View Full Version : Cable Size
Pirate Pete
03-02-2010, 16:53
Hi all out there.
Last night on the chat
http://www.polyboatowners.com/chat.htm
We got onto the subject on what size cable should you run, in this case from the Main Cranking Battery to a VSR Relay & then to 2 AUX Batteries for a 24V electric. About 5m away.
The VSR charges 1 Aux Battery for 1 min & then the other once the Main is charged. (13.7 V according to the web site )
http://www.bepmarine.com/Trolling-Bank-VSR-Module-166-1471.html
The motor used on this boat is a Suzuki 80HP 4 St with an Alternator output of 27A.
http://www.hainessuzukimarine.com.au/content.aspx?pageID=24&mainID=3
I dont know the size of the Main Cranking battery but mine is a Century N70ZN with a CCA of 600A.
From what I can find the recommended charge voltage for valve-regulated-lead-acid batteries (VRLA) are typically charged to between 2.26 and 2.36V/cell. So a 6 cell 12 volt battery should be charged at 13.56 to 14.16 Volts.
Im a spaarkie by trade & all my paper work & calcs are based on AC & 230 / 400V Cable.
I had a quick look on the net last night & found this site.
http://www.rpc.com.au/products/efn/efn133.pdf
That gives a chart to use to determine cable size allowing for a 5% voltage drop. ( 5% of 14.16v is 0.708v ) If I use this chart I will get 13.45V to the VSR so I would need to use a larger size than what is recommended.
http://i233.photobucket.com/albums/ee312/Polyboatowner/12VoltCableSize.jpg
The chart allows for the out & back current path so it is the distance the cable has to run ie. 5m not cable length 5 out & 5 back.
I hope that so far I have made sense.
Now here comes the part that Im a bit scratchy on.
HOW MUCH CURRENT does the AUX Battery Draw to charge ??????
Is it 27A the max output from the Alternator or some what higher because the Main Battry will also be trying to put charge into the AUX. ( The VSR’s wiring diagram wants 70A Fuses )
If I say its 27A the alternator can put out then looking at the chart 30A (next closest to 27A) needs a 13.6mm Cable to run that current that distance. ( again here is where Im in trouble as in Australia the closest to that is 16mm )
Most of the people on the site seemed to think that this was way too big a cable & thought 6mm was heaps big enough.
If I say its 70A the max current set by the fuse. ( This is what my training tells me, the fuse is there to protect the cable & stop FIRES ) then I need to look at the chart at 80A (next closest to 70A ) then you would need to run a 25.7mm cable ( again here is where Im in trouble as in Australia the closest to that is 25mm )
So what is the right cable to use???
Say again all after "Hi"
Slowly this time! Remember I'm a gasman not a sparkie.
And do they make cables for ordinary mortals to use that are 25mm (that's an inch in real numbers)? Couple of metres of that and the boat'll sink!
Pirate Pete
03-02-2010, 17:47
And do they make cables for ordinary mortals to use that are 25mm (that's an inch in real numbers)? Couple of metres of that and the boat'll sink!
That is the cross sectional area Doug not the width. So the a 25mm cable will be about 13mm across including the insulation.
Pete
With the Tri, the crank battery was about 8 foot from the motor. A 28hp diesel.
The trouble I had was the earth was too thin to carry the current required.
The cable I purchased and was recommended by the auto sparkie and Battery world was about the same diameter as a jumper lead cable (i dont have a set so I cant measure it.)
If you are only charging a battery through a VSR, I cant see you needing anything larger than that as the drawer is not that large. The run to the leccy is not going to be long from there.
Pirate Pete
03-02-2010, 18:53
Most of the people on the site seemed to think that this was way too big a cable & thought 6mm was heaps big enough.
If I say its 70A the max current set by the fuse. ( This is what my training tells me, the fuse is there to protect the cable & stop FIRES )
All my training says the fuse is there to protect the cable NOT the appliance attached to it.
If you have a 70A fuse as per the instructions then you must have a cable capable or taking 70A 100% of the time.
For what ever reason you draw more than 70A ( a short etc. ) then the cable will have to carry more than 70A for a period of time so the fuse element can heat up & melt.
If the cable is too small it wont be capable of carrying 70A & it will heat up quite possibly catch fire.
