Y Capacitor Limit Calculator
Y Capacitor Limit Calculator
How large a Y capacitor you are permitted, which is almost never the size you want for filtering. Enter the limit from your own standard or platform specification and this computes the current your capacitance produces, the largest capacitance that limit allows, the common-mode impedance it presents and what the safety limit costs you in decibels of attenuation.
Leakage, the largest capacitance allowed, and what it costs you in dB
two 2.2 nF Y2 capacitors, line and neutral to earth, tested at 1.1 × 230 V and 50 Hz, against a limit of 3.5 mA typed in from the reader’s own standard
One line for the safety limit, one for what it costs you
Zshunt = ESR + j(2πf·ESL − 1 ÷ 2πfC)
insertion loss = 20 log10 | 1 + (Zsource ∥ Zload) ÷ Zshunt |
fSRF = 1 ÷ (2π√(ESL · C)) and above it the Y capacitor is an inductor
- V
- the voltage across the capacitance under the condition the standard tests, which is not always the normal condition — IEC 60990 opens the neutral, in both polarities
- ESL
- the series inductance of the part AND of the loop you mounted it in. It is what stops the attenuation rising forever, and it is usually the mounting rather than the capacitor
- Zs ∥ Zl
- a shunt element can only work against the impedance it is shunting. In 50 Ω / 50 Ω that is 25 Ω; between a stiff bus and a converter it can be a fraction of an ohm, and the same capacitor then does almost nothing
- Y class
- Y1, Y2, Y4 per IEC 60384-14. The class is set by the insulation the position requires — basic, supplementary, double or reinforced — and never by the capacitance
Worked example
two 2.2 nF Y2 capacitors, line and neutral to earth, tested at 1.1 × 230 V and 50 Hz, against a limit of 3.5 mA typed in from the reader's own standard
The test voltage is 1.1 × 230 = 253 V, and 4.4 nF at 50 Hz has a reactance of 723.4 kΩ
So the current into the protective conductor is 2π × 50 × 4.4 nF × 253 V = 349.7 µA, which is 10.0% of the 3.5 mA figure entered
Turned around, that limit would permit 44.03 nF in total — 10.01× what is fitted. On the mains at 3.5 mA the safety limit is often not the binding constraint; at the far smaller limits that apply to medical equipment and to Class II products it is the only constraint that matters
At 10 MHz that capacitance presents 2.989 Ω of common-mode impedance — against 3.617 Ω for an ideal capacitor of the same value, because the 10 nH of ESL and mounting loop puts its self-resonance at 23.99 MHz
And the attenuation it buys: 18.52 dB in a 50 Ω / 50 Ω jig, but only 5.23 dB between the 50 Ω source and 5 Ω load entered here — 13.29 dB of the catalogue figure that does not exist in this circuit. That gap, not the capacitance, is usually why a filter that passed on paper fails in the chamber
Where the constraint comes from, by system
| System | What the Y capacitor does | What bounds it | Document to read |
|---|---|---|---|
| Mains equipment, earthed | passes supply-frequency current into the protective earth conductor | touch current and protective conductor current | IEC 62368-1 clause 5.7; IEC 60950-1 clause 5.1; measured per IEC 60990 |
| Mains equipment, Class II | the same, but there is no protective conductor to take it | touch current through the user | IEC 62368-1 clause 5.7, and the accessible-part requirements around it |
| 28 V military vehicle | injects switching-frequency current into the hull | the platform’s isolation and structure-current rules | MIL-STD-1275F for the bus environment; the platform’s own grounding spec for isolation |
| Aircraft | the same, into the airframe | the equipment’s own qualification and the aircraft’s bonding scheme | RTCA DO-160 section 21 for emissions; the airframe manufacturer’s grounding requirements |
| Spacecraft, single-point ground | creates exactly the structure current the architecture exists to prevent | the grounding architecture itself, before any current limit | MIL-STD-1541A (cancelled, still widely referenced); ECSS-E-ST-20-07C |
Y capacitor classes in IEC 60384-14
| Subclass | Insulation it is qualified to bridge |
|---|---|
| Y1 | double or reinforced insulation |
| Y2 | basic or supplementary insulation |
| Y3 | basic or supplementary insulation, with no impulse test specified |
| Y4 | insulation in equipment where the rated voltage is below 150 V |
The capacitor you are allowed is not the capacitor you want
A common-mode filter works by giving the noise current a low-impedance path back to where it came from. On the mains, that path is the protective earth conductor, and the capacitors that provide it are the Y capacitors from line and from neutral to earth. They are the most effective single element in a common-mode filter and they are the one element whose size is decided by somebody else.
