Marine Wire Size & Voltage Drop Calculator
Enter the circuit current, one-way run length, voltage, drop limit, bundle size, and engine-space condition. The calculator checks both voltage drop and derated ampacity, then recommends the smallest listed conductor that passes both tests.
Recommended marine conductor6 AWG (13.3 mm^2)
- Controlling constraint
- Voltage drop
- Minimum size for voltage drop
- 6 AWG (13.3 mm^2)
- Minimum size for ampacity
- 16 AWG (1.31 mm^2)
- Actual voltage drop (V)
- 0.25
- Actual voltage drop percentage
- 2.05%
- Estimated voltage at load (V)
- 11.75
- Wire power loss (W)
- 3.69
- Derated conductor ampacity (A)
- 120
- Ampacity remaining
- 87.5%
Uses copper conductors with 105 C insulation, the listed standard AWG sizes, and round-trip circuit length.
Applied bundle factor 1.00 and engine-space factor 1.00.
How to use this calculator
- Choose the nominal DC system voltage for the circuit.
- Enter the maximum operating current and the one-way cable distance from source to load.
- Choose the allowable voltage drop for the equipment served by the circuit.
- Enter the number of loaded conductors in the bundle and whether the run passes through an engine space.
- Read the recommended conductor and the controlling constraint before selecting terminals or overcurrent protection.
How the wire size is calculated
Marine DC wire sizing has two separate checks. One check limits voltage drop over the round-trip circuit length. The other checks whether the conductor still has enough ampacity after bundle and engine-space derating. The recommended size is the larger conductor required by those two checks.
The voltage-drop area is calculated with copper's constant for ohm-circular-mil per foot:
CM = 2 × K × I × L ÷ ΔVmax
- K is 10.75 for copper at the condition used by the table.
- I is the circuit current in amperes.
- L is one-way cable length in feet.
- ΔVmax is nominal system voltage multiplied by the allowed drop percentage.
The factor of 2 is important because current flows out and returns through a conductor. A 20 ft one-way run is a 40 ft conductor path for voltage-drop math.
The calculator then rounds up to the next listed AWG size from 16 AWG through 4/0 AWG. It does not interpolate a made-up conductor size. Separately, it applies the ampacity table for 105 C copper conductors, multiplies by the bundle factor, and multiplies again by 0.85 when the cable passes through an engine space. Four current-carrying conductors starts the first bundle derating tier, so a circuit can be ampacity-limited even when the voltage-drop number looks small.
What moves the result
Length, current, and voltage-drop limit usually move the recommended size the most. Doubling the one-way run or doubling the load current doubles the circular-mil area needed for voltage drop. Moving from a 10% limit to a 3% limit more than triples the required area, which is why electronics, navigation loads, and other critical circuits often need larger cable than a simple ampacity check would suggest.
Bundle count and engine-space routing affect the ampacity side. A large harness in an engine compartment can require a larger conductor even on a short run because the table ampacity is reduced before it is compared with the load current.
What this calculator leaves out
This tool does not choose a fuse or breaker, check terminal temperature ratings, verify ignition protection, calculate short-circuit current, or decide whether parallel conductors are acceptable. It also assumes copper marine cable with 105 C insulation. If the result is beyond 4/0 AWG, or the circuit is a high-current battery, inverter, windlass, thruster, or propulsion run, treat the result as a prompt for a full marine electrical design rather than a final installation decision.
Worked example
For a 12 V circuit carrying 15 A over a 20 ft one-way run with a 3% voltage-drop limit, the maximum allowed drop is 0.36 V. The voltage-drop formula gives about 17,917 circular mils, so the voltage-drop test rounds up to 6 AWG. The ampacity test only needs 16 AWG because a 16 AWG 105 C conductor is listed at 25 A with no bundle or engine-space reduction in this example.
The larger requirement is 6 AWG, or about 13.3 mm^2. At that size, the estimated drop is about 0.25 V, or 2.05%, the load sees about 11.75 V, and the wire dissipates about 3.69 W.
Common questions
Why does marine wire sizing use the round-trip circuit length?
A DC circuit has an outbound conductor and a return conductor, and both add resistance. If the equipment is 20 ft from the source, the voltage-drop calculation uses 40 ft of conductor path. The calculator asks for one-way length so the measurement is easy, then doubles it internally.
When should a boat circuit use a 3% rather than 10% voltage-drop limit?
A 3% limit is commonly used for critical loads such as navigation lights, electronics, bilge blowers, and other equipment where low voltage can affect safety or operation. A 10% limit may be acceptable for many non-critical loads. The equipment manual, ABYC guidance, and the boat builder's design should control the final choice.
Why can ampacity and voltage drop recommend different wire sizes?
Ampacity is about heat and insulation temperature, while voltage drop is about resistance over distance. A short high-current run may be controlled by ampacity. A long moderate-current run may need a much larger conductor to keep enough voltage at the load.
Do negative and positive conductors both count in a bundle?
Conductors that normally carry load current count for bundle derating. In a two-wire DC branch circuit, the positive and negative conductors both carry current. Equipment grounding conductors that normally carry no current are excluded from the bundle count.
Can automotive wire be used instead of tinned marine cable?
Automotive primary wire may not have the same stranding, insulation rating, oil resistance, or corrosion resistance expected in a marine installation. This calculator assumes copper marine cable with 105 C insulation. Check the current standards and the equipment manufacturer's requirements before substituting cable types.