8 gauge wire on 24v system?

[Reb2973]

I just bought a new Bass Tracker pro19 bass boat. First thing I did was ditch the 12v foot controlled trolling motor since I like hand controlled with foot switches.

I put a used 70lb thrust 24v minnkota on it I got from ebay. Last weekend It stopped working. I took it apart and saw that the brushes had heated up so bad they melted the plastic around them on the brush plate, had to force them loose with a screwdriver. I didnt know I could possibly fix it(figured it was dead) and ordered a brand new maxxum 80lb 24v hand controlled that will be here tomorrow.

My question is, do you think the 8 gauge wire was part of that problem? I know its hard to say what the other owner of that motor put it through. I was trolling on high for awhile and going through some weeds. When I stopped to make a few cast and go again, it turned a few times and was dead.

I just dont want to damage my new 80lb motor. I also purchased the 60amp breaker from minnkota. The wiring is all 8 gauge . I would run 6 gauge but looks almost impossible to route to the back of the boat. So do you think 8 gauge with a 60amp breaker will be ok for my new 24v trolling motor?

Thanks for any help.

 

[jlinder]

Re: 8 gauge wire on 24v system?

I can think of a couple of factors here.

You have a 60A breaker and 8ga wire. 8ga is rated for 40A. If you have a 60A breaker you should be using 6ga.

Of course there is the question of distance and voltage drop. How long is the wire? (you need to add both the distance to the motor and back)

But you had said the brushes melted. A wire that is too small would overheat in the wire, not the brushes. The melted brushes indicate a limitation in that unit.

Sorry if this is not a definitive answer but hope it helps

 

[Silvertip]

Re: 8 gauge wire on 24v system?

Chances are it was running in the weeds that did the damage. Anytime you stall or nearly stall an electric motor it begins to draw a tremendous amount of current. It doesn't take long under those conditions to fry a motor. A friend that I fish with complained about his battery going bad as it didn't seem to provide very long run time. I suggest we remove the prop to look for fish line. Not only did we find many yards of fish line wound tightly around the shaft, but it had also eaten up the seal. That fish line dragged very hard on the prop shaft so the motor was continually drawing much more power than it needed to.

 

[Reb2973]

Re: 8 gauge wire on 24v system?

ok thanks guys.
I dont know much about the breakers and all. My friend suggested I get one and I saw the minn kota 60amp that said it was good for 12,24 and 36v trolling motors.

So if I run the 8 gauge with a 60amp breaker and the motor starts to get bogged down in some weeds, will the breaker trip before damage to the trolling motor happens?

I just installed the breaker this morning. Figures I got the wrong one..huh?
I would say the wire distance is about as long as the boat,cant really see it. So probably 19ft.

Now that I think about it, that 60amp breaker popped up as a recommended item to purchase with the motor.

 

[Silvertip]

Re: 8 gauge wire on 24v system?

You really need to understand why a breaker is installed and where it is installed. It is installed at the battery and its purpose is to protect the circuit - not necessarily anthing that circuit feeds (in this case the motor). As was pointed out, 8 gauge wire is not capable handling 60 amps of current. So if 50 amps of current passed through those wires the wires would melt and catch fire before the 60 amp circuit breaker would open. The same principle applies to fuses. Forget the 60A breaker -- on 24 volts you don't need that high a rating. You need to find what the maximum current rating is for the motor you have. The MinnKota web site is the place to find that info. You then install a circuit breaker that is only slightly higher than that maximum current rating. Regardless what that rating is, the wire you use must also be able to handle that current. Without doing the research for you, I think 8 gauge wire is fine and 40 amp breaker is sufficient. your motor may have overload protection built in. Any overload may cause the damage you had.

 

[Reb2973]

Re: 8 gauge wire on 24v system?

Well now that makes sense. I thought the breaker was to protect the motor. I was just digging in the garage and found a 50amp breaker from my old boat. I will do some searching on what my new motor draws.

So if the breaker is not meant for protecting the motor, couldnt I just get a breaker according to wire size,rather than what my motor would draw?

Then I just need to know what the motors max amp draw to make sure the gauge wire i use can handle that?

Does that sound like I have it right?

Thanks again for the help.

 

[Reb2973]

Re: 8 gauge wire on 24v system?

This is driving me nuts. The guy above says 8gauge is rated for 40amps and the chart i found online says its good for 80amps.
http://www.boatus.com/boattech/cable.htm

Can someone just tell me what 8 gauge can handle and what 6 gauge can handle please?

 

[Silvertip]

Re: 8 gauge wire on 24v system?

