12v basics

[Col_T]

An imaginary scenario:-

A 12v appliance needs 12 amp cable between the battery and the appliance, but the boat owner only has 8 amp, 6 amp and 4 amp. The recommended solution is to buy 12 amp, but an 'acceptable' solution is to run 2 parallel lengths of 6amp between the battery and the appliance. If parallel lengths of 4 and 8 amp were used, would each cable 'see' the same current e.g. 6amp, thus over-loading the 4 amp cable, or would the current divide in proportion to the cable ratings for some reason - resistance, perhaps?

 

Another 12v basic question. I've seen 'discharge at C/5' used in some threads - I think this is short-hand for 'apply a load that will discharge the battery in 5 hours', so that C/5 is a heavier (higher current) load than C/10 - correct? Also, does the same idea apply to charging?

[Murflynn]

you are over-simplifying the matter. 

the quoted amps rating for a cable is only a measure of the maximum recommended current to avoid overheating. 

in order to identify the correct cable size you need to make a voltage drop calculation for each cable, and will certainly need a higher rated cable in many situations in a typical narrowboat; of course adding more cores of copper wire (e.g. by using 8A plus 4A cable) will decrease the cable resistance and the voltage seen by the appliance can be found by the calculation. 

much of the 12v cable we use is commonly used on cars, where the lengths of the runs are limited, in a boat the runs are typically 2 or 3 times the length.

Edited November 29, 2017 by Murflynn

[Richard10002]

Given that every cable is made up of lots of strands, one could assume that the current doesn't know whether an individual strand is wrapped inside one rubber tube or another.

Thus, the logical answer is that the current and the cable/s work it out for themselves, such that the current is evenly distributed.

[Chewbacka]

But in answer to your question, cables will tend to share current as if one cable gets warmer it’s resistance will rise so reducing the current with the other cable increasing it’s share of the current.  However it is never a good idea to run boat cables warm or hot.  As said above, most cables on a boat are sized for power loss (volt drop) as power is much more precious than cable cost.

[Chewbacka]

22 minutes ago, Col_T said:

An imaginary scenario:-

................Another 12v basic question. I've seen 'discharge at C/5' used in some threads - I think this is short-hand for 'apply a load that will discharge the battery in 5 hours', so that C/5 is a heavier (higher current) load than C/10 - correct? Also, does the same idea apply to charging?

Regarding discharge, you are correct, but be aware that a current that reduces battery charge by say 50% in ten hours is not twice the current that would reduce the charge in 20 hours.  Batteries are much more complex than that.   As to recharging, the quicker you put the current in the quicker it will recharge, but the chemistry involved will limit the rate of charging, and the max rate of charging falls rapidly as the battery charges, which is why getting that last 10% of charge takes too many hours.

[David Mack]

9 minutes ago, Richard10002 said:

Given that every cable is made up of lots of strands, one could assume that the current doesn't know whether an individual strand is wrapped inside one rubber tube or another.

Thus, the logical answer is that the current and the cable/s work it out for themselves, such that the current is evenly distributed.

No. The voltage across the cable is common, but the resistance of individual strands, and the connection to each end of each strand will vary. The current should be distributed in inverse proportion to the total resistance.

But in a single cables the individual strands are in contact along their length, so current can redistribute from one strand to another, which will tend to even out any variations. If the strands are in separate rubber tubes this cannot occur.

 

[Tony Brooks]

I would also point out that you should really fuse to protect the cable but if you needed 12 amp cable then the appliance would be drawing 8 amps or more so if you fused to protect a thinner cable the fuse will blow so you would have to fuse for the load, not the cable. Then if one of the cables made a poor connection or went open circuit the other cable would overheat yet not blow the fuse - the boat is then at risk of fire.

[Jen-in-Wellies]

The cable length voltage drop is even worse in boats than cars as  ot only are most boats longer, but we need to run a cable there and back, doubling the length of wire. Cars can use the bodyshell as a low resistance return path. In practice we need to calculate acceptable voltage drop for each wire run when deciding wire size and the device it is going to run. Some things, like tunnel lamps are less bothered by a bit of voltage drop than things like 12V fridges. Current carrying capacity and ohmic heating of the wire usually gives a smaller wire size than voltage drop calculations. Cables are often marketed as 8A or whatever wire,  but this is for automotive use and not so applicable to boats. Good explation here. https://www.12voltplanet.co.uk/cable-sizing-selection.html

Jen

Edited November 29, 2017 by Jen-in-Wellies

[Jess--]

a good rule of thumb for boats where cable runs are longer and volt drop is an issue is to use cable that is rated for double (or more) the required current....
so for your 12A load I would be looking for cable rated at 24A (probably easier to find 25A cable)

[Col_T]

Thanks for the answers, folks.

