Saturday, October 9, 2010

How to: wire and connections

Boat projects frequently involve electrical wiring.  And in my experience, it is owner-installed wiring that poses the greatest risk on many boats (it certainly was on Eolian).  So, how can an electrical project be done without putting the boat at risk?  Last time, we talked about breakers.  Today let's focus on wire and connections.

First, the wire.  Although houses are wired with solid wire, solid wire should never be used on a boat, not even for the 110V circuits.  Houses don't move (well, except in California).  Boats do - they sway, they pound, and they vibrate.  Solid wire will eventually fatigue and break, probably inside the insulation where the break will be hidden.  The ends will still be in contact, and will spark, generating heat, and possibly a fire.  You should always use marine-rated stranded wire.  This wire will be made of strands that are finer than regular stranded wire, so that it is more flexible and resistant to fatigue.  Modern marine-rated wire will have strands which are individually tinned (coated with a thin layer of metallic tin)  to hold back corrosion.  It's easy to tell if the wire is tinned - it looks silvery instead of coppery.

Next: support.  Now that we have the right wire, we still need to minimize the effects of boat movement and vibration on it.  In order to prevent fatigue breaks at the supports, the wire should be supported frequently along its length.  The supports should be padded if they are metal (which should be stainless steel)- no padding is necessary if they are nylon or other plastic.  Where the wire penetrates bulkheads or other surfaces, the edges of the holes need to be padded to prevent chafing of the insulation.

And finally:  the part of a circuit that is most likely to be the cause of a fire is not the wire itself, but the connections.  We are using stranded wire, and stranded wire does not do well when clamped under a screw - the strands tend to separate and squeeze out, making a risky connection.  Therefore on boats, the ends of the wires will get crimped-on connectors, which can be safely clamped under a screw.  With a crimp-on connector, the strands are trapped and cannot escape (there are some kinds of equipment which use a connector that, when a screw is tightened, traps the wire between two pieces of copper - these can be safely used with stranded wire).

Crimp-on connectors come sized (and color-coded) for specific sizes of wire - this is both because the hole into which the wire is inserted for crimping must fit the wire, but also because the crimp-on connector is itself part of the circuit, and must be capable of carrying the same current load as the wire.  Ring terminals are the most secure, but in some places they may not work (standard 110V outlets, for example - these have trapped screws).  Where rings won't work, flanged spade connectors can be used.

Some advocate soldering crimp connectors - it makes a secure electrical and mechanical connection even more secure.  But when you solder the connector, solder wicks up the wire for a little distance from the connector, making a hard spot where fatigue can set in.  I don't solder my connections.  If I have any doubt at all about whether corrosion could set in, I use adhesive-lined shrink connectors.  Oh, and get a good crimping tool.  Those ones stamped out of sheet metal are not adequate for any but the smallest connectors.

Look at some commercial crimps.  You will see that the edges of the metal are folded in, making a profile that looks like a capital 'B', laid on its side.  Yours should look like this too.  Yes, it does matter which way the connector is positioned in the crimping tool.  Practice until you are making good crimps.

In an ideal world, I'd say that there should never be more than one connector under a screw.  In the real world, well it is likely to happen.  If there are multiple terminals under a screw, it is important to pay attention to the order in which they are stacked.  Take the starter terminal on the engine, for example.  Quite often, the primary connection for 12V to the engine (dashboard instrument power, alternator sense, etc.) will be made to the same terminal that carries battery power to the starter.  Under no circumstances should the small-wire terminal be positioned between the battery cable terminal and the starter body - that would cause the 100+ amps the starter draws (Eolian's draws 200 amps) to pass thru the small-wire terminal - way more than it is rated for.  Where both of these connections must be made at the starter terminal, the best practice would be to position the battery terminal in place, screw down the nut tightly, and then position the small-wire terminal and add a second nut.  Be sure that all connections are clean, metal-to-metal contact.

When you are all done, check your work.  No connection should ever get hot to the touch (warm perhaps, but not hot).  Pay particular attention to the ones carrying high current.  Check them under sustained full load (if it is a 110V circuit, please de-energize it before touching it!).

Next time:  Shore power issues.


  1. THHN wire (stranded wire available at Home Depot and the like, often used inside conduit and for the internal wiring of washing machines and the like) is also NEC and Coast Guard approved for marine applications. It is very suitable for dry locations.

  2. Dry locations, on a boat? There are precious few of those.

    Actually, the THHN designation refers only to the insulation. There is both solid and stranded wire with the THHN designation, and even the stranded version has few, coarse, untinned strands. I won't use it


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