Dont forget this cable could be running past you petrol tank that is now days likely to be plastic just like the rest of the poly boat you are in.
The thing I dont have is the current carrying capacity & volt drop tables for 12 DC operation so I dont know if these calcs are correct.
If we can get these details correct then we can put up the relevant info in the "Resources Section" so Im hoping some one on the site has the info for the rest of us.
I would love to answer this question, but I am on my second bottle of white. I can calculate the sizes of wire required. Need length, maximum allowable voltage drop. Then there is the tricky thing about temperature. As copper heats up it resists current, so there is derating for temp as well.
Pete is kinda correct about the fuse thing. A fuses primary purpose in life is to prevent fire, thats it. I have a chart plotter here at the moment that someone asked me to repair. It had no fuse inline, one of the surge protection devices shorted out and there was a little fire in the sounder! Lucky the copper tracks in the sounder acted as a fuse.
Anyhow, I will reread this psot tomorrow and see if I answered some questions or just made things worse.
"Pete is kinda correct about the fuse thing"
Note to self - don't get Pete to rewire my house.piratesign :D
Pete is 100% correct, load must be less than the rating of the fuse which must also be smaller than the current carrying capacity of the cable. Pete i would not and have never used less than 16mm2 cable when linking batteries, which is basically happening when your voltage sensitive relay is energised. I say this because if you have a big stint on the electric and pull significant charge out of the aux batteries when you do start motoring again, and your main battery reaches the required voltage and energises the vsr you will have a large potential difference between the batteries drawing a high current.
sorry for draging on but i cant really shorten it down. :)
Ultimately your outboard never charges your aux batteries your main battery does, though your outboard charges your main battery. And that is why i think 16 or 25mm2 cable is not a bad idea(higher ccc, less voltage drop and lower conductor temp).
I have set up a few dual and tri battery systems in 4x4's with larger alternators (70-110Amp) i am unsure how this set up will work for you with only a 27Amp alternater i think it might be a little ambitious, but i really hope that i am wrong.
hope that helped even the slightest, cheers
Ok, I will try and explain this the bast I can. I will do it over multiple posts in an attempt to help people understand a bit better. Once everyone gets it, I will move on.
The amount of current your battery will draw from any systems depends on a few things, but the primary reasons are:
1) State of charge of the battery.
2) Charging Voltage.
If you hook a fully charged battery to a flat battery, they will tend to equalize, that is the charged battery will lose some charge, and the flat battery will gain some charge. A fully charged battery will not charge a fully flat battery. The reason for this is that the fully charged battery does not have a high enough voltage, and hence cannot supply enough current to charge the flat battery.
If we increase the voltage, then the flat battery will draw more current. This is basic ohms law. If for the exercise we pretend that a battery at one give state of charge is a fixed resistance we can prove this theory. Pretend the battery is exactly 1ohm when its flat. Ohms law states current = voltage / resistance.
12V / 1ohm = 12 amps
Increase charge voltage from 12V to 13.8:
13.8 / 1ohm = 13.8 amps
This is why it is so important to factor in cable size when charging batteries. If the cable is to small, you get a large voltage drop, and not enough voltage charging your batteries.
Pirate Pete
04-02-2010, 10:43
Thanks Andy & McGyver
This is the info we all need. As I said Im not up on my DC Theory & therefore dont know the resistance of a flat battery etc.
My Cranking Battery says that it has a CCA of 600A
The definition of the Cold-Cranking Amperage (CCA) of an automotive battery is the amount of current a given battery can deliver for 30 seconds at zero (0) degrees F without dropping below a specified cutoff voltage (manufacturer-specific, but usually 10.5 volts).
This can flow into the flat Aux battery for a split second when the relay switches over. You may have seen this type of thing when hooking up jumper leads. It takes a lot of current to make a spark that big with only 12V. You may have also seen the cables quiver if they are laid side by side if they are separate cables. This is why you should never put the earth lead on the battery but on a good earth on the block away from the HYDROGEN created by flattening the battery in the first place.
Keep up the good work guys. We all need to know this info so we dont kill ourselves.
fishmagnet
04-02-2010, 11:23
When I saw the title of this thread I thought maybe the Mrs ha sbeen bragging about me again.
