Current transformer (CT) sizing calculator
Pick the primary rating of a current transformer (CT) for a panel or feeder from the load current — with a safety factor — and read the standard ratio and the secondary current at full load.
CT primary selection
Secondary rating
Selection: required = load × safety factor, then the smallest standard primary above it — from 50 A up to 5000 A. Secondary current at full load: I_s = load / ratio, and the CT ratio is primary / secondary.
About CT accuracy classes
Metering CTs are rated for accuracy at their rated primary — classes 0.5 and 1 are used for panels and feeders (0.5 for billing-grade measurement), while class 3 is only for coarse monitoring. Protection CTs are rated by composite error under fault: class 5P10 guarantees 5% composite error up to 10× rated primary, class 5P20 up to 20×. A meter-class CT must never be used for protection, and vice-versa.
AO Ctrl stocks the current transformers, panel meters and protection used around a motor or transformer installation:
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Frequently asked questions
How do I select the primary rating of a current transformer?
Multiply the load current by a safety factor (the tool defaults to 1.25, typically 1.2–1.5), then take the smallest standard primary above it from the IEC 61869-2 series — 1, 2.5, 5, 7.5, 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 75, 100, 125, 150, 200, 250, 300, 400, 500, 600, 750, 800, 1000, 1200, 1250, 1500, 2000, 2500, 3000, 4000 or 5000 A. For a 100 A load at the default factor the required rating is 125 A, so the chosen standard CT is 150 A.
What does a CT ratio like 150/5 mean?
The ratio is the chosen primary divided by the secondary rating — a 150/5 CT steps 150 A on the primary down to 5 A at full scale, a ratio of 30. For a 100 A load the secondary current is 100 / 30 = 3.333 A, leaving 50% headroom over the load. With a 1 A secondary the same primary gives a 150/1 CT (ratio 150) and a secondary of 100 / 150 = 0.667 A.
Why use a 1 A secondary instead of the common 5 A?
A 1 A secondary CT develops a much smaller burden voltage on the wiring run, so it is preferred for long or high-impedance secondary circuits — with a 5 A CT the lead resistance eats a larger share of the burden budget. 5 A secondaries remain the standard for panel meters and short runs. The tool recalculates ratio, secondary current and margin when you switch.
What accuracy class does a CT need for metering versus protection?
Metering CTs are accuracy classes 0.5 or 1 for panels and feeders (0.5 for billing-grade) and class 3 only for coarse monitoring. Protection CTs use composite-error classes like 5P10 or 5P20, meaning 5% composite error up to 10× or 20× rated primary under fault. Never mix the two — a meter-class CT saturates under fault current and a protection CT is not accurate enough for revenue metering.
Worked example
A feeder carries 100 A. With the default safety factor of 1.25 the required primary is 125 A, so the smallest standard CT is 150 A — a 150/5 CT with ratio 30. At full load the secondary carries 100 / 30 = 3.333 A, and the margin over the load is (150 − 100) / 100 × 100 = 50%. At its rated primary (150 A) the secondary is exactly 5 A.
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AO Ctrl provides this tool for reference only.