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CT Clamp Selection Guide: Choosing the Right Current Transformer for 100/5A to 400/5A Systems
- 3 hours ago
- 9 min read
A current transformer that is one size too small will saturate, run hot, or give useless readings. One that is too large may look safe on paper but leave the meter measuring at the bottom of its range, where accuracy suffers. CT clamps are simple devices, but selecting the right one takes more than matching “100/5 A” on a label.
A current transformer, often called a CT or CT clamp, converts high current in a cable or busbar into a lower current that a meter, relay, or energy monitor can read. In many commercial and industrial low-voltage systems, that output is 5 A at full rated primary current. That is why ratings such as 100/5 A, 200/5 A, and 400/5 A are common.
This guide explains how to choose the right CT clamp by current rating, core type, cable diameter, accuracy class, burden, and meter compatibility.
How a 5 A current transformer rating works
A CT ratio tells you how the primary current relates to the secondary current.
A 100/5 A CT outputs 5 A when the primary conductor carries 100 A. A 200/5 A CT outputs 5 A at 200 A. A 400/5 A CT outputs 5 A at 400 A.
The relationship is linear within the CT’s normal operating range.
CT rating | Primary current | Secondary current at full load | Ratio |
100/5 A | 100 A | 5 A | 20:1 |
200/5 A | 200 A | 5 A | 40:1 |
400/5 A | 400 A | 5 A | 80:1 |
If a 200/5 A CT is installed on a cable carrying 100 A, the secondary current will be about 2.5 A. If the same CT sees 40 A, the output will be about 1 A.
That matters because many meters read best when the CT operates in a healthy part of its range. A CT that spends most of its life at 5% or 10% of rated current may still work, but its readings may be less reliable than a better-matched CT.
Match the CT rating to the expected operating current, not just the maximum possible current.
Start with the real load current
The first step in CT selection is to find the current you need to measure.
Use the actual circuit rating and load profile where possible. The main switch rating alone can mislead you. A 250 A supply may only run at 80 A most of the day. A pump circuit may sit low, then spike during start-up. A commercial tenancy may have a low overnight load and a much higher daytime load.
For metering, choose a CT where the normal current sits comfortably inside the measurement range. A common target is to have normal loads sit somewhere between about 20% and 100% of the CT rating. Exact performance depends on the CT model and accuracy class, but this is a useful selection habit.
When 100/5 A is the right choice
A 100/5 A CT suits smaller feeders, submains, and equipment circuits where the current normally stays below 100 A.
Good examples include:
Small distribution boards
Mechanical services circuits
Small three-phase loads
Submetering for light commercial tenancies
Solar or battery circuits where rated current is within range
Do not choose 100/5 A if the circuit can sustain more than 100 A. Short peaks may be acceptable if the CT and meter are rated for them, but regular overload will reduce accuracy and may create safety issues.
When 200/5 A is the right choice
A 200/5 A CT is a common choice for medium-sized feeders and distribution boards. It gives useful resolution at typical commercial loads while allowing more headroom than a 100 A CT.
It may suit:
Medium submains
Larger mechanical equipment
Distribution boards with mixed lighting, power, and HVAC loads
Three-phase circuits with expected current under 200 A per phase
For many sites, 200/5 A is the practical middle ground. It is large enough for common commercial loads but not so large that light-load readings become poor.
When 400/5 A is the right choice
A 400/5 A CT is used for larger feeders, main switchboards, rising mains, and sizeable plant loads.
It suits circuits where the measured current can approach 400 A and where lower-rated CTs would overload. It may also be used when future load growth is likely, but avoid oversizing too much. If the normal load is only 30 A, a 400/5 A CT is usually a poor metering choice.
For main incomers, check the switchboard rating, maximum demand, service fuse size, and any metering requirements before selecting the ratio.
Choose between split-core and solid-core CT clamps
The next decision is the core type. Most low-voltage metering CTs are either split-core or solid-core.
Split-core CT
Opens on a hinge or removable section so it can be fitted around an existing cable without disconnecting it. Best for retrofits, energy audits, and switchboards where cables are already terminated.