Yes -- but again, if the motor at maximum current draws more than the wire is capable of handling you still have melted wires. The ampacity chart I checked shows #8 wire can handle 40 amps at up to 23 feet which is far short of the 38 foot round trip you have. Runs of just a couple feet, 8 gauge wire can handle 55 amps at 95 degrees C. That maximum number means the breaker should be 40A maximum for a 38 foot run. I'm going to guess that your 24V motor draws somewhere between 29 and 34 amps maximum. However, that 34 amps may not be at locked rotor (stalled motor which would be higher). Installing a 50A breaker would be incorrect for the current carrying capacity of that length of 8 gauge wire so the wire would heat before the breaker opens. The reason you had a 50A breaker before is the current draw of 12V motor is higher than a 24V motor. If my assumption is close on current draw for your 24V motor, a 40A breaker, 8ga wire, and a 38 foot run would work but be at the very high end of the 2% acceptable voltage drop limit. Remember, the breaker would not open unless the motor was drawing more than 40 amps. If locked rotor current is higher than that, the breaker would protect the motor. If not, it won't. If you install a 50A breaker, that would protect the motor but you have again selected a breaker inappropriate for that length of 8 gauge wire. Confusing? Yes it is and that's why houses burn down. People don't realize the relationship between the circuit and the load or loads it must feed.

 

[Reb2973]

Re: 8 gauge wire on 24v system?

Thanks silvertip,

I will order a 40amp breaker to use with the 8 gauge wire. Is there a way to see if the 8 gauge is at its limit? I was thinking when I take the boat out to try the new trolling motor, maybe I can run it on high for a few and then feel the wires back by the batteries and see if they are heating up. Should they get warm at all?

If that would be the case I guess I will have to find a way to route 6 gauge. Would the 40amp breaker still be ok for 6 gauge?

Sorry for all the stupid questions, i just want it right so i can be done with it and go fishing.

 

[TerryMSU]

Re: 8 gauge wire on 24v system?

I would be inclined to agree with Silvertip. Several pieces of info that may be of interest.

1. Most circuit breakers are calibrated to trip (open) at 1.5 to 2 times rating. Thus your 50 amp breaker will likely trip between 75 and 100 amps (if the current lasts for several seconds.)

2. The rating of wire size is somewhat subjective. It really depends on two factors. First (and most critical) is what current will begin to compromise (as in melt) your insulation. This cannot be exceeded, but even this is dependent on conditions. In a bundle, it takes less current to melt the insulation than as a separate wire. In longer lengths, the wire current capacity does not change. What changes is the amount of voltage that is lost in the wire. Longer wire means more voltage drop. What even further complicates the issue is that increased voltage drop in the wire results in less voltage at the motor and starving the motor. Starving the motor is like stalling it and that means it draws even more current. Confusing.... Darn right. I have a masters in electrical engineering, and it is still confusing to me. That is why you get so many answers.

So... See what wire size Minkota recomends for the length of run. Use the breaker size that they recomend for your motor. This may be less (and probably is less) than the wire size allows. That is OK.


TerryMSU.

 

[Reb2973]

Re: 8 gauge wire on 24v system?

Yeah its a bit confusing. Wish my boat came with 6 gauge to start with. Im even starting to think about moving the batteries to the front compartment with some 4 gauge wire and be done with it,ha ha. I really do have a spare front compartment not in use,but im sure that would mess up the ride of the boat.

 

[Silvertip]

Re: 8 gauge wire on 24v system?

No need to get wrapped around the axle on this. I went to the MinnKota web site and here is some installation data to chew on.

Wire Size: http://www.minnkotamotors.com/support/faq.asp?pg=general&pt=6

To determine how much current your motor draws, I went to the Motor Guide web site and came up with 32.5 amps and I have no reason to believe your Minnkota would be any higher since this figure is for an 80# motor, not a 72. Regardless, the numbers are close.

Your existing wiring will be just fine with a 40A breaker and it would also protect the motor. 6 gauge wire would have resulted in less voltage drop but 8 awg is fine. If you were using a 55# 12V motor you would have some potential issues. See how easy that was.

http://sites.mercurymarine.com/pls/portal/docs/PAGE/MOTORGUIDE/MOTORGUIDE%20SUPPORT/ONLINE_MANUALS/MANUAL_CONTENT/MM6900%20-%202004%20-%20OWNER'S%20MANUAL_0.PDF

 

[jlinder]

Re: 8 gauge wire on 24v system?

This is driving me nuts. The guy above says 8gauge is rated for 40amps and the chart i found online says its good for 80amps.
http://www.boatus.com/boattech/cable.htm

Can someone just tell me what 8 gauge can handle and what 6 gauge can handle please?

Reb2973 -
I did go to the site you mentioned and it does indeed say an 8ga wire will handle 80 amps. It also says you can run 45 amps through a 12ga wire. I am very suprised about that.

I was quoting figures from standards used for wiring on land, which I find very useful. Those standards say 20A for 12ga and 40A for 8ga.

I can tell you if you put 45A through 12ga wire or 80A through 8ga it will become very hot. In your house you would be looking for a fire.

There are some wires with insulation rated for a much higher termperature that can take more heat, but I would not do it.

I wonder if there is anyone else on this board who has seen someone say you can put that much current through these gauge wires?

