# AC Ammeter Installation

Discussion in 'The Projects Forum' started by manofgresley, Sep 13, 2010.

1. ### manofgresley Thread Starter New Member

Sep 13, 2010
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Hello

I am a newbie to this site, please go easy on me.

I have a project to measure the amps in a 240 vac single phase circuit to a maximum of 16 amps.

I am thinking of purchasing a cheap panel mounted ammeter 0 - 20 amp deflection off E Bay. My question is this : -

1) Can i fit this directly in Parallel, or do i need a CT or Shunt device with it ? if so what size CT/Shunt would i need.

Regards Ray

ps. A drawing would be a bonus if anyone as one

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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I think there is some confusion here. AC current or DC current?

240VAC will need an AC amp meter, which is a whole different animal from a DC amp meter.

AC meter should come with a current transformer, especially at the large currents you mentioned(over 5amp)

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3. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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If you are sure this is an AC meter and it only has two posts for connection then the current transformer is inside.

You will 'break' a line. Either the Hot lead or the Neutral lead, one or the other, doesn't matter which, but just one of them. Cut the wire and hook up the meter to the two ends. Simple dimple you're done!

manofgresley likes this.

Jul 7, 2009
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A 20 A meter should work fine. You'd wire it in series with either leg. In other words, the ammeter is in series with the load. Note you do not want to put an ammeter in parallel with the full line voltage, as that will destroy it. The page http://www.allaboutcircuits.com/vol_1/chpt_8/4.html (for DC ammeters) shows you how to calculate a shunt resistor; this works for AC circuits if you use RMS currents and voltages and the voltages and currents are essentially sine waves.

Note the approach with a shunt resistor only works if you have an ammeter that has a smaller full scale current than what you're trying to measure. However, many meters are based on a movement that measures small currents (say, in the mA to μA ranges) and have an internal shunt resistor built in. If you can open the meter up, you can find this resistor and replace it with an appropriately-valued one.

When you have the meter open, you may also be able to take out the old scale and make a new one using a laser or inkjet printer.

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5. ### manofgresley Thread Starter New Member

Sep 13, 2010
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Many thanks for the current advice (no pun intended).

My circuit is 240 volts AC, maximum 20 amps. Sorry i did mean Installed in SERIES.

Regards

Ray

6. ### windoze killa AAC Fanatic!

Feb 23, 2006
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Just a reminder. DISCONNECT THE POWER FIRST. Sounds obvious but you know the human race in general are a dumb bunch of animals. Some are ever a dark brown/black colour (note: Not a racist comment) after playing with electricity.

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7. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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So true - just google 'electrocution' and view the image results for proof!