We think this tool will be helpful to personnel of all skill levels to quickly perform calculations and eliminate many unsuitable choices. The final decision on wire and circuit protection should come after a review based on additional sources, such as the ABYC standards or one of the many books in the field.


All marine electrical systems present a risk of fire and other hazards. Blue Sea Systems makes no representation or warranty that this calculator will be suitable to address your particular situation or requirements.

Blue Sea Systems has developed this calculator for use only by individuals having the appropriate working knowledge and understanding of marine electrical systems. This calculator is based in part on the ABYC’s tables and standards contained in E-11 “AC and DC Electrical Systems on Boats,” but it is not intended to be a substitute for calculations based directly on these or other industry standards that may apply. This tool is a prototype, and may contain errors of entry or calculation.

This calculator does not account for all possible variables and factors relevant to the selection of wire size and circuit protection. Such variables may include overloads (such as changing the size of light bulbs or adding additional loads to circuits), wiring errors (such as loose connections or poor crimps), heated terminations (such as motor terminals, heating appliances, or lighting fixtures), data input errors, unusual environmental heat sources, and inadequate or defective wire insulation, software defects, and/or malfunctions of a browser or server computer.

This calculator is not a substitute for the expertise of a marine electrical professional. Under no circumstances should this calculator be used as the sole basis for selecting wire size or circuit protection. Any wire size or circuit protection tentatively selected with this tool should be reviewed for adequacy, before installation, by a professional applying the applicable industry standards.

Blue Sea Systems expressly disclaims responsibility for any use of this calculator that results in inadequate wire size or circuit protection.

To use this tool, you must accept the terms of this disclaimer.

I accept the terms of this disclaimer.

DC Wire Selection:

Information required to meet ABYC Standards:
Additional Derating Factors:

DC Circuit Protection Selection:

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Cold Cranking Amperes (CCA)/Marine Craking Amperes (MCA)

CCA and MCA measure the power capacity of a battery. CCA is the discharge load in amps which a battery can sustain for 30 seconds at 0°F/–18°C without falling below 1.2 volts per cell (7.2V on 12V battery). MCA is similar, but is measured at 32°F/0°C and results in a higher number for the same battery.

The amount of current that a breaker or fuse can interrupt without malfunction is its Ampere Interrupt capacity (AIC). The required AIC for a circuit breaker or fuse is determined by the power capacity of the battery.

Main or Branch circuit

The main circuit breaker is the first breaker in a circuit connected in series with the battery. The circuit connected to the main circuit breaker is the main circuit. All subsequent breakers connected in series with a main circuit breaker are branch circuit breakers. The circuits connected to branch circuit breakers are branch circuits. There are different Ampere Interrupt capacity (AIC) requirements for main and branch circuits. Small circuits that are connected directly to the battery for 24 hour service are technically main circuits.

Circuit Voltage

The nominal voltage in the circuit for which you are choosing wire and circuit protection. Lower voltage systems tolerate less voltage drop.

Load Current

The amount of current used by the load that the circuit is feeding. See ABYC E-11 for guidelines to calculate loads for a feeder panel.

Length of Conductor

The total length of the conductor—the combination of supply wire and return to complete this circuit.

Allowable Voltage Drop

Voltage drop is loss that occurs due to current flowing through the resistance of a conductor over its length. ABYC recommends 3% or less drop for critical loads and panel feeders, 10% for other loads. For some loads, you may wish to choose other allowable voltage drops, for example, 5%.

Voltage drop is the critical factor in most 12 Volt installations.

Wire Insulation Temperature Rating

Insulated wire is classified by the maximum permissible operating temperature in degrees Celsius. Common values are 105°C, 90°C, 80°C, 75°C. If the wire is allowed to operate above this temperature continuously, the insulation may melt, or be otherwise damaged. Wire rated less than 60°C is not recommended.

