Electrical Connections
by Don Casey
Whether adding new electrical gear or rewiring something already aboard,
the requirements for a reliable and safe electrical connection are the
same. You need the right wire, the right terminal fittings, and a couple
of inexpensive hand tools. Wire
A wire could hardly find a more hostile environment than aboard a boat.
On-board wiring is salted and doused, shaken and whipped, crushed and
abraded, exposed to sunlight, subjected to heat, and coated with petroleum.
Neither lamp cord nor house wiring will long endure these conditions.
Choose your wire carefully. Never use solid wire on a boat. Wave-or
motor-induced oscillations eventually fracture solid wire. Boat wiring
must have the flexibility stranding provides. Boatbuilders save a few
dollars using type 2 wire, but a boatowner should use only the most
flexible wire, called type 3.
The wire must be copper, but even copper corrodes in the marine environment,
and corrosion is the primary cause of electrical failures on a boat.
Plating each strand of the wire with a thin coat of tin-called tinning--dramatically
improves corrosion resistance. The additional cost of tinned wire is
nominal, the benefits substantial. Under normal circumstances use only
tinned wire.
Stranded single-conductor wire is called hook-up or primary wire. Since
most after-construction wiring requires two wires, duplex wire is more
convenient and provides the added safety of a second layer of insulation.
The best choice for most 12-volt wiring projects is duplex safety wire,
where the twin conductors are red (positive) and yellow (ground). Making
the ground wire yellow rather than black reduces the likelihood of confusing
a DC ground wire with an AC hot wire-also black. Boat
cable
In recent years wire designed for the marine environment has become
widely available to boatowners. This wire is known as boat cable. Unfortunately
the Underwriter's Laboratories standard that defines boat cable, UL
1426, is less stringent than commonly thought. Boat cable can be type
2, tinning is not a requirement, and the heat rating of the insulation
can be quite low. When you select boat cable, type 3 is better, tinning
is essential, and you want the highest heat rating-designated on the
jacket as BC5W2 (105¡C in a dry environment, 75¡C wet). Tinned boat
cable from a reputable supplier is your best choice for all wiring needs.
Size
As with water through hose, electricity flows more easily through larger
wire. It is essential to size wire for the maximum current flow you
expect it to carry. If the wire feeds a single item, the current requirements
will be shown on a label or plate on the appliance, or in the accompanying
manual. If the rating is in watts, divide it by 12 (assuming a 12-volt
electrical system) to convert the rating to amps. When the wire is part
of a circuit that supplies several appliances, the potential current
through the wire is the sum of the current requirements of every appliance
on the circuit. For example, if a circuit is comprised of six 25-watt
cabin lamps, the wire will be carrying about 12 amps ([25 watts Ö 12
volts] x 6) when all the lights are on.
To size wiring, you need the wire length from the power source to the
appliance(s) and back to the power source. Doubling the straight line
distance to the battery or electrical panel is not adequate. You must
determine the actual length of the wire by measuring along the path
it will follow-up, over, and around. It is not unusual for a wire run
to be more than twice the straight-line distance.
Some voltage is used up pushing the current through the wire. This loss,
called voltage drop, should not exceed 3%. The table shows what size
wire is required to deliver adequate voltage to the other end. Use the
potential current draw to select the row, the round trip wire length
to select the column. The number where these two intersect is the wire
size you need. Electrical wire appropriate for marine use will have
the gauge designation printed on the insulation. The smaller the gauge
number, the larger the wire diameter. Always buy wire at least a foot
or two longer than your measurement. You can easily shorten the wire
after it is installed, but lengthening it requires a highly undesirable
splice. Each wire should be a single continuous run between terminals.
Connections
Other than chafe or lying against hot metal, wires rarely experience
failures in the middle of a wire run. Almost all wiring problems occur
at the connections. Never twist wires together to make a connection,
and never wrap a bare wire around a terminal screw. You will minimize
wiring problems if you terminate all wire ends with crimp connectors.
Selecting the proper connector requires that you match it to the wire
gauge and to the size of the terminal screw. Ring terminals are your
best choice unless the terminal screw is captive. In that case, use
flanged spade connectors.
Use butt connectors for appliances supplied with wire leads instead
of terminals. Step-down butt connectors let you connect heavy supply
wires to lighter leads. To simplify servicing, it can be a good idea
to make the connection with blade or snap connectors instead of butt
connectors. Three-way connectors are useful for tapping into an existing
circuit.
Terminals used on a boat must always be copper, never steel or aluminum,
and like the wire, they should be tin-plated to resist corrosion.
Tools
Nine times out of ten, stripping insulation with a pocket knife results
in nicked wire, which opens the door to corrosion. And you simply cannot
make a dependable crimp connection with a pair of pliers. Every boatowner
should own a wire stripper and a good-quality crimper. The cost is nominal.
Strippers sold by auto supply and hardware stores are for SAE wire,
which is about 10% smaller than AWG, so these strippers will nick AWG
wire. Be sure the wire stripper you buy is intended for AWG wire.
An inexpensive pliers-type crimper is more than adequate for a few connections,
provided you make a few practice crimps first. But if you will be making
a lot of connections, a ratchet crimper offers the benefit of consistency,
even in the hands of an amateur. Crimping
Remove only enough insulation for the wire to reach the end of the barrel
of the terminal. Grip the terminal in the correct die in the crimper,
fully insert the wire into the terminal, and squeeze. If the barrel
has a seam, the crimp indent should be opposite.
Better terminals feature a brass sleeve that can be crimped over the
insulated wire to add mechanical strength. This type of terminal is
usually installed with a double crimp tool. If your crimper doesn't
have a double-crimp die, crimp the terminal to the wire first, then
reposition the crimper and crimp the sleeve to the insulation.
There is only one accepted field test for a crimp terminal-pull on it.
Test every crimp terminal this way. Without using any tool, grip the
terminal and the wire and try to separate them. If they come apart,
the crimp was bad.
Be sure to run your wire as high in the boat as possible-to keep it
dry-and support it at least every 18 inches. For more
information about electrical connections, consult Sailboat Electrics
Simplified by Don Casey.
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