For safety’s sake, the ABYC recommends
always “rounding up” when using voltage drop
tables. In this case, that means we have to
round up from 2 amps (actual) to 5 amps, the
lowest value on the table. We also have to
round up the length, from 61.2 feet (actual) to
70 feet (the next higher value on the table).
This table shows that we should use 8 AWG
wire for both the DC positive feed to the light
and the DC negative return.
If this seems excessive, we can use the
ABYC’s circular mil area formula (from Stan-
dard E-11) as an acceptable alternative to the
table. Let’s work the numbers. Our factors for
the problem are:CM (circular mil area)(KIL)E,
where:
K (constant for the resistivity of copper) 10.75
I (load current in amperes) 2
L (total circuit length in feet) 61.2
E (acceptable voltage drop of 3% for 12 V)
0.36
Therefore:CM(10.75 2 61.2)0.363,655Next, compare the calculated CM value to
the circular mil table from ABYC E-11 shown
opposite. You’ll see that even when rounding
up to the next nearest size from 3,655 CM, we
hit the 14 AWG row at 3,702 CM. That’s
good news; the circuit wiring is compliant.
There is often a huge difference between
the CM calculation method and using the
tables in E-11, as the above example illustrates.
In fact, I rarely use the tables, and almost
always perform the calculations using actual44 electrical systems troubleshooting
values, with no rounding. It often results in a
smaller wire size requirement that still meets
standards. For a boatowner, this can save sev-
eral dollars on a single circuit. For professional
technicians, it can save tons of copper per year!
Does the TDR play a major role in this?
You bet. Because you can check the actual
length of the wire runs, you can confirm com-
pliance in factory-installed, aftermarket, and
repaired wiring. No one should be able to pull
a fast one on you by fudging wire sizing.Value and Utility
The tools described in this chapter are versatile
and can be used for any type of wiring or cir-
cuit, AC or DC. (Just be sure the power is off
when using them, since these tools can’t func-
tion with power running through the wiring.)
The tone-generating circuit tracer is a highly
affordable tool. And while a bit more expen-
sive, the AEMC Fault Mapper is also a tone
generator, so instead of purchasing both tools,
you can add the AEMC model TR02 receiver
to the TDR and have the benefits of both
methodologies described in this chapter com-
bined into one kit.
In the case of the TDR, you may need to
do a little additional homework to gather more
VOP values, but if you’re a marine electrician,
you have the wire lying around on rolls. All
you’ll need are minimum 60-foot lengths to
start building your own library of VOP values.
Once you’ve done that, yet another value of the
Fault Mapper will stand out—the ability to
check inventory by checking how much cable
is left on the roll! I’ll never use an ohmmeter
for this type of work again.