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volts now, efficiency, cable size and voltage drop.
Why 24-Volts?
In very plain terms, each load found aboard your vessel,
from bilge pumps and navigation lights to the electric motor
that starts your engine and the reading lamp in your berth,
calls for an appropriately sized wire through which electricity
is supplied. Guidelines and tables available from the Ameri-
can Boat and Yacht Council (ABYC) (
www.abycinc.org
) guide
marine electricians through the cable selection process. Be-
cause resistance in cabling causes a loss or drop in power,
all efforts are or should be made to minimize its effects
(voltage drop is the difference between the voltage meas-
ured at the battery and the voltage measured at the load, be
it light, pump, thruster or other equipment). Thus, the afore-
mentioned tables offer calculations for a 3% and 10% volt-
age drop when sizing wiring. The former, 3%, is for critical
loads such as bilge pumps, navigation lights and navigation
and communication electronics, while the latter, the 10%
table, may be used for less critical loads such as cabin light-
ing, thrusters, domestic water pumps etc.
Why does voltage drop matter? If, for example, the voltage
at the ship’s battery is measured at 12.6 volts, essentially a
full charge without a charge source such as the engine alter-
nator or battery charger operating, the voltage measured at
a device wired for a 10% voltage drop, say a bilge pump in
this case would be a mere 11.34 volts. A bilge pump de-
signed to run most efficiently at 12.6, and often more, volts
will suffer diminished capacity when operated at the lower
voltage. If, however, that same pump were operated using a
properly sized conductor, which provided a 3% voltage drop,
it would operate much more efficiently (but still at some-
thing less than full capacity) at 12.23 volts.
Going the Distance
The details of wire sizes for the above examples make for
interesting reading. Let’s say the pump is located 60 feet
away (wire lengths for voltage drop calculations are always
measured there and back, 30 x 2, and as the actual length
of wire used rather than the linear distance from power sup-
ply to electrical consumer) from the main 12 volt distribu-
tion panel, not an unrealistic figure on a 40 foot vessel fo
example. If the pump calls for a maximum current of 10
amps, the 10% voltage drop cable will be sized at #10 (t
individual conductors, one positive and one negative), ea
of which is 0.195 (about 3/16) inches in diameter.
The same pump and cable run, wired for the correct 3% v
age drop, would call for # 6 wire, which, at 0.330 (about
5/16) inches in diameter represents a substantial increa
in size, weight and cost (#6 wire costs about three times
much as #10 wire) over the 10% voltage drop figure.
The 3% and 10% voltage drop calculations aren’t mandat
on recreational vessels, however, the prudent boat builde
and mariner follows these recommendations as if they w
mandates. By the same token, the 10% recommendation
just that, a recommendation. Many builders wisely opt to
use the 3% rule throughout the vessel, even for non-critic
loads? Why? Enhanced efficiency. What good is a large b
tery bank with sophisticated charging and monitoring ca
bilities if 7% of its capacity is forfeited in the form of volt
drop, which is transformed into heat by increased resista
of the undersized cable? In extreme cases, the results of
ignoring the recommendations, particularly for critical ge
could be catastrophic. Once again, consider the handi-
capped bilge pump during flooding, or a dimmed navigati
light, which results in a collision, or a VHF radio that won’
operate for a distress call when the batteries are weak.
Now that the reader understands the concept of voltage
drop and related cable sizing issues, let’s make a leap to
next electrical level; 24 volts. The previously mentioned 1
volt, 10 amp bilge pump is now transformed to a 24 volt
pump operating on a vessel equipped with a 24 volt elect
cal system. Without getting too technical, Ohm’s Law, the
guiding force that dictates much of what marine and lan
bound electricians do, tells us that a pump, or any other
vice, that draws 10 amps at 12 volts will draw just 5 amp
at 24 volts; double the voltage, half the amperage, it’s th
simple.
Fortuitously, the ABYC tables that are used to calculate v
age drop are also available for 24 volts electrical systems
this case, a 24 volt, 5 amp load located 60 feet (rememb
12-Volt Bilge Pump Installation
Comparative Wire Sizes