Bow and stern thruster wiring: cable size and fuse, in AWG

What Sleipner's cable table says

Sleipner is unusual in publishing a cable table at all. It is more unusual in printing an AWGAmerican 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. equivalent beside every metric size.

Its current guides print two sizes for each length of run: a minimum and a recommended one. The table below uses the recommended size, the same one each product page shows. In all 15 bands read here, the AWG size is the nearest match for the metric size beside it.

The drop column is the useful part. It is computed at the top of each band, where the run is longest and the drop is worst. There, Sleipner's recommended cable lands between 3.4% and 7.8%.

Of the 15 bands, none meets ABYC's tighter 3 percent figure, and none exceeds the 10 percent one at the very top of the band.

A run shorter than a band's limit does better, which is why the makers publish bands.

Sleipner also publishes a minimum battery for each model, in cold cranking ampsCold cranking amps: how much current a battery can deliver in a short, hard burst, which is the rating starter batteries carry. A thruster draws like a starter, so the makers size its battery this way rather than by capacity.. Where the battery should live, what capacity it needs and why sagThe 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. costs you thrust have their own page: bow thruster batteries.

What Sideshift publishes: a start current, a cable size and a distance

Sideshift's external unitsA thruster that bolts to the outside of the hull, up near the bow or on the back of the boat. Its propeller hangs in the water below it. Nothing is cut through the hull, so there is no tunnel. carry their own motor controller. The maker publishes three things about the cable to it:

• a start current per series
• one AWGAmerican 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. size per series
• how far away the battery may sit

The installation manual says the total cable run from the battery to the motor controller must not exceed 10 ft. The table computes the drop over that run.

Inside the manual's 10 ft, the maker's own cable holds the drop to 0.6% to 3.6%, even at the start current.

That is inside ABYC's 10 percent allowance on every series. It is under the tighter 3 percent on three of the four.

The start current is a peak that lasts a moment, so the steady figure is lower. The maker does not publish the steady figure.

Keep the batteries within the manual's 10 ft and the maker's size computes inside the allowance.

The start currents run from 200 A to 550 A across the four series. The battery Sideshift asks for, and why it wants a starting battery rather than a deep-cycle one, is on the battery page. The Sideshift review reads the rest of the manual.

What VETUS publishes: a table per manual, built to half the drop its text allows

Every VETUS installation manual ends with the same table, headed "Battery capacity, battery cables". It gives:

• the smallest and largest battery the unit may have
• the total run of positive plus negative cable, in bands
• the cable size for each band in mm², with an AWGAmerican 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. size beside it
• the main fuse, with its part number

The text above the table says the drop "must not be more than 10% of the voltage supplied".

The table does not use that figure. Of the 42 rows, 27 come out between 4.3% and 5.1%. Those are every brushed unit but the BOW28548D, every 12 and 48 V BOW PRO but the BOWA0304, and the smallest Boosted unit. The drop is computed at the stated current, at the top of every band, on the metric size printed.

The other 13 Boosted units are built tighter, 0.3% to 3.3% of their 24 or 48 V bank. Two rows follow neither pattern, and the notes below say why.

A run inside a band does better than its top, so VETUS's minimum cable carries more margin than its text asks for. Of Sleipner's 15 bands, 8 compute to more than the five percent rows' highest.

The fuse VETUS names

VETUS prints a number. Every installation manual's table names a slow blow main fuse with its part number. On the brushed units it sits at 60% to 73% of the stated draw. For example, the BOW7512D draws 550 A and takes a 355 A fuse, ZE355.

Every BOW PRO but one runs the other way. There the main fuse is above the nominal draw, 104% to 222% of it. On the 110 to 250 mm units a smaller thruster fuse sits in the controller under it. The BOWA0651, which draws 237 A at 12 V, takes 300 A and 250 A.

The one exception is the BOWB420. It has a 500 A fuse on a 520 A nominal draw, 96% of it, and no thruster fuse. The full set is in the table below.

The AWG column needs the same reading as Sleipner's, and it points the other way. Where Sleipner's AWG size is the larger conductor, VETUS's is the smaller. On every size but 150 mm², it names the AWG size below the metric figure by area.

Two manuals are the exception. In the 110 mm and 125 mm BOW PRO manuals the aught sizes (1/0 to 4/0) are written "AWG 0, 00, 000, 0000". Those two give 50 mm² as AWG 0, the nearest size. Every other VETUS manual gives AWG 1.

