I had not given wire sizing, with its flow-on purchase of components and tools, a lot of thought before I set out to write about putting them all together.
A lot of information, written in books or supplied by manufacturing companies, are for equipment and materials easily sourced overseas. A lot of these components are physically measured differently than they are here.
I think it is also a sign of the times where most of our tools are imported and it appears they have been manufactured for sale in a global marketplace, so they are not an exact fit to products produced here. This does not make it easy when we head off to the chandlery to purchase bits and pieces to do an install or repair job.
So, as I collected information, I assembled this assortment of information into a format that I hope will make it easier for us to get our head around. I know most of us still head off to the local chandler when we want marine wire, so I have tried to incorporate the most commonly stocked wire sizes into the calculations for current handling and crimp terminal sizes.
As the electrics on the average cruising boat become more advanced the one thing that has always been a requirement for reliable operation is the need for electrical installation or repair work to be done neatly and efficiently. To do this correctly you will need to plan the job, have the right tools, wire and the correct terminal fittings.
It does not matter whether you are installing new electronics, electrical gear or rewiring something already on board; the necessities for dependable and secure electrical connections are the same: do it right the first time, using the correct gear.
Planning the job will save you a lot of hair-pulling later. Take time and explore your options to decide how the new equipment will be installed.
That plan should include where the equipment will be installed, what interfaces to the equipment and will access to the newly-installed equipment be reasonably easy.
Where is power to the equipment going to be run from and how many amps are required? These are a very important questions that will need answers for proper installation.
The next important consideration is the length of the cable run. Not just an ‘as the crow flies’ type of measurement, but an ‘all the curves and up and around and through cabinets and cable trunks’ type of measurement.
By taking current draw in amps and cable length into the calculations the correct size wire can be purchased and run. But do not buy cable just big enough for the job, you don’t want the wire to be running at one hundred per cent of current carrying capacity. Go the next size up for redundancy and remember the bigger the wire the less resistance it will offer a load, so the easier electricity will pass through it and the cooler it will run.
A win/win situation for all.
The term ‘wire gauge’ refers to the conductor cross-sectional area of a wire. Wire gauge can be used in calculations to determine electrical resistance and the current handling capability.
In Australia there are several major standards for describing wire gauge: trade cable diameter, metric wire gauge (MWG) and American wire gauge (AWG). Brown and Sharpe (B&S) is another commonly used measure of wire gauge, B&S is equal to AWG. B&S has been commonly misquoted as battery and starter wire, or perhaps this label is used to sell a product that is not sized correctly.
Cable diameter is commonly used to market wire in Australia. This is a trade unit referring to the diameter of the outside of the insulation, closer investigation is needed to find the cross-sectional area of the wire in millimete squared (mm²) MWG before calculations for current carrying capacity can be carried out.
Metric wire gauge is commonly used in countries outside of the United States. When using MWG a cross-sectional area in mm² is quoted to describe the wire cross sectional area.
American wire gauge is most commonly used in the United States and in most cases is stamped on the jacket of good quality marine cable and wires. The AWG is determined by the cross-sectional area of the conductor; the same as MWG, however AWG is described using a list of defined standardised gauge numbers. AWG gauge numbers work in the opposite direction to MWG, with the smallest numbers representing the largest diameter wires. For example 4.59mm² MWG is roughly equivalent, but on the small side, to 10AWG; 1.84mm² MWG is equivalent to 15AWG.
B&S is commonly used for marketing heavier wires and cables in Australia, I guess this is where the confusion and tie up with batteries and starter came about. B&S follows the same gauge descriptions as AWG and is simply another way of explaining the wire gauge.
As an example 2AWG = 2B&S, 4AWG = 4B&S and so on. See Table 1.
As we all know wire could hardly find a more unforgiving environment than on board a yacht. On board wiring has to be able to deal with the high humidity and consistent vibration and be able to recover from the odd submersion as well as heat and cold from all manner of sources.
