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Marine Voltage Drop Calculator

Marine DC circuit inputs
12, 24, 32, or 48 V battery systems are typical.
Source to load. The round trip is included automatically.

Uses the ABYC E-11 sizing formula with K = 10.75 for copper boat cable, the same basis as the published ABYC 3% and 10% tables. Copper conductors only; distance is one-way and the round trip is included. Voltage drop only; check ampacity separately.

Results

Voltage drop
0.31 V
Percentage
2.59%
Voltage at load
11.69 V
ABYC limit
3%

Within the ABYC E-11 3% limit for critical circuits

Show calculation details
Method
ABYC E-11 (K = 10.75)
K factor (copper)
10.75 Ω·cmil/ft
Circular mils
10,380
Round-trip length
30 ft

Voltage drop = K × I × L ÷ CM, where K = 10.75 Ω·cmil/ft for copper (the ABYC E-11 constant), I is the load current, L is the round-trip conductor length, and CM is the conductor's circular-mil area. This is the same formula the published ABYC 3% and 10% sizing tables are computed from.

Calculates DC voltage drop on boat wiring using the ABYC E-11 sizing formula with K = 10.75 for copper boat cable, the same basis as the published ABYC 3% and 10% tables. Select the circuit type and the tool checks the run against the applicable limit: 3% for critical circuits, 10% for non-critical, and recommends the next cable size when the run exceeds it.

How to use this calculator

  1. Enter the system voltage (12, 24, 32, or 48 V are typical) and the load current in amps.
  2. Enter the one-way distance from the source to the load. The round trip is applied automatically.
  3. Select the conductor size. Sizes run 18 AWG through 4/0, the range marine cable is sold in; copper only.
  4. Choose the circuit type: critical (3% limit) for panel feeders, bilge blowers, navigation lights, and electronics, or non-critical (10% limit) for general cabin loads.
  5. Read the drop, percentage, and voltage at the load. If the run exceeds the limit, the calculator suggests the next size that meets it.

Standard reference

This tool implements ABYC E-11 (AC and DC Electrical Systems on Boats), the standard that governs small-craft wiring in the US. The National Electrical Code governs premises wiring ashore, including marina and dock wiring under Article 555; aboard the boat itself, ABYC E-11 applies. E-11 is revised on a rolling cycle (the 2025 revision is current as of this writing); the K = 10.75 copper constant and the 3% and 10% voltage-drop limits for critical and non-critical circuits are longstanding E-11 provisions, unchanged across recent revisions. Unlike this site's NEC-based DC calculators, which use NEC Chapter 9, Table 8 resistance at 75°C, this tool deliberately matches the ABYC published tables; the two methods differ by roughly 20% and the difference is explained below.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

Marine voltage drop: formula, examples, and common mistakes

The ABYC formula

ABYC E-11 sizes conductors for voltage drop with a circular-mil formula. Rearranged to solve for the drop on an existing conductor:

VD = K × I × L ÷ CM
K = 10.75 (copper), L = round-trip length in feet

Where I is the load current in amps, L is the total conductor length out and back (this calculator doubles your one-way entry), and CM is the conductor's circular-mil area. K = 10.75 ohm circular mils per foot is ABYC's constant for copper boat cable, and it is what the widely reproduced ABYC 3% and 10% tables are computed from. DC has no phase, power factor, or reactance, so the calculation is pure resistance.

Worked example

Navigation lights draw 10 A on a 12 V system, 15 ft from the panel (30 ft round trip), on 16 AWG (2,580 circular mils). Navigation lights are a critical circuit, so the 3% limit applies.

VD = 10.75 × 10 × 30 ÷ 2580 = 1.25 V
VD% = 1.25 ÷ 12 = 10.4% (limit: 3%, fails)
Required CM = 10.75 × 10 × 30 ÷ 0.36 V = 8,958
Next standard size: 10 AWG (10,380 CM) = 0.31 V = 2.6% (passes)

The 16 AWG run that looks fine for ampacity drops more than 10% of system voltage, and the fix is three sizes up. That result, 10 AWG for 10 A at 30 ft round trip on 12 V, is exactly what the published ABYC 3% table shows, which is the point of using the ABYC method here.

