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Author Topic:   Primary Battery Distribution Wire Size
Swellmonster posted 06-21-2009 05:40 PM ET (US)   Profile for Swellmonster   Send Email to Swellmonster  
What is the ground cable diameter and length on a 20 outrage?
jimh posted 06-21-2009 05:58 PM ET (US)     Profile for jimh  Send Email to jimh     
What is a ground cable?
Swellmonster posted 06-21-2009 07:06 PM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     
The black negative cable from the battery in the center console that runs to motor block.
jimh posted 06-21-2009 07:43 PM ET (US)     Profile for jimh  Send Email to jimh     
Wire is not normally measured by diameter. Wire is normally measured by its American Wire Gauge or AWG. In primary power distribution on a boat, the overriding concern is voltage drop. You have to size the cable so that the voltage drop does not exceed the required maximum voltage drop allowed for the current flow.

If you tell me the length, the maximum voltage drop, and the current anticipated, we can compute the wire gauge needed. If this conductor will be used to operate the starter motor in your outboard motor, check your motor installation manual for guidelines on wire size and distance.

If you are asking about a particular Boston Whaler boat made in a particular year, and what the original conductor size and length were, you should direct your inquiry to Boston Whaler customer service. Or, check the electrical schematic diagram. The wire size should be indicated. The length seems somewhat straightforward to discover.

HAPPYJIM posted 06-22-2009 12:40 AM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
What size was the old cable that you removed? Most cable or wire is stamped at intervals along the wire. If the wire is the same length as the original go with that size wire. If it is longer, go to the next larger size.

You can figure all of the current carrying capacity by adding up all of you electrical components but you will probably use a separate ground wire for the buss in the console. That will be a much shorter wire and will be no where near the current draw of the motor starter.

jimh posted 06-22-2009 08:42 AM ET (US)     Profile for jimh  Send Email to jimh     
You have to select the wire gauge based on the voltage drop, not on the current capacity. In low voltage circuits, roughly any circuit below 50-volts, the current capacity of the wire should not be the determining factor in selecting the conductor. You have to select a conductor based on the voltage drop. This means you will use a larger conductor, one whose current capacity is greater than the circuit.
Swellmonster posted 06-22-2009 10:08 AM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     
I didnt pull the old cable yet. Im using that to pull the new one in. But if i cant find out the right lenghth before I buy a new cable, I will have to pull out the old one first.
Blackduck posted 06-22-2009 01:31 PM ET (US)     Profile for Blackduck  Send Email to Blackduck     
Go ahead and pull the old one first, just attach a strong piece of twine to it first, and use it to pull in the new cable. That new cable should be a least 2 gauge, if it is as long as a think it is.
HAPPYJIM posted 06-24-2009 12:15 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
Let's see if I understand your question.

You are just replacing the battery ground wire with a new one.

If you have no other new equipment in the rear of the boat, you can use the same size wire. If there is additional equipment in the rear, you must find the current draw of that equipment and add it to the current draw of the starter, then use wire that has sufficient current carrying capacity. You should not have to figure voltage drop because of the wire being the same length.(If I had that right)

You will run a separate wire to the console buss to power all of the other electrical/electronic equipment. Simply add up all of the current capacities of the equipment and go with a size wire that will handle the current carrying capacity of all of the equipment. That length of wire will only be 3 foot or so. I would not worry about voltage drop with that short of a run. Considering it is such a short run, the difference in price for the next size larger would not be a big deal. If it were mine, I would go one size bigger anyway and you will be covered if you add any big current equipment like a margaritaville Slurpee machine.

jimh posted 06-25-2009 08:43 PM ET (US)     Profile for jimh  Send Email to jimh     
NO, no, no. Do not calculate the wire size based on current carrying capacity.

You have to calculate the wire size based on voltage drop.

HAPPYJIM posted 06-25-2009 10:16 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
Without going into a boat load of electrical theory, here is a calculator that can be used to figure what size cable to use. Make sure to use marine wire. Going up 1 size is common practice if you aren't counting pennies. The chart is for solar panels but 12 volts DC is 12 volts DC.

1. Make sure you select 12 volts DC

2. Amperage is the same as current draw. Add it up and enter.

3. Enter how long the cable will be.

4. Keep 3% because it is best.

HAPPYJIM posted 06-25-2009 10:18 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
Well that was pretty's the site.

tim1198 posted 06-28-2009 06:57 AM ET (US)     Profile for tim1198  Send Email to tim1198     
You're both right. The wire gauge is related to a power requirement of your electrical system.


