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AC Ammeter installation

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New Member
I purchased an A/C Ammeter with a 50 amp scale and a donut. I installed the donut around the 2 heavy conductors bringing in the 50 amp 220 volt power and ran the 2 thin wires from the donut to the 2 terminals on the ammeter. Nothing is showing on the ammeter.

Question 1 - should only 1 wire of the 220 volts be going thru the donut?
Question 2 - is the polarity of the 2 thin wires to the meter a consideration?
Waht else should I be looking for to get this working.


New Member
question one; only one wire through a ct(doughnut)
question two; If the meter goes backwards then the polarity is wrong
note; when two phases are put through the ct the meter will read the difference in amps between the two conductors


Extending on kreed's (correct) reply:

The correct name of the "donut" is a Current Transformer, or CT. The basic principle, is that it gives you a small current, proportional to NET current passing through the middle. For example, if a particular CT has a transformation ratio of 800/5, it will give you 5A on the secondary, for every 800A passing through it.

It is mostly used in commercial and industrial installations, which use more than 100A per phase.

Most of the new electronic meters have small CTs fitted internally.

As kreed said, only one wire goes through. If you put both wires through, you have the same current passing is both directions, which gives a net current of ZERO.

It is this principle, which is used in Earth Leakage Circuit Breakers (ELCBs, also known as RCDs, Core Balance Relays or Safety Switches). Both the active and neutral conductors pass through a small CT inside the breaker, and normally, the net current is zero. However, if there is leakage to earth, there will be less current in the neutral than in the active. This is detected by the CT, which operates a trip mechanism, when the difference in current reaches a preset threshold (usually 30mA).

After it trips, the next thing that happens is that you go over to your switchboard, and thank the little device for saving your life :lol:


Active Member
Hi ls44,

Yes, just one wire goes through the doughnut.
(unless this is a special application, like already said
the CT can be used to show a difference also)

The small wires to the meter:

As this is probably a very old unit,
the Ammeter is probably a moving iron type.
You can tell a moving iron type from a moving coil type
by looking at the dial. The graduations are not linear
on a moving iron type, they are linear on a moving coil
type. It doesn't matter which way round the small wires
go on the moving iron type.
If you have a moving coil type, and i dont think so,
then additional components will be needed to make it work.

If the load is removed from the CT during use,
the terminal voltages can get very high, many thousands
of volts. These units are more dangerous than they look.
In normal use, the voltages are quite low.

Regards, John


Active Member
More on CTs and Ammeters.

They were usually kept as a working pair, that is
the same Ammeter and CT were usually kept together.

Most of the CTs i recall were intended to deliver
5 Amperes for their rated setting, and the meters
would have a tiny little 5 on the outer edge of
the dial, corresponding with the rated current.

I spose any working current would do to indicate
on the meters, but it was always 5 Amperes from
what i saw.

Regards, John
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