Battery cabling

[Ex- Member]

I've been through several battery cabling related threads, and in most cases it seems that quite heavy cable is usually recommended 50mm +. What I would like to know is what's exactly required and not what specific boaters have necessarily fitted.

 

We have a 3kw inverter but will not be using any individual 240 volt equipment anywhere near that current draw. I have calculated our likely usage as follows with highest usage equipment first.

 

Small halogen cooker 800 watts Max 7 amp hrs Max use 10 minutes

 

Twin tub washing machine 250 watt Max 21 amp hrs max use 15 minutes any one time.

 

Table top Freezer Iceking 168kwh 19 watts 1.6 amp hrs 12 hours a day

 

Fridge 117KW per year 13 watts 1.1 amp hrs 24 hours a day

 

None of the above include inverter losses as I don't know how to calculate.

 

The halogen cooker will only be used when solar power is plentiful, but would not be used with twin tub or any other high power equipment.

 

Comments appreciated

 

So even if using the halogen oven at 800 watts and with fridge & freezer running we're still under 840 watts = 70 amps Via the inverter.

 

If all 12 volt stuff was running at the same time you can add another 160 watts = 13 amps

 

So our max draw is around a KW say 83 amps through the inverter and 12 volt system combined. Based on that what would be the required cable thickness for connecting/linking batteries. I do have a 1.5 metre length of cable required between battery bank posts and distribution unit & shunt.

[Chalky]

Cabling on boats is not usually sized for the current it has to carry but to reduce the volt drop which has a more significant effect on the electrical system than the current has on the wire. If you sort out the volt drop the cable is usually more than capable of carrying the required current (but you still need to check). You're usually interested in keeping the cable resistance as low as possible thus it has a large copper area.

 

As an example a car may use a 0.75 mm csa cable to run a headlight. The cable run is 4m and the body acts as the earth return. On my boat to run the same headlight the round trip is about 25m and I'm using 6mm csa cable. A smaller cable would carry the current without any problems however the volt drop would make the lamp very dim. This the bigger cable to reduce the volt drop - it's all just Ohm's law.

 

The other way round it is to increase the voltage which reduces the current and thus allows smaller cables.

Edited May 12, 2012 by Chalky

[MoominPapa]

Size the cables for the maximum output of the inverter, it make no sense not to.

 

Note, for instance, that though the fridge averages 13W, it actually switches on and off, and will take a huge (kW) surge as the thermostat switches on. If the cables are too small, the volt-drop as the surge happens will drop the voltage to the inverter and you risk it shutting down on low battery volts, even though the battery voltage is in fact fine.

 

MP.

[nicknorman]

I've been through several battery cabling related threads, and in most cases it seems that quite heavy cable is usually recommended 50mm +. What I would like to know is what's exactly required and not what specific boaters have necessarily fitted.

 

We have a 3kw inverter but will not be using any individual 240 volt equipment anywhere near that current draw. I have calculated our likely usage as follows with highest usage equipment first.

 

Small halogen cooker 800 watts Max 7 amp hrs Max use 10 minutes

 

Twin tub washing machine 250 watt Max 21 amp hrs max use 15 minutes any one time.

 

Table top Freezer Iceking 168kwh 19 watts 1.6 amp hrs 12 hours a day

 

Fridge 117KW per year 13 watts 1.1 amp hrs 24 hours a day

 

None of the above include inverter losses as I don't know how to calculate.

 

The halogen cooker will only be used when solar power is plentiful, but would not be used with twin tub or any other high power equipment.

 

Comments appreciated

 

So even if using the halogen oven at 800 watts and with fridge & freezer running we're still under 840 watts = 70 amps Via the inverter.

 

If all 12 volt stuff was running at the same time you can add another 160 watts = 13 amps

 

So our max draw is around a KW say 83 amps through the inverter and 12 volt system combined. Based on that what would be the required cable thickness for connecting/linking batteries. I do have a 1.5 metre length of cable required between battery bank posts and distribution unit & shunt.

 

 

There is no precise "right answer" but one of the key facts you haven't mentioned, is how far it is from the inverter to the batteries. It is not just how much current the cables can handle, but also long the cable run is, and hence how much voltage drop there is. Again, there is no "right answer" but more voltage drop means more wasted power, and the inverter complaining when under load earlier in the battery discharge cycle than it otherwise might do.

 

I would also be concerned to design a circuit around what you intend to use now, rather than what the inverter is capable of putting out. Even though you may stick to your max power demand, one day someone will come onto the boat (maybe a new owner, or maybe a guest) and plug in their 2KW hair dryer / fan heater / electric kettle or whatever.

