I bought a used boat that had been outfitted with a trolling motor, 55# thrust 24 volt system. The seller left the trolling motor mounting plate and wiring on the boat. I want to install a minn kota 55# 12 volt motor. The wire from the battery to the plug (about 16ft) is a number 8ga. The motor has a #6ga wire lead. I want to break the negative and install a stomp switch with about 8ft of cord. The switch I purchased looks like it is a number 10ga wire lead. Am I heading for trouble? Will the wiring system overheat, etc. Please give me some help on this. I don't know if the negative on a 12 volt system has to be the same size as the feed.This is my first trolling motor.Thanksjimpittman
1. Determine the maximum amp draw of your motor and round up to the nearest 10. I believe a 55# MinnKota 12V has a maximum draw of 50 amps - but check that for your particular motor.2. Measure the WIRE distance from your battery to the motor and multiply by 2. Yes, the negative wire must be the same size as the positive wire. The current makes a round trip from the battery to the motor back to the battery.3. The following chart shows the voltage loss per ft. of wire of various sizes (AWG) at 40 and 50 amp maximum draw.40amp:4AWG = .01V6AWG = .016V8AWG = .0256V10AWG = .0408V12AWG = .0648V50amp:4AWG = .0125V6AWG = .02V8AWG = .032V10AWG = .051V12AWG = .081V4. Select a wire size that will give less than 1 volt drop at the wire distance from step 2.Example:Lets say your motor has a maximum amp draw of 50 amps and the wire between your battery and the motor is 12 ft. We'll try a 10AWG wire first, using the 50 amp table.12 ft. X 2 = 24 ft.24 X .051 = 1.224VThis is over 1 volt drop so it's unacceptable. Using an undersized wire will decrease the voltage at the motor, decreasing power, and could damage the electric motor.Let's try an 8 guage wire:24 X .032 = .768V.This is under 1 volt drop so it will work fine.You can mix two (or more) different sizes of wire but just make sure the total voltage drop doesn't exceed 1 volt. Use the same calculations as above but calculate each different wire size and add the voltage drops together.Hope this helps.
Another note:Where you have a length of one size wire on one side, say the negative, and a different size wire for a different length on the other side, positive, like you will have by installing the stomp switch, use the smallest sized wire of the combination for your calculations.If you need more help, find the maximum amp draw of your motor and get an accurate measure of wire distances and come on back. I'll go through it with you.
Thanks for the reply Crappie Cruncher and newman.My motor manual says this motor will draw 1.0 ampere/hour. I don't know what this means. How do you find the maximum amp draw. My battery is located about 12 feet from the motor.The positive and negative are both #8 all the way to the plug. I will break the negative just behind the plug and run #10 to the stomp switch and back to the plug about 10 feet.can you do any calculations based on this info.Also if I run #8 instead of #10 to the switch, except for a short pig tail to tie in to the switch, and back to the plug, does this help?Man I didn't know this stuff was this complicated.Thanksjimpittman
According to the Minn Kota web site specifications a 55 lb. thrust motor has a miximum draw of either 43 amps or 50 amps, depending on your model. In any case, we round up to the next even ten amps - 50 amps.1. You have 12 ft., batt to motor, X 2 = 24 ft.2. You have 10 ft., plug to switch, X 2 = 20 ft.Using 8 AWG for 1 and 10 AWG for 2:24 X 0.032 = 0.768 volt drop.20 X 0.051 = 1.02 volt drop.Total = 1.788 volt drop. Not good!Using 8 AWG for 1 and 8 AWG for 2:24 X 0.032 = 0.768 volt drop.20 X 0.032 = 0.64 volt drop.Total = 1.408 volt drop. Still not good!Using 6 AWG for 1 and 8 AWG for 2:24 X 0.02 = 0.48 volt drop.20 X 0.032 = 0.64 vold trop.Total = 1.12 volt drop. Again, not good!Using 8 AWG for 1 and 6 AWG for 2:24 X 0.032 = 0.768 volt drop.20 X 0.02 = 0.40 volt drop.Total = 1.168 volt drop. Nope!Using 6 AWG for 1 and 6 AWG for 2:24 X 0.02 = 0.48 volt drop.20 X 0.02 = 0.40 volt drop.Total = 0.88 volt drop. Good!Using 6 AWG looks like the only way to get the voltage drop under 1 volt.If your connectors/plugs/etc. won't accept a 6 AWG wire, you can splice a short piece of 8 AWG to make the connection. A short piece won't cause any problem.
Thanks Crappie Cruncher for all the time you took to research those wire sizes for me. It was nice of you.I think that I am going to run new #6 wire to the plug and if I still want the stomp switch I will run #6 SO cord to it. This will leave only a short pig tail for each the plug and switch to be spliced in. One other question while I am using you. What evidence will I have that voltage drop is too high? Will something burn up before I damage the motor, or will it all happen at the same time?Thanks and hope to see you on the water.jimpittman
The only indications would be warm wires (maybe) and if you measured the voltage at the motor it would be low. Nothing is going to explode or catch on fire and I doubt you would even be able to tell if the wires were warm by touch, especially through fairly heavy insulation.The coils of the motor are where the problem would lie. A voltage drop indicates that some of the electrical current is being converted to heat energy. Excessive heat melts the waxie insulation around the wires coiled up in the motor. This leads to shorts between coils of wire which leads to motor failure. This is, of course, worse case.Many, including possibly the offical Minn Kota folks, would argue that no damage to anything is likely unless the voltage drop is severe - several volts - and that the only effect would be reduced power output from the electric motor and shorter run-time from your battery (decreased voltage = increased battery drain). This might be true - but it might not. The offical Minn Kota and MotorGuide requirements require less than 1 volt drop in the system. That requirement is probably conservative but it was made a requirement for some reason. The cost of using the correct size of wire is negligable and anyway, would you run your outboard de-tuned and accept your 25hp running with all the power of a 15hp?Short pig-tails spliced in to attach plugs, switches, etc. will not matter. Make sure your splices are really solid and protected from corrosion or they can become very high resistance points creating a point of large voltage drop themselves. Solder them if you can.An alternative might be to relocate the battery to shorten-up the wires. But, I'm sure you already thought of that. In many boats it's not that easy to relocate the battery.Let us know how it all works out.
This message is for Crappie Crencher. Just wanted you to know. I replaced the #8 wire with #6 and ran #6 wire for the switch. Tried the system out Wednesday. What a nice rig. Everything works great. I am so pleased.Thanks for all of your help.jimpitt6man