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Basic electricity for boat builders, boat repairers and owners. How to plan the electrical systems.

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.

You should start by reading and studying Basic Electricity;  There are many books, on-line resources and courses available. see IKE'S LIST-Electrical

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
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

If it is a much larger boat, a cruising sailboat or powerboat, you may have even more equipment.

Anchor windlass
Microwave oven.
Chart plotter

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;

Divide the devices into continuous duty; 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.  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.  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: 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:

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:

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
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.

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. 

circuit3.jpg (54309 bytes)

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.

A Typical Buss

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.

Copyright 2010 All Rights reserved.  Revised 01/10/2017  

Links to Offsite Refereences:
Wiring Your Boat