Next we can work out the fuse board needed and the size of the connection to the Domestic batteries.

It is back to those drawings to detail the fuse and the current the circuit is going to need

Example Lights 1 - Maximum current 9 3W lights = 27W / 12 = 2.25A

Smallest cable size (this is the maximum current that the circuit can take without multiple other fuses) as earlier there was found to be a need for 2mmsq it make economical sense to use 2mmsq everywhere 1.5mmsq is shown on the drawings. 2mmsq has a maximum current capacity of 25A. The current to be carried is 2.25A and the Maximum that can be carried 25A.  Knowing what is available means a fuse of 5A to possibly 10A

Fuse Board List

1. Lighting 1 - 5A to 10A, current - 2.25A
2. Passageway Lights - 5A to 10A, current - 2.25A current
3. Lights 2 - 5A to 10A, current – 3A
4. USB Sockets - 10A to 15A, current - 8A
5. Drainage Pump - 15A to 20A, current – 15A(5A)
6. Water Pump -- 15A to 20A, current – 15A (5A)
7. 12V Sockets 12VS1P - Fuse 10A to 20A, current -10A
8. 12V Sockets 12VS3P - Fuse 10A to 20A, current - 10A
9. 12V Sockets 12VS4P Fuse 10A to 20A, current - 10A
10. Fridge 10A to 15A, current - 5A
11. Cigar Lighter socket 10A to 15A, current - 10A
12. Headlight LED - 5A to 10A, current - 0.7A

• Headlight Tungsten - 10A to 15A, current - 4.6A

So the fuse board needed is a 12 way (J assuming only one headlight). It is sometimes sensible to allow a few fuses for expansion in the future.

Cable selection from the batteries via the Domestic Isolator to the Fuse-board

The total current with everything on and running at maximum is nominally 76A. Now this is unlikely to happen but can/ought be prepared for. For example the cables from the Fuse board to the Domestic batteries is a 3-metre run; 25mmsq will give a volt-drop of 0.35V or 2.93%. If we worked on the basis that only 50% of items were going to be used at anyone time that would be 38A and 25mmsq would give a volt drop of 0.18V/1.46% which meets the target for a 2% or less volt-drop in normal operation at the Fuse-board.

Unfortunately, some fuse-board manufacturers are a bit slow in coming up to date with the increased electrical use on boats and the only connection to their fuse-boards is a 6.3mm male spade connector. The crimp connectors for these spades have a maximum cable size of 6mmsq. The current capacity of 6mmsq is 50A unlike the 25mmsq that has a current capacity of 170A; also the volt-drop of 6mmsq is at 38A 0.72V/5.97%. That is nearly 3 times our target of 2% volt-drop at the fuse-board.

The common 12-way 12 way circuit breaker switch panel (fuse-board) is conveniently set out as two columns of 6 fuses/breakers this give us the ability to feed each column from the top and bottom. So we could use four 6mmsq cables, which would give us a volt-drop of 0.18V/1.49% that is almost the same as the 25mmsq and gives a similar current carrying capacity overall using four 6mmsq.

This can be achieved in one of two ways, running 4 6mmsq cables from the domestic isolator fusing each 6mmsq at the Isolator at 30A. That tends to be a bit messy, my preferred way is to run 25mmsq, fused at 100A, from the Domestic Isolator using to a small busbar by the fuse-board. Then from busbar, using 6mmsq, to the fuse board to the top and bottom of each of the two columns. The 6mmsq being fuse as it leaves the busbar at 30A, This will give a volt-drop battery to fuse-board of about 1.5% allowing us to keep the volt-drop to any DC electrical item to less than 0.6V/5%.

Now we need to balance the two columns of the fuse-board, so that each pair of 6mmsq feed cables handles nearly the same current and one pair of 6mmsq is not handling considerable more than the other pair.

1. 12V Sockets 12VS3P - Fuse 10A to 20A, current - 10A
2. 12V Sockets 12VS4P Fuse 10A to 20A, current - 10A
3. Passageway Lights - 5A to 10A, current - 2.25A
4. USB Sockets - 10A to 15A, current - 8A
5. Drainage Pump - 15A to 20A, current – 15A(5A)
6. Headlight LED - 5A to 10A, current  - 0.7A

Maximum current of fuses 1 to 6 - 35.95A

7. 12V Sockets 12VS1P - Fuse 10A to 20A, current -10A
8. Lighting 1 - 5A to 10A, current - 2.25A
9. Lights 2 - 5A to 10A, current – 3A
10. Fridge 10A to 15A, current - 5A
11. Cigar Lighter socket 10A to 15A, current - 10A
12. Water Pump -- 15A to 20A, current – (15A) 5A

Maximum current of fuses 7 to 12 - 35.25A

So by moving fuses and their items from one column to the other one, one section will handle 35.95A and the other will handle 35.25A. This is the nearest in real life you are likely to get the two currents. This difference of 0.7A is acceptable and the two sections and their 6mmsq feed cables will not be over stressed and the volt-drops will be acceptable.

