Boat electrical systems are central to a boat's functionality. The conductors are what pulls everything together. Understanding the two core factors that determine the required size of conductors - ampacity and voltage drop - is of fundamental importance for any boat owner or electrician.
What is Ampacity?
All conductors have a miniscule amount of resistance. When you pass an electric current (amps) through a resistance, you generate heat. For a given cross-sectional area of copper, the higher the amps the more the heat. Ampacity refers to the maximum number of amps that can safely flow through a conductor of a given size before it becomes dangerously hot.
There are some complicating factors! The insulation around a copper conductor has a melting point. The higher the melting point, the more amps that same piece of copper can carry before the insulation melts. Put another way, the higher the melting point of the insulation the higher the ampacity of a given size of conductor. In the boat building world, the insulation temperature rating for conductors ranges from 60°C to 105°C. A given size of conductor with 105°C insulation has an ampacity rating that is almost twice the same size conductor with 60°C insulation.
The ampacity is determined for a specific ambient temperature. Anything that raises the ambient temperature preheats the conductor and lowers the amps the conductor can carry before the insulation temperature rating is reached. In the boat world we derate (lower the ampacity) of a conductor if it is installed in a hot environment such as an engine room, and if it is bundled up with other conductors (because heat from one conductor will be transmitted to the others).
So we have four factors to consider when determining how many amps a conductor can safely carry (its ampacity): the size of the copper conductor, the temperature rating of the insulation, the ambient temperature, and whether or not the conductor is in a bundle with other conductors. Note that the length of the conductor is not a factor to consider when it comes to ampacity. The longer a conductor the greater the surface area for heat dissipation so the two balance out and length is immaterial.
What is Voltage Drop?
In contrast, length is a critical factor with voltage drop whereas the insulation temperature rating, ambient temperature, and bundling are irrelevant. Even though copper conductors have a very low resistance, the longer the conductor the more the cumulative resistance. Resistance robs the circuit of voltage.
Let’s say the voltage at the battery is 12.6 volts and we have a 10% voltage drop through the conductors to a device. We will ‘lose’ 1.26 volts in the wiring and will only measure 12.6-1.26 = 11.34 volts across the positive and negative conductors at the connection to the device.
The more the voltage drop, the more the performance of the device at the other end of the circuit is likely to be affected. In the boat world, with one or two exceptions we set the maximum allowable voltage drop in DC circuits at 10%. Some are set as low as 3%. The only fix for excessive voltage drop in conductors is to shorten the length of the circuit (which is generally impractical) or to add more copper – i.e., a larger cross-sectional area for the conductor.
Wire Sizing Apps
Conductors are sized based on which of ampacity and voltage drop requires the larger conductor. In low voltage circuits (e.g., DC circuits) it is almost always voltage drop. In higher voltage circuits (e.g., AC circuits) it is almost always ampacity.
The ABYC and the ISO have tables for calculating conductor ampacity and voltage drop. You can find these here: ISO tables and ABYC tables. But also check out our wire sizing apps. These make wire sizing easy. They enable users to rapidly determine minimum conductor sizes in different situations, and to plug in different factors to determine the thresholds at which conductor sizes change.
For more in-depth information, check out Nigel's article on Overcurrent Protection on Boats and of course our courses on Marine Electrical Systems!