Buy to VETUS's AWG column and at 25 mm² you have 15% less copper than the metric column asks for. The band tops then compute to as much as 6.0%. That is still inside the manual's 10 percent, because the table started at half of it.

If you want the cable the table was built to, buy the metric size, or go one AWG size up from the one printed.

The table has changed over the years. The December 2019 revision of the BOW55 manual gave each voltage one band. At 12 V it was 70 mm² (printed "AWG 00") to 12 m, 40 ft, which computes to 9.2% at the stated 375 A. At 24 V it was 35 mm² to 23 m, 9.7%. That table was built to the ten percent the text states.

The May 2026 revision prints 11 bands per voltage at the same 375 A, and the same 70 mm² now reaches only 6.4 m. The older table also printed the 24 V fuse as 125 A beside the part number ZE250, which is the 250 A fuse. The new one prints ZE125.

If your manual came in the box a few years ago, check the current one on vetus.com before you buy cable.

Two more things in the manuals. The BOW PRO tables name two fuses: a main fuse at the battery and a smaller "thruster fuse" inside the motor controller. The manual says this of the thruster fuse: "This fuse protects the controller and motor against short circuit/overloading and it must remain present under all circumstances".

The main fuse figure comes with a condition: "The recommended value of the main fuse applies exclusively to a power supply system in which the bow thruster is the only power consumer." The 110 mm manual also has an odd row. Its BOWA0304 row's feet column is not its meter column converted. There 48.2 m is printed as 140.4 ft, and every band in that row is off by the same share. The drop for that row is computed from the meters.

The larger manuals, read 15 September 2026, add these:

• The 48 V BOW28548D manual prints the 24 V BOW16024D's table line for line: the same 560 A, battery, fuse and bands. At twice the voltage the same cable loses half the share, so its bands compute to 2.5%.
• In the 185 mm Boosted manual the BOWB065 is built to five percent like the brushed range, 4.9%. Its four siblings on the same page sit at 2.3% to 3.3%. The same manual's BOWB130 row prints one band backwards, in both the meter and the feet columns. It reads 13.7 to 19.7 m for 150 mm², then 19.7 to 18.6 m for 2 × 95 mm².
• The 300 and 400 mm Boosted manuals print one fuse, not two, and it sits closer to the draw than the smaller units'. It runs 96% to 116% of the nominal figure, and on the BOWB420 it drops below: 500 A on a 520 A draw. On every smaller BOW PRO the main fuse sits above the draw with a thruster fuse in the controller under it.
• The 125 mm BOW PRO manual prints a larger current for the smaller unit, at both voltages. The BOWA0361 (36 kgf) draws 240 A against the BOWA0421 (42 kgf) at 218 A.
• A Boosted unit's bank is charged from the boat's 12 or 24 V system, and the 185, 300 and 400 mm manuals print a second table for that feed cable. These are looser than the thruster tables: at the charger's printed current the 24 V feed rows compute to 7.4% to 11.1% at their worst band top. The 250 mm manual prints no feed table.
• The 185 mm manual prints one feed table under both a 12 V and a 24 V heading. At the charger's 80 A the 12 V row computes to 18.4%, well over the manual's own 10 percent. That table's feet column is also not its meter column converted (40 m printed as 35.1 ft).
• The RIMDRIVE manual (2017) prints no band table. It gives a cable range per model (RD125: 35 to 50 mm²; RD160: 50 to 70 mm²). It names a 200 A main fuse (ZE200) and a 425 A fuse in the connection unit. It has one rule for the larger size: "Use the largest cable diameter for cable lengths of more than 10 m ( 33 ft) and/or for an expected continuous use of more than 5 minutes."
• The BOW PRO user manual (June 2026) prints one fuse per model. It matches the installation manuals above on every one of the 22 models they share. Where an installation manual gives two fuses, it matches the controller fuse. Where it gives one, it matches the main fuse. The October 2024 revision printed four of them differently: BOWA0304 125 A, now 80; BOWA0651 300 A, now 250; BOWB210 300 A, now 250; BOWB285 355 A, now 425.

The VETUS review reads the rest of these manuals. The battery page has the minimum and maximum battery for each model and what changed between revisions.

