Selecting Proper Wire Size: Engine Starting

Electrical and electronic topics for small boats
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Selecting Proper Wire Size: Engine Starting

Postby jimh » Wed Oct 23, 2019 2:34 pm

There are many discussions about moving a source of electrical power to a new location which will put the power source farther away from its electrical loads: for example, moving a 12-Volt DC storage battery farther away from the outboard engine it will try to crank over and start. Whenever a power distribution circuit is modified, the conductor resistance should be reviewed with an eye to maintaining the necessary minimum voltage drop in the power distribution wiring itself.

Limiting voltage drop is particularly important in lower voltage power distribution, and especially in 12-Volt DC circuits, which are typically about the lowest voltage of electrical power that is distributed. The reason for greater concern with 12-Volt DC power distribution is those circuits often use high-current. The voltage drop in a conductor is directly proportional to current. Since good design calls for a limit of wiring voltage drop to 3-percent or less, it is clear that with 12-Volt power a limit of 3-percent means only 0.36-Volts of drop in the wiring can be tolerated.

In order to know the voltage drop in a conductor, only the current (in Amperes) and the resistance (in Ohms) are needed. The load current is generally known by the design of the system, or the current can be measured with an Ammeter. The resistance of the wire is usually so small that direct measurement of the wire resistance is impossible without very sensitive and carefully calibrated instruments; you cannot just use a $50 DVM to measure resistance values that are much, much less than 1-Ohm.

The resistance of copper wire can be assumed from the size of the conductor. In the USA, wire conductors are usually characterized by the American wire Gauge (AWG). The resistance value is usually given for a 1,000-feet length of wire, as this avoids having numbers with many decimal places that begin with a string of zeros. For example, a copper wire of 10-AWG is specified to have a resistance of just about exactly 1-Ohm/1,000-feet.

Values for copper wire are usually given in tables from reliable references. Here is an excerpt of one table of values:

    AWG   Ohms/1000-feet
    0000 0.048
    000 0.0618
    00 0.078
    0 0.0983
    1 0.124
    2 0.1563
    3 0.197
    4 0.2485
    5 0.3133
    6 0.3951
    7 0.4982
    8 0.682
    9 0.7921
    10 0.9989
    12 1.588
    14 2.525
    16 4.016

The table is based on the AWG cross section area and uses a resistivity value of copper of 1.724 ×10-8 Ohm-meters at 20-degrees-C. Actual wire will vary somewhat based on the purity of copper. The table was originally published in "Electric Circuit Fundamentals," 2nd Edition, Thomas L. Floyd, (a 661-page reference book that retails for $170), and I assume it is a reliable source.

I don't bother with wire sizes smaller than 16-AWG because according to USCG federal regulations and ABYC standards, the smallest wire size that can be used in boat wiring for power distribution is 16-AWG.

A very common instance for a battery to be relocated to be further from its load in boating is for an outboard engine starting battery to be moved out of the stern of boat to reduce weight there, often based on increasing weight of the outboard engine itself. Because the current flow in an engine cranking motor is extremely high, typically hundreds of Amperes, there is very great concern about voltage drop in the wiring. For this reason, the conductors must be increased in size whenever their length is increased.

The precise value of current that will flow or the absolute maximum voltage drop that can be tolerated are generally not known for outboard engine starting circuits, but the engine manufacturer usually publishes guidelines about what wire size must be used for certain wire lengths. Here is a table from Evinrude in which they specify wire size for the battery cables for various lengths:

    Recommended Conductor Size for Engine Starting
    Feet Minimum AWG
    1 to 10 4-AWG
    11 to 15 2-AWG
    16 to 20 1-AWG

Source: "Evinrude E-TEC Installation and Predelivery" guide, page 16.

We can make an inference about the maximum value of resistance in the battery cables just based on these recommendations. We can compute the resistance in the cables (for one conductor) based on the length and resistance-per-1000-feet for the wire gauge recommended.

For a maximum of 10-feet, 4-AWG can be used. The resistance of that conductor will be

    (0.2485-Ohms/1000-feet) × 10-feet = 0.002485-Ohms

For a maximum of 15-feet, 2-AWG can be used The resistance of that conductor will be

    (0.1563-Ohms/1000-feet) × 15-feet = 0.0023445-Ohms

For a maximum of 20-feet, 1-AWG can be used, The resistance of that conductor will be

    (0.125-Ohms/1000-feet) × 20-feet = 0.0025-Ohms

Looking at the three values of resistance, we see that the highest value occurred at the longest length, 0.0025-Ohms at 20-feet. This indicates that the resistance maximum must be 0.0025-Ohms. Any cables used must have equal or less resistance.

If the battery cables are to be made longer than 20-feet, the wire gauge must be further increased. The method to calculate the wire gauge based on a maximum permitted resistance is also easy. We know the resistance at some shorter distance, so we just scale up the distance to 1,000-feet to find the resistance at 1,000-feet as used in most tables.

For example, if the wire length must be increased to 25-feet, what conductor size will produce a resistance of 0.0025-Ohms or less?

For the given values, 0.0025-Ohms and 25-feet, we need a conductor whose resistance per foot will be:

    0.0025-Ohms/25-feet = (0.001-Ohm/1-feet) × (1000/1000)
    = 0.1-Ohm/1000-feet

We enter the table and look for a wire gauge with a resistance-per-1000-feet of 0.1-Ohm or less. The suitable wire is 0-AWG with a resistance per 1,000-feet of 0.0983-Ohms. Now we can deduce that for the particular engine (an Evinrude E-TEC) to be properly wired with 20-feet-long cables, the wire size must be increased to 0-AWG.

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Re: Selecting Proper Wire Size: Engine Starting

Postby jimh » Thu Oct 24, 2019 9:16 am

With engine starter motors, when electrical power is initially applied to the start motor the rotor is stalled. This causes a very high current flow. A typical in-rush current to a stalled starter motor is on the order of 500-Amperes. We can compute the voltage drop using that figure. For battery cables that adhere to the calculated maximum resistance of 0.0025-Ohms, the total resistance in two conductors (positive circuit and negative circuit) will be 0.005-Ohms. If 500-Amperes flow, the voltage drop will be

    Voltage drop = 0.005-Ohms × 500-Amperes = 2.5 Volts

When the battery is asked to deliver 500-Amperes there will also be some sag in the terminal voltage of the battery due its internal resistance. Battery internal resistance increases with decreasing temperature. Internal resistance also varies with state of charge. Also, the battery voltage decreases with decreasing state of charge. For a battery at 50-percent state of charge the terminal voltage with no load will be about 12.06-Volts. If the ambient temperature is cold, say 40-degrees, the internal resistance will be higher than normal. All of these factors tend to cause the battery to be unable to deliver as much power in cold weather as in warm weather.

If we assume an battery terminal voltage of 12.0-Volts and an internal resistance of 0.005-Ohms, then the terminal voltage of the battery under load will drop by 2.5-Volts, sagging from 12.0-Volts to 10.5-Volts.

The voltage at the engine starter motor is now only

    12.0 − 5.0 = 7.0 Volts

The starter motor must be able to develop enough torque at this voltage to begin to turn against the load of the engine flywheel. If the motor can begin to turn, the motor will no longer be stalled and the current will decrease. This will cause more voltage to be delivered to the motor, producing more torque. The motor will accelerate and begin to crank over the engine. If the motor remains stalled, the engine will not be cranked over.

The initial in-rush current of the stalled starter motor becomes the most important factor in determining if the engine will begin to crank over.