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CT Sizing & Saturation Calculator

IEC 61869-2 current transformer ratio, itemized burden and accuracy-limit-factor (ALF) / knee-point voltage (Vk) check. Saturation and transient analysis, metering accuracy class, thermal & mechanical withstand, multi-ratio taps and REF/differential ratio matching are subscriber features.

AAJappsEngineering Calculation Report

CT Sizing & Saturation Calculator

IEC 61869-2

Generated

For preliminary engineering use only — always verify results against project-specific standards and a qualified engineer's review before use in a real design.

CT ratio & rating

Rated primary current Ip

Rated secondary current Is

Required accuracy limit factor(ALF)

Knee-point voltage Vk(0 if unknown)

Secondary winding resistance Rct

Secondary burden

Every relay, meter or indicating instrument sharing the CT secondary circuit contributes its own VA at rated Is — they sum since they sit in series on the same loop.

Lead (cable) length

Lead conductor CSA

mm²

Lead return factor(2 = go-and-return loop)

Saturation & transient analysis

Subscriber

Fault current If

System X/R ratio

Fault clearance time tf

Metering accuracy class

Subscriber

Metering class

Thermal & mechanical withstand

Subscriber

Rated short-time thermal current Ith

Rated time for Ith

Rated dynamic withstand Idyn(0 = 2.5×Ith)

Multi-ratio tap selection

Subscriber

Available tap primary currents

REF / differential ratio matching

Subscriber

Check current(A primary)

CT sizing result

CT ratio

400/1 A

n = 400

Total burden Rb

2.780 Ω

2.78 VA at rated Is

ALF actual ✅

28.4×

Required ALF = 20×

Vk required

105.6 V

Actual Vk = 150 V

Burden breakdown

Device	VA @ In	Resistance
Protection Relay	2.50 VA	2.500 Ω
Lead resistance (20 m, 2.5 mm²)	—	0.280 Ω
Total	2.50 VA (relay/meter)	2.780 Ω

Fault current table

Fault level	Primary (A)	Secondary ideal (A)	Secondary at ALF limit (A)
0.2× In	80	0.20	0.20
0.5× In	200	0.50	0.50
1× In	400	1.00	1.00
2× In	800	2.00	2.00
5× In	2,000	5.00	5.00
10× In	4,000	10.00	10.00
20× In	8,000	20.00	20.00
50× In	20,000	50.00	20.00