Solid-core CT
Has a closed magnetic core. The cable must pass through the CT before termination, or the connection must be dismantled. Best for new builds, panel manufacturing, and high-accuracy permanent installations.
Split-core CTs are easier to install
A split-core CT can be clipped around a conductor with minimal disruption. That makes it useful where shutdown time is limited or where cables cannot be easily disconnected.
The trade-off is that split-core CTs have a join in the magnetic path. Good models can still be accurate, but they are more sensitive to installation issues. Dirt, a damaged hinge, or a core that does not close fully can cause poor readings.
Use split-core CTs when access and speed matter, but check the latch closes securely.
Solid-core CTs are often better for permanent metering
A solid-core CT has a continuous magnetic core, so it usually offers better performance for the same size and cost. It is also less likely to suffer from a poorly closed core.
The downside is installation. The conductor must pass through the CT window. That usually means fitting it before cable termination or arranging an outage to disconnect and re-terminate cables.
For new switchboard builds, embedded metering, and long-term revenue-style monitoring, solid-core CTs are often the cleaner choice.
Check cable diameter and CT window size
Current rating is only half the fit. The conductor must also physically pass through the CT window.
Check the outside diameter of the insulated cable, not just the conductor size in square millimetres. Cable insulation, sheathing, bending radius, and lug placement all affect whether the CT will fit.
For three-phase circuits, each phase normally needs its own CT. Do not place all three phases through one metering CT unless the application specifically calls for residual or earth leakage measurement. For standard power and energy metering, each CT goes around one phase conductor only.
Before ordering, confirm:
The cable outside diameter
The CT internal window diameter or aperture
The space around the cable inside the switchboard
Whether the CT can close without pressing against other conductors
The bend in the cable near the termination
The direction the CT leads need to exit
A CT that technically fits the cable may still be awkward to install if there is no room for the hinge, latch, or lead exit.
Understand accuracy class before you buy
CT accuracy tells you how close the secondary current is to the ideal ratio under stated conditions. For metering, common classes include Class 1 and Class 0.5, though available classes vary by product and standard.
A lower number usually means better accuracy. For example, Class 0.5 is generally more accurate than Class 1 when used within its specified range and burden.
For basic monitoring, Class 1 may be enough. For detailed energy management, billing allocation within a site, or performance tracking, Class 0.5 or better may be preferred.
Do not choose accuracy class in isolation. Accuracy depends on:
CT class
Current level
Connected burden
Lead length and cable size
Frequency
Correct installation
Meter input type
A high-accuracy CT can still read poorly if it is connected to the wrong meter input or installed backwards.
Check burden and lead length
Burden is the load connected to the CT secondary, usually expressed in VA. It includes the meter input burden plus the resistance of the CT secondary leads.
If the burden is too high, the CT may not maintain its rated accuracy. Long thin leads can add enough resistance to matter, especially on 5 A CT circuits.
To check burden, review:
CT rated burden, such as 2.5 VA, 5 VA, or 10 VA
Meter input burden
Distance between CT and meter
Secondary cable size
Number of terminations or test links in the circuit
For longer runs, a CT with a higher VA rating or larger secondary cable may be needed. Keep CT secondary wiring as short and direct as practical.
Confirm meter compatibility
This is where many CT clamp installations go wrong. The CT and meter must match.
A meter designed for 5 A CT input expects a current transformer such as 100/5 A, 200/5 A, or 400/5 A. A meter designed for 1 A CT input needs a different CT ratio, such as 100/1 A or 400/1 A. Some energy monitors use voltage-output CTs, such as 333 mV types, and they are not interchangeable with 5 A CTs.
Before installation, confirm these items on the meter:
CT secondary input rating
Supported CT ratios
Number of phases and CT inputs
Wiring diagram for three-phase systems
Whether CT ratio can be programmed
Phase voltage reference for each CT
Required polarity direction
If the meter is programmed for 200/5 A but the installed CT is 400/5 A, the displayed current and energy will be wrong. The same applies if CTs are assigned to the wrong phase voltage.
For three-phase metering, each CT must match the correct voltage phase. A CT on L1 should be paired with the L1 voltage reference. Mixing phases can cause low power factor readings, negative power, or incorrect kWh totals.
