When selecting wire for a boat project, it can be surprisingly complex. US Coast Guard regulations (33CFR183) requires that the maximum current allowable in a given wire be limited by:
Each of these factors reduce the current carrying capability of the wire. For example; when a wire goes through an engine room, the temperature of the room is sufficiently high that it reduces the wire's current-carrying capacity. Therefore, even if only part of the wire goes through an engine room, the entire wire is subject to the reduction for engine room use. The first calculator addresses this characteristic.
A second factor in boat wiring is that for DC use, a wire will lose voltage depending on how much current is flowing and the distance of the wire. In extreme cases, the voltage drop may be so severe that the equipment to be powered by the wire will not function properly. In most cases, the wire size will have more than enough current capabiltiy, but be limited by its voltage drop. The second calculator is useful to determine this characteristic. If you do not understand high power DC voltage drops, you may wish to review my High Power Cabling Tutorial.
The first calculator then provides the required wire size that will be safe for the intended load, taking into account temperature of the wire's insulation, engine room heat, and heat generated by bundling wires (above 50 Volts). The second calculator provides an acceptable voltage drop for DC circuits taking into account the intended load and the length of the circuit.
When selecting a wire for DC use, both calculators must be applied; and the calculator providing the largest wire is the required wire. For AC use, only the first calculator is needed as the voltage drop in AC circuits as a function of wire lengths found in a boat is negligible.
Note that most so-called "Marine Grade" wire is rated at 105° C, but may also be derated to 75° C in Wet environments. It is essential you determine that temperature rating of the specific wire you intend to use.
US Code 33CFR183 is the sole authority for the determination of proper wire and cable used on a boat. The calculators on this page are presented only as a convenience, and are not intended to replace the information contained within US Code 33CFR183.
The calculators on this page are based on both AWG and SAE Circular Mil sizes. USCG 33CFR183 allows the use of both SAE and AWG wire for wiring under 50 Volts, but only AWG wire for use over 50 Volt applications (33CFR183.402).
Any use or information obtained from the calculators on this page must be compared with 33CFR183.435 Table 5 to ensure the results comply with federal regulations. The reader assumes all risks when using these calculators.
Marine Wire Size Calculator.
Voltage Loss Calculator for 12 VDC Circuits.