 

With all the costs involved in having a boat at all, it seems a little inappropriate to penny pinch on cable size for what is presumably a fairly short run. A 3KW inverter would normally be fed by 70mm^2 cables, or 90mm^2 if a long cable run.

[Ex- Member]

There is no precise "right answer" but one of the key facts you haven't mentioned, is how far it is from the inverter to the batteries. It is not just how much current the cables can handle, but also long the cable run is, and hence how much voltage drop there is. Again, there is no "right answer" but more voltage drop means more wasted power, and the inverter complaining when under load earlier in the battery discharge cycle than it otherwise might do.

 

I would also be concerned to design a circuit around what you intend to use now, rather than what the inverter is capable of putting out. Even though you may stick to your max power demand, one day someone will come onto the boat (maybe a new owner, or maybe a guest) and plug in their 2KW hair dryer / fan heater / electric kettle or whatever.

 

With all the costs involved in having a boat at all, it seems a little inappropriate to penny pinch on cable size for what is presumably a fairly short run. A 3KW inverter would normally be fed by 70mm^2 cables, or 90mm^2 if a long cable run.

 

Of course good point. It wasn't so much penny pinching, I've moved the battery bank, replaced with all new batteries etc. I originally used 35MMs cable and have quite a lot left all in very good condition so would like to utilise it. I could double up I suppose making everything 70MMs if that's possible. I do have enough 70MMs to link the batteries and will be wiring them in the optimum arrangement where all cable lengths are the same from the batteries to the distribution posts.

 

I forgot the start up power of items too, silly me. Been concentrating on the actual new installation and ripping old stuff out rearranging etc so forgetting the technical side

 

So Would 70MMs suffice. The inverter is 2.5 metres away from the batteries but have 1 metre of neg and pos at 100mms connected to a 500 amp junction box which the battery leads are connected to at 1.5 metres long currently in 35MMs cables, but as I suggested I'll double these up and complete the rest in 70MMs

Edited May 12, 2012 by Julynian

[Robbo]

I've been through several battery cabling related threads, and in most cases it seems that quite heavy cable is usually recommended 50mm +. What I would like to know is what's exactly required and not what specific boaters have necessarily fitted.

 

Don't know what size is required, but don't use anything less than what's needed for the size of the inverter, electrical fire's on boat's are common and if the Insurance company finds you not used the right size cable your insurance may be void.

 

So Would 70MMs suffice. The inverter is 2.5 metres away from the batteries but have 1 metre of neg and pos at 100mms connected to a 500 amp junction box which the battery leads are connected to at 1.5 metres long currently in 35MMs cables, but as I suggested I'll double these up and complete the rest in 70MMs

 

If you do double the cable up, remember to double the fuses, but the fuses have to be rated for the cable and not the combined cable.

[Ex- Member]

Don't know what size is required, but don't use anything less than what's needed for the size of the inverter, electrical fire's on boat's are common and if the Insurance company finds you not used the right size cable your insurance may be void.

 

 

 

If you do double the cable up, remember to double the fuses, but the fuses have to be rated for the cable and not the combined cable.

 

Cheers Robbo, fuses not yet bought, so no problem there.

 

 

 

[Richard10002]

Just been through all this with a 2.5Kw inverter charger. 8m total run requires 95mm2 cable, (and a 400A fuse). No point under speccing the wire and fuses. What happens when someone who doesn't know plugs a kettle, toaster, Hoover etc. in?

[Robbo]

Cheers Robbo, fuses not yet bought, so no problem there.

 

Double check with a electrician before hand too see if there are any other issues with doubling cabling up before you go ahead thou. I only really know the basics, and one is fuses are there to protect the cable not the device.

[smileypete]

Hi,

 

35mm2 PVC is good for 240A, a 5m length passing 240A will drop about 0.6V (so it's losing 140 watts in heat if your inverter is putting out ~2.5kW)

 

But how big is your ammeter shunt, 200A? Would be good to protect that too, in which case 35mm2 and 200A breaker should do. If you do have a 200A breaker, 35mm2 between batts will be fine.

 

And if you have enough cable to spare it wouldn't do any harm to double up the longest run between batt bank and inverter.

 

cheers,

Pete.

smpt.

Edited May 12, 2012 by smileypete

[by'eck]

Whilst using multiple cables to increase the current carrying capability & reduce volt drop is theoretically sound, IMHO it looks messy, amateurish and adds mechanical complexity to the system. In the unlikely event you do have an electrical problem causing damage to the boat, an insurance accessor looking for contributory factors may be suspicious of such practice.

 

For your cable run you ideally need 90 or 95mm² cable from batteries to inverter with a 400 or 500 amp fuse. Forget suggesting that you won't use the full inverter power. At some time or other you or someone else on board will & in any case equipment needs to be installed to operate safely at its limits.

