What CT ratio do I need for my smart meter?

The CT primary rating should be the next standard size above your maximum demand current. For example, a 200A three-phase supply needs 200/5A CTs. Standard ratios available: 50/5, 100/5, 150/5, 200/5, 250/5, 300/5, 400/5, 500/5, 600/5, 800/5, 1000/5A. Do not significantly oversize — a 1000/5A CT on a 100A supply will be inaccurate at low loads because CTs are most accurate at 50-120% of their rated primary current.

How many CTs do I need?

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.

What accuracy class do I need for my CTs?

Class 0.5 is the standard for fiscal/billing metering in the UK — it means the CT is accurate to within 0.5% between 25% and 100% of rated primary current. Class 0.2 is used for high-accuracy revenue metering (large commercial/industrial supplies). Class 1.0 is acceptable for sub-metering and energy monitoring (non-billing). Using a higher accuracy class CT than required adds unnecessary cost.

What is CT burden and why does it matter?

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).

What wire size should I use for CT secondary wiring?

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.

TradeCalculator

Smart Meter CT Calculator

Select the correct CT ratio for smart metering based on maximum demand. Calculate secondary wiring burden, required CT VA rating, and number of CTs for single and three-phase supplies.

Maximum Demand

Maximum expected demand

Supply Type

Determines number of CTs

Secondary Wire Length

CT to meter distance

Accuracy Class

Class 0.5 for billing

How We Calculate This

This calculator selects the appropriate CT ratio based on maximum demand and calculates the secondary wiring burden to ensure metering accuracy.

Key Calculations

Max current = kW × 1000 ÷ (V × √3) (three-phase)

Wire burden (VA) = I² × R = 25 × (2 × L × ρ)

CT Selection Rules

Primary: Next standard size above max demand current
Secondary: 5A standard (1A for long runs)
Accuracy: Class 0.5 for billing, Class 1.0 for monitoring
Burden: Total must not exceed CT VA rating
Wiring: 2.5mm² up to 10m, 4mm² to 20m

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.