Question for the electric experts. I just upgraded my old motor guide 25lb thrust trolling motor with a new minn kota 40lb thrust trolling motor. It appears the original factory wiring is about a #10 wire. Is this enough for the upgraded thrust. I have been told that if the wire is too small, it may damage the brushes in the electric motor head. I have also been told that no damage will occur, but the motor will  not make it to the 40lb thrust mark. I have some #6 wire a friend at work gave me he salvaged out of a boat he junked, but I would have to tear have my boat apart to fish it through from the back to the front. I fished out of the boat today and it worked fine. Just don't want to damage it.  

6ga. would be much better, battery would run down as fast. You can't tape the 6ga to the 10ga & fish it thru? If you do that make sure you secure your wires together good. You don't want them pulling apart while fishing them thru.

If you have a good digital voltmeter, the next time you're at the lake, connect the DVM across the cables where they connect at the battery. Do this across the cable lugs, not across the battery. Run the TM on max for a short distance, measuring and making note of the voltage. Now connect the DVM across the TM cable lugs/connectors at the bow of the boat. Again make sure you are connecting to the cable in the boat and not after the connectors or anything else you have connecting the TM. Run the motor on max a short distance again. If the readings your get on the DVM at the bow of the boat are the same as the readings you got across the cables at the battery, your cable will work just fine. If the readings are less, even if only a few tenths, then your cable is too small and needs to be upgraded. One or maybe two tenths aren't going to hurt you much but anything more and you are loosing power.

Let me say again, make sure you are reading each end of the cables, if you take the reading with connectors, circuit breakers or anything else between the meter lead and the cable in the boat, you can get a wrong reading because these items can cause resistance and loss of voltage, but that's not because the cable is too small, that's caused by bad connections or other parts too small for the current load.

Don't try to do this at home, the motor has to be under a full load to do this check, free spinning it out of the water does nothing to load the motor.

Boy, you are going to get a lot of different opinions on this. Here is mine.

You will not hurt your motor by running 10 gauge wire to it. What you could hurt is your wiring it self. 10 gauge wire is typically rated for 30 amps. 6 gauge is rated for 50. If your motor draws significantly more current than what the wire is rated for you could have problems.

The biggest thing that could happen is your wire will get hot and burn if you draw too much current from it. A lot of factors come into play before that happens. How much over the rated current and duty cycle (on time versus off time) are just a couple.

The other issue is voltage drop. If your motor does not get enough voltage to run it will draw more current from the battery. This will also cause the wire to heat up and shorten battery life as well.

I personally would go with the bigger wire if this motor where the primary means of moving the boat and it ran for more than 15 minutes at a time.  If the motor were only used for 30 seconds every 10 minutes or so to move along a bank and fish I would not worry about it. The wiring would not heat up enough to cause concern.

Paul

I agree a larger wire is probably needed but acutally 10 guage can handle up to 40 amps and 6 guage can handle up to 75 amps without damage. Notice I said without damage and didn't say without heating up. Yes a 40 lb thrust TM running on max is right on the edge, if not slightly over. If the time is taken to do the voltage test I suggested, it will easily determine if he needs larger wire. If he's not getting a voltage drop, he's not going to be getting enough resistance to produce heat. The more the voltage drop, the more heat the wire will produce and the more performance he will loose. So, if he's not getting a voltage drop he has no problem and will see no loss in TM performance.  I would also figure the original wiring to be closer to 8 guage than 10 guage.

Don't forget that cable length also affects voltage drop.

This is borrowed directly from Powermaster's website:

                     

                                   Recommended Cable Gauge Size:

 AMPS          Up to 4'      4'-7'      7'-10'      10'-13'      13'-16'      16'-19'      19'-22'      22'-28'

35-50           12ga.       12ga.      10ga.      10ga.       10ga.        8ga.          8ga.         8ga.

 

50-65           10ga.        8ga.        8ga.       6ga.         6ga.         6ga.          6ga.         4ga.  

65-85           10ga.        8ga.        8ga.       6ga.         6ga.         4ga.          4ga.         4ga.

85-105           8ga.         8ga.        6ga.       4ga.         4ga.         4ga.          4ga.         2ga.

105-125          6ga.         6ga.        4ga.       4ga.         2ga.         2ga.          2ga.         0ga.    

