About Marine Cable
Marine (tinned) Cable
Automotive cable is cheaper than marine cable and is easier to obtain, so it’s tempting to use automotive cable on your boat – this is a mistake. Un-tinned copper cable corrodes rapidly in the marine environment, turning black so that some connections become unreliable. The current capacity of the cable is also reduced. With marine cable each strand of copper is coated with a thin layer of tin which prevents the copper from corroding in damp conditions.
Voltage Drop
All cables have an electrical resistance; this causes a voltage drop to occur as some of the current is converted into heat. In an extreme situation the temperature increase could result in a fire hazard. The thicker a cable is, the better it conducts electrical power and the higher the current (amps) it can safely carry. By the same token, thicker cable, given the same current (amps) will result in less voltage drop than thinner cable. Furthermore any loss of voltage increases in proportion to the distance travelled by the current; this must be measured along the entire circuit, i.e. double the cable length.
Any rise in cable temperature caused by bundling of cables or by external factors such as ambient air temperature will also result in an increase in resistance, e.g. additional allowances should be made for cables in hot engine rooms.
The significance of any voltage losses is greater with low voltage (12v and 24v) systems. So, more care must be taken when working with 12 volt or 24 volt systems to ensure that voltage losses are minimised, a 3% loss is generally considered a necessary maximum for items such as navigation lights, electric fridges, electronics, inverters, macerators, bilge pumps, bilge blowers and high current devices. At 12v a 3% loss equates to 0.36v or less while at 24v this would be loss of 0.72v or less. A 5% voltage drop would be acceptable for less critical items such as cabin lights, bait pumps etc.
Cable thickness is graded either by the cross sectional area of the conductor in mm², or an AWG number (American Wire Gauge).
Choosing the correct cable thickness
Each cable that you install into your boat’s electrical system needs to be of sufficient thickness to avoid an unacceptable voltage drop and ensure that no fire hazards occur. To do this, consult the appropriate 12 volt or 24 volt Project Notes Cable Thickness Guide
Follow this procedure:
Work out the total current loading for each cable in Amps. Amps can be calculated by dividing watts by the voltage.
Measure (or estimate) how long the cable run will be and double this number to arrive at the total circuit length.
Once you have these figures you can use either the 12v or 24v table (separate note) to select the correct cable.