If a Current Transformer (CT) reads low, then the secondary is probably overloaded. Burden is the load which may be imposed on a transformer secondary by cables and connected devices without causing an error greater than the stated accuracy classification. Lower burden and proper sizing can improve the accuracy of the CT. It is a good idea to check your CT burden if the CT primary rating is under 200A.
Burden is expressed in ohms impedance or volt-amperes for current transformers. The standard burden limits are defined by ANSI C57.13.
Current Transformers
CT's carry an ANSI burden designation that is a "B" followed by the ohms limit (e.g., B0.1). Below is a typical CT specification in our catalog:
1. CT primary to secondary ratio
2. Allowable burden in ohms (0.5 ohms max)
3. Allowable burden in volt-amperes (12.5 VA max)
4. Accuracy at burden above (e.g., ±1.2%)
You can see from the above example that if the total burden (wire plus device) is only 2.5 VA, the accuracy of the reading will be ±0.6%. If a higher burden (up to 12.5 VA) is used, the accuracy will be ±1.2%.
For lower line currents, the accuracy at 10% of the CT rating (120A through a 1200:5 CT) is double the published value. On the above example, at 2.5 VA burden the accuracy would be ±1.2%. At a 12.5 VA burden the accuracy would be ±2.4%.
Burden Calculation
STEP 1:
Determine the burden of the connected device in VA or ohms impedance. This should be on the device data sheet.
STEP 2:
Add impedance of the secondary wire run. Measure the length of the wire run between current transformer and the burden (e.g., meter, relay, transducer, etc.). Refer to Nomogram No. 2 (next page) and determine the resistance, in ohms, of the wires that connect the secondary of the current transformer to the devices. The Nomogram makes allowances for the return wire. Add this resistance from Nomogram No. 2 to the impedance of the connected burdens. Nomogram No.1 will help convert ohms to VA if desired.
STEP 3:
Make sure the total burden does not exceed the specified limits for the chosen CT.
CT Secondary Wire Sizing Example
A WT-5 Watt Transducer (0.75 VA burden) is to be used with three 600T-122 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 catalog page (Also shown on previous page).
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. Smaller cable could be used if 1.2% accuracy could be tolerated.
Nomogram No. 1
Relationship Between Volt-amperes (VA), Amperes (I) and Impedance (Z). If 2 values are known, the third can be found. Place a straightedge on the two known values and read the third.
Nomogram No. 2
Resistance of Copper Wire. Place a straightedge between Wire Size and Length of Wire Run and read the Ohms of Resistance in the circuit on the center scale.
