Cable and Cable Sizes for 12 volt DC Circuits
One of the most important aspects of replacing or installing any part of a boats electrical system is determining the correct size and type of cable to use for each circuit.
Too small a cable size and you'll run the risk of generating heat in the cable and cause a fire; too large and you'll be wasting money on copper you don't need. Also, what sort cable should you use, plain copper or tinned, standard PVC insulation ? In this section I will give you an insight into how electrical cable is specified and allow to you choose the right one for your application.
Too small a cable size and you'll run the risk of generating heat in the cable and cause a fire; too large and you'll be wasting money on copper you don't need. Also, what sort cable should you use, plain copper or tinned, standard PVC insulation ? In this section I will give you an insight into how electrical cable is specified and allow to you choose the right one for your application.
Cable Type
You will have noticed that cable used in boat electrical systems is very flexible in contrast to the cable you would find in your home, which is fairly stiff. The reason for this is that solid copper core cable in house installations, although quite ductile, is susceptible to 'work hardening' when subject to vibration and mechanical shock, such as is experienced when installed in a boat. This work hardening causes the metal to become more brittle which could, over a long period of time, cause a stiff, solid conductor to crack and fail.
This problem is overcome by manufacturing the core from many small diameter strands of copper wire to make up the desired cross-sectional area, rather than using a single wire. This type of cable is known as 'stranded' cable and provides much more flexibility, which means improved resistance to work hardening making it more suited to use in boat electrical circuits.
You will have noticed that cable used in boat electrical systems is very flexible in contrast to the cable you would find in your home, which is fairly stiff. The reason for this is that solid copper core cable in house installations, although quite ductile, is susceptible to 'work hardening' when subject to vibration and mechanical shock, such as is experienced when installed in a boat. This work hardening causes the metal to become more brittle which could, over a long period of time, cause a stiff, solid conductor to crack and fail.
This problem is overcome by manufacturing the core from many small diameter strands of copper wire to make up the desired cross-sectional area, rather than using a single wire. This type of cable is known as 'stranded' cable and provides much more flexibility, which means improved resistance to work hardening making it more suited to use in boat electrical circuits.
Selecting cable
1. Current carrying capacity
Each component or appliance connected to a circuit will have a current draw associated with its operation and it is important that the cable supplying power to these is capable of carrying the normally expected current, plus a % of safety. If it is not capable then it is likely to result in the cable becoming hot and potentially catching fire.
Although fuses are used in the circuit to protect the cable, the cable itself should be of an adequate rating to prevent this over-heating occurring under normal circumstances.
1. Current carrying capacity
Each component or appliance connected to a circuit will have a current draw associated with its operation and it is important that the cable supplying power to these is capable of carrying the normally expected current, plus a % of safety. If it is not capable then it is likely to result in the cable becoming hot and potentially catching fire.
Although fuses are used in the circuit to protect the cable, the cable itself should be of an adequate rating to prevent this over-heating occurring under normal circumstances.
If we wanted to wire up a light that we know has a power rating of 50W, then using I = P/V the current draw would be 50W/12V = 4.17A when rounded up.
This tells you that you could use a cable with a rating of 4.17A or above, however it is good practice not to design a circuit operating at the upper end of the cable's rating and so you should select a cable with some additional capacity.
In this case 0.5mm² cable with an 11 amp rating would be appropriate.
2. Voltage drop
All elements of an electrical circuit have resistance, including electrical cable, which means that there will be energy loss in the form of voltage drop experienced along the length of the cable. Just as a bulb converts electrical energy into heat and light due to its resistance, and so induces a voltage drop, a copper conductor has resistance and will convert some of the energy it conducts, causing a voltage drop in the same way. The difference is that voltage drop across an electrical load such as a fridge, lights, horn etc is useful as that's what makes it work, but voltage drop along cable and other passive parts of a circuit is not desirable as it's not a useful conversion of energy.
In low voltage systems such as a 12 volt DC boat circuit cable length can have a significant impact on voltage drop. Even a cable run of a few metres for small cross-section conductors can produce significant voltage drops and this problem is demonstrated well on some boats where the tunnel spot light is not as bright as it could be.
If you check the voltage of the tunnel lights lamp connector, you may well find that the voltage is under 12V from the circuit due to the conductor size being too small for the cable run length.
Some owners may well opt to improve their circuits by using cable with a larger conductor over a shorter distance which allows the circuit to provide full voltage to the lamps, often with very significant improvements in lighting brightness.
So we need to select a cable to make sure that the voltage drop is not so large that it will cause problems, but what is acceptable and how do we calculate the right cable size to use?
You can get away with a voltage drop for DC circuits on a boat of around 8-9 %, but personally my preference is under 5%.
We can use the calculation V = IR to calculate the voltage drop for a cable if we know the current draw of the load and the cable's resistance per metre.
For Example
Using the above example of a 50W light we now know it draws 4.17A, so if we were to use a 0.5mm² cable which has a resistance of 0.037W/m and its total length from battery positive back to battery negative was 5m, then the voltage drop would be:
Vdrop = IR = 4.17A x (5m x 0.037W/m) = 0.77V or 6.4%
This shows that although 0.5mm² cable is OK for the expected current draw of the light, it's just about OK for the cable run length as the drop is under 8 %.
So what about 1.5mm² cable with a resistance of 0.013W/m ?
Vdrop = IR = 4.17A x (5m x 0.013W/m) = 0.27V or 2.3%
This shows that 1.5mm² cable (at a current rating of 21A) will be suitable for the cable run length as the drop is well under 3%
There is a general rule of thumb that says if you're unsure whether the cable is large enough for the job, go up a size. This is a bit crude and not very scientific but it's not a bad rule to apply as increasing cable size can't do any harm.
Anyway, enough maths - to make it easy we've developed this handy calculator which will show you the approximate voltage drop based on cable size, supply voltage, current draw and cable length.
Using the above example of a 50W light we now know it draws 4.17A, so if we were to use a 0.5mm² cable which has a resistance of 0.037W/m and its total length from battery positive back to battery negative was 5m, then the voltage drop would be:
Vdrop = IR = 4.17A x (5m x 0.037W/m) = 0.77V or 6.4%
This shows that although 0.5mm² cable is OK for the expected current draw of the light, it's just about OK for the cable run length as the drop is under 8 %.
So what about 1.5mm² cable with a resistance of 0.013W/m ?
Vdrop = IR = 4.17A x (5m x 0.013W/m) = 0.27V or 2.3%
This shows that 1.5mm² cable (at a current rating of 21A) will be suitable for the cable run length as the drop is well under 3%
There is a general rule of thumb that says if you're unsure whether the cable is large enough for the job, go up a size. This is a bit crude and not very scientific but it's not a bad rule to apply as increasing cable size can't do any harm.
Anyway, enough maths - to make it easy we've developed this handy calculator which will show you the approximate voltage drop based on cable size, supply voltage, current draw and cable length.
It should be noted that voltage drop occurs not only along the positive cable to the load but also along the negative return cable, so when you calculate the cable length as the 'one-way' distance of a circuit the calculation of the return distance will be identical, giving you a total cable length which is twice that of your entered value. In practice your return cable length might be shorter, so the distance back to the battery negative would have a lower resistance relative to a positive cable. In this case the actual voltage drop would be less than calculated, but the calculations provides a 'worst case' figure to work with.