What Lewmar publishes: one cable and one run per model, built to about a volt

Lewmar's Gen 2 installation manual has a table headed "Correct cable sizes". It gives each model and voltage three figures: a stated current draw, the smallest cable in mm², and the longest run that size will carry. There is no AWGAmerican 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. column.

The table carries a note about the run: "Cable length is total from battery to thruster and back."

We computed each row at the stated current, on that cable, over that run. Every one of the 15 rows loses between 0.83 V and 0.96 V. That is a fixed number of volts, whatever the system voltage:

• 6.9% to 7.8% on the seven 12 V rows
• 3.7% to 4.0% on the six 24 V rows
• 1.8% to 1.9% on the two 48 V rows

So a 12 V Lewmar at its longest allowed run sits inside ABYC's 10 percent and outside its 3 percent. A 24 V one lands within a point above the 3 percent figure. A 48 V one is under it.

The manual's own floor is looser than its table. It says: "The minimum voltage at motor when running should be 10V for 12V and 21V for 24 V units". That allows 2 and 3 volts of loss.

Read the US size column with care on the big units. Of the 15 rows, seven name a metric size with more copper than a single 4/0:

• On five of them, a pair of one size per leg holds it.
• On the other two, the metric size holds more copper than two 4/0 per leg, past any common marine cable. A shorter run is the common-cable answer there.

Industrial cable above 4/0 exists, but it is not common marine stock, and ABYC's table stops at 4/0.

The fuse column runs the way VETUS's brushed units do above. The ladder prints a fuse for every electric model, and every one is below the stated draw, 54% to 95% of it.

Three 24 V 185 mm units share one 130 A fuse: the 3, 4 and 5 kW, drawing 160 A, 235 A and 240 A. On the 5 kW that fuse is 54% of the draw.

The 300 10.8 kW at 24 V has two fuses on the ladder for one current figure: 400 A on the aluminum-gearbox part number and 500 A on the bronze.

Lewmar's whole battery rule is one sentence: "Battery crank capacity should be at least equal to the thruster current."

The battery page puts it beside the other makers'.

Under the table sits a worked example: "a total of 28 m (92 ft) of cable with a model 250 8.0 kW thruster would need 95 mm CSA (000 AWG) cable".

The row above it says 185 mm² to 20 m. At the row's 500 A, the example's cable over the example's run loses 2.54 V. That is 10.6% of 24 V, over ABYC's 10 percent and 2.7 times the row's own figure.

The example belongs to the previous manual. Issue 2 covered the 140TT, 185TT and 250TT. It printed a table of the longest run for each of eight cable sizes, and its 250TT 8.0 kW cell on 95 mm² reads 28 m.

The Gen 2 manual replaced the table and kept the example. Its "000" is the AWG size nearest 95 mm² by area, 3/0. It is the one place the section gives an AWG size at all.

The old table was built to a share of the voltage, and a bigger one:

• the four 12 V rows lose 1.10 V to 1.93 V, 9.2% to 16.1%
• the five 24 V rows lose 1.73 V to 2.60 V, 7.2% to 10.8%

Both straddle ABYC's 10 percent. Every cell of the old table allows more loss than any row of the Gen 2 table.

Its feet table is not the meter table converted. Of the 49 cells that appear in both, 43 are more than a foot away from a conversion, because each table was worked on its own sizes.

For example, the 140TT 2.2kW's 35 mm² cell is 10 m, 32.8 ft, and its AWG 2 cell is 30 ft. If you own a pre-Gen 2 unit and size from its own manual, you are sizing to that older, looser table.

The Lewmar review reads the rest of both manuals: the warranty's one-year exceptions, the run-time rule, and the boat-length bands the maker does not publish.

What Quick publishes: cable by run length and a fuse, and no current

Quick's BTQ installation manual gives every model two things. The first is a fuse, as a CNL DIN rating (a bolt-down fuse standard). The second is a cable cross-section in three bands of total run.

The cable table is footnoted "L = positive cable + negative cable".

The bands are under 5 m, 5.1 to 10 m, and 10.1 to 20 m. Each gives a metric size with an AWGAmerican 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. size in brackets, and "2 x" where a leg needs two conductors.

Neither of its manuals gives a current draw. Without one, no drop can be computed. The warning in the draw section about working amps from kilowatts applies here as much as anywhere. The table prints what Quick prints and computes only what the table itself can say.

