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Author Topic:   Electrical Distribution Capacity
mrtoyz posted 07-07-2010 01:41 AM ET (US)   Profile for mrtoyz   Send Email to mrtoyz  
I'm planning on re-working and re-wiring my Dauntless taking in to account future upgrades. I plan to use a Blue Sea Contura circuit panel rated at 20A. I am less than a novice and hope you can explain a few things. I'm looking at Cannon downriggers that are rated at 25-ampere maximum draw and recommend a 30-ampere fuse or breaker. Does this then mean that even using correct wire size--unsure what that is so far--that I could not wire them into the 20-ampere panel? Seems logical that I could not, but I'd like to make sure I understand. Same goes for a MinnKota 55 12-volt trolling motor that I believe requires a 50-ampere fuse or breaker.

Appreciate your help.


jimh posted 07-07-2010 08:37 AM ET (US)     Profile for jimh  Send Email to jimh     
A distribution panel that is rated for 20-amperes of current cannot be used to supply electrical current to devices that need 30-amperes and 50-amperes of current.

Your description of the device you intend to use as a "Blue Sea Contura circuit panel rated at 20A" is not clear to me. If you give the Blue Sea part number for the electrical distribution panel you are asking about, we could have a better idea of its capacity.

jimh posted 07-07-2010 08:45 AM ET (US)     Profile for jimh  Send Email to jimh     
The conductor size needed to distribute electrical power is determined by two considerations:

--the maximum current the conductor can safely carry

--the maximum voltage drop that is tolerable in the branch circuit

The maximum current the conductor can safely carry is a function of three factors:

--the size of the conductor, that is, its copper cross section area, which is usually specified as its wire gauge, and in the U.S.A. by its AWG rating;

--the composition of the insulation on the wire and its tolerance to resist deforming as temperature rises; most thermo-plastic wire insulation is only rated to 105-degree-C;

--the position of the conductor relative to other conductors and the air ambient; a conductor that is part of a cable and enclosed in insulation is rated for lower current (due to higher temperature) than a conductor suspended singularly in air.

The maximum voltage drop that is tolerable in the branch circuit is determined by the nature of the load; some loads are rather sensitive to voltage sag and need to be supplied with a source of power without much voltage drop. A voltage drop of three-percent is often used as a critereon. The voltage drop is determined by three factors:

--the size of the conductor;

--the length of the conductor;

--the current drawn in the circuit.

jimh posted 07-07-2010 08:48 AM ET (US)     Profile for jimh  Send Email to jimh     
In electrical distribution where the voltage is below 50-volts, as is the case in a small boat where 12-volt power is being distributed, it is common that the conductor will have to be sized with regard to the maximum tolerable voltage drop allowed, rather than the maximum current capacity. That is to say, a branch circuit might be only carrying 30-amperes of current, which could safely be handled by a conductor of 10-AWG, but due to consideration of voltage drop the conductor size will have to be increased to a larger size in order to limit the voltage drop that will occur.
jimh posted 07-07-2010 08:52 AM ET (US)     Profile for jimh  Send Email to jimh     
A good source of information on wire conductor size for planning electrical distribution on a small boat is found at

The wire calculator is very useful:

mrtoyz posted 07-07-2010 10:10 PM ET (US)     Profile for mrtoyz  Send Email to mrtoyz     
Here are the panels I'm looking at.
davej14 posted 07-10-2010 11:15 AM ET (US)     Profile for davej14  Send Email to davej14     
The panels you are considering are for low current accessories such as navigation lights, radios, etc. They have an integrated 15A circuit breaker so the are not appropriate for your down rigger or trolling motor application.

I question why you would want to wire either of these to a switch panel since they have their own controls. You do need to have a fuse in each circuit but on/off switch control from the dash is not recommended. I wired my downrigger directly to a blue seas fuse panel that is capable of a 30A branch circuit:

For your trolling motor, if it does not come with an inline fuse then you should use something like this wired directly in series to the battery:

If this is confusing then I recommend you seek help with your electrical distribution plan before you start the project.

mrtoyz posted 07-10-2010 07:44 PM ET (US)     Profile for mrtoyz  Send Email to mrtoyz     
The why? Lol cause I dont know better... lol I'm learning.

The panel you use for your downrigger looks large/lots of other fuses. What else do you run to it?

Looks like all the Minn Kotas do have inline fuses.



davej14 posted 07-12-2010 11:58 AM ET (US)     Profile for davej14  Send Email to davej14     
They are not so large as they look at the link. I like the ones with the common bus included:

I use the 12 circuit version to allow for future expansion. It now services:

Scotty downrigger
Compass backlight
Lowrance Sonar/GPS
VHF Radio
AM/FM Radio
Accessory lights in center console
NMEA 2000 Network

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