March 27

Understanding Marine Electrics: Voltage Drop and Ampacity

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. In this blog post, we explore these two factors.

What is Ampacity?

Ampacity refers to the maximum number of amps that can safely flow through a conductor. It is determined by heat generated in the conductor. For a given cross-sectional area of copper, the higher the amps the more the heat. 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. In the boat building world, the insulation temperature rating for conductors ranges from 60°C to 105°C. A conductor’s ampacity tells us how many amps we can put through the copper before the insulation approaches its melt-down temperature.

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). 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?

Voltage drop absolutely is a function of length. Even though copper conductors have a very low resistance, the longer the conductor the more the cumulative resistance. Resistance robs the circuit of voltage. If there is a 10 percent voltage drop through the conductors, in a 12 volt circuit we lose 1.26 volts in the conductors. 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%. 

Conductors are sized based on which of ampacity and voltage drop requires the larger conductor.

Wire Sizing Apps

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!

About the author 

Nigel Calder

Nigel is often referred to as THE guru when it comes to technical systems on boats.

He is a long-time member of the American Boat and Yacht Council (ABYC) electrical Project Technical Committee (PTC) which writes the standards for recreational boat systems in the USA, and has also been involved in European standards development.

Nigel is best known for his Boatowner’s Mechanical and Electrical Manual (now in its 4th edition), and his Marine Diesel Engines (in its 3rd edition), both considered the definitive English-language works in their field.


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