Split-Core Current Transformers: How They Work & How to Choose

Direct answer: a split-core current transformer is a CT whose magnetic core opens on a hinge or clamp, so it can be fitted around a live conductor without disconnecting it. You choose one by accuracy class, window size, primary current range and housing type.

The reason split-core CTs exist is simple: retrofit. On a new build you can thread a conductor through a solid-core CT before termination. On an existing installation you cannot — not without cutting the cable or taking the circuit down. On a live commercial feeder or a running production line, that shutdown is usually the most expensive part of the whole metering project.

Split-Core vs Solid-Core: The Real Trade-Off

The honest engineering position: if you are building new and can thread the conductor, a solid-core CT gives you better accuracy per dollar. The split-core earns its premium the moment shutdown costs enter the calculation.

Why the Air Gap Matters

A split-core has a mating face where the two halves meet. That joint introduces a small air gap into the magnetic circuit, and the air gap is the reason a split-core is typically specified one accuracy class below an equivalent solid-core.

It also explains the single most common field fault: a contaminated or improperly closed mating face. Dust, paint overspray or a partially latched clamp widens that gap and the ratio error rises immediately. When a split-core CT reads low on commissioning, check the joint before you suspect the meter.

How to Size the Window

Measure, do not assume. The window has to clear the actual conductor including insulation — and on a busbar installation, including any bend radius or support hardware near the CT position. A CT that will not close is a returned CT.

• Round cable: measure the outside diameter over insulation, add clearance
• Busbar: measure width × thickness; confirm the window is rectangular, not round
• Multiple conductors: all phase conductors of one circuit only — never mix circuits

Key Specifications to Compare

• Accuracy class: 0.5 for billing-grade metering, 1.0 for monitoring and load profiling.
• Primary current range: our NLH Series covers 5–4000 A across the family.
• Secondary output: 5 A and 1 A are the standard options; confirm what your meter expects.
• Housing material: flame-retardant PC+ABS is the norm for panel installation.
• Frequency: 50 Hz or 60 Hz.
• Burden: the CT must drive the meter plus the cable run — long secondary leads consume burden.

The Safety Rule That Is Not Optional

Never open-circuit the secondary of a current transformer while primary current is flowing. With no burden connected, the secondary develops a dangerously high voltage across its terminals. Short the secondary before disconnecting a meter. This applies to every CT type, and it is the one mistake that turns a routine metering job into an incident.

Where Split-Core CTs Get Used

• Retrofitting sub-metering into occupied commercial buildings
• Tenant billing in multi-occupancy properties
• Temporary energy audits and load surveys
• BMS and energy-management system integration
• Adding measurement to existing industrial panels

FAQ

Can I use a split-core CT for revenue metering?

In many jurisdictions yes, provided the accuracy class meets the utility's requirement — usually Class 0.5. Always check the utility's approved list before ordering, because approval is model-specific in some markets.

Why does my split-core CT read low?

Most often the mating faces are not fully closed or are contaminated. Open the CT, clean both faces, and re-latch. If the error persists, verify the burden and check that the conductor is centred in the window.

Does conductor position inside the window matter?

Yes, though less than the joint condition. Centring the conductor gives the most consistent result, especially at the low end of the measuring range.

Can Yomin supply a custom ratio?

Yes — we manufacture to order, OEM or own-brand, and can build non-standard ratios and window dimensions. Send your requirement for feasibility and MOQ.

Factory-Direct Supply

Zhejiang Yomin Electric has manufactured current transformers since 1996 and ships to 95+ countries. The NLH Series split-core range, the NSQ and MES toroidal series and custom ratios are all built in-house and tested before dispatch. Browse the current transformer range or send your specification for a quote within 24 hours.