There are also 2 factors that limit the size of the wire. You need something big enough so it will not get too hot and you need to limit the voltage drop so the voltage getting to the motor is not too low.

Then there is the question of the size of the fuse. A fuse should not be bigger than the capacity of anything in the circuit it protects. In this case it would be the wires and the motor that need to be rated for the size of the fuse.

But I would say that while this is interesting to know it is probably not where your problem is. The people who advised you about fishing line around the hub and problems with running in the weeds are probably right on target.

The worst thing you can do to a motor is to stall it out, or load it too heavily. The current goes way way up and that is probably what killed your system.

 

[Reb2973]

Re: 8 gauge wire on 24v system?

Thanks again silvertip,

I tried for about 1 hour to find the max amp draw for my new trolling motor but couldnt find it. I will just wait till it gets here and see. Im going with the 8 gauge and 40 amp breaker like you said. Thanks for the help.

jlinder,

thanks for the info. Thats what was starting to confuse me, what you said and then seeing that chart saying way more. I learned alot today about this stuff thanks to you guys, Thanks again.

 

[Silvertip]

Re: 8 gauge wire on 24v system?

For everyone else that is confused about the amount of current that a conductor can carry, you need to understand that the current being passed, the length of the run, the temperature and the type of wire all have a bearing on that number. There are many charts that must be used to deterimine this. 80 amps in 8 gauge wire was shown without regard for circuit length. 80 amps through 8 gauge wire in a circuit that is 38 feet long will suffer a meltdown.

 

[Reb2973]

Re: 8 gauge wire on 24v system?

Well the ups just dropped off my new 80lb but the manual didnt say what the max amp draw was. I went ahead and called them,the lady said the max amp draw on the 80 was 56amps and said to use 6 guage with a 60amp breaker.

The chart they have on their website to figure voltage drop dont even go up to 60 amps. Whats you thoughts on this one Silvertip?

Should I try to upgrade to 6 guage or do you think I will be ok with what we figured.

 

[Silvertip]

Re: 8 gauge wire on 24v system?

Call them back and tell them to put someone on that can read a chart. I have in my formerly nicotene stained fingers, the 2008 MinnKota catalog. Their 80# motors max amp draw is 56 amps. The person you talked with probably looked at the Thrust column next door and saw 80. Even 56 amps sounds high and I will go out on a limb and say that has to be stalled rotor. If it indeed does draw 56 amps, you are in 6 gauge territory and 60 amp breaker.

Page 2 of the following MotorGuide manual shows current draw for the various motors. Although the draw is indicated in kW (kilowatts) all you need to do is move the decimal point three places right to get watts. Then divide that number by 24. As you can see, a 82# motor draws .78kW (780 watts)which equates to 34 amps.

http://sites.mercurymarine.com/pls/portal/docs/PAGE/MOTORGUIDE/MOTORGUIDE%20SUPPORT/ONLINE_MANUALS/MANUAL_CONTENT/MM6900%20-%202004%20-%20OWNER'S%20MANUAL_0.PDF

One or the other of these figures is wrong or the test conditions are very different for the two motors.

 

[Reb2973]

Re: 8 gauge wire on 24v system?

Yeah that seems like a high number she told me. I guess I will be safe though and ordered the 6 guage wire. What was amp rating of 6 gauge? I have the 50amp and 60 amp breaker. I wonder if 60amp breaker is to big?


Well I guess it will be better on everything upgrading my wire,but it sure looks like it will be a pain getting it from the front to the back. Either way, my new trolling motor is getting one trip this weekend on the 8 gauge. I will go easy on it. My new wire wont be here till next tuesday.

Thanks again for all your help silvertip.

 

[Silvertip]

Re: 8 gauge wire on 24v system?

I was a little harsh on the MinnKota rep as I swapped the two numbers in your post so it is I that can't read. The number is 56 amps (I had read 80 amps). Sorry about that. I have checked a number of ampacity charts and here are the overall results: 6AWG wire, at 24V, over 36 feet, carrying 56 amps, will see a 1.638 volt drop so the voltage at the plug would be 22.362. This represents a 6.82% voltage drop. I think you want to stay 5% or less so the way to do that would be to move the batteries up front. That would cut that drop significantly. The boat would probably perform better as well. The 60A breaker is what you need.

 

[jlinder]

Re: 8 gauge wire on 24v system?

You asked how much current a 6ga wire will carry. A basic rule for finding current capacity is this:
14ga will carry 15 amps
12ga will carry 20 amps
Every time you go 4 ga. larger you double the current capacity.
Every time you go 4 ga smaller you halve the current capacity.

So 10ga will carry 30 amps (twice 14ga) and 6ga will carry 60 amps (twice 10ga).

Of course this refers only to capability to carry the current without melting and does not consider voltage drop over length. It also does not take into account that you can get wires with higher temperature insulation ratings, if you bundle the wires you need to lower the rating, if the moon is full and you are looking east ......

But it is a good rule of thumb