Electrical Connections
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. Boat builders save a few dollars using type
2 wire, but a boat owner 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 boat
owners. 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. |
