is 12 gauge wire fat enough for my trolling motor?

[jlinder]

Re: is 12 gauge wire fat enough for my trolling motor?

Regarding how the electrons actually flow - The electrons move from the negative to the positive terminal. If I remember correctly the reason we draw current as flowing from the positive to the negative (the other way) is due to Benjamin Franklin who defined it as such. He did not know about electrons at the time.

Someone asked about why we put the fuse in the positive. Cause that is what we do. We treat the negative as the common and fuse the positive. Back 50-60 years ago some cars were built with a positive ground - the reverse of what we do today. It does not matter, as long as you are consistent.

Of course, all this is great trivia but has no bearing on the questions at hand.

Regarding how current flows in wire that is doubled - it will flow based on the resistance, but you are talking about varying percentages. If you took one wire with one resistance, and another with double the resistance, the lower resistance wire would carry twice the current as the higher resistance wire.

It is not a winner take all situation.

In this case with the two wires being the same I doubt you could find test equipment that would be sensitive enough to be even able to measure the difference.

Doubling wires for house hold wiring is a little different and generally not done. One reason may be that is it alternating current. With AC you need to be concerned that the path lengths match or they would combine with different phase angles. Of course this would not happen unless you had some really different paths for the 2 wires but it is just not typically done.

Regarding how much current you can put through a wire there are 2 things to consider - will the wire heat up and how much power is lost in the wire.

For heating/safety think of this: In house wiring 12GA will carry 20 amps, 14GA will carry 15 amps, and every 4 gauge sizes will double or halve the capacity (e.g. - 10GA will carry twice what 14GA will, or 30 amps). Just a rule of thumb but not far off.

For power lost you are worried about voltage drop. The longer the wire the more the chance you will need to increase the wire size from that needed to prevent excessive heating. For that you can do a google search for voltage drop calculators, but if you only have a 4 ft run, or a total path of 8 ft. I don't think that will be a factor.

Someone mentioned looking at the fuse. The size has to be at or less than the current capacity of the weakest link in your circuit (wire, switches, motor, etc.). If you fuse is 40A then your wire must be rated to safely carry 40A.

Hope this helps

 

[bruceb58]

Re: is 12 gauge wire fat enough for my trolling motor?

Except you are not talking wires woven together, which do act as one conductor. We are talking about two seperate paths, with two seperate connections. The current will follow the path of least resistance.

You may want to laugh at it, go ahead, but please provide a better explaination why it is NOT an accepted practice in any wiring, be it car, boat, house, etc? Please enlighten me.

Then talk to your electrician. See if he upgrades a 15 amp 14ga wired outlet to 20 amps by adding a 2nd run of 14ga, which would be cheaper than installing 12 ga.

The reason it is not accepted practice is because you double the connections. This is not allowed in home wiring. For example, you can't have two wires leaving a circuit breaker. Has nothing to do with your explanation of the resistance of the wire. I am an electrical engineer and have seen the doubling of wires done in certain chassis. They weren't production chassis but it was still done. In fact, there are certain computer graphics cards that use two power plugs because they draw more current than one of the wires can provide.

Imagine you have 2 wires with one wire have a resistance of 0.01 ohm and the other wire having a resistance of 0.02 ohms. Of course the current will divide down the two wires following Kirchoff's rules. The wire is not going to degrade because you are putting current through it though.

 

[bruceb58]

Re: is 12 gauge wire fat enough for my trolling motor?

With AC you need to be concerned that the path lengths match or they would combine with different phase angles. Of course this would not happen unless you had some really different paths for the 2 wires but it is just not typically done

They would have to be extremely long to make a difference.If you just use a rule of thumb as a ns/ft and had a 50 ft difference, the phase angle difference would astronomically small in degrees of phase. We are talking 60Hz here...very slow.

 

[fishrdan]

Re: is 12 gauge wire fat enough for my trolling motor?

We are talking about two seperate paths, with two seperate connections. The current will follow the path of least resistance.

You may want to laugh at it, go ahead, but please provide a better explaination why it is NOT an accepted practice in any wiring, be it car, boat, house, etc? Please enlighten me.

Then talk to your electrician. See if he upgrades a 15 amp 14ga wired outlet to 20 amps by adding a 2nd run of 14ga, which would be cheaper than installing 12 ga.

Current will flow in the path of least resistance. If both conductors are the same length, then they would have the same resistance. If one has slightly lower resistance and attracting more current, the resistance of that wire will go up, making the other wire the lower resistance and attracting more current, basically they will balance out to each other with each carrying 1/2 the current.

