Where should I install current transformers in my circuit?

Current transformer installation locations:

• Main distribution point : CTs installed at main incoming supply (after utility meter, before main breaker). Measures total facility consumption. Position CT toroid around phase conductor (L1, L2, L3) with secondary leads routed to meter terminal block.
• CT positioning in circuit : CT must be in series with circuit being measured. Primary conductor passes through CT toroid core once (single pass). Multiple passes create turns-ratio error; avoid.
• Three-phase installation : Install one CT per phase (3 CTs total for 400V three-phase system). Each CT secondary rated 5A or 1A. All three secondaries wired to meter (DIRIS) three-current inputs. Neutral conductor: no CT required (neutral current = vector sum of three phases, calculated internally by meter).
• Single-phase branch circuits : Individual circuit metering requires separate CT per circuit. Example: sub-metering HVAC zone (one 50A circuit) requires one 50/5A CT.
• Physical installation : - Route primary conductor through CT toroid opening. - Secure CT to cable tray, DIN rail, or cabinet wall (vibration isolation important for accuracy). - Secondary leads (typically 3-core shielded cable) routed separately from power conductors (min 200mm distance for EMC interference avoidance). - Secondary leads kept short (<10m typical); long runs increase voltage burden, may cause meter reading error.
• Polarity marking : CT primary/secondary terminals marked P1/P2 (primary), S1/S2 (secondary). Current direction through primary determines secondary polarity. Polarity error (reversed CT) causes negative meter readings or relay malfunction. Verify orientation before energizing.
• Ground/earth : CT secondary core must be earthed at one point only (typically meter ground terminal). Double-earthing CT secondary creates circulating current, measurement error. If CT remote from meter, ground CT secondary core at CT location via separate earth conductor to main earth bar.
• Split-core CT option : For retrofit (existing conductors cannot be disconnected), split-core CTs available. Hinged toroid opens; existing conductor inserted without disconnection. Slightly lower accuracy (±1–2% error) vs standard solid-core CTs.
• Upstream/downstream positioning : CT location (before or after breaker) depends on application. Measurement: CT downstream of breaker (measures actual load). Protection relay: CT upstream of breaker (detects fault current immediately, faster response). Verify specification per application.

Socomec CTs (DCTWRM 1–3 pass solid-core, DCTSE split-core retrofit options) rated per IEC 60044-1 accuracy class 0.2S (metering) or 5P20 (protection). Optim-Elec verifies CT ratio per circuit maximum demand, confirms secondary burden compatibility with meter specifications. Contact optim-elec.com for CT selection and installation layout.