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30 feet away, there and back) from the source, will require a
truly gossamer #18 wire (again, two conductors, one posi-
tive and one negative), (according to ABYC guidelines, num-
ber 18 wire is too small to be used outside of a harness or
enclosure so most electricians simply go up one size to #16,
the minimum acceptable wire size), which is but 0.110
inches in diameter (about 7/64) for a 10% voltage drop and
#12 wire, 0.16 (about 11/64) inches in diameter, for a 3%
voltage drop. It’s worth repeating, in order to comply with
ABYC guidelines, bilge pumps, among other critical gear,
should always be wired using the 3% voltage drop calcula-
tion. On larger vessels, over 40-feet, that are wired for 12-
volt service only, this can lead to wire sizes that are ungainly
at best and overwhelming at worst.
It’s clear to see from these examples that the switch to 24
volts represents a clear advantage for the installer (as a for-
mer marine electrician, I can attest to the difficulty of install-
ing wires in a large 12 volt system, as opposed to those of a
24 volt system) as well as the consumer. The savings in
weight, expense, cable bulk (and resultant vessel configura-
tion) are undeniable; the move from 12 to 24 volts means
the system’s cables will occupy one-fourth the cross sec-
tional real estate. That’s right, as a result of the doubled
voltage and halved amperage; the cable cross-section is
decreased by a factor of four.
For all of the above-mentioned reasons, new vessel con-
struction in this 36-foot and up range will benefit from the
switch to 24 volt service. Perhaps more significantly, older
vessels undergoing major electrical refits (vessels over 20 or
30 years old that are undergoing systems refits, often re-
quire the replacement of all or most wiring) will benefit im-
measurably from the switch to 24 volts.
Having worked on or supervised a number of large electrical
refits, the reasons for this are ample and clear. Many older
vessels were wired without following the 3% or even 10%
voltage drop guidelines (remarkably and regrettably, this is
still true for some vessels being manufactured today, con-
firm this before you buy). Thus, their wires were undersized
when they left the factory. A proper electrical refit will set
this scenario straight and in the process the wire channels
and chases often suffer from severe overcrowding. Add to
this dilemma the addition of scores of additional electric
and electronic pieces of equipment; enhanced lighting, e
tertainment systems, communication and electronic gear
bow and stern thrusters, davits, inverters etc. It’s easy to
that by adding all of the modern conveniences found on t
day’s vessels to a vessel of yesterday, electrical gridlock
quickly occur.
In one refit I supervised a 49-foot, 30-year old trawler re-
quired the installation of four, high capacity bilge pumps.
The sub-panel for the pumps, located on the lower deck,
fed from the main panel on the main deck. The run from
main panel to the sub panel was roughly 70-feet roundtri
(excluding the run from the sub-panel to the individual
pumps). The simultaneous load of the four 12-volt pumps
must be assumed that they would all run at the same tim
in the event of major down-flooding) equaled approximat
50 amps at 12 volts. Thus, the wires required to service t
sub-panel, at a 3% voltage drop, were 2/0! At 0.65 (21/3
inches in diameter or about as large as the average index
finger, this presented a major challenge for the electricia
The wire simply would not fit in the existing chases. A swi
to 24 volts, with the resultant halving of required ampera
would have decreased the required wire size by a factor o
Wires Exiting Conduit Into Junction Box
Typical Boat Wiring Installation. Smaller Wires Help