Bowthruster battery cable sizes

[Bloater]

I have just installed a Vetus 75kgf bowthruster and need to connect up the battery or batteries which are located adjacent to the thruster.

Reading the Vetus instructions I require cables that are 150mm2 cross section area. To me this seems very excessive. In a previous boat I has a bowthruster that had much smaller cables (something like 35mm2). I can get 35mm2 cables made up very easily, but not for the recommended size. I can't even visualise the bigger size.

Are Vetus being too particular? Are they over specifying the cable size? I plan to use either a single 120Ah AGM battery or 2x 90Ah AGM batteries. The cable run will be under 1 metre.

What cable size does the forum recommend using?

 

[rosssavage]

I would recommend the size Vetus recommend, because they recommend it....

You've just spent (presumably..) some £k on it, why scrimp on £50 of cable?

They've done the maths, worked out volt drops and safety factors and arrived at 150mm2. Use that

 

[All_at_Sea]

The cable run seems very short, do you have your battery bank well forward? The run includes out and back, so at least 2 M on your measurements. If it is this short then the drop won't be too bad, for the cables the biggger the better as the power these units use in short bursts is big.

 

[davidej]

Is 150 mm2 really required if a separate battery is fitted 1m away from the thruster - sounds more like the requirement if you are using the main batteries and a 6-10m (each way) run.

I would have thought the OP could use something much more modest.

 

[Bloater]

Each battery will be mounted on a platform directly on top of the thruster tunnel, almost touching the motor so the cable runs are very very short. In fact if I use two batteries I will be putting one each side of the motor so both (total) runs would be 1 metre maximum.

So what size cable do you actually have? I have seen plenty of boats with bow thrusters fitted but I've never seen any 'monster' cables. Does anyone ever fit the recommended size, or do manufacturers fit what they think they can get away with?

 

[pvb]

Reading the Vetus instructions I require cables that are 150mm2 cross section area. To me this seems very excessive. In a previous boat I has a bowthruster that had much smaller cables (something like 35mm2). I can get 35mm2 cables made up very easily, but not for the recommended size. I can't even visualise the bigger size.

Are Vetus being too particular? Are they over specifying the cable size? I plan to use either a single 120Ah AGM battery or 2x 90Ah AGM batteries. The cable run will be under 1 metre.

The Vetus instructions specify 150mm2 cable for up to 18m of cable run, and this is to limit voltage drop to a max of about 10%. They could have usefully quoted different cable sizes for shorter runs. Interestingly, in the Vetus catalogue, they quote 70mm2 cable for up to 8m cable run. Obviously, if your cable run is only 2m total, you can use smaller cables. I'd suggest 35mm2 would be fine, and should result in a voltage drop below 0.7v, which is acceptable.

However, the same instructions you refer to specify a minimum 143Ah battery (although the current Vetus catalogue suggests a minimum 120Ah battery). Regardless, I wouldn't run a 550A thruster on a single battery. Your suggestion of having 2 x 90Ah batteries is much better.

 

[Talulah]

The Vetus instructions specify 150mm2

Where could you actually source that size of cable from?

 

[bammylovechops]

You could try this site, it gives a cable calculation table.

http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

Worth checking twice and buying once

 

[pvb]

Where could you actually source that size of cable from?

I'm sure it's available somewhere, but most people would just use 2 or more smaller cables in parallel.

 

[markdj]

If you visit the Merlin Power Store website, they have a free program for calculating cable size and volts drop.

I'd go for less than 10% volts drop and if you only have a short run ,then 5% is better.

 

[Bloater]

What I find interesting is that the instructions state the thruster draws 550A, yet they specify just a 355A fuse.

I'm not worried about the voltage drop here, its more of a case that I don't want the wires to melt.

At the moment I'm thinking of 2x 90Ah batteries, each seperately wired by 35mm2 cable. It should do but it is not what Vetus say is required.

 

[lw395]

550A start up surge perhaps?
A fuse will blow after several seconds or more at its nominal current, it will take 10x that for a few milliseconds. There are different flavours of fuse with different time factors, slow blow, fast blo, anti-surge. rswww.com has links to manufacturers date for this kind of thing.
As well as volts drop, I would be a little concerned about temperature rise in the cables, especially if they are well enclosed. We normally expect bow thrusters to be used sparingly, but stuff happens! When you really cock up in a marina, you don't want an electrical fire as well!
Again, looking an manufacturers websites will help on allowable max temp and temp rise/current. The max temp of insulation varies a lot.
In a short cable run, a lot of heat will be lost via the ends of the cable, which helps.

 

[pvb]

What I find interesting is that the instructions state the thruster draws 550A, yet they specify just a 355A fuse.

I'm not worried about the voltage drop here, its more of a case that I don't want the wires to melt.

At the moment I'm thinking of 2x 90Ah batteries, each seperately wired by 35mm2 cable. It should do but it is not what Vetus say is required.

They specify a 355A slow blow fuse, which will cope with higher current for long enough periods to run the thruster.

If you use 35mm2 cables and wire the batteries separately to the thruster, you'll get around 0.3v drop, which certainly won't result in the cables getting too hot.

 

[VicS]

Be sure if you reduce the cable size to less than that recommended by Vetus on the basis that volts drop will be much less over a short run that you do not reduce it below the minimum size that can carry the bowthruster current safely.

Also consider the cable size in relation to the fuse value. The cable used must be able to safely carry a current of at least the fuse rating.

2 by 35mm²/AWG 2 might just be a little too small IMHO

2 by 40mm² /AWG 1 would be better

2 by 50mm² / AWG 0 better still

Volts drop on a couple of 1m cable runs at 550amps even using 2 by 35mm² will be less than 0.3 v so not the problem with such short lengths as it would be with longer cable runs.