[dmr]

We have pathetic little 35mm2 cables in our battery box, and I confess it was me who put them there.

We had a BSS inspection a few weeks ago and the BSS man said "good to see you are using nice thick cables rather than the thin things lots of boats use".

90mm2??? learn to use less electricity????

.........Dave

[Arthur Brown]

Pairs of cable well installed is good where you approach Ith considerations as the heat generated in the cable can be dissipated from both surface areas rather than just one. This means that a pair of 35mm^2 will carry more current than a single 70mm^2. the temperature will be less so the volts drop will be less.

[by'eck]

Pairs of cable well installed is good where you approach Ith considerations as the heat generated in the cable can be dissipated from both surface areas rather than just one. This means that a pair of 35mm^2 will carry more current than a single 70mm^2. the temperature will be less so the volts drop will be less.

 

As I said the theory is sound, but consider the extra chaffing possibilities of that extra surface area. Then there is the fusing issue. Clearly you could treat them as one & fuse as a single cable. What happens if a single cable is broken though, the other having to carry twice the current.

 

All these scenario's are highly unlikely I know but why add this even small risk. The KISS principle still holds sound

 

If the temperature of a single cable is an issue you are using inadequate cross-section.

[Ex- Member]

Thanks for every ones comments, really helpful.

 

I've done a bit of jiggery pokery this morning and have struck lucky.

 

Some time ago I had 2 x 70MMs cables made up for the old battery bank both 1.3 metres long, + and - of course. For the new location of the bank though they would not be long enough, 8 inches short. So today I had a rethink and the obvious "as usual" was staring me in the face, simply relocate the battery posts terminal ends in a different location and hey presto I've got the 70MMs cables to fit perfectly. The terminals are now not in my preferred location but nothing wrong with their new location. So now the 2 x 35MMs cables I was going to use can now be cut down and used to link and connect the batteries to the terminal ends, all equal lengths of course. I also have some useful lengths of 35mm so won't have to buy any extra cable at all.

 

I'm constructing my own terminal ends from M10 Brass studding & nuts & washers. What helped me greatly with relocation the battery terminal ends were these lugs.

 

http://www.cablecraft.co.uk/ProductGrp/copper_tube_angled

 

So it looks like I can follow Pete's suggestion as I do have a 200 amp shunt already fitted. The use of a 200 amp fuse also assists in the 70MMs +ve cable being able to reach it's location as I can create a 90 degree bend at the stud connection where as having to loop the cable it wouldn't have reached. Happy days

 

So now total cabling from inverter to battery output terminals is down to 2.3 metres, I metre of that being the cable supplied with the inverter which is BIG, probably shortening by 8 inches will not make a big difference, but every little helps according to TESCO Also the 70mm cables run lays mostly on the slipper stern base plate so will always cool there.

 

I'll get some photos on later if time permits.

[bottle]

cable supplied with the inverter which is BIG,

 

Personally I would wire all the batteries etc. in the same size (or bigger).

 

Your boat, do it the way you want.

[Ex- Member]

Personally I would wire all the batteries etc. in the same size (or bigger).

 

Your boat, do it the way you want.

 

 

I found out earlier the inverter cables are 120MMs 600 watts, there's no way I'm wiring the lot in that

[Richard10002]

What is the total return distance from inverter to batteries? This will dictate the size of the cable. I think you may have provided the length, but in a convoluted way.

 

It may be that you can use 70mm2, fusing it appropriately. Have you had a look at one of the voltage drop calculators?

Edited May 13, 2012 by Richard10002

[Richard10002]

Try this. Round the wire size up or down as appropriate.

 

http://www.calculator.net/voltage-drop-calculator.html

[bowten]

Try this. Round the wire size up or down as appropriate.

 

http://www.calculator.net/voltage-drop-calculator.html

Remember, always over engineer on wiring.

[Ex- Member]

What is the total return distance from inverter to batteries? This will dictate the size of the cable. I think you may have provided the length, but in a convoluted way.

 

It may be that you can use 70mm2, fusing it appropriately. Have you had a look at one of the voltage drop calculators?

 

Hi Richard

 

Great calculator, like it

 

Well the results are for the 1 metre of inverter cable are as follows.

 

Votage drop: 0.026<br style="color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; ">Votage drop percentage: 0.22%<br style="color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; ">Votage at the end: 11.974

 

The inverter cables go to a 500 amp battery switch + and to a 200 amp shunt - at the same length 120mmS

 

I then have + and - cables at 70mmS from battery distribution posts at 1.3 metres. Result

 

Votage drop: 0.067<br style="color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; ">Votage drop percentage: 0.56%<br style="color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; ">Votage at the end: 11.933

 

If I average the cable lengths and thickness over the 2.3 metres I think it would equal 110mmS

 

Votage drop: 0.074<br style="color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; ">Votage drop percentage: 0.62%<br style="color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; ">Votage at the end: 11.926

Bearing in mind their calculated on a load current of 100 amps.