In summary then, if you must parallel cables then use multiples of the same size cable. For clarity, I would always buy the correct size cable, even if it means the job has to wait!

[Chewbacka]

10 minutes ago, Tony Brooks said:

I would also point out that you should really fuse to protect the cable but if you needed 12 amp cable then the appliance would be drawing 8 amps or more so if you fused to protect a thinner cable the fuse will blow so you would have to fuse for the load, not the cable. Then if one of the cables made a poor connection or went open circuit the other cable would overheat yet not blow the fuse - the boat is then at risk of fire.

True, but in the case of a boat cable, if you use two cables of the same cas for volt drop reasons, the current carrying limit for an individual cable will often be greater than the max size of fuse required to avoid nuisance fails.  So if you used two parallel 2.5mm2 cables for a 12a circuit (to limit volt drop) you may use a 16a fuse.  However the max rating for a single 2.5mm2 cable is more than 16a, so would be safe in this example.  However if you used a thick and a thin cable together, then it could be very different.  But if you need to ask that question you probably don’t know enough to do the sums, so get help.

[WotEver]

19 minutes ago, Tony Brooks said:

I would also point out that you should really fuse to protect the cable but if you needed 12 amp cable then the appliance would be drawing 8 amps or more so if you fused to protect a thinner cable the fuse will blow so you would have to fuse for the load, not the cable. Then if one of the cables made a poor connection or went open circuit the other cable would overheat yet not blow the fuse - the boat is then at risk of fire.

^^^^ this. 

It’s why I disagree with Victron when they say to double-up 50mm² cable for their 3kW inverters. Far better and much safer to use one 90mm² cable. Or bigger, if you really wish. 

1 minute ago, Chewbacka said:

True, but in the case of a boat cable, if you use two cables of the same cas for volt drop reasons, the current carrying limit for an individual cable will often be greater than the max size of fuse required to avoid nuisance fails.  So if you used two parallel 2.5mm2 cables for a 12a circuit (to limit volt drop) you may use a 16a fuse.  However the max rating for a single 2.5mm2 cable is more than 16a, so would be safe in this example.  However if you used a thick and a thin cable together, then it could be very different.  But if you need to ask that question you probably don’t know enough to do the sums, so get help.

A good point well made

[Chewbacka]

I agree with you comment about big inverters as a single 50mm2 cable will be overloaded and get very hot supplying the 250a required at 3kW.  Maybe the recommendation is based upon the volt drop being so big that the inverter shuts down, or that no one draws 3kW for very long.  Either way I think it is a bad idea, and should not be encouraged.

[nicknorman]

5 minutes ago, Col_T said:

Thanks for the answers, folks.

In summary then, if you must parallel cables then use multiples of the same size cable. For clarity, I would always buy the correct size cable, even if it means the job has to wait!

Not exactly. If you use 8A and 4A cable in parallel, the current will be divided roughly in proportion to the current rating. This is because the 8A cable will be roughly half the resistance of the 4A cable - ie twice the cross sectional area. Since the voltage drop over the entire run of the cable must be the same for both cables (Kirchov’s second law, for Tony’s benefit!), the current in each cable will be inversely proportional to the resistance. I say “roughly” because the current rating of a cable depends on the properties of the insulating sheath, and even if these are the same, to the ratio of cable area to circumference (cooling occurs only at the outside surface) which of course is not linear.

Anyway, having put the science bit to bed I’d agree that although it should be OK in theory, issues with fusing make it a bit of a bodge and so yes, definitely use one cable for preference, or failing that two identical cables each fused.

4 minutes ago, Chewbacka said:

I agree with you comment about big inverters as a single 50mm2 cable will be overloaded and get very hot supplying the 250a required at 3kW.  Maybe the recommendation is based upon the volt drop being so big that the inverter shuts down, or that no one draws 3kW for very long.  Either way I think it is a bad idea, and should not be encouraged.

On the other hand Victron have been at it for quite a long time and sold thousands of inverters with instructions to use two 50mm cables. Perhaps it is just a matter of not catastrophising things. How often does one 50mm cable fall off? Has it ever happened? I suspect not.

[WotEver]

5 minutes ago, nicknorman said:

Kirchov’s second law, for Tony’s benefit!

Thanks

Now go and work out when it’s relevant