As discussed in earlier articles, marine cables have many different characteristics than their counterpart in the construction industry. The cables used in a marine environment need the strength to withstand the continuous vibrations that are experienced in a boat. Another important factor that we need to take into consideration is the distance of our electrical equipment from the power source.
All cables have an inherent resistance (in electrical terms resistance is the opposition to the flow of electrons). The longer a cable of the same gauge is run from the power source, the higher the resistance it poses to the path of electrons; thus degrading the voltage. This is called voltage drop. This loss is experienced in heat emanating from the cable. When the power source is a 12 volt battery bank, the situation is exacerbated since there is little margin for any voltage drop. Recall that a full 12V battery is 12.8 to 12.6 volts and at the halfway point is only 12.2 volts. Only 0.4 volt represents 50% percent of the battery capacity. Voltage drop at appliances reduces performance, is a common cause intermittent failures and can lead in rare cases, to fires. To solve this we have to look into cable sizes.
Cable size is an extremely important factor when wiring any low voltage (12 or 24 volt) boat. The motto of many marine electricians is “keep it short, keep it big”. The bigger the cable the longer the run it permits us to do with acceptable levels of voltage drop. One of the two cable selection criteria is that the cable should be sized to handle at least the maximum amperage rating of the equipment thus making sure that under no circumstances the equipment will draw more amps than the cable can take. This is especially the case when talking about electric motors that can vary their amperage needs dramatically depending on the effort that they are putting out.
The second criteria is voltage drop should be never more then 10%, but more commonly no more then 3% voltage drop on a cable run. This is not necessarily a problem when AC voltage is involved (3% of 120 volts is 3.6 volts) but in a DC voltage of 12 volts it does not take much to reach the maximum drop allowed.
Weight and cost are always an important factor to consider but this is the price that has to be paid for a better and safer boating experience.
For future reference, the table that denotes required American wire gage for the distance required in marine boats can be found on page 33 of the law resource of the American Boat and Yacht Council (ABYC)