Electric motor, what wire to extend power cable?

[shaun_s]

hey guys,
i got a new 44 lb bow mount watersnake with a pretty short power cable. my battery is under the front cast deck and still doesnt reach.
i need to extend the power cable, but not sure if there is a specific cable or size of cable i need to buy to avoid any power loss?
has anyone else had to extend their cable?
also is it necessary to wire in a circuit breaker?
cheers for any help

[TheRealAndy]

8 gauge (or 8 B&S) from Bias. You should have a breaker wired in close to the battery. Not sure what current the water snake draws, but from memory i bought a 50A breaker for my 55lb motorguide.

[NormC]

Apart from safety, the objective is to limit voltage drop between the battery and the motor. Voltage drop is a factor of cable length (the longer the higher the drop), cable thickness (the thinner, the bigger the drop) and current (the higher the current, the bigger the drop.

I've always used at least 10mm squared cable for this purpose and currently use 13mm squared (6 B&S). 8 B&S cable is 8mm squared. In my view, 8 B& S will be OK if the overall distance is not more than 2 metres. Any longer, I'd be using thicker cable.

I also use a circuit breaker and strongly recommend you do the same. Put it as close as possible to the positive terminal of the battery. I use the 60 Amp breaker near the bottom of this page, which can also be used as a switch to isolate the battery when not in use.

http://www.sidewinder.com.au/page27.html

You can get cable etc from the same business via mail. Derek Bester owns the business and is very helpful and reliable.

Norm C

[shaun_s]

thanks heaps guys that info is very helpful
exactly what i needed to know

[TheRealAndy]

I have a 3 metre run of 8 guage and its not a problem. I have 2 anderson plugs in series and as Chris Ryan will confirm I had a fire on my boat at 1770 thanks to corroded andersons!! They all thought I was onto a fish when I was yelling out that we have a fire!

Anyway, I am a sparky and I had known that there was already problems beforehand and I ignored it. Make sure that all you connections are good and clean and are coated in something that will stop corrosion.

[bearessentials]

voltage drop is the reason i brought a 24v electric motor for my boat... i only have a 4.2 m boat, but the battery's are located down the back. i ran 16mm squared flex cable, but this is probably a bit of over kill.. voltage drop is a big killer of electric motors, some more so than others..

[TheRealAndy]

Voltage drop is pretty easy to calculate. Just need to know the distance and cable size.

[NormC]

Just to ensure there is an understanding of the impact of wire size, distance and current on voltage drop, this is the formulae to calculate it.

Voltage drop = Length of conductor in metres x Current in amps x 0.017 divided by conductor cross section is sq. mm

Length of conductor is the RETURN LENGTH. That is, if your pair of cables from battery to electric motor is 3 metres, the return length is 6 metres.

As an example, lets say the cable pair is 3 metres long, the return length is 6 metres.
I'm not sure of the current draw on your motor, but for the moment, lets say that flat out, it is 40 Amps (won't be far wrong).
8 B&S cable has a cross sectional diameter of 8 sq mm

So the calculation is 6 x 40 x .017 / 8

The voltage drop is therefore .51 volts.

Doesn't sound much, but (depending on type of battery), a fully charged battery is around 12.8 Volts. At 70% capacity it is around 12.3 Volts (.5 Volts less than fully charged).

You might think that this only means a loss of 30% of the battery's capacity. But (again depending on type of battery), it is normally not recommended to take most deep cycle batteries below 50% of capacity as doing so severely shortens their life. At 50 % State of Charge, most batteries are around 12.06 Volts. So you have lost close to 70% of the battery's usable capacity through voltage drop.

With this formula, you can do some research, then do your own calculation, based on your circumstances.

Changing just one thing - from 8 B&S cable (8mm sq) to 6 B&S (13mm sq) will reduce the voltage drop from .5 Volts to .3 Volts, a decrease of 40% in Voltage drop.

I'm not looking to start a debate on what works and what does not. Just making you aware of the facts and importance of wire size in 12 Volt systems when dealing with high currents.

8 mm wire will work fine. 13mm will work better. The difference will be in increased time you can run your motor from your battery.

Hope this helps.

Norm C

[TheRealAndy]

Voltage drop for a 44lb motor is going to be roughly 0.2V per meter using 8g cable at full thrust. Most of the time you wont be operating at full thrust, so drop is even less.

The 8g stuff is fine. Anything over 10% drop I would consider unacceptable.

There will be no noticable difference on running time.

For what its worth, I would be more concerned about connections. In my recent incident I lost a 1/2knot of top end speed because of corrosion in the Anderson plug. You will probable get more voltage drop in connections and the breaker than you will in the cables.

[NormC]

Voltage drop for a 44lb motor is going to be roughly 0.2V per meter using 8g cable at full thrust. Most of the time you wont be operating at full thrust, so drop is even less.

The 8g stuff is fine. Anything over 10% drop I would consider unacceptable.

There will be no noticeable difference on running time.


Just keep in mind that if you have a 10% voltage drop, your fully charged battery of 12.8 Volts, will deliver 11.52 Volts to the motor. This is the equivalent of just 20% capacity for most deep cycle batteries and possibly not enough to drive the motor. A less than fully charged battery (in real life, that's where most batteries sit), will be seen by the motor as close to dead flat. Running time on such a set up would be dramatically below an optimum set up.

Small numbers make big differences in 12 Volt systems. 10% voltage drop does not sound like much, but in 12 Volt systems it is the difference between a good efficient system and an inefficient one, delivering well below optimum performance.

Remember that just because your system works, does not mean it is at it's optimum. Many people have less than optimum set ups without knowing it. Their set up works and they learn to live with the limitations without ever fully understanding that with minor changes, it could be much better. A small change like heavier or shorter cable (costing very little) might add up to 50% to available battery power. What a difference to a days fishing that could make.

Anyway, enough from me on this. There is enough info on this thread now for people to make a judgement on how to best select wire size and length for this application.

Norm C

[TheRealAndy]

11.5 v is fine. I have a 130 ah battery not connected to the outboard charger and I can get 3 full days use of my 55lb motorguide before I need to charge it.

I estimate I had about 2V+ drop in that anderson (probably more, enough to make it smoke in the end), and I only lost 1/2knot of top endspeed. May not be optimal but I still managed ok in NQ currents on big tides (only just).

If you are worried about 1/2knot at full thrust, then you need to be considering a larger electric.

[shaun_s]

thanks for your input guys
appreciate it