Be Careful With Boat Electrical Work

I am not a certified marine electrical provider.  But, I do have electrical credentials.  I put myself through college working as a journeyman electrician at a very large electrical sign company.  And, I have maintained an interest in things electrical for years - especially aboard Snickerdoodle.
And, first of all, I don't consider electrical circuits too complex for the average person with some common sense.
Like many of us, Snickerdoodle came with a basic factory installed 12VDC (that's 12 volt direct current) system for running lights, interior lights, and extra circuits ready for electronics like 2-way radio, depth sounder, etc.  Unfortunately for me (and I suspect many sailboat owners) there was no electrical schematic diagram provided with the boat or in the owner's manual.  There was a switch panel with several circuits indicated; and the switch panel included an in-line fuse for each circuit.  But, that was about it.
Fairly soon after buying Snickerdoodle, I wanted to add a 2-way radio, depth sounder, an auto pilot, and other electronic devices.  These all are powered by the 12 volt battery.  I also wanted a backup for the battery that would help create a "house" battery system.
Of course, to keep the batteries charged up, I needed a battery charger.   And, since I had shore power available, I began thinking about putting in a shore power system and circuits for a battery charger and extra circuits for plug in appliances like a microwave oven.  That was when I realized that I needed to be careful to not mix the 12 VDC (battery powered circuits) with the 120VAC (shore powered circuits).  And, this can be confusing because of wire colors used.  What follows is some thoughts about these two very different electrical systems.

A simple 12VCD circuit uses a red wire for the positive (hot) or + side of the circuit and a black wire for the - or ground side of the circuit.    A "proper" circuit will also have a switch on the hot side of the circuit and probably also a fuse so-as to not overload the circuit.  Simple, right???
As we add 12VDC circuits everything is pretty similar... but becomes more complex.

So, here we have three 12VDC circuits made up of three red wires and three black wires.  When extra circuits are added, a switch panel with switches and fuses or breakers is often added.  And, a buss bar to connect the ground wires is often used as well.  But, the circuits are still red for + hot and black for - ground.
When shore power is added though things start to become more problematic.  This is because 120VAC circuits use three wires instead of two that you have been using for the 12VDC.  BUT, and this is a BIG BUT, the 120VAC circuits also use black colored wire for the "hot" or "lead" wires.

These 120VAC shore power circuits connect the shore power plug with appliances or wall outlet inside the boat.  There is a black wire that creates the "hot" or lead side of the circuit; a white wire (shown as a dashed line) that makes the common or return side of the circuit; and a third wire that is the ground wire.  The ground wire might be bare copper or a green covered wire.  As with other circuits, there should be a switch and a fuse or breaker in the circuit.

When we add more shore power circuits, we add three wires each time a circuit is added.  So, here we have three black "hot" "lead" wires; three white common wires; and three copper/green ground wires.  There is also a buss bar for the white common wires and a second buss bar for the ground wires. 
And, everything works just fine as long as we remember whether the circuit we are working with is for the 12VDC "house" circuits or if the circuit is for a 120VAC shore power circuit.
The PROBLEMS OCCUR when we forget what circuit we are working with and connect a black 12VDC ground wire with a 120VAC shore power circuit.  Mixing 12 volt direct current with 120 volt alternating current will not work.  And, if you do this, you will almost certainly destroy your 12VDC electronic device.  This is easier to do than you might think.  The black wire buss bar can easily be confused with the 120VAC switch panel.  And, wire nuts are convenient... but it is all too easy to use a wire nut on two very dissimilar black wires.
So, be careful and keep the 12 volt and the 120 volt circuits completely separated.  Remember that a 2-way marine band radio is a 12VDC electronic device and a microwave oven is probably a 120VAC appliance; while an AM/FM stereo radio may be one or the other... make sure you know which it is.  If it has a wall outlet plug, it's 120 volt...... if the radio has a red wire and a black wire coming out the back, it's 12 volt.
Again, this is not difficult... but it can be confusing.  The wires look about the same.  So, be careful.

Bill Holcomb
BarnacleBillHolcomb@gmail.com