The power factor in a single phase circuit is cos φ = P/VI where P power read by an wattmeter, V = voltage read by a voltmeter and I = current read by an ammeter. All the three instruments have a limiting error of ± 1%. The power factor of the circuits can be known if only one meter i.e. an electrodyna-mometers type power factor having an limiting error ± 1% used. The preferred method is to use a single instrument like a power factor meter rather than using 3 instruments because the error with using wattmeter, voltmeter and ammeter is