The last column is the table's own shape. At one current, the loss over a run is the run divided by the copper. So meters per mm² at the top of a band measures the loss that band accepts, up to the current the maker leaves out.

On the long band it is 1.5 to 2.3 times what it is on the short band. Quick accepts between one and a half and two and a third times the loss at 20 m that it accepts at 5 m. A short run on Quick's minimum cable carries the margin, and a long one does not.

VETUS's tables above work the other way. On 40 of its 42 rows, the bands spread by no more than 1.3 times. The two that spread more, the BOWB385 and BOWB420, are the rows whose table offers two sizes for one run.

Quick's AWG column rounds down like VETUS's. It uses five pairings: 35 mm² as AWG 2, 50 mm² as AWG 1, 70 mm² as AWG 2/0, 95 mm² as AWG 3/0, 120 mm² as AWG 4/0. All of them name the AWG size below the metric one by area. They run from 85% of the metric copper on 50 mm² to 96% on 70 mm².

One cell, the BTQ 3030048's middle band, prints "2 x 70 mm2" beside a bracket of "AWG 2/0" with no "2 x".

The BTQ250 and BTQ300 manual (REV006C) prints one "Section of wire" per model and no run length. Set that against the three-band table's 10.1 to 20 m band for the same model.

It is the same on three models (BTQ 2524024, BTQ 3025024, BTQ 3030048). It is smaller on four (BTQ 2512012, 2 × 95 mm² against 2 × 120 mm²; BTQ 2512024, 120 mm² against 2 × 70 mm²; BTQ 2514024, 120 mm² against 2 × 70 mm²; BTQ 3027024, 2 × 95 mm² against 2 × 120 mm²). On the BTQ 2512012 and BTQ 3027024 it is the three-band table's middle band.

The REV006C fuse column agrees with REV003A on six of the seven models. The BTQ 2524024's fuse cell prints "(2 x AWG 3/0)" where REV003A prints 500 A.

The fuse is the one current-shaped figure Quick prints, and it is not a draw. Two makers above print both. On each of them, every brushed unit's fuse sits below its stated draw: VETUS's at 60% to 73% of it, Lewmar's at 54% to 95%.

Read Quick's 130 A on the BTQ 1102512 as the fuse it is. Get the draw from the installer or the maker before sizing anything from it.

The Quick review reads the rest of both manuals, the run-time rule that lives in the control panel's manual, and the three sets of boat-length bands.

What Max Power publishes: a cable table at two currents, and a fuse sized to the cable

Max Power's CT100/CT125 manual gives each of its two units two current figures, two pages apart. The first is on the performance page, where the data "is measured with an approx. consumption of 680A and 11V for the CT100, or 430A and 22V for the CT125".

The second is in the fuse table, which gives the same units a "Maximum Amperage" of 550 A and 330 A. The cable table is four bands of total run, both legs counted, in mm² with no AWGAmerican 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. column.

We computed the drop at both currents:

• At the maximum amperage, the CT100's bands hold 4.2% to 5.3% of 12 V, and the CT125's 2.4% to 4.0% of 24 V.
• At the measured consumption, they hold 5.1% to 6.5% and 3.1% to 5.1%.

Every band is inside ABYC's 10 percent at both currents. Only one of the eight, the CT125's shortest, is under the 3 percent figure at the maximum amperage.

The manual says how the fuse is sized: "on the basis of the cable sections in the circuit and NOT the amperage drawn by the appliance".

Both fuses sit below both currents on both units. That is the same direction as every VETUS brushed unit and every Lewmar row above.

Of the eight bands, four name more copper than a single 4/0. A US owner running a CT100 more than 5 m there and back is into a pair of one size per leg.

Past 10 m, the manual's size holds more copper than two 4/0 per leg, past any common marine cable. A shorter run is the common-cable answer there.

The Max Power review reads the rest of the manual. It covers the battery the manual asks for, the one an owner was told to fit, the run-time rule and the selection chart.

What each maker publishes, and what none does

Six makers put figures on this page. Sleipner, VETUS, Lewmar and Max Power published a current and a cable table each, Max Power with two currents. Quick published a cable table and a fuse with no current. Sideshift published a start current, a cable size and a distance.

Here is what this page holds for each brand: