Cables must be capable of carrying the current running through them or they will overheat.
Cables must also have the correct insulation covering for the environment in which they are being used.
For AC wiring we would recommend as a minimum the use of 2.5mm square Acrtic Blue 3 core multistrand cable (or equivalent) for all connections to shore and also for internal AC wiring.
Multi-strand Automotive/Marine Cable
Solid single core cable, such as the standard household type is not allowed for new builds. The flow of electricity through copper cable can cause it to "work harden". Therefore, solid single core cable can break in the case of vibration and flexing so only multi-strand cable is now permitted.
It is written In both the AC and DC systems specifications of the Recreational Craft Directive, that multi-strand copper conductors are to be used. The cross section area and number of strands of the cables is determined by temperature and application. If you are building a new craft, reference to the RCD is essential.
Interestingly, the BSS allows for existing solid cable so long as it is secure and shows no sign of fatigue. As the Scheme moves along with latest developments and restricts the use of solid cable we would suggest, that if your older boat has any, to change it.
Specialist tinned marine cable is available for the belt and braces approach. Tinned cable has the copper strands within the insulation coated in tin which makes them much more resistant to corrosion. It is worth considering such cable for engine compartments and battery banks where the environment is pretty harsh.
We’ll say it again, refer to the BSS, your local examiner or marine surveyor and electrician if you are in any way unsure concerning cable specification.
By looking in our Products and Services Directory
you will find suppliers for electrical products who specialise in DC systems such as those traditionally found in motor vehicles. These suppliers have long experience in the marine market so can also advise you on what cable type to use for particular applications. So can most chandlery
Choosing the Correct Cable
Standard cable is sized using the metric system. You will see a range of numbers used to describe the properties of the cable:
such as 21/0.30mm 1.5mm2 21A
In this instance the cable consists of 21 strands of .3mm copper, has a cross sectional area of 1.5mm2 and has a maximum continuous load rating of 21amps.
Occasionally, the diameter of the cable is also quoted. It is important to remember that for specifying load rating it is the mm2 or cross sectional dimension of the cable that is important.
We’ll try and give you a helping hand here with regards to choosing the correct cable for your DC system.
There are a couple of basic criteria to satisfy.
Ensure the cable is correct for the voltage. Obvious maybe but don’t assume all cable works for all voltage. The rating is for the insulation and will be on the cable drum or available from your supplier.
Regarding the insulation, make sure it is suitable for the environment you are using it in. Get appropriately resistant products for harsh environments such as bilge pumps and engine bays.
Finally make sure that the cable is rated for the current you are passing through it. Refer to the Volt Drop
section above for calculating mm² cable cross section.
Use the manufacturers rated amp carrying capacity as a guide but the final decision on cable size is always determined by the voltage over the distance when designing DC systems. To work that out, refer to our easy guide to calculating volt drop.