
ContinuousWave Whaler Moderated Discussion Areas ContinuousWave: Small Boat Electrical Sizing conductors

Author  Topic: Sizing conductors 
jwestwood 
posted 01312014 10:39 AM ET (US)
I have read many of the posts here concerning sizing conductor wiring when batteries are moved forward to a console location. This is what I plan to do to help clean up the aft space. Do you, Jim, or others here have a suggestion for sizing starter conductors for an Outrage 21 with twin 1988 Yamaha 90's. As always, thanks for the help. 
jimh 
posted 02022014 10:23 AM ET (US)
As I have mentioned many times, in distribution of 12Volt power the conductor will almost always have to be selected on the basis of voltage drop, not on the basis of current capacity. One chooses the conductor size so, for the particular length of conductor, the voltage drop is limited to the desired tolerance. The tolerance for voltage drop seems to be threepercent in the most rigorous case, and as much as 10percent in more lenient cases. The threepercent tolerance would be used for loads that are sensitive to voltage, such as a navigation lighting circuit. (The light output from an incandescent lightbulb varies greatly with voltage, and low voltage will produce a significant dimming of the light output.) For loads that can tolerate a greater voltage drop, the 10percent tolerance can be used. Some modern electronic devices are happy to operate over a wide range of input voltage, as they contain internal voltage convertors and regulations for their power circuits. The voltage drop is a function of the resistance of the conductor and the current being carried. If we consider the nominal voltage of our system to be the voltage of a fullycharged battery, we can use 12.6Volts as the system voltage. A threeprecent voltage drop would then be 12.6Volts x 3/100 = 0.378Volts The voltage drop in a conductor is equal to the current times the resistance. I demonstrate by example: A circuit is to carry 3Amperes of current to a load that is 15feet from the battery. What is the maximum resistance in the conductor for a threepercent voltage drop in a 12.6Volt system? The maximum voltage drop will be 0.378Volts, as previously calculated. From Ohm's Law we can write E = IR and rearrange to R = E / I
R = 0.378Volts/ 3Amperes R = 0.126Ohm This resistance is for a circuit of 15feet out and 15feet back, or a total wire length of 30feet. 0.126Ohms/30feet = 0.0042/foot Most tables of wire resistance give the resistance of 1,000feet of the wire, so we have to scale this up by a factor of 1,000 which gives us a value of 4.2Ohms/1,000feet as the maximum wire resistance we could use. Now we consult a table to find a suitable conductor. A good table is http://www.powerstream.com/Wire_Size.htm Inspecting the table, we see: 16AWG = 4.016Ohms/1,000feet Thus a total conductor length of 30feet of 16AWG will distribute 12.6Volts DC to a load of 3Amperes without incurring more than a threepercent voltage drop. Note also that in boat wiring a conductor of 16AWG is the smallest recommended conductor to be used for any electrical wiring (with some exceptions) according to the recommendations of industry groups like the ABYC and 14AWG is mandatory according to certain federal regulations (and forgive me for being too lazy to look up and cite the appropriate section of our government's voluminous Code of Federal Regulations). Note that 16AWG is listed as having a current capacity of 22Amperes for short chassis wiring use. This example demonstrates how voltage drop is more significant in selecting a conductor size, as the current in this example was only 3Amperes, or 13percent of the rated maximum current. 
jimh 
posted 02022014 11:47 AM ET (US)
For many cites of regulations regarding wire gauge and type per federal regulations, see an earlier discussion at http://continuouswave.com/ubb/Forum6/HTML/001447.html which has a number of hyperlinks to primary sources. 
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