In most cases, the manufactures recommended wire is 8 gauge for most 24-volt systems and 6 gauge for a system of 20 feet or longer. To account for voltage-drop, heat and current changes resulting from low batteries I recommend 6 gauge wire and 4 gauge for over 15 feet. My additional reasoning for this recommendation is that your system will degrade between maintenance to remove corrosion and replace connectors.
I think to get the whole picture we are going to need to start with a different question, and then we can answer the original question in detail.
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How many amps does my 24 volt trolling motor draw?
The main way to find how many amps your trolling motor draws is to look in the back of your manual. You can also look up product specifications on manufacturer websites, but this information can be buried in their site and hard to find. I went through the big three manufactures’ information and generated a table. I created the table including the highest demand motors in each voltage so it would be more useful and give you an idea of how the thrust relates amperage.
My motor is a Minn Kota 80 lb Riptide 24 volt. If you look at the chart, it is right in the middle of my chart. You can see on the chart that the maximum amp draw for my motor is 56 amps, per hour. Keep in mind this is manufacture information, and they might want to be on the safe side of the numbers. I put the ammeter on my power cable while running my motor at full power and it was lower, how much lower, I will leave it at lower, so we use the best wire per the information we have.
How do I calculate what gauge wire I need for my 24 volt trolling motor?
I went through all the information on the ABYC wiring guidelines, which are in line with the US Coast Guard Code and got the formulas to generate a table. The leading manufacturer calls for a maximum of 5% voltage drop, but the standard tables are categorized at 3% or 10% voltage drop.
I did more research on electric motors, and it appears that a drop in voltage of more than 5% can cause damage to a standard electric motor. As a result, I had to build tables. If you wanted to use a standard table, you would need to use a 3% voltage drop table to avoid any chance of electric motor damage.
I did the calculations and compared the numbers to the manufacture’s information. The middle of the table wire gauge size matched exactly on the 24 volt motors, but the short length and the long length did not.
The formula generated a smaller gauge wire for the shorter length system connections and a larger gauge wire for the longer connections. From my observation, it appears that the manufacture upsized the wire on the shorter lengths to be uniform with the trolling motor lead and to prevent a smaller wire from acting as a fuse which could cause a fire.
To be safe, always go up to the next size wire. (example if the chart calls for a 6awg use a 4awg) If you go up in wire size, the worse that can happen is that it will cost you a little more money. If you undersize your wire you could damage your motor, control board, or have a boat fire. None of the things I listed are a problem if you can swim, right?
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What is the correct size breaker for my 24 volt trolling motor or fuse?
If we go to table one in the last column, you will see a manufacturer recommended breaker size, which is either 50 amp or 60 amp. The most important factor when choosing a fuse is not to allow more current through the fuse then the wire can handle.
You can go back to the table and see that if a wire is 10 gauge, it should be a 50 amp breaker/fuse and if it is 8 gauge wire or larger you will use a 60 amp breaker/fuse. I would only use 8 gauge wire or larger so I can avoid using a 50 amp breaker/fuse completely.
I have provided tables that I made from manufacture data and ABYC wiring standard formulas but when in doubt always use bigger gauge wire. Also, if you use a 3% voltage drop table, you will be safe. Lastly, never oversize your breaker.
US Coast Guard Electrical Codes link below
ABYC E-11 electrical manual purchase page link below