Hope that is helpful to you all and that it has enabled a bit more learning to go on.

In the Appendix you will find: the explanation of how the two-way switch operates, the complete one page drawing for the Lights, and the one page drawing for the sockets, copies are available from me.

Now we get down to the rules that have to be followed so the wiring is done to meet the RCD requirements and recommendations.

cable insulation colours

Black cable may only be used for negative cables and the cable must be the same colour throughout its entire length – so if it is a negative it must be black all the way, not a red with a bit of black at each end

If the boat has a three-phase AC system (unusual with our boats but possible) the black is used for an AC live so yellow should be used for all the negatives not black.

Positive cable – a positive cable (battery volts) may be any colour except

• Black,
• Brown,
• Blue,
• Green,
• Green/yellow,
• If a 3 phase boat yellow because that will be negative.

recommendation or good practice

Red is used for all positives except switch positives i.e. the wires between two 2-way switches; the cable from the switch to an item (light, pump etc) should be a different colour to red. The means when testing if it is red it should be at battery volts, any other colour that is allowed to be a positive that will need a switch switched to be at battery volts.

back to the rules:

securing cables and what can run with what or not run with what

All cables should be secured every 300mm, unless they are supported in a conduit or ducting. Where there are only DC cable in the duct they do not need to be secured, if other cables are also in the duct, AC for example, secure every 300mm to keep the two cable types at least 100mm apart.

DC cables should not be run within 100mm of AC cables or gas pipes unless protected in a conduit. (Recommendation I tend to use split-corrugated conduit secured every 300mm to 1000mm) within the duct.

To make this clearer there are two situations

• A Gas Pipe in the picture

All cables closer than 100mm to a gas pipe have to be in conduits be they AC or DC.

• No Gas pipe in the picture

If AC and DC cables are closer than 100mm one of them has to be in conduit to separate them away from the others.

All cables should be protected anywhere they can be abraded by the vibration of the boat etc, This particularly means where the cable crosses an edge of metal or wood, even plastic, goes through a hole in a bulkhead (wall) be it wood or metal etc or where they cannot be secured at 300mm intervals, conduit must be used.

I think that is about it but anyone shout if I have missed anything

finally

You should now have all the information to purchase the cables you need with the terminations, the correct size of fuse/switch board and do the DC installation of the boat from the Batteries via fuse board to the electrical items.

in the appendix are

• How two 2-way switches can control things from two different positions.
• A copy of the full one page drawing of the Lighting circuits
• A copy of the full one page drawing of the Sockets etc.

Copies of the originals are available from me via the 12 Volt Boating Group 

where are things going to go? - the drawings

The start is not what batteries you are going to have; it would be lovely if it were. That would make it too easy and this is a boat and boats are rarely easy.

The starting point is with the electrical items and where they are going to go. How we connect them to the batteries that are going to supply the power needed.

Begin with a scale outline drawing of the boat. Mark in where the cabin walls are going to go, doorways and what the rooms are. Then mark the drawing in 1 metre sections stern to bow. If you have a side view also mark that in 1-metre sections vertically from the base plate up.

Now breathe and make several copies of the master diagram. It is now is the time to involve the other half.

Starting with the lighting, mark the positions of the lights on the diagram, giving them a number etc. Don’t worry about cable routes etc just mark the lights position. Where their the switches are going. Mark the switches in someway. Do not forget about wall lights. Lights over the cooker and kitchen sink, Bathroom sink lights etc in the bathroom.

Also mark on the drawing where you would like 12vDC table lamps and standards lamps. Again mark and number. Join up each light that will be operated by the same switch/switches and link to the switch/switches and you end up with a diagram like this.

Using another boat outline, mark where the water pump is going and its local switch so it can be turned off when the tank is empty. Same with the shower pump and it maybe a good thought to think about the shower pump being an automatic pump so only one hole is needed through the hull. But this may not be possible if the kitchen is on the other side of the boat.  Don’t forget about the Bilge pump it needs to draw its power from the Domestic bank not the Starter battery. Remember to give them an identity.

