Starter Cable Size Question

[rwidman]

Originally Posted by a64pilot

Only savings is cost, and the wire is as good as it will ever be the day you buy it, from that day on resistance will slowly increase. One day having cables that are "too big" may be what saves the day when you have to start with a low battery or they merely get old.
I see wires like anchors, what does "too big" hurt?

Wire resistance does not increase with age.

I agree on the wisdom of not going cheep with undersized or marginal wire or cable but not the reason you posted.

[ramblinrod]

What frightens me most about this thread is that after so many posts, there are still several factors that influence proper cable size selection not mentioned.

The correct answer is...

Refer to marine electrical standards before ANY wiring modification AND follow them EXPLICITLY.

If you are not going to take the time to READ AND UNDERSTAND the electrical standards AND IF YOU ARE NOT ALREADY SKILLED in safe and reliable marine wiring practices, hire a certified Marine Electrical Technician.

Reading a few posts on the internet does not mean one can safely wire a boat.

Poor electrical wiring, especially on unfused battery circuits can risk personal injury including death. Though electrical wiring may not be brain surgery (it may be harder) not only may a botched job kill you, it may also burn your perfectly good boat to the waterline, whereas if your lobotomy goes wrong, your boat (and all the women and children in it) will still be safely floating. ;-)

[a64pilot]

Originally Posted by rwidman

Wire resistance does not increase with age.

I agree on the wisdom of not going cheep with undersized or marginal wire or cable but not the reason you posted.

Not even as it oxidizes? Even tinned wire eventually corrodes?

http://www.madsci.org/posts/archives...0823.Ph.r.html

[ramblinrod]

Originally Posted by a64pilot

Not even as it oxidizes? Even tinned wire eventually corrodes?

Re: Does wire wear by losing conductivity or gaining resistance with age?

I think the point is that "age" does not affect resistance, other factors, that may be associated with time, (increased corrosion for example) may.

A wire, resting motionless in a vacuum, passing no current, will be the same resistance 100 years from now as the day it came off the production line.

[rwidman]

Originally Posted by ramblinrod

I think the point is that "age" does not affect resistance, other factors, that may be associated with time, (increased corrosion for example) may.

A wire, resting motionless in a vacuum, passing no current, will be the same resistance 100 years from now as the day it came off the production line.

And wire does not wear out from passing current (that's within its rating). Wire installed in buildings a hundred years ago is still performing as intended. Marine wire, properly installed in a boat would be the same except that it hasn't been around a hundred years so we have no experience to draw on.

It should be noted that non-marine, non tinned wire is used to power submersible pumps in residential wells with an indefinite lifespan. It is submersed in water 24/7 with no ill effects.

[rwidman]

Originally Posted by Marqus

I like Boatwright's advice.

From a theoretical perspective, using a longer cable for the positive link vs negative link to the starter motor, has the same effect as using a thinner cable for one of the links - the voltage drop on that link will be greater than on the other.

As long as the total voltage drop during engine start is within your tolerance, you can adjust the length and/or thickness of each link to suit your requirements (it will involve calculation using max DC current and copper cross section of each conductor).

Usually, it's quicker to just calculate copper thickness using the overall path length, but there is absolutely nothing stopping you from calculating voltage drops on two segments of the current path, and just adding them.

Even quicker is to not calculate anything, just look it up in a voltage drop table such as the one in the West Marine catalog and on their website.

[westwinds]

Lets get some data to work with on wire sizing. What kind of engine does the boat have, that is how big is the starter and therefore the current that it draws? If no specific engine, lets go with 500 amps for a several hundred horsepower engine like an old Detroit 6-71 diesel and get some real numbers worked up.

[a64pilot]

OK a boat that is used in salt water and not the vacuum of space the wire may increase resistance as the wire ages, therefore I see no harm at all in oversizing wires, it buys you some cushion.
Like the wires used in my original lighting, they are way oversized now that I am LED, does that cause any problem? Of course not.

[kmacdonald]

Use the smallest wire that meets current carrying capacity and Voltage droprequirements. Bigger adds weight.

[mitiempo]

Originally Posted by kmacdonald

Use the smallest wire that meets current carrying capacity and Voltage droprequirements. Bigger adds weight.

