Putting CTs in Parallel - multiple CTs on one port

Issue
A single meter is required to monitor multiple loads.

Product Line
ION Meters
PowerLogic Meters

Environment
3090SCCTs

Resolution
*Warning: Installation and maintenance of this device should only be performed by qualified, competent personnel that have appropriate training and experience with high voltage and current devices. Failure to follow the instructions / warnings in the Meter Installation Guide can result in permanent damage to the meter, severe injury or death.

Two or more CTs can be landed on a single port of an ION or PowerLogic meters, provided the following is considered:

*Putting CTs in parallel:

1. All of the transformers (CTs) must have the same nominal ratio, regardless of the rating of the circuits in which they are connected.
2. All transformers which have their secondary leads in parallel must be connected in the same phase of the primary circuits.
3. The secondary leads must be paralleled at the meter and not at the current transformer.
4. There should be only one ground on the secondary side of all transformers at their common point at the meter.
5. Use modern current transformers with low exciting currents and, therefore, little shunting effect when one or more current transformers are floating at no load. (Three of more floating current transformers might have an effect that should be investigated).
6. The secondary circuits must be designed so that the maximum possible burden on any transformer will not exceed its rating. The burden should be kept as low as possible as its effects are increased in direct proportion to the square of the total secondary current.
7. A Common voltage must be available for the meter. This condition is met if the circuits share a common bus that is normally operated with closed bus ties.
8. Burdens and accuracies must be carefully calculated.
9. If adjustments are made at the meter to compensate for ration and phase angle errors, the ratio and phase angle error corrections used must represent the entire combination of transformers connected as a unit.
10. The watt hour meters must be of sufficient current capacity to carry without overload errors the combined currents from all the transformers to which it is connected.
11. Low-voltage, low-burden-capability current transformers are not suited to this application since the burden imposed on parallel secondary may be very high.
12. Meter voltage usually is equipped with a throw-over relay to avoid loss of meter voltage in the event the normal supply is de-energized.

*Figure 1.0: Multiple CTs connected to a single port on a ION Meter.

More Information

Source for 1-12 list and Figure 1.0 below:

*Source: [Handbook for Electricity Metering, 10th edition: Edison Electric Institute - Pg. 274, 2002, Washington, DC]