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  Wire gauge to use, 1983 Revenge

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Author Topic:   Wire gauge to use, 1983 Revenge
JMARTIN posted 02-10-2013 12:48 PM ET (US)   Profile for JMARTIN   Send Email to JMARTIN  
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 02-10-2013 01:34 PM ET (US)     Profile for Tom Hemphill  Send Email to Tom Hemphill     
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 02-10-2013 02:28 PM ET (US)     Profile for jimh  Send Email to jimh     
Use 8-AWG, a continuous run, no splices.
jimh posted 02-10-2013 02:30 PM ET (US)     Profile for jimh  Send Email to jimh     
In distribution of 12-Volt power the current rating is not a consideration. The calculation of voltage drop for a conductor of about 10-feet 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 12-Volt power distribution.
JMARTIN posted 02-11-2013 12:30 AM ET (US)     Profile for JMARTIN  Send Email to JMARTIN     
Tom, no fuse, to the fuse block.

Thank you Jim,

John

jimh posted 02-11-2013 08:10 AM ET (US)     Profile for jimh  Send Email to jimh     
Example of voltage drop calculation for distribution of 12-Volt power:

If using 8-AWG cable, the resistance-per-1000-feet is 0.7421. We assume a current of 25-Amperes. The voltage drop will be

25 x 0.7421 = 18.5525-Volts-per-1000-feet

The maximum tolerated voltage drop is to be three percent or

12 x 0.03 = 0.36-Volt

Using 8-AWG cable, what is the maximum length of cable in the circuit before 0.36-Volt drop occurs when 25-Amperes of current is flowing?

Length = (1000-feet / 18.5525-Volt ) x 0.36-Volt = 19.4-feet

Because the cable is an out-and-back run, if a three-percent voltage drop is to be maintained the maximum distance which 8-AWG can be used to distribute 12-Volt power for a load of 25-Amperes is 9.7-feet .

An 8-AWG cable in open air is rated to carry 60-amperes. 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 10-percent voltage drop.

12 x 0.10 = 1.2-Volt

Length = (1000-feet / 18.5525-Volt ) x 1.2-Volt = 64.7-feet

Because the cable is an out-and-back run, if a 10-percent voltage drop is to be maintained the maximum distance which 8-AWG can be used to distribute 12-Volt power for a load of 25-Amperes is 32.3-feet .

jimh posted 02-11-2013 08:14 AM ET (US)     Profile for jimh  Send Email to jimh     
In electrical power distribution, a device to provide over-current 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 8-AWG 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] 8-AWG cable can carry 60-amperes. A circuit breaker rated for 60-amperes or less should be used at the source to provide over-current protection for the 8-AWG cable.
tmann45 posted 02-12-2013 11:34 AM ET (US)     Profile for tmann45  Send Email to tmann45     
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/allowable-amperage

[To download and install a pre-compiled 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 02-12-2013 11:37 AM ET (US)     Profile for tmann45  Send Email to tmann45     
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 02-12-2013 12:30 PM ET (US)     Profile for jimh  Send Email to jimh     
Wire current rating can be improved if special insulation is used, such as Teflon. The cost is not usually justifiable. Since in a marine 12-Volt 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 weight-saving application: spacecraft.

jimh posted 02-12-2013 01:03 PM ET (US)     Profile for jimh  Send Email to jimh     
If an 8-AWG conductor were used to feed a secondary power distribution panel that was only rated to handle a current of less than 60-Amperes (or less than the current handling capacity of the wire, if a specialized wire were used), then the over-current protection device should be down-rated to match the capacity of the secondary power distribution panel. For example, if the secondary power distribution panel being fed by the 8-AWG were only rated for 25-Amperes, then the over-current protection at the primary power source should be not more than 25-Amperes.

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