houses /industry etc does not require such flexibility & for the same current capacity has fewer but thicker copper strands

If the cable copper conductor dia was only around twice that of standard house power circuit cable ,it is far too light for quick & full van battery charging due to excessive voltage drop.

I do hope it was FUSED at both batteries with a fuse size suitable for it's current carrying capacity It could easily over heat sufficiently to melt the insulation & cause a fire

Cupie · Guru

oldtrack123 wrote:
Cupie wrote:
I often come across the very regular discussions on the correct size of cabling for caravan battery feeds.
I usually wonder why no one mentions the use of 240v power cables. Perhaps this may be a cost effective option.
When one of my mates was wiring his set up I offered a length of very heavy power cabling that I picked up on a mine site that I was was working at. It was probably twice the size of household powerpoint cabling.
He was delighted & has been using it for many years.
Not sure if he stripped off the outer grey insulation. The conductors were multi stranded & somewhat flexible.

HI
Electric cables used in vehicles including vans should flexible multi strand to cope with vibration & movement That is why 12V auto cable has so many relatively fine strands
Normal 240V cable used in fixed wiring houses /industry etc does not require such flexibility & for the same current capacity has fewer but thicker copper strands
If the cable copper conductor dia was only around twice that of standard house power circuit cable ,it is far too light for quick & full van battery charging due to excessive voltage drop.
I do hope it was FUSED at both batteries with a fuse size suitable for it's current carrying capacity It could easily over heat sufficiently to melt the insulation & cause a fire

Thanks all for the replies ..

I reckon that the multistrand flexibility aspect is spot on.

I would like to see comparative figures on resistances of the 240v power cable & the recommended stuff for van battery charging. Had a quick look on the net but couldn't find any info. As it is only an academic exercise I didn't persevere.

The installation was fused. I noticed a large spoofy battery distribution block in use with a series of circuit breakers.

oldtrack123 · Guru

Hi
The resistance of either will be entirely dependent on the cable's cross sectional area of copper
Tables are available which gives resistance per metre or this is one of many formula can be used to calculate voltage

Continuous Current rating is dependent on cable cross sectional area, insulation grade, installation methods[ open air or enclosed] ambient temp.

VD=L [total length of both pos & neg] x AMPS [to be carried] x0.017 / cross sectional area of copper conductor [in mm2]

-- Edited by oldtrack123 on Monday 4th of July 2016 02:58:47 PM

Hylife · Guru

All conductors (wire) have a measurable resistance per distance.

Like a length of water pipe, the smaller the pipe the greater the effects of reduced flow over a longer distance.
So the bigger the wire diameter, the less the resistance.

Different metals have differing resistance but many are too brittle for automotive use. eg, high tension wires on pylons are made of aluminium, not copper.

The sum of resistors in parallel is the value of the individual resistors divided by the nunber of resistors.
So, the more wires (strands) the less resistance.

Voltage has a relationship to current and resistance known as Ohms Law.

Ohms Law says: V = I/R
Or, Voltage = Current divided by resistance.

Your voltage input is usually fixed by your alternator/tug battery.
Your resistance will be fixed based on your chosen wire size.
Your current may be fixed for powering a device, but for charging it will increase or derease depending on the batteries state of charge.

Therefore, depending on the voltage fed into one end, your voltage out the other end will vary based the current you try to pull out.
The higher the current draw the greater the effects of resistance.
The lower the input voltage, the greater the effects of resistance.

So, bigger wire (with more strands is better than less or a single strand) means less resistance and therefore less voltage drop over a long run for any given voltage and current draw.

The method used for mains electricity transmission over long distances with minimal losses is to transform UP your voltage to a Very High Voltage, send that over a long run, and then transform it down to a usable voltage at the other end. Those big boxes on the poles that hum are transformers doing just that task.

A 100 volt drop over 50 kilometers means a lot at 230 volts but next to nothing at 300,000 volts.
Trucks use 24 volt systems specifically because of their need for long wire runs rather than 12 volts like cars.

Aus-Kiwi · Guru

HV . Transmission lines are alloy with stainless steel cable core . 11kv can be very thin 16mm hard drawn copper 7/16 or 7/14 in old imperial size ..Yes volts and amps are always "proportional"., The higher the volts the lower the amps .. LV wire should be saddled to prevent movement . Generally 7 strand or single core 2.5mm & under .. 7 strand to 16mm ..

PeterD · Guru

Or to put it another way, there is more current in the style of cable we talk about than in house wiring cable.

House style cable looks big but that is just an optical illusion. You see the big fat cable but most of the diameter is insulation and not the copper. The insulation around house cable is required because the voltages are greater. The insulation is also doubled so that if there is movement in the framing the abrasion of the cable will only go through the first layer of insulation. Lower voltage cable get away with much thinner insulation. The smaller looking size does not mean less copper in them.

The smaller size strands in the cable has been well covered above.

Aus-Kiwi · Guru

When I say wire size as in Xmm it's the actual copper size . Coming from domestic elect . If your wiring another battery to battery cable ? Not too sure what to use ? Just get welding cable wire . It will take a direct short and flatten both batteries in seconds . Not that I recommend testing it . Yes the SDI ( single double insulated) cable outside is mainly for mechanical, UV protection.. Twin and earth is the same in building or 240 / 415 LV usage .. Welding cable . 1 AWG is roughly just over 50mm square . Which is what we use as temp bonding on power pole etc as temp bonding. Up to 400 amp or so protection fuse . On 12v it's the spot on .

-- Edited by Aus-Kiwi on Tuesday 5th of July 2016 03:38:48 PM