Direct vs CT Metering: Current Paths, Burden and Cabinet Planning
We follow the load current and measurement signal separately, then use simple illustrative calculations to show why CT ratio, secondary wiring and cabinet layout belong in one project discussion.
Start with where the load current flows
We separate two metering arrangements by the current path. In a direct-connected arrangement, load current passes through the meter's designed current path. In CT-operated metering, a current transformer senses the primary conductor current and provides a corresponding measurement signal to a compatible meter. The primary conductors continue to carry the load current.
| Arrangement | Load-current path | Measurement path |
|---|---|---|
| Direct-connected | Supply conductor to the specified meter current path, then onward to the load | The meter measures current through its designed current circuit and also receives the appropriate voltage information |
| CT-operated | Supply conductor through the CT sensing location, then onward to the load | CT secondary measurement circuit to the specified meter; the metering arrangement also includes the required voltage sensing |
We use CT to mean current transformer. Its measurement circuit is different from a power transformer that supplies machinery. These conceptual paths explain the equipment roles; the approved project diagram defines the actual connections, protection and metering arrangement.
Plan direct-connected equipment around its complete interface
We begin a direct-metering layout with the selected service and meter arrangement. Meter current capability, voltage, phase, conductor arrangement and terminal or socket interface must suit the application. A socket-mounted design also requires the matching contact form, retention and specified service features; a board-mounted meter brings different fixing and terminal-cover requirements.
The enclosure provides housing and access, while any socket, terminals, main device and outgoing distribution have their own duties. An empty box dimension does not establish the current rating of a fitted service assembly. Our meter-box component guide explains those boundaries.
We keep installation geometry beside the electrical information. Cable bends, terminal access and sealable areas can govern the layout even when the selected meter body fits comfortably inside the proposed box.
Plan CT metering as a measurement chain
We connect the CT location, primary conductors, secondary route, test facilities and meter position in one layout discussion. The meter receives a measurement signal rather than carrying the full primary load current through its current inputs. The selected system determines the required current and voltage interfaces.
For conventional current-output CTs, the secondary current is related to primary current through the specified ratio. Other sensing technologies can use different outputs and require their own compatible instruments. We therefore identify the actual device and meter interface instead of treating every current sensor as an interchangeable 1 A or 5 A CT.
CT ratio, primary conductor capacity and cabinet assembly current rating remain separate fields. We coordinate those fields with the utility or project metering requirements; there is no single universal ampere threshold at which every installation must change from direct to CT metering.
Use ratio and burden calculations for the questions they answer
An ideal ratio example
For illustration, consider a conventional CT specified as 400/5 A. Its nominal current ratio is 400 ÷ 5 = 80. At an assumed primary current of 200 A, the ideal secondary current is 200 ÷ 80 = 2.5 A. This arithmetic explains the signal relationship; it does not establish actual accuracy, saturation behavior or a supplied EverBen CT specification.
A separate secondary-lead example
Assume the complete outgoing-and-return secondary wiring loop, including its relevant connections, has a resistance of 0.2 ohm. For a resistive loop, burden at a stated secondary current follows VA = I² × R. We calculate wiring burden separately from the meter and other connected devices.
| Compatible measurement system considered | Assumed loop resistance | Arithmetic | Resistive wiring burden |
|---|---|---|---|
| 5 A nominal secondary system | 0.2 ohm | 5 × 5 × 0.2 | 5 VA |
| 1 A nominal secondary system | 0.2 ohm | 1 × 1 × 0.2 | 0.2 VA |
These are two different compatible-system choices, not a setting to change on an existing installation. The CT ratio, meter input and associated equipment must match the chosen system. At the 2.5 A secondary current in the first example, the same resistive loop dissipates 2.5 × 2.5 × 0.2 = 1.25 W; nominal burden comparisons must still use the rating conditions specified for the selected devices.
We add the instrument and other circuit burdens using their specified conditions, then coordinate the result with the CT burden and accuracy requirements. Accuracy class is a separate parameter: a correct arithmetic ratio alone does not ensure the complete measurement chain meets the required accuracy.
