From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: Measuring Amp usage of a Motor Home. Date: Tue, 21 Dec 2004 22:43:53 -0500 Message-ID: <jlphs01e847kan0b9o4s2bpirsnftfq51q@4ax.com> That thread got sorta wild. Here are the basics. You have basically 3 ways to measure the AC current: Direct measurement Clamp-on meter Current Transformer (CT) meter. Direct measurement includes a 30 (or dual 50 amp) meters or a current shunt in the power line with a suitable millivolt meter attached. I don't recommend this method because high current direct measurement meters are somewhat expensive and because the meter is at line voltage. If you happen to bump the thing and the front comes off, there are line voltage parts there for the touching. The one exception I'd make to this rule is if you find an industrial 270 deg pointer swing utility-grade meter. These are rugged enough and expensive enough not to have that problem. For occasional measurements, a clamp-on meter works fine. Chicom made meters can be had in the <$50 price range. If you're willing to spend about $100, Fluke makes a very nice little slip-on unit. It has a U shaped jaw that does not have to close around the wire. Just poke it on the wire and it reads. Another alternative in the clip-on area is a clip-on CT. These are used for inputting current flow into data loggers and the like. The output is usually 1 mv or 1 ma per amp. Match it to an appropriate meter and you're ready. You can buy, say, a 50 mv meter with a 50 amp face or you can make your own face. Here's a nice little program that will let you easily print a new meter face for your analog meter: http://tonnesoftware.com/meter.html There is a standard in industrial and utility power measurement that offers the third method, the 5 amp output current transformer (CT). A ct is rated in the format in:5. "in" is the amount of current passing through a conductor passed through the center of the CT to make 5 amps flow in the output circuit. A 200:5 CT would produce 5 amps out when 200 amps flowed through the wire in the hole. Two major advantages of a CT. It is isolated from the line voltage and 5 amp in meters are very commonly available with just about any face calibration. A 50:5 CT and a 50 amp meter face would do the trick. Two for 50 amp service. I like to use a higher range meter on 30 amp service because you're more likely to overload the service than with 50 amp service AND you can get away with the overload for a short period of time before the breaker trips. Small, low accuracy CTs are quite cheap, usually <$20 retail. they are widely available from the surplus outfits. Meters are a bit more expensive, running in the $60-$100 range new. But they're also widely available surplus. Just look around on the net. BTW, if you use a clamp-on meter, here's a little trick to making the measurement easily. Get a male and female connector like on your power cord. Connect them together with about 1 ft long pieces of #10 stranded THHN wire, a black, white and green piece for 30 amp, black, red, white and green for 50 amp service. You can buy this by the foot at Lowe's, etc. When you want to measure the current, plug this little cord in the pedestal, plug your RV plug into the other end and then clamp the meter around the strand of wire you're interested in. Black and Red are the hot leads, white is neutral and green is ground. A further trick I use on mine is to make the green wire longer so I can loop it into a 10 turn bundle, secured with cable ties. Clamping the meter around this bundle multiplies the sensitivity by 10. This let me measure ground current with 10X more sensitivity. Handy for finding sneak current. One amp on the meter would be 100 ma through the conductor. John |