If you are DIY’ing your narrowboat electrics, buy the best you can afford.

Invest in a high quality ratchet crimping tool rather than the commonly used flat pressed metal product found in basic kits. It will significantly improve the quality of the connection and save you time and money in the future.

Connections are universally colour coded by their gauge and the power handling ability of the cable. Use the right connector for the right job and do not cut corners here. Use shrink sleeving over all exposed connections to help prevent moisture ingress.

Be careful when clipping cable runs or conduit. It’s a miserable experience trying to remove or add to a cable that is tightly clipped behind the centre of a long panel or bulkhead. The Boat Safety Scheme states secure clipping at 900mm intervals for conduit and secure fixing for cables every 300mm intervals.

Be aware that the Recreational Craft Directive has a whole section devoted to the installation of both AC and DC electrical systems in small craft. For example, DC conductors (wiring) should have crimped terminal ends, i.e. no bare wires to screw or stud terminals. Therefore you should familiarise yourself with what the regulations are before you undertake any work.

Call in a professional if you are in any way unsure.

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 narrowboat 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 narrow boat, 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.

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 narrowboat 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.