So, the longer wire will experience voltage drop due to the increased resistance in the wire. At the same time, your electronics will still require a certain amount of power (or current at 12V)

The net is: For the lengths you're dealing with (under 20 ft), the voltage drop and additional electronics "probably" will not matter. I would use the same sized wire and would be comfortable adding a few more feet of length without concern. Reasons are:

1. The wire is already sized for the biggest power hog (starter), and everything else is relatively small.

2. You'll get more resistance drop across connectors than an additional few feet of copper wire.

3. Electrical systems are typically designed with a margin factor of 2x-4x maximum power requirement.

So, unless you're adding an air conditioner or some massive power hog, stick with the same wire size. Good luck.

jimh posted 06-28-2009 08:58 AM ET (US)     Profile for jimh  Send Email to jimh     
Actually, only one of us can be right because our techniques lead to different solutions. When you select the wire based on its voltage drop you will get a wire that is larger than if you select on current capacity when the system voltage is below about 50 volts. I will say this for the fourth time: in a system where the voltage is below 50 volts, select the conductor size based on consideration of the maximum voltage drop that can be tolerated, not on the current capacity of the wire.

It has nothing to do with the relationship expressed in the formula

Power = Voltage x Current

That is not a consideration. It has to do with the resistance of the wire and the voltage drop which results from current flow in the wire. The relationship is

Voltage = resistance x current

The charts for current capacity are based on rise of the wire above ambient from heating. The heating is from the voltage drop.

Again, when you are making any calculation about the size of the wire needed for the primary battery distribution on a small boat, calculate based on the voltage drop in the conductor, not on the conductor current capacity.

jimh posted 06-28-2009 09:45 AM ET (US)     Profile for jimh  Send Email to jimh     
Let me explain further with an example. Often an example that illustrates the problem provides better understanding than a simple statement of the problem.

Let's suppose we have a conductor connecting two elements of a circuit, and a certain current, I, flows in this conductor. The conductor has a certain resistance, R. When the current flows there will be a voltage drop in the conductor that is

E = I x R

Let us suppose that the voltage drop is 1-volt. This voltage drop is only related to two elements: the current flow in the wire and the resistance of the wire.

Now we first assume that the voltage in this circuit is 120-volts. We assess how much voltage drop we have sustained:

1/120 = 0.83-percent voltage drop

In a 120-volt circuit we have only a small voltage drop. We put 120-volts into the conductor at one end, and we get 119-volts at the other end. We hardly notice the difference. If we are trying to operate an electric motor that is rated for 120-volts, we can be quite certain it will operate with 119-volts.

Next we assume the voltage in this circuit is 12-volts. We assess how much voltage drop we have sustained:

1/12 = 8.3-percent

In the case of the 12-volt circuit, we have lost a significant amount of the voltage we are trying to distribute. We put in 12-volts at one end of the conductor and we only get 11-volts at the other end. If we are trying to run the starter motor for an outboard engine, we may find that the electric motor will not crank over very fast with only 11-volts applied to it instead of the full 12-volts.

HAPPYJIM posted 06-28-2009 02:11 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
Before you disconnect the existing wire, measure the voltage at the battery and then at the end of the cable where it connects to the starter. If the voltage is around 11.5 VDC or better go with the same size wire. You need a fairly good quality volt meter to measure this.

Or you can do what is very common practice. Go 1 size larger wire than what you have now. It will probably cost an additional 5 bucks which is nada.

The smaller diameter wire has more resistance which causes more voltage drop.

Large diameter wire has less resistance which causes less voltage drop.

Most people are not electrical engineers and have little desire to figure voltage drop. A whole slew of info is needed to figure voltage drop with the resistance of a given length of wire to start. If you showed up at West Marine and asked them what the resistance of 20 foot of #2 multi-strand wire was, You will probable get a blank stare.
Then you need a basic understanding of Ohm's Law to take all of the info gathered and with various formulas you can come up with the proper wire size by figuring voltage drop.

With wire lengths of less than 20 feet, voltage drop will not be a big factor if the proper size wire is used when figuring current carrying capacity. It is much easier to add the current of all of your electrical and electronic components to come up with a total current. Then you look at a chart of current capacities of marine wire and pick the size wire. As I said before, It is common practice to go 1 size larger than needed. For your console wiring, this will amount to maybe a buck.

