Measuring Multiple Circuits with One CT

Overview

One current transformer (CT) can be used to measure several different circuits if all the circuits are powered from the same phase of the electrical service. This technique is useful when you want to combine a number of individual circuits into one measurement. It works best with smaller gauge wires that are long enough to thread through the CT. This technique can be used with a combination of single phase and three phase circuits.

Using this technique to measuring several circuits with one WattNode^® and one set of CTs can significantly reduce costs. The down side is the increased risk of running a circuit through the CT in the wrong direction or mixing up phases. These errors are more difficult to detect because the error of each circuit will only be a fraction of the total power. Another concern is reduced accuracy at low current levels, discussed below.

When planning a project where individual branch circuit load currents will be added together in one CT, an electrical panel schedule is a handy tool to help figure out which circuit phases can go into which CT. Once the circuits are determined, select a CT with a full scale current rating equal to or slightly larger than the total load current. Panel schedule forms can be downloaded from the Electrical Panel Schedule Forms page.

Key Points

All the circuits being measured with one CT must be on the same phase.
All circuits must pass through the CT in the same directiontowards the source of power.

Example 1

To measure the total amount of energy that an HVAC systems uses for heating and cooling a building, first list all the circuits and the current that they draw on each phase as follows:

Table 1: HVAC System Loads
Load	ØA amps	ØB amps	ØC amps
Compressor 208V 3Ø	60	60	60
Supply Fan 208V 3Ø	20	20	20
Condenser Fan A 208V 1Ø	20	20
Condenser Fan B 208V 1Ø		20	20
Electric Heater 208V 1Ø	30		30
Humidifier 120V 1Ø	20
Controls 120V 1Ø			10
Total current per phase	150	120	140

Add up the maximum rated current of each load, on each phase, and select a CT with a current rating equal to, or larger then, the highest phase current. In this case, a 150A CT is the closest match.

Example 2

An other application that can benefit from this technique is measuring the energy used for lighting. If all the circuits in an electrical panel are lighting circuits, then a WattNode and set of CTs can simply be installed on the mains feeding the panel.

Unfortunately, in many situations this will not work because there are other circuits in the panel besides lighting circuits. A WattNode could be installed on each lighting circuit and the individual measurements added together to come up with the total lighting energy. This approach gives the best accuracy but can be prohibitively expensive.

A number of lighting circuits can be measured with one WattNode and one CT for each phase. This approach is more economical and can be reasonably accurate as long as the total lighting load does not drop below 10% of the CT’s full scale rated current.

For example, to measure the total energy used for lighting in a 277 volt 1 Ø panel, first list all the circuits as follows in Table 2:

Table 2: Lighting Loads

Load	ØA amps	ØB amps	ØC amps
Main Entrance Lights	20
Conference Room Lights A		20
Conference Room Lights B			20
Room #100 Light	20
Room #101	20
Room #103		20
Room #104			20
Total current per phase	60	40	40

Add up the maximum rated current of each load, on each phase, and select a CT with a current rating equal to, or larger then, the highest phase current. In this case, a 70A CT is the closest match.

CT Installation

Typically, the CT is first installed on the circuit with the largest wire diameter. Then other circuits on the same phase are added. Be sure to thread all the circuits through the CT so that the CT arrow or label “THIS SIDE TOWARD SOURCE” faces towards the current source: generally the utility meter or the circuit breaker.

Reduced Accuracy At Low Current

The accuracy of industry standard CTs is typically rated ±1% from 10% to 130% of rated current. With all the loads turned on and the current is high, accuracy is good, but when many loads are off or in a low-power state, accuracy can be reduced. In the example above, the 150 A CT is accurate down to 15 amps. If all the loads are off except the 10 amp Controls load, accuracy may be reduced.

In general, using one CT to measure multiple circuits can save a lot money but you my pay a price in reduced accuracy.
Accu-CT^® current transformers, which are rated ±0.75% all the way down to 1% of rated current, are the best choice for measuring multiple circuits with one CT.