Install CTs in the correct direction
Most CTs have polarity markings. These may appear as P1 and P2, K and L, or an arrow showing source-to-load direction.
For standard load metering, the CT direction usually follows current flow from supply to load. If the CT is reversed, the meter may show negative power or incorrect energy flow. Some meters allow software correction, but it is better to install the hardware correctly.
Also check the secondary terminals, often marked S1 and S2 or k and l. Keep polarity consistent across all phases.
A few practical checks help prevent errors:
Fit CTs around a single phase conductor only
Keep all CTs facing the same source-to-load direction
Match each CT to the correct voltage phase
Confirm the meter ratio setting before energising
Label CT leads at both ends
Use shorting links or approved test blocks where required
Never open-circuit the secondary of a current-output CT while primary current is flowing. A CT secondary can produce dangerous voltage if left open. CT circuits should be installed and tested by a licensed electrician or qualified technician under the rules that apply in Australia.
A practical selection process
Use this simple process when choosing between 100/5 A, 200/5 A, and 400/5 A CT clamps.
Confirm the meter input
Check whether the meter needs 5 A, 1 A, or voltage-output CTs. If the meter is not a 5 A CT meter, none of the 100/5 A to 400/5 A options will be correct.
Find the expected current
Use measured load, maximum demand data, circuit rating, or equipment full-load current. Select a CT rating that covers the real operating current without excessive oversizing.
Choose the CT ratio
Use 100/5 A for circuits up to 100 A, 200/5 A for medium circuits up to 200 A, and 400/5 A for larger circuits up to 400 A. Leave sensible headroom, but keep normal load high enough for useful accuracy.
Pick split-core or solid-core
Use split-core for retrofits and easier installation. Use solid-core where the cable can be passed through the CT and better long-term accuracy is preferred.
Measure the cable
Confirm cable outside diameter and switchboard space. Make sure the CT window, hinge, and lead exit all fit.
Check accuracy and burden
Select an accuracy class suitable for the job. Then confirm the CT rated burden can handle the meter and lead length.
Verify wiring and programming
Match CT direction, phase assignment, secondary wiring, and programmed ratio before taking readings as valid.
Quick comparison of common 5 A CT sizes
CT ratio | Best suited to | Avoid using it when | Key check |
100/5 A | Smaller feeders and subcircuits | Current may regularly exceed 100 A | Normal load is not too close to the limit |
200/5 A | Medium distribution boards and plant circuits | Normal current is very low, such as small loads under light use | Meter resolution is suitable at lower loads |
400/5 A | Main feeders and larger three-phase circuits | The circuit normally runs far below 400 A | Cable diameter and CT window size are large enough |
Common mistakes to avoid
The most common CT selection error is oversizing. A 400/5 A CT may seem safer than a 100/5 A CT, but it can reduce useful accuracy on a lightly loaded circuit.
Other common problems include:
Installing a 5 A CT on a meter that expects 1 A or mV input
Choosing a CT window that does not fit the insulated cable
Clamping around more than one phase conductor
Reversing CT polarity
Mixing CT phase order with voltage phase order
Ignoring burden and using long secondary leads
Leaving a CT secondary open while current flows
Using protection-class CTs where metering accuracy is needed
Protection CTs and metering CTs are not the same thing. Protection CTs are designed to support relays during fault conditions. Metering CTs are designed to measure normal load accurately. Choose the type that matches the task.
The safest choice is the one that fits the whole system
A good CT clamp selection is not based on current rating alone. The right device must match the load current, meter input, cable diameter, core style, accuracy class, burden, and installation conditions.
For smaller circuits, 100/5 A often gives the best measurement range. For medium feeders, 200/5 A is a common and practical choice. For larger mains and high-current circuits, 400/5 A provides the needed capacity. Split-core CTs make retrofit work easier, while solid-core CTs often suit new boards and permanent metering.
Before ordering, check the meter manual, measure the cable, confirm the expected load, and plan the wiring path. That short check can prevent inaccurate readings, wasted parts, and unsafe CT secondary wiring.