Keep phase and polarity identities attached to every signal
We use consistent phase and circuit identifiers from the primary conductor through the CT reference, secondary terminals and meter channels. The meter combines current and voltage information, so matching the correct phases and polarity is essential to meaningful power and energy measurement.
A label that identifies a CT but omits the associated phase or meter channel leaves part of that relationship unresolved. We carry the identifiers into the layout, terminal schedule and metering diagram together. Where multiple accounts or feeders are present, their destination references remain distinct as well.
Changes to conductor routing, CT orientation or meter location can affect this information. We record those changes in the project documents so qualified commissioning personnel work from the final arrangement, without relying on the appearance of similar devices.
Set out the service interface and supply responsibilities
We discuss the serving utility's or project owner's selected metering arrangement before settling the cabinet layout. Service voltage and phase, meter format, required measurement method, access and sealing areas shape the equipment interfaces. Requirements differ between installations, so the actual project documents remain the basis for the configuration.
| Item | Reference to record | Supply or responsibility field |
|---|---|---|
| Enclosure and mechanical supports | Arrangement drawing and mounting details | Our agreed enclosure scope; included supports identified in quotation |
| Meter and any socket | Selected device and socket form | Utility or project-assigned provider to be identified |
| CTs and primary interfaces | CT references, ratio, mounting and conductor arrangement | Assigned provider and installation responsibility to be identified |
| Test facilities and secondary connections | Selected test equipment and terminal schedule | Included equipment and responsible commissioning party to be identified |
| Voltage sensing and protective devices | Approved metering and circuit diagram | Device supply and connection responsibility to be identified |
| Main or disconnect equipment | Specified device and assembly configuration | Agreed equipment boundary and site responsibilities |
We use this schedule to distinguish physical enclosure work from measurement-device supply and site activities. If the metering assembly also includes main equipment, our meter-main configuration page explains the additional distribution and protection interfaces.
Translate the measurement method into cabinet space
We compare primary cable space with the separate measurement route. CT window or mounting geometry must suit the selected primary conductors, while supports and the enclosure arrangement must accommodate the specified mechanical and electrical duties. The secondary route needs its own terminal organization and access to the selected test facilities.
Moving a meter away from the CT compartment can increase secondary lead length and burden. It also changes cable routing and service access. We therefore treat meter relocation as a measurement-chain change as well as a new mounting position.
Our CT cabinet configuration page develops the enclosure, supports and document inputs. For a three-phase service, the phase and conductor arrangement also needs to remain explicit; the number of phases is separate from the number of accounts or meter positions.
Plan commissioning and later service around the selected system
We organize handover information around the final device references, CT ratios, meter inputs, circuit identifiers and approved drawings. The project team defines the required inspections, metering checks and utility involvement for the selected arrangement. Equipment identification and the agreed records keep later support discussions tied to the actual installation.
Secondary-circuit work needs a defined service arrangement
An energized conventional current-output CT secondary must not be left open. We include the specified testing and shorting provisions in the layout discussion so qualified personnel can use the correct equipment and authorized procedures. This article explains planning relationships; it provides no live connection or disconnection sequence.
When a meter, CT or lead route changes, we revisit interface compatibility, burden and the associated drawing revisions. Retaining the original equipment references is useful for identifying what changed and which checks belong in the revised project scope.
Bring the current paths and equipment schedule into one brief
We can start with the service arrangement, intended measurement purpose and any utility metering drawing already available. Include the proposed direct or CT method, meter reference, primary conductor information and enclosure location. For CT metering, the selected ratio, output type, required accuracy and planned secondary route give the discussion greater precision.
Our next step is to connect those inputs to the physical layout and supply schedule: what carries load current, what provides a measurement signal, which components are included, and where access is needed. This gives your purchasing, electrical and installation teams one consistent description of the required cabinet interfaces.
If the method is still undecided, share the known service and project conditions. We can discuss the enclosure implications as the responsible metering team develops the final arrangement.