Brucesta
04-02-2010, 11:24
i'd be looking at using 25mm2 cable for any battery links to a Leccy or a dual battery system for that initial current draw on a well used battery and for starting purposes. It's all been explained in the posts above. i'd forget about the CCA of the battery especially being a aux battery as you are not starting your car which needs the grunt, boat motors need a lot less CCA than a car, never forget chucking the 4WD battery on to my mates boat, geez she spun that alternator quickly!!
Andy, thanks for pulling me up there about the main battery charging the aux battery my wording was terrible. You are right its not charging the aux batteries its just a transfer of charge/current from main to aux, which then is replenished by the alternator charging the main battery.
cheers
OK, it's dumb question time.:rolleyes:
If I hook my auxiliary battery up to a charger overnight in my garage etc, do I have to disconnect it from the VSR (i.e. am I likely to destroy the VSR by accidentally applying a higher voltage than it is normally expecting on the "outlet" side)?
Up until now, I have always disconnected the VSR when charging my aux battery overnight but I am considering wiring in an Anderson plug to permit quick and easy hook up of the charger to my aux battery during those quick turnarounds (off the water at last light or after and back on before first light) the next morning; probably between 10PM and 4:00AM down here in Tassie in mid-summer). I am just a bit concerned that I might damage the VSR or even confuse my charger (it's a CTEK 25000) if it tries to charge two batteries.
Hopefully, the VSR will allow me to only charge the aux battery in this situation but I'm just wondering whether someone much more knowledgable is able to confirm this.
OK, it's dumb question time.:rolleyes:
If I hook my auxiliary battery up to a charger overnight in my garage etc, do I have to disconnect it from the VSR (i.e. am I likely to destroy the VSR by accidentally applying a higher voltage than it is normally expecting on the "outlet" side)?
Up until now, I have always disconnected the VSR when charging my aux battery overnight but I am considering wiring in an Anderson plug to permit quick and easy hook up of the charger to my aux battery during those quick turnarounds (off the water at last light or after and back on before first light) the next morning; probably between 10PM and 4:00AM down here in Tassie in mid-summer). I am just a bit concerned that I might damage the VSR or even confuse my charger (it's a CTEK 25000) if it tries to charge two batteries.
Hopefully, the VSR will allow me to only charge the aux battery in this situation but I'm just wondering whether someone much more knowledgable is able to confirm this.
Lesson 2 :) VSR...
When charging a battery, lets say you supply it with 15V. If its flat, it will draw a lot of current, this typically pulls the voltage that you read across the battery down close to what the battery was BEFORE you conencted the charger.
So if you battery reads 11V before you connect the charger, then chances are it will draw a lot of current, and pull the charger voltage down to 11V also.
A VSR works by sensing this voltage. When the battery is charged, the voltage rises. A VSR might wait until the voltage rises to about 13V before it switches on, allowing the second battery to begin charging...
If you connect your charger to the AUX battery you will not damage the VSR. Most likey the VSR will be switched off, because you are not charging your main battery. This means the aux battery will be isolated from the main battery.
Thanks for that, Andy. I will go ahead and run a lead from my battery to an Anderson plug mounted somewhere quick and easy to plug into for overnight charging. Will also need to make up some sort of connection for my charger with and Anderson plug on it but once done, it will make life a lot easier.
Anderson plugs are a great invention. I have a lead coming from my dual battery switch to someplace that doesn't require you to disassemble the boat. I can plug in a battery charger or a little solar panel with the greatest of ease.
SuperCheapAuto and BCF carry them (at inflated prices)
My battery charger has two sets of leads - one with alligator clips and the other with eyes to be screwed onto the battery terminals. I cut them off and put an Anderson plug on instead.
A little bit of initial effort and I save myself the frustration of accessing the batteries waaaaaaaaaaaaaaay under the transom. Now I just have to remember to top them up. The next ones are going to be sealed, no maintenance ones.
i'd be looking at using 25mm2 cable for any battery links to a Leccy or a dual battery system for that initial current draw on a well used battery and for starting purposes. It's all been explained in the posts above. i'd forget about the CCA of the battery especially being a aux battery as you are not starting your car which needs the grunt, boat motors need a lot less CCA than a car, never forget chucking the 4WD battery on to my mates boat, geez she spun that alternator quickly!!
