Measuring Current on DC Circuits


On most RVs, there isn’t a nice wiring diagram with fusing, grounding and splice locations, wire colors, and printed circuit identifiers like we expect on a car.  When something isn’t working, it’s often not obvious what circuit it’s on; when a fuse blows, we often don’t know what’s on the circuit that might have caused it.

It’s also pretty common for fuses to be oversized, not so much for the wiring, but such that if a problem were to occur, a fuse isn’t going to be very quick to blow.  It’s not easy to work with a standard multimeter to connect inline, and measure the current with the load turned on.

Here’s what I use:

EXTECH Automotive 30A Current Tester
EXTECH Automotive 30A Current Tester


This is one of my favorite automotive electrical diagnostic tools beyond a multimeter.  It’s a really simple current meter that plugs in where the fuse goes, and the fuse plugs in to the side of the connector.  I can see that my water pump peaks at just under 5A, and the furnace at 10A.  Same thing for lighting circuits, the refrigerator, etc.  This also allows you to put together an estimate of what you need in terms of batteries, solar, generator runtime, etc. if you were to run something for a particular amount of time.

It’s not quite as sophisticated as the Kill-A-Watt meters used for 110V power, but does it’s job quite well.

4 Comments Add yours

  1. Ron corwin says:

    I was thinking about you charging your lithium batteries. You have stated that your inverter is not suitable for this. I can’t remember if you have solar panels or not. If so you could connect a do power supply to the input and use that to supply the controller. Even put it on a switch to disconnect your panels. Just a thought. Not sure if they would even play well together.

    1. Dave says:

      The inverter as it was delivered would have tried to charge the batteries to about 54.6V–when maximum charge for the pack is only 50.4V. I am still using it though, just with my controller interrupting power to the input side of the inverter when the battery is charged (or, more accurately, only supplying power to the charger when needed).

      Once I had a couple of months in that mode, I added just a little bit of logic to change the thresholds, more or less forcing it to top off in the wee hours of the morning. Doing that, it’s really rare that it ever charges during the day, which lets me go ahead and run both air conditioners on a 30A hookup (more on that later–the air conditioners are now talking to the charger).

  2. Bob Winsor says:

    Dave, thanks for this. I just ordered one on Amazon. I’ve often wanted to test a load on various circuits in the car or the rv. I knew I could do it with my cheap volt/ohm meter but it’s a hassle. This tool should come in handy. I actually think there is something draining my chassis batteries too fast now. I’m going to see if I can use this to diagnose that issue. Only problem is the Freightliner uses big ole truck fuses I think. I’ll have to make some sort of adapter maybe.

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