Electrical Systems Planning
Basic electricity for boat builders,
boat repairers and owners. How to plan the electrical systems.
Planning the Electrical System:
If you are building boats for the purpose of sale,
you should have an Electrical Engineer or Certified Marine Electrician design and install the electrical system.
This will save you a lot of headaches in the long run. It will work better,
last longer and have fewer problems. But should you want to do this
yourself then it is extremely important to have a good understanding of
electrical systems. The basics of electricity and electrical systems are not hard to
understand. But if the system you are installing goes beyond simple DC then you should go to a
professional. Keep in mind, most fires on boats are electrical fires, and AC can kill. Do it right or not at all.
An excellent article by Owen Youngblood on
Wiring Your Boat, from the Metal Boat Quarterly.
https://newboatbuilders.com/docs/WiringYourBoat.pdf
You should start by reading and studying
Basic
Electricity;
https://newboatbuilders.com/pages/electricity1.html There are many books, on-line resources and courses available.
see
IKE'S LIST-Electrical
https://newboatbuilders.com/pages/links_electrical.html
Once you have an understanding of the basics and the
terminology, then you can begin planning the electrical system on your
boat. The following only applies to DC systems. AC systems use another
similar process. Please keep in mind, this is not a detailed
tutorial of the process. This is only a general overview of the process.
1. Determine your needs. What electrical
equipment will you be using?
The engine
Bilge Pump
Blower
Cabin Lights
Navigation lights
Instruments panel.
What else? If this is a small boat for fishing, maybe you have a trolling
motor and a depth sounder.
Depth Sounder
Trolling Motor
AM/FM/Weather Radio
VHF Marine Radio
GPS
If it is a much larger boat, a cruising sailboat or
powerboat, you may have even more equipment.
Anchor windlass
TV
Microwave oven.
Chart plotter
Computer
Refrigerator/Freezer
The following is an example of how the load
calculations are performed. This list helps you determine the loads (how much power each item uses) and how
many amp-hours are used up. Batteries are rated in
amp-hours;
https://newboatbuilders.com/pages/electricity3.html
Divide the devices into continuous duty; that is, the switch is
always on, and intermittent duty; the switch is only on when needed. Put
continuous loads in Column A and intermittent loads in Column B. Determine
how many hours the equipment is used per day. Multiple the amperes times
the hours to get amp-hours. Then add up the amp-hours in Col A and Col B. If you don't know how to
determine how many amperes an item uses then you need to go back to Basic
Electricity and look at
Power: Amperages and Watts.
https://newboatbuilders.com/pages/electricity5.html Most electric appliances and equipment
have a label listing the rated wattage it uses.
Watts / 12 volts = amps
The following numbers are all hypothetical and for
demonstration only.
Electrical
Load Requirements Worksheet |
Col A Continuous |
Col B intermittent |
Item |
Amperes |
Hours |
Amp Hours |
Item |
Amperes |
Hours |
Amp Hours |
Navigation Lights |
3.0 |
2,0 |
6.0 |
Cigarette lighter |
5.0 |
0.5 |
2.5 |
Bilge Pumps |
5.0 |
0.5 |
2.5 |
Cabin
lights |
3.5 |
3.0 |
10.5 |
Blower |
2.5 |
0.5 |
1.25 |
Horn |
1.0 |
0.1 |
0.1 |
Wipers |
3.0 |
0.3 |
0.9 |
Other Electronic Equipment |
3.0 |
2.0 |
6.0 |
Marine radio |
1.5 |
8.0 |
12.0 |
Trim Tabs |
5.0 |
0.5 |
2.5 |
Depth Sounder |
0.5 |
8.0 |
4.0 |
Power trim & tilt |
5.0 |
0.5 |
2.5 |
Engine Electronics |
2.0 |
12.0 |
24.0 |
Toilets |
3.0 |
0.5 |
1.5 |
Refrigerator |
7.0 |
2.0 |
14.0 |
Anchor Windlass |
40.0 |
0.3 |
12.0 |
Instruments |
1.0 |
8.0 |
8.0 |
Winches |
3.0 |
0.3 |
0.9 |
|
|
|
|
Fresh Water Pump |
5.0 |
1.0 |
5.0 |
|
Total Col A |
23.5 |
|
72.65 |
Total Col B |
73.5 |
|
43.0 |
Col A amp-hours = 72.65 Round to 73
Col B amp-hours = 43
(ABYC says take the larger of 10% of Col B or the largest value in Col B)
= 12.0
Total of Col A + (either 10% or largest of Col B) = 73 + 12 = 85
Total Amp-hours needed is 85. Add 20% for safety =
17 85 + 17 = 102
So what does this tell you? It tells you how
many amps-hours you use, but that is not how many you need to maintain the
batteries for a long life. You want to size the battery so that it does not
discharge below 50% of it's capacity because discharging a battery below
50% of it's capacity is destructive to
the battery. Most designers use 40% discharged as the limit. (The battery is down to 60% of
it's capacity so it has used 40%.)