Wire used on a boat must be copper, though even copper has been known to corrode in the harsh marine environment. The most common cause of electrical wire failures on a boat is primarily corrosion and secondly poor work practices installing terminals and wires.
I am sure we have all heard of tinned copper wire and we will need to pay more for the advantage of having the wires tinned during the manufacturing process. Tin plating each of the multiple strands of the wire dramatically improves corrosion resistance. The additional cost of tinned wire is worth every additional cent because the benefits are substantial.
Under normal circumstances on a boat use only tinned wire, even though the cost saving you could find at the auto electrical section of the car parts supplier for untinned wire does look attractive.
Choose your wire carefully. Never use solid wire or wire intended for wiring a house. Vibrations will eventually fracture solid wire. It does not need to be high frequency vibrations, wave motion is enough to flex all the wires on the vessel even if you do not feel the movement.
Boat wiring must have the flexibility only multiple fine strands provides. The insulation covering the wire used in house construction is different to cable intended for the marine industry, so this is another reason not to use house wire on a vessel.
Marine cable uses heavy tin-plated annealed copper wire formed in such a way as to reduce capillary action of moisture within the cable. The flexible stranding (multiple fine strands) helps withstand fracturing due to the movement caused by the marine environment. Good quality marine cable has high temperature insulation to withstand the high ambient temperatures of enclosed engine bays.
Depending on the brand some offer V75 (75 degrees Celsius) and I have seen other suppliers/manufactures who market V95 cable. Where is your cable going to run? If it is through the engine bay, purchasing the wire with the higher temperature rating specification could be the better option.
Do not use wire in an engine bay if you have no idea of the temperature rating of the cable insulation jacket, it is just asking for trouble.
Most work for rewiring or installations will require two wires: one positive and a ground (return) of the same size, do not fall into the trap of believing the return wire size is insignificant, its size has to be equal to the supply wire. Twin sheath wire where the twin red (positive) and black (ground) conductors are encased in a second layer of insulation. This is convenient and provides the added safety of that second layer of insulation. There is also imported twin sheath wire with red and yellow conductor insulation, this is usually wire made for the American market due to their marine colour code where the black insulation colour is the active wire in an AC circuit. There is single wires and a three core wire for use on extra low voltage work on board, the wire colours vary with supplier but white brown yellow is common.
As discussed earlier, wire sizing in Australia is not easy for the average yachtie to get their head around. Unfortunately this mishmash of labelling requires careful investigation of the product, meeting the correct size specification before handing over your hard-earned money.
The bigger the wire, the easier electrical current will flow through it. Some voltage will be used up pushing the current through the wire. This loss, called voltage drop, should not exceed 3%, for most circuits. However, there is some equipment that will not mind a slightly lower voltage, the only way you will know this fact is to look at the equipment specifications sheet supplied with your equipment.
It is essential to use wire sized for the maximum current flow you expect it to carry. If the wire supplies a single unit, the current requirements will be shown on the label on the unit, or in the installation owner’s manual.
While a 3% voltage drop sounds like it would be easy to achieve, remember 3% of 12.6 volts is a drop of .38V. So the voltage, at the other end of the wire at the appliance, when running under full load should be at a minimum 12.2V.
To arrive at the length of wire used in the calculations for the correct size wiring, you need the wire length from the power source circuit breaker to the unit and back to the power source. As explained earlier not a ‘as the crow flies’ type of measurement. You must determine the actual length of the wire by measuring along the path it will follow: up, down, over and around. I have found in most cases that it is not unusual for a wire run to be more than twice the ‘as the crow flies’ distance!
Not all electrical wire made to Australian standards as appropriate for marine use will have the gauge designation and temperature rating printed on the insulation: on the reel yes, but not the cable itself. Perhaps the Australian Standards need a kick start, this is a really poor state of affairs.
You will find that the best thing to do is to label the wire after purchase so it does not cause confusion at a later date. If you have purchased imported wire remember the smaller the gauge number, the larger the wire diameter.