I set the number of conductors at 2, is that correct?

ETA ignore above incorrect

Well the results are for the 1 metre of inverter cable are as follows.

 

Votage drop: 0.016

Votage drop percentage: 0.13%

Votage at the end: 11.984

 

The inverter cables goto a 500 amp battery switch + and to a 200 amp shunt - at the same length 120mmSThe calculator only goes up to 107mmS so a bit in hand.

 

I then have + and -cables at 70mmS from battery distribution posts at 1.3 metres to battery switch& shunt. Result

 

Votage drop: 0.033

Votage drop percentage: 0.28%

Votage at the end: 11.967

 

The cable average over 2.3metres is I believe 91mmS . Result

 

Votage drop: 0.047

Votage drop percentage: 0.39%

Votage at the end: 11.953

 

Bearing in mind their calculated on a loadcurrent of 100 amps. I set the number of conductors at 2, is that correct?

Edited May 13, 2012 by Julynian

[Richard10002]

Looks like your total length is 4.6m - I think it's 1 conductor, but could be wrong.

 

With a 3Kw inverter, the current would be around 250A when using full power, so you should calculate for that.

 

At the end of the day, it's up to you.... As long as the wires are capable of carrying the max the inverter can draw, and are fused accordingly, voltage drop merely affects whether the unit will work or not.

[Ex- Member]

Looks like your total length is 4.6m - I think it's 1 conductor, but could be wrong.

 

With a 3Kw inverter, the current would be around 250A when using full power, so you should calculate for that.

 

At the end of the day, it's up to you.... As long as the wires are capable of carrying the max the inverter can draw, and are fused accordingly, voltage drop merely affects whether the unit will work or not.

 

Ok Richard thanks for that I'll re-calculate @ 250 amps and apply one conductor.

 

I'm not sure weather averaging 2 different cable sizes over a distance gives a correct result, just did it to see what it produced.

 

 

 

[Ex- Member]

Well the results are for the 1 metre of inverter cable are as follows.

 

Votage drop: 0.080

Votage drop percentage: 0.67%

Votage at the end: 11.92

 

The inverter cables goto a 500 amp battery switch + and to a 200 amp shunt - at the same length 120mmS The calculator only goes up to 107mmS so a bit in hand.

 

I then have + and -cables at 70mmS from battery distribution posts at 1.3 metres to battery switch& shunt. Result

 

Votage drop: 0.17

Votage drop percentage: 1.42%

Votage at the end: 11.83

 

The cable average over 2.3metres is I believe 91mmS . Result

 

Votage drop: 0.23

Votage drop percentage: 1.92%

Votage at the end: 11.77

 

All the above Re-calculated at 250 amps.

 

The on line calculator works in AWG and a maximum of 107mmS - My Inverter cable is 120mmS

 

Calculation on the 70mmS cable was made on 0/2 AWG which is 67.4mmS not the actual 70mmS I've used, so a bit in my favour on bothaccounts.

 

The averagecalculation @ 0/3 AWG is 85mmS where as my average calculation is 91mmS

 

What I find really interesting though is if I used 70mmS for the entire 2.3 metres removing the inverter cables the result is as follows.

 

Votage drop: 0.29

Votage drop percentage: 2.42%

Votage at the end: 11.71

 

So without the supplied inverter cables you could comfortably cable in with 70mmS or the calculators 0/2 AWG 67.4mmS

 

Well within parameters.

 

 

 

VoltageDrop and Cable Size Calculator for ELV Solar Power Systems

Allowable Voltage Drop

 

0 %

 

Run Length

 

2.3 m

 

Current Demand

 

250 A

 

System Voltage

 

12 V

 

Voltage Drop

 

0.00 mV/A.m

 

Recommended Cable Size

 

70 mm²

 

Fromhere http://www.kilowatts...oltage-drop.php

 

Edited May 13, 2012 by Julynian

[by'eck]

VoltageDrop and Cable Size Calculator for ELV Solar Power Systems

Allowable Voltage Drop

 

0 %

 

Run Length

 

2.3 m

 

Current Demand

 

250 A

 

System Voltage

 

12 V

 

Voltage Drop

 

0.00 mV/A.m

 

Recommended Cable Size

 

70 mm²

 

Fromhere http://www.kilowatts...oltage-drop.php

 

 

This second calculator you used is useless. Whatever current/length/volt drop you put in it gives the same 70mm² recommendation.

 

Try this one which also gives the answer in cable cross-sectional area.