I am in complete agreement with you, Jim, IF you are designing a system from scratch and you have runs of wire that exceed 20 feet. That is not what we have here. He just wants to replace the existing wire with a new one.

tim1198 posted 06-29-2009 08:16 AM ET (US)     Profile for tim1198  Send Email to tim1198     
Hi Jimh:
I respectfully disagree with your statement:
'It has nothing to do with the relationship expressed in the formula: Power = Voltage x Current'

I won't go into engineering principles which would bore the heck out of everybody except engineers; I suspect that you may be. I am an engineer also.

You were right in your initial response. But so was HAPPYJIM. The details of this discussion can continue off-line, as an academic intellectual discussion.

My rule of thumb is, without further in-depth analysis: unless you're adding more than 1/3 length (like going from 20' to 26'), I'd stick with the same gauge wire, since engineers already build-in design factors into the wire size. It just isn't that exact on a boat.

Charlie Munger said this to Warren Buffett: "In the end, you'll agree with me because you're smart, and I'm right!"


jimh posted 06-29-2009 08:19 PM ET (US)     Profile for jimh  Send Email to jimh     
The first error in this discussion was to refer to the battery primary distribution negative conductor as a "ground" conductor. A ground conductor is generally a conductor intended as a safety device, and in normal operation it carries no current. It is sized so that it can trip the over-current protection in the circuit in the event of a fault, so generally it is roughly the same size as the current carrying conductors.

The second error was to recommend that the conductor size for the primary battery distribution be selected on the basis of its rated current carrying capacity and matched to the circuit current maximum.

In my statement,

"It has nothing to do with the relationship expressed in the formula

Power = Voltage x Current"

the pronoun "it" refers to the process for selecting the wire gauge in the primary battery distribution wiring. A wire carries current, and so long as its insulation is appropriate, it cares not about the voltage potential to ground.

Again, I repeat ad nauseum, do not attempt to select the size of the primary battery distribution conductors based only on their rated current carrying capacity. You must consider the voltage drop as the most important factor when the circuit voltage is below about 50-volts.

Using a simple metric, if you double the length of the conductors in your primary battery distribution you will also double the voltage drop. To get the voltage drop back to the original value, if you double the length of the conductors you have to double the size of the conductors.

ASIDE to tim1198--Your attempt to close the argument with you having the last word is a good technique, but I am afraid that I won't cooperate. I am inclined to continue on-line.

jimh posted 06-30-2009 09:44 AM ET (US)     Profile for jimh  Send Email to jimh     
If the length of the conductor is very short, then the current carrying capacity is more important than voltage drop, as in a very short conductor there won't be significant voltage drop even at the maximum rated current. Voltage drop develops in proportion to resistance, and resistance is in proportion to length. Rated current capacity is not proportional to length, it is only proportional to diameter and composition of the conductor.

Most of the time the concern about conductor size in a primary battery distribution system occurs when the distance between the battery and the main load, the outboard engine starter motor, is about to be significantly increased by moving the battery from the stern to the middle of the boat. This is precisely the sort of situation where voltage drop becomes important. Engine starting is the most important electrical function on a small boat. Engine starting is very dependent on voltage delivered to the starter motor. It makes no sense to scrimp on the primary battery distribution conductor size and seriously compromise the ability to get the outboard motor started.

White Bear posted 06-30-2009 11:43 AM ET (US)     Profile for White Bear  Send Email to White Bear     
The poor guy just wants to know what size wire to use, not a treatise on electrical engineering. If I remember correctly, the 20' Outrage originally had its batteries located in the splashwell (at least mine did). This question relates to batteries located in the center console, which fact will certainly impact on the length of the wire run. Again, if I remember correctly, when I relocated my batteries to the console, I ran the wires from the new battery switch positiion inside the console to starboard through the duct under the deck, up the hull side inside the removable chase thoughtfully provided by BW to the underside of the gunwale and then aft to a point where I was able to connect with the existing wiring to the engine which I also relocated to the underside of the starboard gunwale. I used crimped terminals and I believe 2 gauge for the new run. Perhaps not perfect from a theoretical perspective, but it has performed flawlessly for over 10 years.
HAPPYJIM posted 06-30-2009 02:43 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
I know this is starting to get a little far reaching for something so simple as replacing an existing ground wire but sometimes it gets so technical that people get turned off by the whole thing and just get disinterested.

From the Ancor Marine Wire web site, color codes for wires include the color "black' to be "DC Negative Conductor" used for "Return, Negative Mains". Most people not in the electrical field refer to the red wire as positive or hot and the black wire as negative or ground. I understand that it is technically not correct but it does happen.

