From: John De Armond
Subject: Re: Measuring Amp usage of a Motor Home.
Date: Tue, 21 Dec 2004 22:43:53 -0500
That thread got sorta wild. Here are the basics.
You have basically 3 ways to measure the AC current:
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:
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.