Ratio's of 100:5, 200:5, 300:5, 500:10, 700:10, 1000:20 are common here at 400V
Very interesting are the 500:10,700:10 and 1000:20 values. Are these metering CT's or protection CT's?
I have done a lot of CT metering work in the North Island and the secondaries were always referred to as 5 Amps for 400 Volts metering. The Watthourmeters are very accurate between 0.5 and 6.5 Amps loading. With current in the 10 to 20 Amps range the current coils in the 5 Amp meter would be saturated and big under readings recorded on the watthourmeter.
Most CT meters class 1 and 2 were rated at 5 Amps ( Sangamo, Ferranti, English Electric, Landis&Gyr ).
Some Email meters had nameplate ratings of 2.5 / 10 Amps. ( the only ones I'm aware off ) which were designed to take 10 Amps from the CT's.
CT accuracies were CM, BM or class 1 and common maximum burdens were 5VA or 10VA for CT's.
A Trivector kWh / kVArh meter would have a burden of about 4 VA.
Auckland Central used thermal MDI meters in series with a standard kWh meter which required 10 or 15 VA rated CT's
Ratio's used were 100,150,200,250,300,400,500,600,800,1000,1200,1600,2000,3000 / 5 Amps.
In installations over 1600 Amps often summation CT's were used because double feeders were provided for in large pumpstations or shopping malls.
The multitap CT's were most accurate on the highest tapping while being tested.( these CT's were e.g. 150/250/400/5A ).
From 2000 onwards through the MARIA standards a lot of multitap CT's are being replaced to single tap class 0.5 or 0.2 CT's for the T.O.U. metering.