DETERMINING
BURDEN FOR CURRENT TRANSFORMERS
CT
SECONDARY WIRE SIZING EXAMPLE
A Watt Transducer
(0.75 VA burden) is to be used with three 1200:5 current transformers. The CT
secondary is 30 ft. one way. To maintain the maximum CT accuracy (0.6%), the
total burden must be kept below 2.5 VA as seen on the current transformer
specification sheet.
If 2.5 VA total is available then
2.5  0.75 = 1.75 VA is available for the wire. Assuming the maximum 5 amp
secondary current, on Nomogram No. 1, align a straightedge on 5 amps (left hand
column) and 1.75 VA (center column). Read the impedance in ohms (right hand
column). This should read 0.07 ohms.
Now go to Nomogram No. 2. Place a
straightedge on 0.07 ohms (center column) and 30 ft. cable length one way (right
hand column). Read the cable size (left hand column), which should read #10 AWG
copper cable or larger.
With the above conditions, #10 wire or larger
will keep the current transformer within its best accuracy limit.
