Install
Bow and stern thruster wiring calculator: gauge and voltage drop
Pick your thruster, or type what it draws, and say how far the battery sits from it. You get the cable size in American wire gauge, the size scale for electrical cable. The number falls as the cable gets thicker: 2 AWG is thicker than 6, and 1/0, 2/0, 3/0 and 4/0 are thicker still., with the battery and the fuse your maker asks for. It can also check the cable already in your boat and show the volts it loses before the current reaches the motor.
It sizes the cable for a stern thruster too. The picker holds 28 units made only for the stern, from ezDrive, Max Power, Sideshift, Sleipner Side-Power and Yacht Thruster, each with the draw its maker publishes.
This is a reference estimate from the numbers you entered, not a substitute for a marine electrician, a surveyor or an ABYC-compliant inspection of your boat. It is provided as guidance only, without warranty, and checking it is yours to do before you rely on it, especially for fuse and breaker sizing. Terms of use.
The short answer
- Start with the maker's draw. The current your thruster draws is the number everything hangs on. Take it from the installation manual for your model at your voltage. Never work it out from the kW rating (the motor's power) divided by the volts.
- Measure the run one way. Measure from the battery to the thruster along the route the cable takes. The arithmetic doubles it for the return leg.
- ABYC gives two drop limits. E-11, ABYC's electrical standard, asks for 3 percent where voltage drop must be kept to a minimum. It allows 10 percent where drop is not critical. A thruster is named in neither list, so you decide.
- The larger size wins. The E-11 minimum is the larger of two sizes: the one the The most current a cable can carry all day without overheating, as the standards table it. It depends on the size of the copper and how hot the space it runs through is. table asks for (the most current a cable may carry all day) and the one the The volts lost in the cable between the battery and the motor. A long or thin cable loses more, and the motor only ever sees what is left. asks for.
- The maker's own figures sit beside ours. Where this site holds them, the maker's cable, run limit, battery and fuse print with a link to the manual they came from. Check your cable against that manual.
Size your cable in four steps
- Get the draw from your manual. Pick your thruster from the list, or type the draw from the installation manual for your model at your voltage. Do not work it out by dividing the motor's kW rating by the voltage. That misses amps, and the wiring page shows how many against Sleipner's own published figures.
- Measure the run. Measure from the battery to the thruster, one way, along the route the cable takes. Tick the engine compartment box if any of the cable runs through it, because ABYC rates a cable lower in engine heat. The run gets doubled, and the section “How the arithmetic works” below says why.
- Choose your drop limit. Read the E-11 minimum and the size for the tighter limit side by side. The section “What the calculator gives you” says what each figure means.
- Check it against the maker. Hold the result up against the maker's cable, run limit, battery and fuse, and against the manual they come from. The section “What it does not tell you” covers the part a cable size cannot.
Already have cable in the boat? Choose "Check cable I already have" at the top of the calculator, then pick the size printed on the jacket. You get the volts it loses before the current reaches the motor.
What the calculator gives you
Sizing new cable gives you three figures, the shopping list.
- The cable. The size the maker's own table calls for on your run, where this site holds that table. A line under it says how that size stands against The American Boat and Yacht Council, the body that writes the standards US boats are built and wired to. A surveyor or an insurer who asks whether wiring is to standard means ABYC’s.'s table. Where the maker's size is rated below your draw, the line says what the maker relies on and the size that has ABYC's full margin. Where no maker's table is held, the cable is the E-11 minimum.
- The battery. The bank the maker asks for, in cranking amps.
- The fuse. The fuse the maker's manual names.
ABYC's own figures sit in the Details fold under them. There are three. The first is the E-11 minimum: The American Boat and Yacht Council’s standard for electrical systems on boats, the one US surveyors and insurers check wiring against. Its tables set the most current each cable size may carry and the most voltage a run may lose. sizes a cable by its ampacity table and by voltage drop, and takes the larger. The second is the same rule at ABYC's tighter drop limit. The third is the heating time: how long the copper takes to reach its insulation rating at your draw with no cooling, beside the run your maker allows. A chart of every cable size opens under the fold.
Checking cable you already have works the other way. Add the size printed on the jacket. You get the volts it loses before the current reaches the motor. The same maker, heating, battery and fuse figures sit beside it.
The ABYC arithmetic runs off the maker's stated draw and ABYC E-11, and the result shows it line by line. The maker's cable, the battery and the fuse are the makers' own figures. Each one links to the manual it came from, and this site computes none of them.
How the arithmetic works
Voltage drop
The voltage a cable loses is its resistance times the current. A longer cable loses more. A thicker one loses less. Copper's own figure sets the rate.
The length is the total run, positive plus negative. That is why the form asks for the one-way distance and doubles it.
Sizing new cable solves that sum for the area, then names the smallest AWG size at or above it. Checking fitted cable solves the same sum for the volts instead. Above 4/0 the answer becomes two conductors per leg, of the smallest size two of which do the job. That is what the makers' own tables do on a long run.
ABYC's two limits
ABYC E-11 asks for 3 percent on circuits where voltage drop must be kept to a minimum. It allows 10 percent where drop is not critical. A thruster is named in neither list, so you decide.
The ampacity table
E-11 also has an ampacity table. Ampacity is the current a conductor carries all day without its insulation overheating. Section 11.15.1.2.7 of the July 2025 edition says to check the size against that table and the drop table, and take the larger where they conflict.
The same section says a motor's cable should meet or exceed the maker's own sizing. So the size the standard points to is the heavier of the maker's cable and the E-11 minimum.
The E-11 minimum here is that larger size. Where one conductor is not enough, two per leg are counted together. The 3 percent figure takes the larger size the same way, so on a short run with a big draw the two figures can be the same cable. The wiring page shows where the makers' own sizes sit against the ampacity table: mostly below it at short runs.
The heating time
The heating time is why a maker's table and ABYC's table can disagree about one cable. ABYC's rating is continuous. A thruster runs for seconds to a few minutes, and what carries a burst is the copper's own heat capacity.
The calculator works out how long the named cable takes to warm from the table's 30 C ambient to its 105 C rating at your draw. It assumes no heat leaves the cable at all, which is the case the IEC 60949 equation describes (the adiabatic case, in the standard's own word). The result prints that time beside the run your maker allows.
A real cable sheds some heat over minutes, so the true time is longer. The figure is a floor, and the run limit is the maker's rule.
The fuse
The fuse protects the cable. ABYC puts it at the battery end, within 7 inches of the source. The fuse is the maker's figure, and the site computes none.
ABYC treats a motor's fuse apart from a light's. The fuse, or the thruster's own protection, must keep a jammed motor left switched on from starting a fire. The wiring page explains why the fuse is rated below the draw, and what ABYC asks of two cables per leg.
Marine battery cable is stranded and usually Cable whose copper strands are coated in tin, so they do not corrode in salt air. Most marine battery cable is tinned., and both raise resistance a few percent over the solid annealed copper the arithmetic assumes. Copper also gets more resistive as it warms. A computed size is the floor: if you are between sizes, buy the larger one.
What it does not tell you
The volts at the motor assume the battery holds its full voltage. It will not. A battery under a thruster's load The drop in a battery’s voltage while a heavy load is drawing from it. The battery recovers when the load stops, but the motor only ever sees the sagged figure. on its own, before the cable takes its share. That is why the makers print thrust at a sagged voltage.
The battery page works that half through in the makers' own figures, and says where the bank should live.
The calculator also gives no percentage of thrust lost. The volts and the makers' cutoff voltages are published numbers. A thrust figure worked from them would be ours, and it would look more exact than it is.