On a 40' sailboat I doubt a pound or so makes much difference. I would much sooner have the advantage of faster engine starting that comes with larger gauge wire than being a few pounds lighter.

I have never sized wire by weight.

[twinboat]

Originally Posted by Boatwright

However......

Your starter is bolted to a cast iron engine block which should also be grounded. If one ground wire is serving both starter and engine, often attached at one of the starter mounting bolts or other point close to the starter, it should be the same size as the positive wire. If there is a second separate engine ground it will act in parallel to your starter ground and the starter ground can be reduced accordingly.

The starter bolted to the block does not always guarantee a path to ground, if the block is grounded. Some starters have an isolated negative post.

I installed an aftermarket starter, that had a negative post on the back of the starter. It would not work, unless I ran a cable from the starter negative post to the engine block. I could have moved my negative cable to the starter motor but that would have left my engine un-grounded.

[Maine Sail]

The higher the voltage at the starter motor, the more current it develops and the faster it spins the engine over. This means shorter starts and less overall work for the starter motor. having the right size wire can mean the difference between a 4 second loaded to unloaded cranking duration and a 0.75 second loaded to unloaded starting duration.

On top of wire voltage drop we can't forget about battery voltage sag under the load the starter creates. If the battery is sagging to 8-9V on in-rush, and many do, and the cable is dropping another 2V - 4V (and many do) then the motor is going to be pretty anemic, and many are.

Even an .8kW starter motor from a Westerbeke 30B Three, with a fairly healthy battery, will draw about a 370A in-rush and this is just firing it on a bench without even starting a motor. If the battery is in poorer health the starter may not even be able to develop 300A of initial cranking current. The voltage delivered to the starter motor plays a major roll in the performance of the starter motor.

1/0 on negative and 2/0 on positive is not the main concern it is the total circuit voltage drop that matters. You would need to calculate VD for the 1/0 run and the VD for the 2/0 run, and then figure your total circuit voltage drop. Also remember that wire voltage drop does not account for all the terminals, switches etc. so the number you get for wire resistance is only added to when you have a circuit with multiple terminations switches etc. in the path.

[ramblinrod]

Originally Posted by rwidman

And wire does not wear out from passing current (that's within its rating). Wire installed in buildings a hundred years ago is still performing as intended. Marine wire, properly installed in a boat would be the same except that it hasn't been around a hundred years so we have no experience to draw on.

It should be noted that non-marine, non tinned wire is used to power submersible pumps in residential wells with an indefinite lifespan. It is submersed in water 24/7 with no ill effects.

Nobody said anything about wearing out. We were talking resistance. Resistance changes with temperature. Temperature changes with current passing through cable.

[Marqus]

To illustrate using Maine Sail's conservative 370A starter current (if your starter current is higher, the voltage drop will increase):

6 feet of 1/0 copper cable will have a voltage drop of 0.21volt
6 feet of 2/0 copper cable will have a voltage drop of 0.17volt

So, hypothetically, if you have one 6ft link of 2/0 followed by a 6ft link of 1/0, the total voltage drop (due to wire length) to the starter will be around 0.4volt.

If both links were 2/0 copper, the total drop will reduce to around 0.34volt.

[Frankly]

I don't understand the thinking that the starter positive conductor should be the same size as the return (negative). On Cbreeze the single conductor from the battery ground bus to the engine block is a length of #2/0 (chosen 11 years back because it was just laying in my scrap wire box). The positive conductor is #2 and fused at 200 amps (type ANL). The runs are not long and the 3GM starts pretty easily.

The 2/0 also serves as the return for the 100 amp alternator. As the starter and the alternator are not in operation at the same time just makes good sense to me (money, weight, environmental, or whatever your cause is). I did include a short #4 jumper from the alternator case to the next hole in the engine block to make sure I had a low resistance path for the alternator current.

I do not have a dedicated starting battery just 3 sets of GC batteries with one set way up fwd connected with #2 for both legs. To keep an eye on the batteries, couple of times a year I cycle starting the Yanmar from each individual set. Last month while cycling the batteries through a nearly full discharge, I cold cranked the engine from the fwd set with the resting terminal voltage of 11.7 volts. She hesitated a bit but still busted right off.