Now that I'm ready to install a permanent cable to permit easy charging of my aux battery, I have been doing some calculations. Given that cable diameter means more to me than the cross-sectional area of the cable I worked backwards from the 25mm2 cross-sectional area.
I don't think the formula for calculating the area of a circle has changed much in the last 45 or so years since I learned it in high school, I used that to work backwards.
Area of a circle = pi x (radius)^2
If the area = 25mm2, the radius is only about 2.8mm, making the diameter just under 6mm (not allowing for the insulation).
That's impressive stuff for an old phart, Barry. Have you got the numbers for next week's PowerBall?
I am sorry Doug ,but my Stones Green Ginger wine inspired mind tells me that, 1 Inch is actually 25.4mm.laugh1laugh1laugh1 get it right man.
Probably will be 6 numbers between 1 and 45 Giffo.
If you could narrow it down a tad to say 6 numerals I would be very appreciative !.It is bloody pissing down here Barry, and I want to go fishing as I am on holidays.How is the weather in Tassie ?
Too damn good today. I was in the Highlands this morning and clear blue sky there was not a breath of wind; far too calm to go "sharking" for trout on Great Lake ("sharking" involves actually sight-fishing to trout by spotting them in the face of a wave) and far too crowded to go fishing in an area known as the Nineteen Lagoons (because tomorrow is a public holiday in southern Tassie, every man and his dog was in the Highlands).
Pirate Pete
07-02-2010, 22:06
Now that I'm ready to install a permanent cable to permit easy charging of my aux battery, I have been doing some calculations. Given that cable diameter means more to me than the cross-sectional area of the cable I worked backwards from the 25mm2 cross-sectional area.
I don't think the formula for calculating the area of a circle has changed much in the last 45 or so years since I learned it in high school, I used that to work backwards.
Area of a circle = pi x (radius)^2
If the area = 25mm2, the radius is only about 2.8mm, making the diameter just under 6mm (not allowing for the insulation).
That is mathematically correct Barry but cable is made up of round wires & your calc hasnt allowed for the little gaps between each wire. Im back at work Monday so I will get my little book out & tell you the right measurements. :):)
Is it just wire size for charging that are being considered, or will you be running loads through the same cable, ie cranking from the aux battery?
How long are the cable runs?
If you are doing nothing but charging, then the cables will not have to be that big.
The main values that should be considered when calculating wire size is:
1) The resistance of the wire in question. this can only be had from the manufacturer, however chances are the grades of copper they use will not vary to much so you can work on standard resistance of the supplied cross sectional area of the copper. The total resistance of the wire depends on the actual lenght of the cable.
2) The current being drawn through the wires.
3) The maximum allowable voltage drop that is acceptable in the system.
4) for correctness I will include temperature, however I would leave this right out of the equation.
It will be just for charging Andy and the cable run will be no more than 1.5m. I was giving it some thought overnight and I won't need to go any heavier than the output cables on my charger (nor will I go any lighter, of course).
Brucesta
08-02-2010, 09:59
might be able to get away with 6mm twin sheath auto cable or slightly bigger, there isn't going to be a massive inrush like when you start your boat.
Pirate Pete
08-02-2010, 17:50
Is it just wire size for charging that are being considered, or will you be running loads through the same cable, ie cranking from the aux battery?
How long are the cable runs?
If you are doing nothing but charging, then the cables will not have to be that big.
The main values that should be considered when calculating wire size is:
1) The resistance of the wire in question. this can only be had from the manufacturer, however chances are the grades of copper they use will not vary to much so you can work on standard resistance of the supplied cross sectional area of the copper. The total resistance of the wire depends on the actual lenght of the cable.
2) The current being drawn through the wires.
3) The maximum allowable voltage drop that is acceptable in the system.
4) for correctness I will include temperature, however I would leave this right out of the equation.
I set up the circuit we were talking about in the first post Andy. Read that & check the pdf file for the VSR & note that it says to use a 70A fuse. There for any cable that is used must be capable of carrying 70A
Based on my reading of the chart, something with 6mm copper diameter should be more than adequate.
I set up the circuit we were talking about in the first post Andy. Read that & check the pdf file for the VSR & note that it says to use a 70A fuse. There for any cable that is used must be capable of carrying 70A
Yes, but the alternator can only supply 27amps, so a 70amp fues is like using a nail in the fuse holder of your sounder... I would be stepping down to 30Amp fuses.
