
ContinuousWave Whaler Moderated Discussion Areas ContinuousWave: Small Boat Electrical Wire gauge to use, 1983 Revenge

Author  Topic: Wire gauge to use, 1983 Revenge 
JMARTIN 
posted 02102013 12:48 PM ET (US)
What gauge wire should run from my A/B switch box to the main power bar behind the helm? It starts out as a 10 then is crimped on to a 12 to finish the journey for some unknown reason. John 
Tom Hemphill 
posted 02102013 01:34 PM ET (US)
Is that run of wire protected by a fuse or circuit breaker? If so, the wire needs to at a minimum support its amp rating. 
jimh 
posted 02102013 02:28 PM ET (US)
Use 8AWG, a continuous run, no splices. 
jimh 
posted 02102013 02:30 PM ET (US)
In distribution of 12Volt power the current rating is not a consideration. The calculation of voltage drop for a conductor of about 10feet or longer will typically require using a wire of larger size than would be required to handle the current. Voltage drop trumps current rating in 12Volt power distribution. 
JMARTIN 
posted 02112013 12:30 AM ET (US)
Tom, no fuse, to the fuse block. Thank you Jim, John 
jimh 
posted 02112013 08:10 AM ET (US)
Example of voltage drop calculation for distribution of 12Volt power: If using 8AWG cable, the resistanceper1000feet is 0.7421. We assume a current of 25Amperes. The voltage drop will be 25 x 0.7421 = 18.5525Voltsper1000feet The maximum tolerated voltage drop is to be three percent or 12 x 0.03 = 0.36Volt Using 8AWG cable, what is the maximum length of cable in the circuit before 0.36Volt drop occurs when 25Amperes of current is flowing? Length = (1000feet / 18.5525Volt ) x 0.36Volt = 19.4feet Because the cable is an outandback run, if a threepercent voltage drop is to be maintained the maximum distance which 8AWG can be used to distribute 12Volt power for a load of 25Amperes is 9.7feet . An 8AWG cable in open air is rated to carry 60amperes. We see that the current rating of the wire is much greater than will be needed in the power distribution circuit. We can also calculate the length for 10percent voltage drop. 12 x 0.10 = 1.2Volt Length = (1000feet / 18.5525Volt ) x 1.2Volt = 64.7feet Because the cable is an outandback run, if a 10percent voltage drop is to be maintained the maximum distance which 8AWG can be used to distribute 12Volt power for a load of 25Amperes is 32.3feet . 
jimh 
posted 02112013 08:14 AM ET (US)
In electrical power distribution, a device to provide overcurrent protection, such as a fuse or circuit breaker, should be used at the source of the power to protect the conductors carrying the power distribution. In the example under discussion, the recommended 8AWG cable distributing power to the helm from the main battery power distribution should be protected by a circuit breaker whose rating is lower than the current capacity of the wire. A [typical] 8AWG cable can carry 60amperes. A circuit breaker rated for 60amperes or less should be used at the source to provide overcurrent protection for the 8AWG cable. 
tmann45 
posted 02122013 11:34 AM ET (US)
If you use "marine grade" (105 deg C insulation) wire like Ancor, it has a higher ampacity than the stuff you get at Home Depot/Lowes. The actual safe current to run thru it depends on ambient temperature and number of conductors in the bundle. For exampe 8 AWG 105C wire has an ampacity of 80 amps if used at 30 C or less and two or less energize wires in the bundle. See http://www.marinco.com/page/allowableamperage [To download and install a precompiled binary exectuable application that works on certain versions of the Windows operating system and performs] neat wire size calculations, see http://www.midcoast.com/~aft/index2.html 
tmann45 
posted 02122013 11:37 AM ET (US)
The wire size calculator does not take you directly to the calculator download page, you have to click on the "WireSizer3.2" link on the left side of the page to get there. 
jimh 
posted 02122013 12:30 PM ET (US)
Wire current rating can be improved if special insulation is used, such as Teflon. The cost is not usually justifiable. Since in a marine 12Volt distribution we will always be using a wire of larger diameter than will be required to carry the current safely, there is little value in using specialized wires that can handle more current for a given wire size than the standard wire. In situations were weight is a concern, specialized wire insulation like Teflon may permit a conductor to be rated for more current. The weight saved in copper may be a useful benefit. This is often seen in aircraft applications. Teflon wire may have been used first in the ultimate weightsaving application: spacecraft. 
jimh 
posted 02122013 01:03 PM ET (US)
If an 8AWG conductor were used to feed a secondary power distribution panel that was only rated to handle a current of less than 60Amperes (or less than the current handling capacity of the wire, if a specialized wire were used), then the overcurrent protection device should be downrated to match the capacity of the secondary power distribution panel. For example, if the secondary power distribution panel being fed by the 8AWG were only rated for 25Amperes, then the overcurrent protection at the primary power source should be not more than 25Amperes. 
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