102 amp hours = 40% of desired amp-hours
Divide 102/0.40 = 255
amp-hours.
So you need 255 amp-hours. You can use one
battery rated at 255 amp-hours or more, or two batteries at 128 amp-hours
wired in parallel. However, you will probably not find a battery rated
exactly at that amount. Pick one with a slightly higher rating or pick
multiple batteries (of the same rating each) that add up that amount.
The above example is only to show you
how to do it. The numbers in the table were just random. Real values may
be vastly different. Your solution will be different
This determines how big your battery bank will be.
It also helps to determine the size of the fuses or circuit breakers for each branch of the electrical system.
See Pacific Sea Breeze:
How to size and use your battery bank.
http://www.pacificseabreeze.com/tech-library/batterysizing/1batcrg.htm Also, do you need a starting battery
for the engine and a separate bank of house
batteries? How are you going to charge the batteries? Batteries and
Chargers: https://newboatbuilders.com/docs/BatteriesandChargers.pdf You need a charger that will recharge the battery to at
least 90% of it's capacity in about two hours.
Next you need to determine where
the equipment will be on the boat.
Draw
a diagram showing the general arrangement of the boat and where each item
of electric equipment will be.
This
determines how long each wire has to be and helps in determining the wire
size. It also helps to determine the route of the wire through the boat. Remember, wire
cannot go through solid objects like pipes and vent ducts, and other equipment. It
has to go around such things. It should not just be strung through the boat and it should not be in the
bilge. It should be fastened down at 18 inch intervals or less.
And don’t forget the return run. There are two wires; positive and negative.
2.
Where will your battery bank be?
Put this
on the diagram. It needs to be in a dry warm place. It should not be in the bilge, or anywhere exposed to
water, but because batteries are very heavy they need to be low in the boat. They need to be in a space that is ventilated.
Most boats have them near or in the engine space, this is good but there are rules about the placement of the batteries
near fuel lines and other equipment. Electrical Regulations:
https://newboatbuilders.com/pages/elect.html
3.
Where will the fuse or circuit breaker panel be?
Put this on the diagram. Again, there are rules about placement of fuses and circuit
breakers in relation to the source of power.
Electrical Regulations:
https://newboatbuilders.com/pages/elect.html
4. On/OFF Switches? Where will they be?
Some are on the instrument panel. Others are near the piece of equipment. Some are built into the equipment. Show
on the diagram where they will be. Also, will you have more than one on/off switch? Example: Cabin lights,
do you want a switch at both ends of the cabin, or will each individual light have its own switch?
This really gets complex with navigation lights, especially if
you have combination anchor/running lights with more than one bulb in the
fitting, or more than one filament in the bulb.
5. Where will the battery shutoff switch be?
This needs to be very close to the battery bank but easily
accessible. It especially needs to be where you can shut it off in the case of an engine room fire or
flooding.
6.
What items need to be directly wired to the battery and not through the switch?
There are some things on a boat that you do not want to shut off
when you turn off the battery switch, such as the automatic bilge pump, or
an automatic fire fighting system.
7. Determine wire sizes for each wiring run.
This is determined by a combination of the amperage (the load) and
how many items are on the circuit.
See Wire
Size:
https://newboatbuilders.com/pages/elect3.html
This also determines the size of the fuse or circuit breaker.
If you have a very long run you may have to go up a size in the wire to prevent
voltage drop. https://newboatbuilders.com/pages/elect4.html
Next you need to draw a schematic. What is a schematic? It is a diagram of
the electrical system showing all of the equipment, the batteries, the
engines and fuse blocks. It shows all of the connections between them. It
does not show where these are on the boat. It does not even look
like the boat. It only shows the electrical system. Below is a
simple schematic. It shows where items are on each circuit.

There are symbols
used by engineers and electricians for each device, but if you don't know the symbol do not worry. Just draw a box and label it with the name of the
device (inverter, switch, whatever) and show the wires connecting to it.

8. Use your diagram and other information to
begin drawing a schematic of the system. Show the ground.
On most boats the ground is the engine block.
Also determine if you are going to have a grounding buss for the DC system. This is a green wire that runs through
the length of the boat that connects all of the metal cases of the electrical equipment to the main ground. This is not the same as a
bonding system.
9. On the schematic show where busses and fuse blocks go.
A buss is a solid metal conductor that has many posts on it for circuits coming off the buss, but
only one connection to the source of power. In a DC system if you use busses, you need both positive and
negative busses. See the photo for a typical buss.
10. After you get the schematic laid out, show the schematic to someone with a marine electrical background and ask them
to look for any problems.
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reserved. Revised 04/29/2021
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