When purchasing wire for a job, buy wire at least 600mm longer than your measurement. You can easily shorten the wire after it is run, but lengthening requires a splice. Each wire should be a single continuous run between terminals, this helps keep the moisture out of the cable length.
Wire sizing reference
Table 2 shows what size wire is required to deliver adequate voltage to the other end. Use the maximum current draw of the equipment to select the row and the round trip wire length to select the column. The number where these two intersect is the wire size you need.
I have tried to simplify the wire sizes to the ones commonly held in stock at chandlers. The numbers with mm (e.g. 6mm) is the normally advertised (trade measurement) cable size, this is not the cross sectional area.
Wire sizes without the mm are B&S measurements. Cables less than 8B&S are available in the white twin sheath; cable sizes 8B&S, 6B&S and 2B&S are available in a twin with black sheath but not normally stocked in most chandleries, but should be available so shop around.
For better protection when using single layer insulated cables the cables will need to be run in conduit, cable trays, loom tube etc.
The right tools
The cost of the tools nowadays is nominal, however there are some very good trade quality tools out there costing a whole lot and for very good reason: they are made for day in/day out work. For the amount of work most of us do on our vessels I do not think the outlay would be justified.
Beware of clever advertising, I have seen tools that I would say are really poor quality, advertised as tradie tools. The slightly better tool with several additional functions are marketed as professional tools. Do your homework before splashing the cash and you should be able to get tools that are easy to use and provide trouble-free service. I would even go as far as asking the sales assistant for a demo.
Yes I bought a crimp tool that was the be all to end all; what a disappointment when I got it out of the box at home and found I would need handle extensions to make it work. What upset me the most was that I had to pay a restocking fee when I returned a tool that clearly did not work.
We all should have a good quality insulation wire stripper in the toolbox. Stripping insulation with an inappropriate tool can result in a nicked conductor, or damage to the insulation. Nicking through the tin coating opens the gate to corrosion at a later date.
On the other hand, if the stripper damages the insulation, there is the possibility of this hindering your ability to complete a proper crimp and the flow on result: a compromised join.
Something to be bear in mind is the strippers sold in auto supply, hardware and tech stores can be for a variety of wire types. To clarify, I mean wire manufactured to different standards and the way they are measured.
You may also find your strippers are marked with SOL and or STR, confusing unless you know that SOL stands for solid wire and STR is short for stranded wire. We should never use solid wire on a boat, so buy stranded wire and use the STR marking slot.
The next hurdle is going to be whether the strippers are for AWG or MWG, or has the tool been made/marked to be for both. Now the thing here is who did the calculation and did they round up or round down when doing the cross reference.
The best thing you can do, once you find a set of strippers you like, is to do some test strips to make sure you do not nick the conductors or reduce the size of the wire by accidentally trimming the conductors and mark them up for your use.
I found a set that works well and is marked with both AWG and MWG, I still have to be careful when using them to make sure I do not scrape the tin off several of the conductors, but I know they are not going to reduce the size of the conductors by nicking off strands of wire.
Table 1 has a reference between the different sized wires so selecting the right slot on the stripper can be done easily.
A crimp tool is another necessity. You absolutely cannot make a dependable crimp connection with a pair of pliers or a set of multi-grips. However, reasonably inexpensive, non-ratchet, plier-type crimpers normally sold for the automotive industry, if used correctly can make satisfactory crimp connections.
There are different styles of crimpers, it is not a one style does all. Some are for insulated terminals only and others feature jaws/dies for insulated terminals and non-insulated terminals.
One important rule is to know how to use the tool, so make some practice crimps. However, you cannot beat a ratchet crimper, this style of tool offers the advantage of not letting you produce a crimp that does not have the required tension. That is,as long as the wire and terminal sizes have been matched correctly.
A ratchet crimper offers the benefit of consistency if used correctly. Just so you are aware, having a better tool does not guarantee perfect results every time, but practice first and you will be producing good crimps in no time.
