Smart Meter CT Calculator

Pick the next standard primary at or above your maximum-demand current — but ideally with some headroom. A 200A demand sized to 200/5A CTs runs the CT at 100% of rated, which is the top of the accuracy window with no margin for growth; many installers size up (e.g. 250/5A) so the CT runs at the more accurate ~60–80% of rated and has spare capacity. Standard ratios: 50/5, 100/5, 150/5, 200/5, 250/5, 300/5, 400/5, 500/5, 600/5, 800/5, 1000/5A. Don't grossly oversize either — a 1000/5A CT on a 100A supply reads only 10% of rated and is inaccurate at low load, because metering CTs are specified 5–120% of rated primary (Class 0.5S from 1%) per BS EN 61869-2. Use our Maximum Demand Calculator to determine your peak current.

Single-phase supply: 1 CT on the line conductor. Three-phase 3-wire (no neutral, e.g., motor supplies): 2 CTs on any two phases (Blondel's theorem). Three-phase 4-wire (with neutral): 3 CTs, one on each phase. The meter uses the CT signals to calculate power. Half-hour data meters typically use 3 CTs for 4-wire supplies to capture all phase power and reactive components accurately.

Per IEC 61869-2 (BS EN 61869-2), a Class 0.5 CT holds a ratio error within ±0.5% at rated current, with accuracy specified across 5% to 120% of rated primary current (test points 5/20/100/120%). UK fiscal and half-hourly billing metering now generally requires the 'S' (special) variant — Class 0.5S — which is specified down to 1% of rated current, so it stays accurate at low load (Elexon Codes of Practice, e.g. CoP5). Plain Class 0.5 suits general metering, Class 0.2/0.2S high-accuracy revenue metering on large supplies, and Class 1.0 sub-metering and energy monitoring (non-billing). Don't pay for a higher class than the application needs.

CT burden is the total impedance of the secondary circuit — including the meter, wiring, and any other connected devices. It is expressed in VA (volt-amperes) at rated secondary current. If the actual burden exceeds the CT's rated burden, accuracy degrades because the CT core saturates. Typical meter burden is 2-5VA. Secondary wiring adds burden proportional to wire length and inversely to wire size. Total burden must not exceed the CT's VA rating (typically 5-15VA).

CT secondary wiring size depends on the distance from CTs to meter and the secondary current (usually 5A). For runs up to 10m: 2.5mm² copper is adequate. 10-20m: 4mm². Over 20m: 6mm² or larger. The wire adds burden = I² × R, where I is 5A and R is the total send+return resistance. A 20m run in 2.5mm² has R = 2 × 20 × 0.00741 = 0.296Ω, giving burden = 25 × 0.296 = 7.4VA — which may exceed the CT's rated burden. Use 4mm² instead.