What gauge wire to use?

[Masito]

I bought a used 12' Sears aluminum boat with a 4.5 Johnson and Minn Kota 12 v., 30 lb. thrust motor. My hope is to put the trolling motor battery as far forward to the bow as possible to distribute some weight more equally. May someone tell me what gauge wire I must use to run from the battery at the front of the boat to the trolling motor on the transom?

 

[wifisher]

Re: What gauge wire to use?

not my chart so I am not responsible for any errors, but seems to be about right.
When in doubt go bigger!!!

 

[jhebert]

Re: What gauge wire to use?

Pay no attention to that chart shown above.

In 12-Volt DC power distribution systems you must calculate the wire size based on voltage drop. Using the maximum current capacity of the wire is not useful in 12-Volt DC power distribution. The voltage drop in the conductor is the most important factor, and in a 12-Volt DC system the voltage drop limit will be reached before the maximum current carrying capacity of the wire is reached. Calculate the wire size based on voltage drop. There are many on-line calculators that can do this for you. I give a hyperlink to one below

http://www.marinco.com/files/applets/wirecalculator.html

 

[Silvertip]

Re: What gauge wire to use?

Way over engineering. This is a 12 foot boat, it has a 30# thrust motor that draws 30 amps. 8 gauge wire is more than adequate. And unless the motor gets hung up with weeds to the point of stalling it, it will never draw 30 amps except for a fraction of a second during startup.

 

[wifisher]

Re: What gauge wire to use?

Pay no attention to that chart shown above.

In 12-Volt DC power distribution systems you must calculate the wire size based on voltage drop. Using the maximum current capacity of the wire is not useful in 12-Volt DC power distribution. The voltage drop in the conductor is the most important factor, and in a 12-Volt DC system the voltage drop limit will be reached before the maximum current carrying capacity of the wire is reached. Calculate the wire size based on voltage drop. There are many on-line calculators that can do this for you. I give a hyperlink to one below

http://www.marinco.com/files/applets/wirecalculator.html

If you look at that chart it accounts for wire length, not maximum current capacity. Having looked at the chart and checked some of the figures on it, I trust it more than a calculator on a web page that is computing an unknown formula.

 

[fishrdan]

Re: What gauge wire to use?

I'm with ST, for a 30# trolling motor 8ga wire will be fine.

But,,, if you are ever going to buy/use a 40 or 50# trolling motor, I would go with 6ga wire.

 

[jhebert]

Re: What gauge wire to use?

If you look at that chart it accounts for wire length, not maximum current capacity. Having looked at the chart and checked some of the figures on it, I trust it more than a calculator on a web page that is computing an unknown formula.

There is no basis to trust the chart regarding voltage drop. It never says what voltage drop it allows for. You'd have to spend an hour trying to deduce what the voltage drop tolerance might be. Nor is there any basis to assume the chart uses a better formula than the calculator. So on both counts there is no advantage to the chart, and on the voltage drop there is advantage to the calculator.

In most cases it is better to use wire with a conductor size that is too large rather than too small. On a small boat, for a single circuit, there is little cost concern if the wire is a bit too large.

Since the electric motor already has some cables attached with a particular wire gauge, you really need to figure that resistance into the calculation of voltage drop. The guy who designed the motor probably figured it was going to connect to a battery directly. If the wiring from the motor has a lot of voltage drop in its branch of the circuit, you'll have to use an even larger conductor from the battery so you don't add too much resistance in that branch.

 

[Masito]

Re: Waiting for ice out

Re: Waiting for ice out

I can't thank you all enough for such valuable information. Now, I can take each deduction into account and formulate an educated decision. Certainly, the debate has raised educated questions, however, I am far more ahead than I was asking the guy at the Auto Zone or Tractor Supply. The forum, consisting of such experienced boaters, is such a wealth of information, I could never thank any of you enough! I am so grateful! Thank you.
I will decide to install an MKR 19 60 amp circuit breaker, as close to the battery, as instructed. Then, I believe I will use an 8 ga. wire to connect the motor. Once finished, I will sit in the garage, stare at the boat, drink beer and wait for the lakes to ice out!

 

[Silvertip]

Re: Waiting for ice out

Re: Waiting for ice out

just one more item. A 60 amp breaker is way higher than the current capacity of the wire so essentially the wire becomes the fuse rather than the breaker. Your motor draws a maximum of 30 amps. therefore a 40 amp breaker would be better. Yes -- in the future if you install a bigger motor you must change the breaker and possibly even the wire depending on how big you go. Such is life. If that is even a remote possibility, then it makes sense to upgrade the wire to 6 gauge but the breaker must still be rated for a little over the expected currect draw otherwise the motor wiring can become the fuse. For now, I repeat, 8 AWG is all you need and a 40A breaker is sufficient.