measuring DC current, make sure you understand your DVOM’s directional component.
Some come with a plus sign and an arrow,
showing which side of the meter should face
the battery’s positive terminal. Consult your
manual. Second, you’ll need to measure
current through just one conductor — particularly, the positive conductor (usually red).
On marine DC systems, you’ll sometimes
find duplex wire, which contains both the
positive and negative (usually black or yellow)
conductors inside a single sheath; you’ll need
to separate them to do this test.
1. Identify the positive conductor of the
circuit you’re testing.
2. Set your multimeter to DC amps, at the
3. Open the jaws of the amp clamp, then
encircle the positive conductor. You should
get a reading to record.
For more detail on these and other tests, see
Ed Sherman’s Powerboater’s Guide to Electrical
Systems or take the online course “Basic
Marine Electricity and Marine Corrosion
Prevention,” created by Ed Sherman and
yours truly ( www.abycinc.org).
Tim Murphy is the co-author of Fundamentals
of Marine Service Technology (ABYC, 2012).
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An amp clamp