Engine Room

Engine room space is assumed to have a higher ambient temperature of up to 50°C while all other spaces are assumed to have ambient temperatures of 30°C. In a higher ambient temperature environment, such as an engine room, there is less temperature range between ambient and wire insulation temperature rating. Therefore, the higher temperature of an engine room limits the capacity of wires and components.


When current flows through wire, heat is generated because of the resistance that the current encounters in the wire. As current flows through a wire, the temperature of the wire increases until heat flows out to the surroundings at the same rate that is generated. Large wires are slower to heat up to their final value. Therefore, it helps to know how long current will flow in a circuit before current stops flowing and heat is no longer generated. When the duration of current flow is long, larger wire may be recommended under certain circumstances.

Type of Load

All loads are vulnerable to short circuits. However, variable load circuits, such as those that supply motor loads (for example, windlass), panel feeders and AC receptacle circuits, are subject to increases in current that could lead to an overload condition without anything being broken or a fault occurring.

Conduit or Sheath

Jackets or conduits around wires provide additional physical protection. However, they increase thermal insulation resulting in temperature rise. Wires clad in conduit or sheath will get hotter when current passes through them compared to wires not clad in a conduit or sheath. This condition is exacerbated when current is continuous.

Terminated on Fuse

Fuses can create considerable heat when current passes through them, especially when current is continuous. Wire directly terminated at a fuse is subject to this additional heat and may fail at the termination even if the rest of the wire is within its temperature limits. Larger wire may be recommended when it is terminated on a fuse and the duration of usage is long.

Thermal Insulation

Wire may be surrounded by extra thermal insulation such as foam surrounding a conduit between hull and liner. When wire is surrounded by insulation, thermal insulation increases and temperature rises.

DC Circuit Wizard Help

When the mouse cursor is on a data entry field or section heading, an explanation of that data entry field or section heading is displayed in this window.

Wire Selection

Wire size must be sufficient to prevent excessive voltage drop and also must have sufficient Ampacity to prevent overheating. A solution must meet both requirements.

Satisfying the voltage drop requirement usually determines wire sizes for longer wires at low voltage. At higher voltages, or for short lengths of wire, the wire must still be heavy enough to avoid overheating.

You may select a wire without filling in DC Circuit Protection Selection parameters.

Circuit Protection Selection

You must fill in DC Wire Selection parameters, as well as these DC Circuit Protection parameters, to select circuit protection.

Information required to meet ABYC standards

These are the factors used to determine wire size according to ABYC Standards. They are required to choose either wire or circuit protection, whether or not you are using Blue Sea Systems’ Additional derating factors below.

Additional derating factors

These factors are not considered in the ABYC Standards for wire sizing and they are not required to be entered in the Circuit Wizard to determine a wire size that meets ABYC requirements.

However, Blue Sea Systems’ engineers have identified Additional derating factors from experience in the field and experimentally in our laboratory as factors that can significantly influence the proper wire size required in a circuit. Some of these factors are under consideration by ABYC for future inclusion in the Standards.

Wire selection for ampacity including additional derating factors

This wire selection is the result of applying ABYC Standards factors and additional derating factors.

Wire selection for ampacity using only ABYC Standards

This wire selection is the result of applying only ABYC Standards factors

Mimimum wire size required to meet voltage drop

This wire selection is the result of calculating only the allowable voltage drop with no consideration for ampacity.

Recommended wire

This wire selection is the result of applying ABYC Standards factors, additional derating factors, and allowable voltage drop.

Minimum rating of circuit protection device

Smallest rated circuit protector that will not overheat in normal use or cause nuisance trips.

Capacity of selected wire

Estimated amperage capacity of the selected wire, given the installation conditions. Also the highest rated circuit protector to use with that wire.

Optimal rating of circuit protection device

Optimal circuit protection rating that is suitable for the circuit load and will protect the wire.

Minimum AIC of circuit protection device

The minimum AIC rating of the circuit protection device that can be used in this cirucit.

©2006-2012 Blue Sea Systems. All rights reserved. Portions based on ABYC Standards. Icons from Silk icon set. R20.