Marine Electrical Wire Size and Voltage Drop Calculator

Estimate a marine DC copper conductor size from system voltage, maximum sustained current, routed cable length, allowable voltage drop, insulation rating, engine-space heat, and cable bundling. Inputs stay in this browser tab.

Planning estimate — not an installation approvalHave a qualified marine electrician verify cable construction, circuit protection, fault current, terminals, routing, ignition protection, equipment instructions, and current rules. Incorrect conductor sizing can cause heat, fire, equipment malfunction, or loss of essential systems.

Circuit and cable conditions

DC circuit

Use the circuit's nominal voltage, usually 12, 24, or 48 V DC.

Include simultaneous loads and an appropriate design margin.

Source to load only; bends and routing included.

The model doubles this for the return conductor.

Confirm the limit for the actual equipment and installation.

Percentage of nominal system voltage.

Ampacity conditions

Use the rating printed in the cable specification.

Engine-space heat applies a temperature correction.

Count loaded conductors grouped in a bundle, sheath, or conduit; see the scope note below.

Privacy: calculations are performed locally. Input values are not sent, saved, or included in analytics events.

Sizing result

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How to use the calculator

  1. Enter nominal DC voltage and the maximum sustained current the circuit is designed to carry.
  2. Measure the actual one-way cable route. The calculator uses twice that distance for the positive-and-negative circuit.
  3. Select a circuit-specific voltage-drop limit. Do not assume 10% is acceptable for voltage-sensitive or essential equipment.
  4. Enter the cable insulation rating and installation conditions that reduce ampacity.
  5. Use the larger of the voltage-drop and ampacity sizes, then obtain an installation-specific review.

What the result means

The recommended AWG is the first listed conductor that passes both modeled tests. The result separates them so you can see whether voltage drop, allowable current, or both control the size.

  • Voltage-drop size limits modeled loss along the complete circuit.
  • Ampacity size carries the entered current after selected temperature and bundling corrections.
  • Terminal voltage is nominal source voltage minus modeled cable drop; real source sag and connection loss are not included.
4/0 limit: if no listed size passes, the tool does not design parallel conductors. That requires installation-specific engineering, protection, and termination checks.

Formulas and assumptions

For stranded copper, the voltage-drop screen uses a copper K-factor of 10.75 Ω·cmil/ft:

Required circular mils = 10.75 × current (A) × round-trip length (ft) ÷ allowable drop (V)

Actual drop for a selected size reverses the same relationship. Standard AWG circular-mil areas determine the next available size. The ampacity screen uses the USCG Boatbuilder's Handbook Table 5 values for the selected insulation temperature, then applies the handbook's engine-space and bundle correction factors.

Adjusted ampacity = Table 5 ampacity × engine-space factor × bundle factor

The model assumes one copper positive conductor and one copper negative conductor of the same size, steady DC current, and the entered route length. The cited federal bundle rule addresses circuits of 50 V or more; for lower-voltage circuits, this selector is a conservative what-if screen and not a claim that the federal factor applies. The federal subpart also has a defined vessel scope. Use the standard and correction rules that actually govern the installation. The model does not include connector resistance, corrosion, source-voltage sag, motor starting current, duty cycle, AC circuits, aluminum conductor, or parallel cable sets.

Frequently asked questions

Should marine wire be sized by voltage drop or ampacity?

Both. The conductor must carry the design current under its installation conditions and keep voltage drop within the chosen limit. This tool recommends the larger resulting size.

Is marine cable length one way or round trip?

Enter the one-way routed distance from the source to the load. The tool doubles it to represent the complete positive-and-negative DC circuit.

When should 3% or 10% voltage drop be used?

Three percent is a common starting point for critical circuits such as electronics, navigation lights, bilge blowers, and panel feeders. Some non-critical loads may use 10%. Equipment instructions and current standards control.

Why do engine spaces and cable bundles need larger wire?

Higher ambient temperature and closely grouped loaded conductors reduce heat dissipation. Correcting ampacity for those conditions can move the minimum to a larger conductor.

Does this calculator select a fuse or circuit breaker?

No. It does not select overcurrent protection or check interrupt rating, inrush, termination limits, fault current, ignition protection, or the permitted distance from the power source.

Can I use ordinary building wire on a boat?

This calculator only models conductor area and ampacity. Use cable construction, insulation, stranding, oil resistance, flame resistance, support, and termination methods accepted for the vessel and jurisdiction.

References and limits

Safety limitThis educational screen is not a substitute for the current ABYC standards, applicable law, manufacturer instructions, or a qualified marine electrician's design. Standards and product ratings can change.

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