Starter Cable Size Question

[Terra Nova]

Originally Posted by Terra Nova

...the TOTAL distance from battery + all the way back to battery -. That is what determines cable size.

Originally Posted by Frankly

I don't understand the thinking that the starter positive conductor should be the same size as the return (negative)...

See above.

[rwidman]

Originally Posted by Frankly

I don't understand the thinking that the starter positive conductor should be the same size as the return (negative). ..............

I already answered this once but here it is again:

The current through the entire circuit is the same. For example, if the starter draws 200 amps then 200 amps must flow through the positive conductor, the motor and the negative conductor.

[rwidman]

Originally Posted by S/V Alchemy

Correct. There is some logic in keeping the cable sizes identical and of equal length, as well..........

Again, the same current flows through the entire circuit. There is absolutely no electrical difference between having a one foot positive conductor and a ten foot negative conductor compared to a ten foot positive conductor and a one foot negative conductor. It's eleven feet of cable either way.

[rwidman]

Originally Posted by 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, .......

Again, nope. The connections may oxidize and increase resistance if not properly done but the resistance of the wire will not change.

[a64pilot]

I guess you have never stripped wire back and found it black well away from the ends?
OK, so the terminal ends increase in resistance, how does that not increase the resistance of the circuit?

I stand by that your circuit is the best it will ever be the day its new, and only one thing can happen, and that is it will increase in resistance.
What do you think often cause fires in old wiring systems ?

Why do people rewire old boats if the wire lasts forever?

[Frankly]

I understand Ohm's law, Kirchoff's Current Law, and Kirchoff's Voltage Law (MSEE). There is no prostitution of those laws that would indicate that both the positive and negative starter motor cables should be the same size.

They should be able to safely carry the current, and the voltage delivered to the motor terminals should be at an acceptable level. That the positive and negative cables be the same size is pure BS (that was the question by the OP).

[rwidman]

Originally Posted by Frankly

I understand Ohm's law, Kirchoff's Current Law, and Kirchoff's Voltage Law (MSEE). There is no prostitution of those laws that would indicate that both the positive and negative starter motor cables should be the same size.

They should be able to safely carry the current, and the voltage delivered to the motor terminals should be at an acceptable level. That the positive and negative cables be the same size is pure BS (that was the question by the OP).

Here is the OP's post:

What size negative cable should I be running from my starter back to my house bank if the run from the house bank requires 2/0 on the positive side? Can it be 1/0?

So the cables don't have to be the same size but if the positive cable needs to be 2/0, the negative cable cannot be smaller.

There is never any harm (electrically) in using a larger cable or wire than needed but there could be a problem with physical size.

There is no prostitution of those laws that would indicate that both the positive and negative starter motor cables should be the same size.

I'm not sure what you are trying to say or who you are trying to impress with that statement, but in this series circuit, there is no advantage to using different sized cables so a normal person would use the same size cable throughout the circuit.

Would you make up an anchor rode with half 1/2" chain and half 1/4" chain?

[mitiempo]

Originally Posted by 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.

The wire gauge calculator I use is here: AWG by wire length/amps calculator

Assuming a 12' total run with battery voltage at 12.7 and 370 amp load it gives 3/0 wire size for a 3% drop. 3% of 12.7 is .38 volts.

[Marqus]

Originally Posted by Frankly

I understand Ohm's law, Kirchoff's Current Law, and Kirchoff's Voltage Law (MSEE). There is no prostitution of those laws that would indicate that both the positive and negative starter motor cables should be the same size.

They should be able to safely carry the current, and the voltage delivered to the motor terminals should be at an acceptable level. That the positive and negative cables be the same size is pure BS (that was the question by the OP).

Strongly articulated perhaps, but you just can't fault that statement.

If you know the overall voltage drop you can or want to tolerate, feel free to use different cable lengths and/or cables of different thicknesses.

Just sum your voltage drops - calculation can be quite easy as Mitiempo well illustrates.

[ramblinrod]

Originally Posted by mitiempo

The wire gauge calculator I use is here: AWG by wire length/amps calculator

Assuming a 12' total run with battery voltage at 12.7 and 370 amp load it gives 3/0 wire size for a 3% drop. 3% of 12.7 is .38 volts.

10% voltage drop tables are acceptable for starter circuits.

Will reduce voltage to the starter slightly, but will cost much less, for negligible difference in performance.

[ramblinrod]

Originally Posted by Marqus

Strongly articulated perhaps, but you just can't fault that statement.

If you know the overall voltage drop you can or want to tolerate, feel free to use different cable lengths and/or cables of different thicknesses.

Just sum your voltage drops - calculation can be quite easy as Mitiempo well illustrates.

With the exception that one cannot exceed the maximum current carrying capacity of any wire.

But, for all intents and purposes, it is generally easiest to just calculate the round trip cable run distance and pick the size off the table for the max current expected.

[Terra Nova]

Originally Posted by ramblinrod

10% voltage drop tables are acceptable for starter circuits.

Will reduce voltage to the starter slightly, but will cost much less, for negligible difference in performance.

Stick with the 3% table of values.

[mitiempo]

Originally Posted by Terra Nova

Stick with the 3% table of values.

I agree. I believe ABYC requires 3% drop for motors. Even if they don't it is the better choice as the losses from connections will add to the drop.

[dlymn]

Originally Posted by a64pilot

I guess you have never stripped wire back and found it black well away from the ends?
OK, so the terminal ends increase in resistance, how does that not increase the resistance of the circuit?

I stand by that your circuit is the best it will ever be the day its new, and only one thing can happen, and that is it will increase in resistance.
What do you think often cause fires in old wiring systems ?

Why do people rewire old boats if the wire lasts forever?

I think the poster was saying that the flow of 'electricity' is in the core of the copper wire and not on the surface. oxidation on the surface of a wire will make little difference to the resistance of the wire. Corrosion that eats into the wire, reducing its diameter significantly and destroying the conductivity of the circuit does have an impact. That corrosion is most likely to occur where there is a 'mechanical' connection and where there is moisture and air. A poorly crimped terminal connector will readily corrode on a boat.

[ramblinrod]

Originally Posted by dlymn

I think the poster was saying that the flow of 'electricity' is in the core of the copper wire and not on the surface. oxidation on the surface of a wire will make little difference to the resistance of the wire. Corrosion that eats into the wire, reducing its diameter significantly and destroying the conductivity of the circuit does have an impact. That corrosion is most likely to occur where there is a 'mechanical' connection and where there is moisture and air. A poorly crimped terminal connector will readily corrode on a boat.

Skin effect only pertains to AC.