I have a technical question for the electrically gifted out there.
I want to install two AC amp meters to monitor my portable generator
load, and my research has shown that there are two types of AC amp panel
meters available - those that use a current shunt and those that use a
I'm familiar with the workings of each, but I don't know which is better to
use in my application.
My initial thoughts were that the CT would be better, as it has no direct
connection to the circuit being measured, whereas a shunt is placed in line
with the circuit - and that seemed like a drawback,
But the more I read the less I know.
Current shunts have very low resistance and as long as they are used below
their maximum rated value they seem to have no problems.
Accuracy is important, I guess, but only in a relative way. I plan on
monitoring the current on each leg of the 220 supplied by the generator so
I can keep the load balanced and below the generator's max rating.
The current I'll be measuring will be below 20 Amps.
So which is 'best'?
I'm defining best as the one that will cause the least amount of power loss
in the line, but there may be other considerations that I'm not aware of.
Neither one will cause an appreciable power loss (i.e., one that you
would notice or could even measure). A shunt, as you seem to understand,
is simply a very low-value resistor that produces a voltage drop
proportional to the current flowing through it; the meter (actually a
voltmeter) measures this voltage and displays the corresponding current
in amps. As long as you use a shunt rated for the highest possible draw
(say, 30 amps), there's no danger of it failing.
I'd think the shunt method would be marginally more accurate, but it
probably doesn't matter for your application, where I'm guessing you
just want a rough idea of how much power you're drawing. So I'd go with
the easiest and cheapest solution. (Unfortunately, I can't give you
advice on that, but I'm sure that others here can.)
Made From Pears: Pretty good chance that the product is at least
On Sat, 04 Apr 2009 01:05:51 -0500, firstname.lastname@example.org wrote:
Since 1954. Very good.
I'm sure they sell good products and do a good job, and that there
remanufactured meters are real bargains, I really am, but what do you
suppose this means: "Our accuracy is traceable to the National
Institute of Standards and Technology (NIST)."
BTW, their slogan is "Meters you can believe in!" I wonder if that
preceded or followed "Change you can believe in".
Hmmm. All their remanufactured meters are watt-hour meters, the
glass-enclosed things that sit outside one's hom. And they're at
least 67 dollars and require a base. Maybe there is something I
haven't found, but is this really the company you had in mind. Watt
hour meters don't read amperage, except by timing how fast that disk
spins around. Of course that could be easier for the OP. If the
black section of the silver disk appears at the same time on each
meter, he knows the amperage is the same. But I don't think it's what
OP, if you would spell the word as ammeter, you would have much better
luck searching for them or information about them.
As to which kind you want, I was only acquainted with shunt meters.
Multimeters for testing are like that. Ammeters in cars are like
that. Although I now have a vague feeling that those big
glass-cylinder enclosed watt-hour meters on the sides of our homes are
transformer type. sci.electronics.repair is a possibility.
It means what everybody who does any instrumentation calibration that
aren't NIST means--they use test instruments which have been calibrated
by a series of measurements that do go back (eventually, and very
indirectly) to the NIST standards.
Basically, the use Agilent/Tek/whatever test gear that is periodically
calibrated. The labs that do those calibrations use test
gear/procedures that in turn have been calibrated by their vendor(s) or
other labs (internal or external). And so on until eventually one
reaches a measurement that did originate from direct comparison to NIST.
That's all there is to use; they don't loan the platinum bar (altho the
reference is the platinum bar any longer and that has to do w/ length,
not current, but the idea is the same)...
Yes, although I had known that the proper term was ammeter, I for some
reason can't retain that information in my head. I'm sure it comes from
the term Ampere, named after, um, Mr. Ampere, (one of the giants whose
shoulders we stand on) but then why isn't a Voltmeter called a Volmeter?
anyway...to the point:
I've decided to go with meters that have a build in shunt. Price is about
the same as for external shunts or external CTs, but with a lot less space
requirements and perhaps a bit more safety (less exposed energized bits of
I did, however, like the suggestion to look for a pre-made meter set
designed just for this purpose - If I can find one of those at a price in
the ball park of what I can do from parts (a distinct possibility) I will
go that route.
Let's look at this some more. If the resistance of the shunt were 1
ohm and the current was 15 amps, E=IR shows that the voltage drop
would be 15 volts. Plainly, the drop is nowhere near that. it's less
than a volt, probably only a small fraction of a volt, so the
resistance of the shunt is less than 1/100th of an ohm, probalby much
less. So that's why shunt-based ammeters are fine.
The shunt is not some thin piece of wire that has a resistance
measureable on the 10 ohm scale and that might break some day. It's
pretty thick and the meter across it is really a galvanometer I think,
a voltmeter designed for very small voltages. (Actually, I think
almost all meters are galvanometers, with different resistors and
maybe a diode to determine how much the needle moves, etc. and when.)
There would be very little but some impedance in the main circuit with
a transformer based ammeter also. Probably about the same very small
fraction of an ohm. Because you can't get something for nothing.
This is a common after market item, as two watt meters in a single
sealed box, and often prewired for easy instal. it allows you to
monitor and keep the unit balanced, Google it, 40-60$ will get you a
good kit at many generator sites. For 300 or less you can get prewired
transfer panel kits, that are complete with meters, cable, plugs,
exterior box, sockets etc. I got a Generac kit free at Lowes with a
cheap 5500w gen.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.