In Swellmonster's 3rd post, he did say that he was just replacing the existing wire with a new one. All I did was suggest using the same size wire as long as no additional equipment was added and if there was more equipment, to add that to the starter motor current to get a total ampacity so the correct wire could be bought.
I also injected that if the console buss wiring were to be updated that a total of the current of the equipment be totaled for determining wire size. This is very basic electrical theory and practice.

The length of wire inside the console is almost no concern of voltage drop because it is such a small run. I can think of no equipment that would be used on a small pleasure craft that would require a precise voltage requirement.

I worked, for 2 years on a Coast Guard Cutter as an Electrician with 24 VDC diesel/electric system. I then went aviation and worked on C-130 electrical systems for 8 years. The DC voltage was 24 volts on aircraft. Much of that time was spent on Research and Development of new navigation and electronic systems that required wiring to existing busses in the aircraft. I say this in the sense that we never figured voltage drop in circuits because there were charts drawn by engineers to show what size wire to use if the total current carrying capacity was figured. Below is such a chart.

We also used calculators to figure wire size. The first step was to determine current draw of all of the new equipment and get a total. Then we would plug that info into the calculator and find the size wire needed. Below is one such calculator.

Some of those same aircraft are still operating today. None have ever crashed but most have been mothballed as they were wired 30+ years ago.

So.........I will concede defeat to you in that you run a fine site full of the best information available for Boston Whaler freaks. But behind your back I will quietly be figuring my wire size by my old method of adding current of equipment and checking charts for what wire size I need to buy.

jimh posted 06-30-2009 09:24 PM ET (US)     Profile for jimh  Send Email to jimh     
Figuring the proper wire size is not "a treatise on electrical engineering."

The wire calculator mentioned above (by HAPPYJIM) takes into account voltage drop.

Ancor's wire size calculator includes voltage drop:

I don't understand why anyone would decide that ignoring voltage drop in calculating wire size is a prudent approach.

When I turn the key, I want my outboard motor to crank over. I don't want it to fail to start because there is too much voltage drop. This is not about arguing; it's about getting your motor to start on that stormy day when your battery is a little below normal and you need to get going.

Swellmonster posted 07-02-2009 12:24 PM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     
Marine max said it generally a 2 guage. I guess its just that simple, as no mods have been installed.
If I do add a margarita slushey machine :) I will make sure it is a two stroke so I can blame my merc smoke on the blender. LOL
Blackduck posted 07-02-2009 01:14 PM ET (US)     Profile for Blackduck  Send Email to Blackduck     
Geez, I told you that a week ago, I don't know what all of the mumbo jumbo that went on for the last week was all about. It really isn't that complicated.
jimh posted 07-03-2009 08:33 AM ET (US)     Profile for jimh  Send Email to jimh     
If "mumbo jumbo" here means simple electrical circuit design, then, yes, there was some "mumbo jumbo" that went on. I do not think that using AWG-2 conductors is the solution to all electrical problems, so sometimes it might be necessary to consider other sizes of conductors.

The best place to get arbitrary and possibly inaccurate answers is probably not in SMALL BOAT ELECTRICAL.

Blackduck posted 07-03-2009 09:49 AM ET (US)     Profile for Blackduck  Send Email to Blackduck     
Yes Jim, but size 2 wire is the correct size for this app. There isn't anything unusual here, and I just can't see confusing the issue. What is it that Tom Clark likes to say, KISS, keep it simple-
Blackduck posted 07-03-2009 10:06 AM ET (US)     Profile for Blackduck  Send Email to Blackduck     
And Jim, I would not give anyone "arbitrary" advice on anything. I try to help where and when I can, giving the best information I can. And once again, thank you for this site, Walter
jimh posted 07-03-2009 09:42 PM ET (US)     Profile for jimh  Send Email to jimh     
It remains to be seen if 2-AWG is the right answer for this application, since we never did get the length of the conductors or the current draw of the starter motor. But 2-AWG is a good solution for many battery primary distribution systems. If the conductor is larger than necessary there is no harm, except to the pocketbook of the guy buying the wire.

In general I think it is better to discuss the problem than to just give the answer. I still find it hard to embrace the concept that simple relationships or formula involving nothing more than multiplication or division are looked on as over-doing the answer.

new2whalering posted 07-06-2009 03:23 PM ET (US)     Profile for new2whalering  Send Email to new2whalering     
May I never ask the time for fear that I will be instructed on how a watch is built

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