I could just leave it here as a general piece about crimp tools but, there always is a but, crimp terminals come in a variety of styles. The problem is recognising the type of crimp you have so you can match it to the correct tool. Hopefully, the next paragraphs will shed some light on the styles and what tool to use.
There are five terminal styles: insulated, insulated shrink tube, uninsulated, cable lugs and uninsulated specialist connectors.
Insulated terminals can be easily identified by the coloured plastic boot on the end of the terminal. It does not just stop at just a plastic boot however, depending on the brand some have additional tinned copper sleeves under the plastic insulator.
The colour of the insulated sleeve on the terminal will denote what size wire the terminal is made for.
If you have not had a lot of exposure to crimp terminals you may not realise the plastic sleeve serves a couple of purposes. Besides insulation and identification, one important aspect of the sleeve is that in a properly-crimped insulated terminal the sleeve acts to relieve the stresses placed on the wire where it exits the insulation jacket. This will stop unnecessary stress on the wire due to vibration or movement when connected to a terminal block. Without strain relief the wires may fall off the back of the terminal after a short service life.
Insulated adhesive-lined shrink tube terminals are colour coded like their cousins, the difference is the shrink tube is transparent and a larger diameter than the standard insulated terminals.
These terminals are not a bad idea on a boat, they offer a level of strain relief not achievable with standard insulated terminals. However, they do not create a watertight seal at the end of the cable, but they will stop water entering between the wire insulation and the tubing.
Greater care will need to be exercised when using shrink tube terminals, so the tube is not damaged. A single jaw crimp tool must be used, not the dual jaw/die as would be used with standard insulated terminals.
The other way to achieve a heat shrink strain relief is to apply shrink tube over the crimped insulated terminal and shrink it down. See Table 3 for colour coding.
Uninsulated lugs come in a few different styles, some are made for specific wire sizes and hole diameters on the ring terminals. When using uninsulated terminals, if the crimp barrel has a seam, the crimp indent should be made on the opposite side, mostly the seam is easy to see.
Uninsulated terminals can be used to form a very robust connection: see photos 12 and 13 on page 46.
Firstly, crimp on an uninsulated terminal to form a mechanical connection on to the wire, then solder the tip of the wire to the terminal. When the terminal cools slightly install adhesive-lined shrink tube. Once shrunk down it will create a good strain relief that under normal conditions is a watertight seal on the end of the cable.
There is a bit of conjecture about solder being not as good a conductor as copper. While this may be, the connection is already very good due to the crimp, the solder is just additional and is excellent in the marine environment because it
can keep moisture out of the end of the wire terminal.
Other terminals are made for use with specialist crimp tools, these terminals are made for plastic boots to be fitted over the cable insulation before crimping the terminal on, then slid over the crimp once finished. These crimp tools can also be used to install the specialist terminals when assembling multiple contact quick connect plugs and housings.
Strip enough insulation for the wire to reach the end of the barrel of the terminal inside the insulated end. Grip the terminal in the correct crimper slot, fully insert the wire into the terminal.
Now is the time to eyeball your creation before squeezing the trigger on the crimp tool. Make sure enough wire is inserted into the terminal barrel, if you are using fully insulated terminal and cannot see the end of the wire try the wire for size in an open terminal to be sure. Too much insulation stripped off can be as bad as not enough and could lead to a compromised crimp job.
When using insulated terminals no bare wire should protrude out from the terminal. Have another quick look, hold the wire into the terminal and squeeze tightly. See photos .
As discussed in the terminal section, the insulated terminals plastic sleeve will need to be crimped on over the insulation of the wire to add mechanical strength. Insulated terminals are usually installed by a crimp tool with a double jaw/die. If your crimper does not have a double crimp jaw, crimp the terminal to the wire first, then reposition the tool and crimp the sleeve to the insulation.