Now the bit I find the most difficult, where to put the 12V power points. Think about where you want 12V power points for charging etc. Mark them on the drawing then add where you want USB charging points and mark them.  Then go back and look, have you put one in the kitchen area? If not I would suggest add a 12V power point and a USB. Think about someone cooking from a recipe online.

Bedrooms, I suggest that you make sure both sides of the bed have access to a USB charger for the mobile phone, and Kindle. Do not forget the navigation lights. I know they are not a requirement on the UK canals but rivers etc require them. Regardless of that a red & green light come towards you at night gives you the beam of the boat and its outer edges. It making it a bit easier to pass in the dark.

Below my version of the outline with the sockets etc marked and numbered if more than one.

Now is the time to think about how you are going to get the cabling from the stern of the boat to the bows and everything that needs power. Do not just think about the 12V DC cabling but also the AC mains cabling. There are various ways to do this, some people make a duct under each of the gunnels to take the cables. Others put a duct down each side of the boat in the ceiling. Along the edge where the ceiling meets the sidewall. You will also need a route from Port to the Starboard side of the boat. You also need to think how you are going to get cabling up and down the boat vertically. For switches this can be done inside some conduit behind the panelling. Once you have decided how and where it is going to go, mark it on the outlines.

The next stage is building the circuits, adding the cabling. But first you have to decide where the electrics cupboard is going. In it or adjacent will go the 12VDC Fuse board (Distribution Board) any inverter, charger etc. So don’t skimp on its size Mark the fuse board on all the outlines.

Now the next thing to do is to draw the electrical diagram. Lets start with the lights, first split the lights up roughly into two halves or even three or four if there are a lot of lights or long cable runs; so that each fuse has lights from every area of the boat. The each section will run from its own fuse, i.e. Lights 1 fuse and Lights 2 fuse. Doing it this way means that if one of the lighting fuses blows there is still lighting available close to hand and one does not have to stumble around the boat looking for a torch before the lights are back on. Then you can sort out what caused the problem and sort it out.

To draw the electrical diagrams it is easier to use one of the free drawing apps. I use draw.io, all the drawings in this article, as well as the ones I put on the group are drawn on draw.io. It allows for a clear simple drawing that can be altered and bits moved around with no problems.

So lets get started; the first thing I do is a symbol chart for everything that is going to be on the drawing, switches of the various types, the different types of lights etc. No complex electrical symbols that you do not know or understand just simple symbols. A Symbols list is to remind you what they are and what they do. Do not worry if you cannot think of everything you need or you miss some. They can easily be added to the Symbols list as you go on. But do it and do not rely on your memory. When you have to find faults the drawing becomes the map of what is there. Also when you come to sell the sell the boat the new owners will be impressed that there is a set of electrical drawings. It could get that sale, where it was only 50/50 in the buyers mind before. It shows that care has been taken of the boat from day one. How often do you hear boaters grumbling that all they have a jumble of cables and no drawings to tell them what should do what, please make sure you do Electrical Drawings for your boat, it is not difficult and will make installing them easier as well as fault finding is easier for whoever follows you.

So lets start with Lighting one. I normally start at the stern and work my way forward doing a separate section of the drawing for each fuse. I split the lighting into two or three sections so that the volt-drop is at a reasonable level and never less than two sections. Then if one lighting fuse blows there is light in every part of the boat albeit reduced. This is going to need to be three sections to keep the volt-drop to reasonable limits.

Start with the two rear deck lights. The most complicated electrical circuits that you will encounter in the whole of your wiring of your boat. There is the need to be able to switch them on and off from two different positions. If it is confusing I have written an explanation of how it works in the Appendix of the article.

Label each of the cables, as an example, the positive cable from the Lighting 1 Positive busbar to the common of the first of the two-way switches. I have called it L1P1 as it is for the first lighting section on the drawing.

Having drawn the first of the lighting diagrams you need to work out how long those cables are. Remember if the boat is a Narrow Boat, it will have an internal beam of 2 metres approximately.  Your drawing will have marker lines spaced at 1 metre apart. Using these to measure the L1P1 cable from the switch the through the cable ducts back to fuse board, which is where the Busbars will go measuring the distance, mark the distance on the cable on the drawing.  Do not forget the ups and downs the cable has to travel. Now continue to measure and mark the length of every cable on the drawing. Label the cables from the lights to the Busbars both negative and positive and you will end up with a drawing that looks like this.

©Graham Mills, 2020