Instrument Transformers and the Meter Multiplier

Instrument Transformers and the Meter Multiplier

By Steve Lindsay


Peak Demand Alta Series Current Transformers


Utility meter shops are concerned with many things, but when it comes to metering and instrument transformers, ratio and the meter multiplier are very important for accurate billing purposes.  So how does a meter tech determine the meter multiplier of a particular service? It really boils down to basic math.

Every current transformer, or CT, has a ratio. A typical CT ratio might be 200:5 or 400:5. What this means is that a CT will step down higher current levels to a level that the meter can safely handle.  In the case of a 400:5 CT, 400 amps will be stepped down, or transformed, to 5 amps.  This relationship is constant and it is linear.  If there was 200 amps flowing through a CT with a ratio of 400:5, the secondary would read 2.5 amps – half the current on the primary would produce half the current on the secondary.

The watthour meter multiplier is very similar to the ratio in that it is a very simple math calculation.  If we use the same 400:5 CT as above, our meter multiplier would be 400 divided by 5, or 80.

Not all meter installations use just CTs, though.  Quite a few also have voltage transformers, or PTs (potential transformers).  Installations that have both CTs and PTs still rely on basic math though.  Voltage transformers work on the same premise as CTs in that they step down higher voltages to usable levels. Most PTs have a secondary voltage of 120 volts.  A PT with a ratio of 4:1 would step down 480 volts to 120 volts.

How does this impact the watthour meter multiplier?  Let’s assume you have a 480 3-phase service. The installed CT has a ratio of 400:5 and the PT has a 4:1 ratio.  The CT multiplier would be 80 and the PT multiplier is 4.  Since a watt = amps x volts, the watthour meter multiplier in this case would be 80 x 4 or 320.

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