With a 100-120ah battery, you probably dont want it to be drawing more than about 25-30amps on charge anyway. If the flat battery is hooked directly to a charged battery i doubt you would see this much current either.
For the record, I have a second battery in my 4wd, and its hookd via a VSR to the main battery. I have 40amp fuses at each end of the cable. My alternator outputs 136Amps at 5000rpm. I have never had a blown fuse.
Pirate Pete
08-02-2010, 18:57
Yes, but the alternator can only supply 27amps, so a 70amp fues is like using a nail in the fuse holder of your sounder... I would be stepping down to 30Amp fuses.
With a 100-120ah battery, you probably dont want it to be drawing more than about 25-30amps on charge anyway. If the flat battery is hooked directly to a charged battery i doubt you would see this much current either.
For the record, I have a second battery in my 4wd, and its hookd via a VSR to the main battery. I have 40amp fuses at each end of the cable. My alternator outputs 136Amps at 5000rpm. I have never had a blown fuse.
This is the type of info we need Andy. 2thumbsup 2thumbsup
So the fact the main Battery is fully charged its really only the alternator that charges the aux battery not the main in conjunction with the alternator. So going by our chart we would need to run the "13.6mm" cable or in reality a 16mm cable with some 30A fuses.
This is the type of info we need Andy. 2thumbsup 2thumbsup
So the fact the main Battery is fully charged its really only the alternator that charges the aux battery not the main in conjunction with the alternator. So going by our chart we would need to run the "13.6mm" cable or in reality a 16mm cable with some 30A fuses.
They charge will come from the alternator. If you get a dead flat battery or a battery with a short then it may draw more current, but in that situation a blown fuse may not be a bad thing. I regularly run the aux battery flat in the car, much lower than the manufacurers reccomend but I still have not had an issue.
I am not familiar with auto wiring sizes, I used to buy pretty specific stuff direct from the manufactures. I would reckon 16mm2 is still probably overkill, but if thats all you can get then thats what you would need!
Been a while since I have done the maths but I will put out my books later and see if I have still got what it takes!!
Pirate Pete
08-02-2010, 21:57
Im still looking round & there are some interesting comments on this site.
http://forum.candm.com.au/showthread.php?t=2692
and another here with some calcs
http://www.caravanandmotorhomebooks.com/articles/wiring_woes.htm
& another.
http://redarc.com.au/knowledge-base/how-to.../calculate-voltage-drop
& another
http://www.fridge-and-solar.net/dual_bat.htm
Pirate Pete
08-02-2010, 22:37
Reading the above sites there seems to be a big difference in cable sizes.
We 230V sparkies use the mm2 sizing ie.
1, 1.5, 2.5, 4, 6, 10, 16, 25 etc.
Then there is the AWG size which is the American Wire Gauge & the bigger the number the smaller it is. ( This is where the 13.6mm2 in the chart comes from )
Both of the above are the actual copper size.
Then there is an auto cable its size is measured with the insulation as well. This means the actual copper can be less than 1/2 size stated, this will make a big difference to your actual power at your device!!!
The other convenient aspect about hooking up the VSR (particular for my 24v configuration) is that even whenI want to use a 240v charger at home, I need only hook it to my primary cranking battery, and the rest of the circuit will work as if I had the engine/alternator running. This is a lot simpler than having to disconnect the in-series circuit and then hook the charger to both batteries up front.
Best advice I can offer is to get hold of your battery manufacturers sales catalog, they contain both text and graphical information of how a given capacity battery performs under different charging regimes and will state maximum voltages and currents (if fixed current charging is utilised). Part of my background has been pre deployment testing sealed lead acid batteries for telemetry systems for the last 2 decades and I am still learning especially with the new chemistry that is being used to manufacture these devices.
$$ expended, depth of discharge and the correct charging process will all contribute to the life of any battery.
Gravy
dont forget temperature of battery
seems to be one of the main factors leading to premature failure
as for cable size 6mm2 should do it for shortish runs
i allways go 1 size higher than specified
vBulletin® v3.8.4, Copyright ©2000-2012, Jelsoft Enterprises Ltd.