From my experiences I have found wires rarely fail in the middle of a wire run. Of course there are exceptions to the rule here: a nick in the insulation letting in moisture to corrode the wire; chafe due to the lack of securing points; crush damage or perhaps lying against the hot metal of the motor, or heater. So otherwise, almost all wiring problems occur at the connections.
As discussed in the terminal section, selecting the proper connector requires you to match it to the correct size wire gauge and to the size of the terminal block screw or busbar stud. I have found ring terminals are your best choice unless the terminal screw is captive, then the best connector to use will be a flanged spade connectors.
A lot of chandleries supply nylon/plastic screw connector strips, while these look like an easy way to make a connection they are nothing but trouble in the long run. Simply tightening the screw can cut part way through the conductors.
This is not to say all of these strips are bad, there are some that have protective shield to stop screw damage but these are hard to get. If you must use these connectors use a crimped pin terminal or ferrule over the wires to protect against damage. But in the long run using terminal block strips with screws and ring terminals on the wire is a more reliable way of making connections.
While it is easy to say that terminals, terminal blocks and busbars used on a boat must always be copper, it may not be easy to achieve unless you source your supplies from reputable businesses. Take along a magnet to check terminals, terminal blocks and busbars before purchase.
If you already have terminals in your spares you need to check them out, if all else fails scrape off some of the tin coating and leave the terminals on a tissue soaked with seawater for a week, remembering copper does not rust. While this may sound a bit strange the cheaper terminals for the marine and auto market are sometimes not the best quality and many people have been duped into thinking they have a bargain.
So never use steel or aluminum and, like the wire, the terminals, terminal blocks and busbars should be tin-plated copper to resist corrosion.
You are likely to come across an install where the equipment is supplied with wire leads instead of terminals, eg. a radio. Having a terminal block would not be convenient for the connection of power, external speakers and perhaps GPS or AIS. Inline connectors let you connect supply wires together easily, there are several sizes in the insulated crimp range. To aid servicing at a later date it could be a good idea to make the connection with blade or bullet snap connectors instead of the fixed inline connectors/cable joiners.
Here a few no-no’s, unless it is an emergency ‘get us home’-type repair: never twist wires together to make a connection and never wrap a bare wire around a terminal. If you must, put it under a washer and do the screw or nut up.
While three-way, snap-on connectors are useful for tapping into an existing circuit, they are a quick way to introduce corrosion into perfectly good wire. They rely on insulation displacement for contact then, in doing so, inevitably nick the tin plating off the conductor. Then the design of the snap-on connector will not supply enough of a water block for the marine environment.
Wire, terminals and busbars can corrode in the marine environment but the corrosion is accelerated when the damp wiring is able to form an electrical circuit through the fine layer of moisture. Care should be exercised and drip loops put in place to minimise the water becoming a return path for the electrical current.
One thing that is often forgotten during installations is securing the cables. This is to guard against chafe, vibration and in some cases heat and pinch points.
What is a pinch point you ask? It is a point where, due to some mechanical movement, wire can be trapped in the mechanism or crushed when the machine is in use. Well that could be a locker door, a rudder stop, an autopilot arm, cogs in the steering pedestal etc.
As a general rule, tying the cables to a secure point every three hundred millimetres is very good and will certainly limit vibration-induced cable failure. This may be extended to a greater distance but I would not go much past 500mm unless necessary.
The wire run should start at a fuse or circuit breaker. The fuse should be sized to protect the wire in the run, this will be a larger size fuse than the appliance or unit requires for protection. Install a fuse to protect the appliance at the unit, this will help reduce the voltage drop and the heat in the circuit breaker/fuse at the beginning of the wire run.
If you must join wire with a crimp connector in the middle of a run, install adhesive-lined heat shrink tube, or wrap the lot up with self-amalgamating, self-sealing, rubber tape to supply a layer of moisture protection.
I have never found liquid electrical insulation to be much good, it does not give much protection. Adhesive-lined heat shrink is a good option. Use rubber tape and shrink tube for a seal as good as the original insulation if done correctly. ≈