Critical vs non-critical circuits

ABYC E-11 holds circuits to 3% when low voltage affects safety or essential operation: panelboard main feeders, bilge blowers, navigation lights, and electronics. A GPS that reboots on engine crank or navigation lights running dim are safety problems, not annoyances. Loads where reduced voltage is only a nuisance, general cabin lighting being the classic case, may drop up to 10%. When a non-critical run lands between 3% and 10%, this calculator flags it as compliant but notes it would not qualify for a critical load, and shows the size that would meet 3% if you want margin.

Why this differs from the NEC-based DC calculators

This site's DC, solar, and 12V voltage-drop calculators take conductor resistance from NEC Chapter 9, Table 8 at its published 75°C basis. ABYC's K = 10.75 corresponds to a cooler conductor, so Table 8 predicts roughly 20% more drop for the same run. Neither number is wrong; they are different standards with different basis temperatures. For a boat, agreement with the ABYC tables is the accuracy bar, because those are the tables a marine surveyor or ABYC technician will check the installation against. For a land vehicle, battery bank, or PV system under the NEC, use the NEC-based tools instead.

Common mistakes

  • Comparing a one-way distance against the ABYC printed tables. The published tables are indexed by round-trip length. This calculator takes the one-way distance and doubles it internally; to compare against a chart, look up twice your one-way run.
  • Sizing for ampacity only. On a 12 V system, voltage drop, not ampacity, sets the conductor size for almost any run longer than a few feet. Check both and use the larger answer.
  • Treating the bilge pump as non-critical. Bilge blowers and other safety-related loads belong in the 3% category. When in doubt, apply the critical limit.
  • Using solid or untinned building wire. Boat wiring is stranded, tinned copper cable (UL 1426). Solid conductors fatigue under vibration and bare copper corrodes in a salt atmosphere.

Frequently asked questions

What voltage drop does ABYC E-11 allow on a boat?

ABYC E-11 works with two limits. Circuits where low voltage affects safety or essential operation, such as panelboard main feeders, bilge blowers, navigation lights, and electronics, are held to 3% voltage drop. Circuits where a dimmer light is only a nuisance, such as general cabin lighting, are allowed up to 10%. This calculator applies whichever limit matches the circuit type you select and recommends the next cable size when the run exceeds it.

Why does this calculator use K = 10.75 instead of NEC Table 8 resistance?

Because the governing standard for boat wiring is ABYC E-11, and the ABYC sizing tables that surveyors and boatbuilders check against are computed from K = 10.75 ohm circular mils per foot for copper. NEC Chapter 9, Table 8 lists DC resistance at a 75°C basis, roughly 20% higher, so a Table 8 based marine tool would disagree with every published ABYC table. The difference is a documented methodology choice, not an error; for land-based DC circuits under the NEC, use the DC Voltage Drop Calculator instead.

Do I enter one-way distance or round-trip conductor length?

Enter the one-way distance from the power source to the load; the calculator doubles it internally, since the positive and negative conductors each carry full current and each drop voltage. The printed ABYC tables are indexed by round-trip length, so when comparing against a published chart, look up your one-way distance times two.

Why is voltage drop so much worse on a 12 V boat than on shore wiring?

Voltage drop is an absolute number of volts, and its impact is the percentage of system voltage it consumes. A half-volt drop is 0.4% on a 120 V shore circuit and 4.2% on a 12 V battery system. Boat loads also draw ten times the current of the same wattage at 120 V, which multiplies the drop again. That is why boat cable steps up in size so quickly with distance, and why a 24 V or 48 V house bank relaxes the problem considerably.

Does this calculator cover ampacity and overcurrent protection?

No. It evaluates voltage drop only. ABYC E-11 also requires the conductor to carry the load within its ampacity rating, with derating for engine spaces and bundling, and requires overcurrent protection sized to the conductor. Check both requirements and use the larger conductor of the two answers. Aluminum conductors are not used for boat wiring under ABYC E-11, which is why this tool is copper only.

Can I use regular THHN building wire on a boat?

Marine practice calls for stranded, tinned copper boat cable rated for the marine environment (UL 1426). Solid conductors fatigue and break under vibration, and untinned copper corrodes quickly in a salt atmosphere. The voltage drop math applies to the conductor size regardless of insulation type, but conductor selection should follow ABYC E-11 construction requirements, not premises wiring practice.

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