125-150          6ga.         6ga.        4ga.       2ga.         2ga.         2ga.          2ga.         0ga.

150-175          4ga.         4ga.        4ga.       2ga.         2ga.         0ga.          0ga.         0ga.

175-200          4ga.         4ga.        2ga.       2ga.         0ga.         0ga.          0ga.         00ga.  

Keep in mind that you cannot go to large but you can go to small and to use quality oxgen free cable.

upgrade the wiring....

the rate is about an amp of draw per pound of thrust at 12 volts. #10 wire is 30 amp wire...running 45 pound motor on it is overworking the wire by about 50%. Electric systems are always working to achieve balance.

keep in mind this basic explanation...

amperage and voltage are inversely proportionate and effected by resistance. by that I mean...voltage goes down, amperage goes up. So at less voltage - you draw more amperage. ( example...at 24 volts a 30 lbs motor will draw about 15 amps...at 12 voltsthe same motor will draw 30 amps...1/2 as much voltage, twice as much amperage).

Voltage is decreased by resistance - so wire offering resistance will lower voltage and increase apmerage. resistance causes heat...and heat causes resistance...and demand factor changes everything.

so while you may read a full 12 volts at the motor with the boat sitting static in the back yard...under load ( with the motor running...or demand on the system) you may see a drop in voltage and increase in amperage and heat ( and thus resistance) over time. As the motor is used and heat increases, so does resistance in the wire, which lowers voltage...and increases amperage...so you may end up pulling more like 50-60 amps thru your 30 amp wire by the end of a fishing day...and the longer you do this, the worse the problem will become.

you will increase battery life ( less heat and less apmerage draw) and motor life  ( same reason) by changing your wiring. every time you heat the wires...you increase the conductive resistance of the wire...and the next time you use it, you will create more heat...which will increase reistance, lower voltage, and increase amperage...it is a self replicating problem that gets worse each time you use the motor.

almost without exclusion...a good rule to follow...is if your electrical system wiring is warm or generating heat, you have a problem that needs fixing. I would bet if you ran around on full power for awhile and then went back and felt your battery connections, they would be warm.

can you damage the motor...YES...the brushes in the motor are amperage rated...if you are running the motor on too small of wire, thus increasing resistance...lowering voltage...and increasing amperage, you will over rate the motor brushes and burn them up. ...and if you read you motor warranty there is a blurb in there about that

use the old wires to fish the new ones....

Stranded wire not solid wire................Al

To just jump in and say "upgrade the wire" is wrong.  He may very well not need larger wire.

First, I would be willing to bet if that's the factory installed wire, it's probably #8.  That was the norm for those that didn't run the larger #6 wire to start with.  He just said it looked like it might be #10, unless he works with it regular, he's totally guessing.

Second, there's a lot of people that have to budget their fishing and spending close to $100 and the work involved might not come quit as easily to some as it does to others.

Third, if he does that simple little voltage test, and there is no voltage drop at the TM end, it doesn't matter what size wire he has, it's big enough.

We have beat the current draw and heat down to a pulp.  If there is no voltage drop, there is no loss, if there is no loss, there is no heat to speak of.  If there is no heat, there is no loss of power, making the wire in the boat perfectly suited for what he's doing.

My advice.

when it comes to electricity you either know what you are doing or you take it to a professional.

professional wiring means spending a few bucks

ametuer wiring means destroyed motors, fire, or worse

Your stuff, your choice

6ga. would be much better, battery would run down as fast. You can't tape the 6ga to the 10ga & fish it thru? If you do that make sure you secure your wires together good. You don't want them pulling apart while fishing them thru.

Been there and done that

18 years as an electrician tells me you need the wires rated capacity to match the full load amperage...there is no "gettting by" with electricity. It is either right...or it is wrong and potentially dangerous.

I have a saying...just because the lights come on does not mean it was wired right!

upgrade the wiring, if necessary, to match the motor's amperage draw. and if you do upgrade do not use house wire. THHN sucks for this kind of thing. use weldeing cable...it is considering more flexible and easier to fish through tight spaces and has more and finer strands per Sq. MM ( electricity flows on the outer surface of each strand...more strands in the same diameter, better "conductivity")