# How to choose cables for different amps

I am trying to make a diagram for caravan electric system.As a part of it I have to recharge my batteries from shore/main current. I am thinking of using 230Ah AGM batteries. I will use a 30A 12V charger to charge my batteries from shore current. But I am not understanding which cable should I use to connect the batteries with the charger?

I have been searching a bit and found that most people use 16mm cable which are 110amp - to charge their 110amp batteries. so from there I get the idea that I need same amp of cable as my batteries.

But then again when I search for 230amp battery cables then search results come up with cables rating with awg.

So, how do I decide on the cable? there are awg, amp, mm for cables.

• The cable needs to be rated for the maximum current, 30 amps (according to your question). Commented Jun 26, 2020 at 9:01
• @Andy Thanks for your answer but that for shore current to charger- for which I am using 183YAG BLUE 3-CORE 2.5MM² cable. But I need the cable size for charger to 230Ah AGM battery Commented Jun 26, 2020 at 9:05
• @shaon What is the charging current from the charger to the batteries? Commented Jun 26, 2020 at 9:12
• @Andrew not sure I understand. But shore current is supposed to be 234/240v. and charger is 30amp 12v , batteries are 12v and 230Ah . Commented Jun 26, 2020 at 9:41
• If this cable is also shared with the load it needs to be sized for the greater of the charge or discharge current. Commented Jun 26, 2020 at 11:36

When choosing the wire gauge for an application there are two numbers you need to consider.

The first is the ampacity of the cable. This is the maximum current that the cable can take without melting the insulation or the copper. This number can be found in various tables on the internet. It varies with the number of cables you have bundled together, obviously if you have a big bundle of high current wires they are going to get hotter than one wire or a pair of wires. It also varies with the operating environment. This number should be considered as an absolute maximum for a particular wire gauge.

The second number is the resistance of your wire run, there and back, this multiplied by the current gives your voltage drop. You need to decide what drop you are prepared to accept. The resistance per metre number again comes from the wire date tables.

The current you are dealing with for wiring up the charger is the current the charger can provide in your case 30A, not the Amp hour rating of the battery, the 230AH means it can provide, for example, 10A for 23 hours.. Several internet sources recommend 10AWG as the minimum wire size for a short run at 30A. This corresponds to 6 mm squared.

• thanks, it cleared up a lot of confusions. But I didnt understand the battery part. How do you know that the battery provides 10amp for 23 hours? cant it also be 23amp for 10 hours? Commented Jun 26, 2020 at 10:52
• Yes it can, that was an example. The amp hour rating is given for a particular load current or time and that should be somewhere in your battery data.
– RoyC
Commented Jun 26, 2020 at 11:15
• @shaon It depends on how much current your stuff takes from the battery. So it could be 10 amp for 23 hours or 23 amps for 10 hours or 230 amps for 1 hour (probably not - that's probably too many amps all at once) or 1 amp for 230 hours. Commented Jun 26, 2020 at 11:23
• The 230Ah battery is limited by it's internal resistance, but it could be a lot of current at once. You should have an inline fuse with ANY direct connection to the battery. Then size your cabling for that fuse value. Commented Jun 26, 2020 at 11:32
• Other way round choose cable first and then select fuse for cable ampacity.
– RoyC
Commented Jun 26, 2020 at 12:16

The cable needs to be rated for the maximum current it has to carry, that's the main value you have to look for. As thiner cables have a higher resistance, this automatically leads to thicker cables required for higher currents. AWG (American Wire Gauge) is a standard that directly refers to a certain diameter.

For 30A you should aim for around $$\4mm^2\$$ or AWG11

• so, for 230amp battery I should be looking at 50mm or 70mm cable? which are respectively 1AWG and 2/0AWG ? Commented Jun 26, 2020 at 9:51
• You have a 230Ah battery, that defines the batteries capacity not the current it will deliver. PS: why the downvote? Commented Jun 26, 2020 at 10:08
• @op.pe lol.. It wasn't me who down voted Commented Jun 26, 2020 at 10:12
• You also need to consider the volt drop along the cable. If the charger is a long way from the battery a thicker cable would be needed to reduce the volt drop. Mounting the charger within, say 2-3 feet, of the battery should prevent the need for this. Commented Jun 26, 2020 at 11:11