CT Ratios Explained (WattsOn-1100 / -1200)

The original WattsOn-1100 and WattsOn-1200 meters were designed to be used with a variety of CT types including mA output, 5A output and mV output types.

WattsOn meters with inputs configured for 5A and 333mV may be used with any CT that generates the appropriate (5A or 333mV full scale) output.

WattsOn meters calibrated for use with mA output CTs (MCT Series, MSCT Series, MS Series) must be used with the specific CT for which the meter is calibrated to maintain maximum accuracy.

To maintain consistency, the CT Ratio is always calibrated in the same fashion, regardless of the CT type being used. This way, the RTU software can be the same no matter which meter model is being used.

The WattsOn is configured to measure all current inputs as a "5A" full scale. That is; it normalizes all current measurements as 0-5A depending on the actual calibrated full scale. For this reason, every CT type is a "CT Ratio" multiplier of xxx:5, even if the CT type is not a 5A variety.

The application is best demonstrated by example:

WattsOn Model CT Type Used Full Scale "CT Ratio" Multiplier
WattsOn-1100-5A "Industry Standard" 400:5 400A 400:5 80
WattsOn-1100-5A "Industry Standard" 1000:5 1000A 1000:5 200
WattsOn-1100-333mV mV output 100A (333mV full scale) 100A 100:5 20
WattsOn-1100-MCTA-200A MCTA 200A 200:5 40
WattsOn-1100-MSCT5-1600A MSCT5 1600A 1600:5 320

The CT Ratio (and PT ratio, if PTs are used) must be correctly set for the floating point registers to be properly calculated. Current, Power and Energy registers are dynamically calculated using the CT Ratio set in the meter. Energy is accumulated without the CT ratio, therefore changing the CT ratio will affect the accumulated energy readings.

It is important to remember that the resolution of the WattsOn readings depends on the CT and PT Ratio settings also. The "basic resolution" must be multiplied by the effective CT ratio to determine the actual system resolution.


Parameter Accuracy
(of reading)
Basic Resolution
(multiply by Ratio for actual resolution)


0.5% 0.1V PT Ratio


0.2% 0.001A CT Ratio
Power (W): 0.2% 0.1W (per phase, 1W per total) PT Ratio x CT Ratio
Power (VA): 0.5% 0.1VA (per phase, 1VA per total) PT Ratio x CT Ratio
Power (VAR): 0.5% 0.1VAR (per phase, 1VAR per total) PT Ratio x CT Ratio
Power Factor 0.5% 0.001  
Frequency: 0.1% 0.1Hz  
Energy: 0.2% 1Wh PT Ratio x CT Ratio