If you use the optional amp clamp, the
unit will display alternator output in amps,
both loaded and unloaded. This information
is useful for performing a load analysis. The
alternator’s job is to supply voltage to nor-
mally running electrical equipment andhave
at least 0.5 V left over to recharge the battery.
An underrated (in amps) alternator will not be
able to do this and will display less than the
0.5 V increase over static voltage with loads
turned on. Again, you can perform this test
with a stand-alone amp clamp around either
the battery positive or negative cable.
Voltage Drop Test
I’ve been endorsing voltage drop as a way to pin-
point weak links in an electrical circuit for years,
but there is still a general lack of understanding
of the procedure and its applications. Many
troubleshooters set their digital volt-ohmmeter
(DVOM) to the ohms scale and check for con-
tinuity at various points throughout a circuit.
Others use a 12 V test light to see if there is
enough potential at a given point in a circuit to
light the bulb. Neither of these approaches is
really adequate, however. An ohmmeter may
show continuity, and even a measurable resist-
ance value, but what does that tell you if you
don’t know what the resistance shouldbe? The
bulb in the test light may glow, but is it glowing
brightly enough? Only by measuring voltage
drop can you really get a handle on the quality
of a circuit and all its termination points.
To measure voltage drop—i.e., how much
voltage is “lost” in a circuit—there has to be at
least some current flow. At one time, this test
couldn’t be performed on an inactive circuit,
but as noted earlier, the EXP generates its own
signal, so this is no longer a problem.
Significance of Voltage Drop
Keeping in mind that conductance is the inverse
of impedance (resistance), excessive voltage drop
is caused by high electrical resistance. Too much
testing batteries, charging systems, and starter circuits 33resistance (which is measurable) can cause a cir-
cuit to perform poorly or not at all. ABYC Stan-
dard E-11 identifies key circuits and the accept-
able levels of voltage drop as a percentage of
nominal voltage. For critical circuits like elec-
tronics, bilge blowers, panel feeders, and naviga-
tion lights, the limit is 3% of the nominal circuit
voltage (generally either 12 or 24 V). For non-
critical circuits, such as cabin fans and electric
heads, the figure is 10%. These translate into the
specific voltage losses shown in Table 2-5.
These numbers represent the maximum
amount of voltage we want to see “go missing”
due to resistance in electrical circuitry, exclu-
sive of the actual load in the circuit. (The lost
voltage actually turns up as heat, the primary
by-product of excessive electrical resistance.)
These values, I believe, are part of the reason
why many electricians have trouble grasping
the significance of voltage drop: the numbers
seem too small to worry about. But in fact,
these small numbers have great significance.
Losses in excess of these amounts are due to
undersized wiring or loose or corroded termi-
nation points—among the most common
problems in DC marine systems. (We usually
don’t worry too much about AC voltage drop
on board because the wire runs in AC circuits
are generally too short to be a major factor.)Using the inTELLECT EXP-1000
The inTELLECT EXP-1000 generates a low-
level signal and calculates the voltage drop over
a given leg of the circuit or over the entire circuit,System Critical Noncritical
Voltage Circuits (3%) Circuits (10%)
12 O.36 V 1.2 V
24 0.72 V 2.4 VAcceptable DC Voltage Loss
(per ABYC E-11)TABLE
2-5