Noreastern, while the others are arguing about wire, I thought I'd go off in a different direction. If you have a computer with a spreadsheet program, things will be a bit easier, though you can do this exercise with a pad of paper and a pencil, to start.
First, since you have a sizeable boat (38' Pearson), you must realize that you will probably want the option to add stuff, at some time in the future, or if you have increased the available number of circuits, a future owner will pay more for your boat.
Write down a list of all the electrical stuff you have, or want for the future. Now look at this stuff and decide what circuit it will be a part of. For example, all cabin lighting
on one (or two circuits). I have two lighting
circuits on my boat, portside and starboard. That way, if I need to shut off power to one side (or if there is a problem), I can still have enough lighting to avoid working completely in the dark. If you need some help here, use the Blue Sea catalog (http://www.bluesea.com/
), which has circuit breaker labels, as a guide. Once you have decided how many circuits you want, then write down, in the spreadsheet, or paper pad, all of the specific lamps or other units that will be powered by that circuit breaker. Now, find out how much current
each unit needs (this can be found in a catalog or by shopping
at a good marine store). Now, group all units that might be located near one another. For example, all of the galley
lamps, the main salon
lamps, etc. This is the point where your planning will take a great leap forward.
Having added up all of the current
(amps) that would be flowing should you have every unit in the circuit on at the same time (example, all portside lighting units) you will have a figure that tells you how big a circuit breaker is called for. You're not done, though, because the wire the current flows through is subject to the electrical resistance of the size wire and the distance the wire has to travel, from the breaker to the unit, then back to the ground buss.
To avoid having used individual wires to each unit, from the circuit breaker, you need to divide your total units into groups, which will be powered by dual strip busses, metal strips with lots of screws. For example, starting at the electrical panel, where the breaker is located, you will have several busses, located in several parts
of the boat. Perhaps one in the galley
, one in the main salon
, one in the forward cabin
, and anywhere else that you have a number of lamps that need wire running in relatively short distances from the buss. The idea here is that current flows from the breaker using a large wire to a buss, via a two-wire cable for the positive and negative sides. This cable can then terminate on the buss, powering both the positive and negative strips, or it can also continue to another buss further away. If you do this, it will result in fewer wires running from the circuit breaker to the individual lamp, and allow a more efficient distribution of adequate power to all units served by the breaker. For example, because you have calculated that you will need 10 amps (all of the lights associated with one buss in the galley) you can the write down the distance the current flows from the breaker to the galley lighting buss. Then you measure the distance for the wire needed to go from the galley lighting buss to the lamp over the stove, and write it down. You do the same with each unit that is fed by the buss.
Now, go to the Blue Sea wiring calculator (http://circuitwizard.bluesea.com/
) and begin to determine the wire sizes needed to conduct sufficient electricity from the breaker to the buss that results in less than 3% voltage drop (the result of pushing electrons through the resistance of copper wire). Then you do the same thing for the individual lamp, measuring the distance from the buss to the lamp over the stove, which tells you that a lamp using 2 amps, getting its power from the galley buss, will also be receiving power with less than 3% voltage drop.
What I'm trying to say here is that you want a large wire to feed the buss, and to tap off the buss with smaller wire to the individual units needing power. You want a minimum of large wires running around the boat to their respective busses, which then branch off with smaller wires to the individual lamp. The calculation is done for you by the wire size calculator. You just need to figure where on your own boat, you want to locate the busses. Make it a protected spot, safe from things that might roll or touch the two sides of the buss accidentally (even though you will be covering the busses with a protective cover). Remember, Murphy's Law has not been repealed, nor likely ever will be.
When you have done all of the calculations for all of the circuits on the boat, you are ready for the next step, which you and anyone else following in your footsteps will be thankful in the future. This requires a labelmaker and your spreadsheet. Give each individual circuit a number (example, Portside Cabin Lights is labelled "1"). Under the heading for this circuit, you list all of the individual busses you will have that are powered by that breaker (example, Galley Lights, Main Salon Lights (portside), Head
Lights, Forward Cabin Lights). Give each of these busses their own number. Now you are almost done. Give the individual units (lights, in this case) their own distinct number). Now, on the spreadsheet list all of the wires served by the breaker, in numerical order (example, the fluorescent lamp over stove is labelled 01-01-01). The idea is that each wire on the boat is coded individually, telling anyone that the unit is served by a specific breaker, which leads to a specific buss, then to a specific unit.
Once every unit is coded, you are ready to do the task of wiring. Because you have calculated the total current for every lamp drawing power from the breaker, you install the correct breaker at the panel. If you are drawing too much from one breaker, you can consider making two circuits instead, to reduce the size of the breaker (but increasing the number of breakers on the panel). Then, because you have calculated the current to the individual buss (from all the unit served by that buss), you can install a feeder cable to the buss that is adequate for your needs. Then, the same for the individual unit power wiring.
So, starting at the panel, you install a dual buss somewhere inside the electrical compartment for each breaker (if there will be several distributed busses throughout the boat, as in our cabin light example). That way, you have a large wire from the breaker to the panel buss, then smaller cables to the individual busses, then smaller wires to the individual unit. As you begin installing your new wiring, you use the spreadsheet, with its codes, to guide you. You label the wire from the breaker (example,"01") to the panel buss. Then, starting in order, you label each wire going off to its respective buss (example, the wire going to the galley is "01-01", the wire to the forward cabin is "01-05". As you begin to do the actual wiring, you do it in order of the code so that you don't forget anything. And, as you do the wiring, you provide a label for each end of that wire (assuming it's more than a couple inches away). That means that our galley stove fluorescent lamp is labelled "01-01-04) and its code is labelled at both ends of the wire. Then, you can check off the appropriate line on the spreadsheet and be absolutely sure you have done the job.
I make my labels about three inches long, and I provide a name and the code, both on the left margin side of label, so that the label has, on the left side, the name "GALLEY FLUOR at the top, and underneath "01-01-04". Then I strip off the adhesive
cover and fold the label in half, over the wire, creating a flag, near the point of attachment to the buss or the lamp. Where things connect to the individual lamp or unit, I use insulated male/female connectors so that I can easily remove the lamp or pump or whatever if it needs service
in the future.
I know this seems arduous, but it will ensure you have adequate power to everything that will be served, that anyone can determine what an individual wire connects with, and that a surveyor
, inspecting your vessel, will see that you really had your act together. Especially, if you show him your spreadsheet with its indicated current requirements, wiring length and wire sizes. Insurance companies really eat it up, also, assuming they've sent someone who really gets it.
Also remember that you may want to add things in the future, so size your breakers and busses for possible future units, which means adequate cable feeders to the busses.
Sorry this is so long-winded. I built my boat over thirty-five years ago when things were simpler, and I was poorer. I had six DC breakers and 240 amp hours of battery
capacity. I now have 16 DC breakers, a 2000 watt inverter
, and 740 amp hours of house bank. I needed a system to ensure I did have the correct wiring and breaker sizes for my needs now and in the future.