If 1 of those wires were to fail,,, the other wire will not be able to carry the load and become a hazard. I suspect this is the reason for not running 2 light wires to equal 1 heavy wire.

Oh yeah, I would never do something like this, except an emergency situation.

 

[jlinder]

Re: is 12 gauge wire fat enough for my trolling motor?

They would have to be extremely long to make a difference.If you just use a rule of thumb as a ns/ft and had a 50 ft difference, the phase angle difference would astronomically small in degrees of phase. We are talking 60Hz here...very slow.

You are right, it would have to be very different lengths to make a difference. But in high power conditions it would not take much phase angle difference to create a problem because of the very low resistance.

Sorry if this confused the issue. Not a factor here.

 

[bruceb58]

Re: is 12 gauge wire fat enough for my trolling motor?

If one has slightly lower resistance and attracting more current, the resistance of that wire will go up, making the other wire the lower resistance and attracting more current, basically they will balance out to each other with each carrying 1/2 the current.

No they won't. Google Kirchoff's rule and you can understnad how the current divides up through path's of differing resistances. The thermal properties of a copper wire are insignificant for the argument here.

Never heard of a wire attracting electrical current...that's a new one!

 

[fishrdan]

Re: is 12 gauge wire fat enough for my trolling motor?

No they won't. Google Kirchoff's rule and you can understnad how the current divides up through path's of differing resistances. The thermal properties of a copper wire are insignificant for the argument here.

Sooo, your saying it's not a big deal that the resistance of a wire will go up as more current is pulled through it, eventually heating the wire if current is excessive? And I thought all along that was the exact reason heavy gauge wire was used for high current loads, starter, trolling motor, etc.....

Never heard of a wire attracting electrical current...that's a new one!

Yes, it's called fishrdan's law,,, hook up 1' of wire and 1,000,000' of wire and see which one will attract more current. Attract probably wasn't the best choice of words, let's just say the path of least resistance...

 

[Mark_VTfisherman]

Re: is 12 gauge wire fat enough for my trolling motor?

thanks for all the help, 8 gauge it is, the last thing I want on my boat is a fire or the chance of getting shocked because I was too cheap to buy some wire. I may have to snake it or run it outside of the rails, but that that's fine.Thanks for all the input.

8 gauge will be OK though I would prob use 6: I tend to figure out what the "right" wire size is and then use the next larger size for high-current wiring in boats or cars. But that is just me. Even the correct sized wire can get warm when run at capacity.

My 2 cents.... the rest of the discussion was fun but sounds like you got what you needed.

 

[NYBo]

Re: is 12 gauge wire fat enough for my trolling motor?

8 gauge will be OK though I would prob use 6: I tend to figure out what the "right" wire size is and then use the next larger size for high-current wiring in boats or cars. But that is just me. Even the correct sized wire can get warm when run at capacity.

My 2 cents.... the rest of the discussion was fun but sounds like you got what you needed.

Bigger wire also generally means bigger connectors, and that's likely to be the spot of greatest resistance. So that's a good way to go. Bigger IS better.

 

[jlinder]

Re: is 12 gauge wire fat enough for my trolling motor?

Another reason to go bigger - you are losing power in your cables.

For example, if your cables are sized so that you have a 1.2 volt drop across them when running that is 10% of your battery's power. That means 10% of your battery's capacity is wasted in heat. Bigger cables will give you a longer run on a single charge.

Of course, there are diminishing returns. The biggest effect comes from when you are going from undersized cables to ones that work right.

Think of how much your battery costs to run. In this case divide by 10 and that will be what bigger cables is worth just for that. (Of course, can't put a price the most important reason for sizing the cables right - safety)

 

[Mark_VTfisherman]

Re: is 12 gauge wire fat enough for my trolling motor?

Another reason to go bigger - you are losing power in your cables.....

exactly! But....

.....For example, if your cables are sized so that you have a 1.2 volt drop across them when running that is 10% of your battery's power. That means 10% of your battery's capacity is wasted in heat. Bigger cables will give you a longer run on a single charge...

I am not an electrical engineer but according to some charts you get the following yields at 12V theoretical at the following wire sizes:

8 gauge @ 30A will lose .4 volt over 10' one way

6 gauge @ 30A will lose .2V over 10' one way

The percentage loss of volts at 12V are surprisingly high when you actually look at the numbers. At least they always surprise me. That's why I like using fatter wire sizes when the Amps are up.

 

[Mark_VTfisherman]

Re: is 12 gauge wire fat enough for my trolling motor?

Here is a quick way to come up with a wire size. Like I said I am not an engineer, so after my last post I did a quick search on yahoo and got this:

12 Volt Calculator

 

[jlinder]

Re: is 12 gauge wire fat enough for my trolling motor?

exactly! But....
I am not an electrical engineer but according to some charts you get the following yields at 12V theoretical at the following wire sizes:

8 gauge @ 30A will lose .4 volt over 10' one way

6 gauge @ 30A will lose .2V over 10' one way

The percentage loss of volts at 12V are surprisingly high when you actually look at the numbers. At least they always surprise me. That's why I like using fatter wire sizes when the Amps are up.

Right. The lower the voltage the higher the percentage of power lost. That is why AC power is distributed at high voltages - to minimize the distribution losses.

 

[bruceb58]

Re: is 12 gauge wire fat enough for my trolling motor?

Sooo, your saying it's not a big deal that the resistance of a wire will go up as more current is pulled through it, eventually heating the wire if current is excessive? And I thought all along that was the exact reason heavy gauge wire was used for high current loads, starter, trolling motor, etc.

There are two reasons. One is voltage drop and the other is heating but the main reason is voltage drop.

 

[jlinder]

Re: is 12 gauge wire fat enough for my trolling motor?

Regarding the question of the resistance going up as the wire heats:

I checked and copper is rated with a temperature coefficient of 0.39% per degree C.

Consider it you run enough current to heat the wire to 80 degrees C or 176 degrees F (pretty hot). That is about 60 deg. C above room temperature. At that temperature the resistance has gone up by 23%.

If you had a 10% voltage drop at room temperature (1.2v) it would be a 12.5% voltage drop, or 1.5v now. And the temperature rise would probably also go up by 23%, or from a 60C rise to a 75C rise.

Yes, it does go up with heat, and 23% is not insignificant, and it is a factor if you are planning to run this wire this hot.

But I would argue that if you are running your power cables so hot that you can use them to heat your coffee (OK, a little exaggeration) the resistance change is not the problem.

 

[triplehooked]

Re: is 12 gauge wire fat enough for my trolling motor?

I think the orig. poster threw away the trolling motor and bought a set of oars........

 

[John_S]

Re: is 12 gauge wire fat enough for my trolling motor?

The reason it is not accepted practice is because you double the connections. This is not allowed in home wiring. For example, you can't have two wires leaving a circuit breaker. Has nothing to do with your explanation of the resistance of the wire. I am an electrical engineer and have seen the doubling of wires done in certain chassis. They weren't production chassis but it was still done. In fact, there are certain computer graphics cards that use two power plugs because they draw more current than one of the wires can provide.

Imagine you have 2 wires with one wire have a resistance of 0.01 ohm and the other wire having a resistance of 0.02 ohms. Of course the current will divide down the two wires following Kirchoff's rules. The wire is not going to degrade because you are putting current through it though.

Yes, a wire length difference will only make a very minor difference in resistance. If the connections are done correctly, they too will be ok, and there would only be minor differences between how much current will flow in one vs the other. If one of those connections are bad or becomes bad over time, the other wire will become overloaded. After re-looking at my original statements, I would say the cycling back and forth would most likely NOT occur.

As far as graphics cards, I have seen that done, but only because the current draw needed could not be supplied by the motherboard. They used a secondary plug to the power supply. I believe, but have never tested, that there was some circuitry such that if you didn't plug into the PS, you would not burn out the motherboard. I also believe the rules for low amp draw 1-5amps are different than 30 amps.

I have seen special cases with running two wires. In those, each was capable of handling the entire load, ie it was there just for redundcy.

 

[bruceb58]

Re: is 12 gauge wire fat enough for my trolling motor?

I also believe the rules for low amp draw 1-5amps are different than 30 amps.

Trust me...they are exactly the same.

 

[John_S]

Re: is 12 gauge wire fat enough for my trolling motor?

Trust me...they are exactly the same.

OK, is there a current load where it does become a safety issue? I thought there was exemptions for 24V and less, but if it exceeded a certain current, then no exemption.

 

[bruceb58]

Re: is 12 gauge wire fat enough for my trolling motor?

I think you are getting Electrical Code mixed up with basic physics. Yes, there are rules how you wire your house with size and type of wiring for a given load and voltage but the physics stays the same if you are talking 12V, 24V or 110V or any voltage.