On the mains, the constraint is a current, not a capacitance. Everything a Y capacitor passes to earth for the filter’s benefit is current in the protective conductor, and product safety standards bound it: IEC 62368-1 in clause 5.7, Prospective touch voltage, touch current and protective conductor current, with 5.7.2 for the measuring networks, 5.7.4 for earthed accessible parts and 5.7.5 for the protective conductor current itself; IEC 60950-1 in clause 5.1, whose subclause 5.1.7 is titled Equipment with touch current exceeding 3.5 mA and whose Annex D gives the measuring instrument. Both defer the measurement to IEC 60990, Methods of measurement of touch current and protective conductor current. Those documents are copyrighted; this page cites them and computes the method, and the limit that applies to you is the one you type in from your own copy. It is not one number — it depends on the equipment class, on whether the equipment is portable or permanently connected, on the market, and on which of IEC 60990’s measuring networks applies.
The single-fault condition is where designs come undone. IEC 60990’s clause 6.2.2 applies faults one at a time, and 6.2.2.3 opens the neutral, with earth intact, in normal and in reversed polarity. A design that assumes the two Y capacitors share the line voltage between them is assuming something the test deliberately removes. Size for the condition in which one capacitor carries the lot.
On an isolated DC platform none of that applies, and something harder does. A 28 V vehicle bus, an aircraft DC bus or a spacecraft primary bus has no earth leakage path of the mains kind, because the return is not bonded to structure at the load. What binds instead is the isolation resistance the platform requires between the power return and the chassis, and the structure current the platform permits — and both of those come from the platform’s own documents. MIL-STD-1275F defines the voltage and transient environment a 28 V military vehicle load must survive, which is what sets the voltage across the capacitor, but it says nothing about isolation or structure current. Those live in the grounding and bonding specification and in the EMC control plan.
Spacecraft are the case where a calculator has to stop. In a single-point-ground architecture — the practice MIL-STD-1541A set out and that ECSS-E-ST-20-07C carries forward in its grounding and electrical bonding clauses — the structure is deliberately not a current path, and every deliberate connection to it is a decision recorded in the grounding diagram. A Y capacitor to chassis inside a unit is a connection to structure at radio frequency. It may be exactly what the architecture calls for, at the unit’s power input, downstream of the isolation barrier; or it may create precisely the structure current the architecture exists to prevent. This page computes the current and the impedance. It cannot tell you whether your grounding architecture permits the capacitor, and anyone who tells you a calculator can has not read a grounding diagram.
Y-class parts are safety components. Only an appropriately rated and approved Y-class capacitor — Y1, Y2, Y3 or Y4 to IEC 60384-14 — may bridge insulation between a supply conductor and earth or an accessible part. The subclass is chosen by the insulation that position requires, not by the capacitance; a Y1 part is not merely a higher-voltage Y2. The whole point of the class is a construction that fails open rather than short, because a short there energises the chassis.
What the limit costs you, in decibels. That is the number an engineer opens this page for. The insertion loss of a shunt capacitor is 20 log10|1 + (Zs ∥ Zl) ÷ Zshunt|, and two things in it are routinely got wrong. The first is the shunt impedance: a real Y capacitor has series inductance, its own and its mounting loop’s, and above the resonance that inductance forms with the capacitance the part is an inductor and the attenuation falls away. The second is the impedance it is working against. CISPR 17 quotes insertion loss between 50 Ω and 50 Ω because that is a repeatable jig; between a stiff battery bus and a converter presenting a low — and sometimes negative — incremental resistance, the same capacitor has almost nothing to shunt and delivers a fraction of the catalogue figure. The chart above plots all three: your own impedances, the 50 Ω jig, and what an ideal capacitor would have promised. Above roughly 10 to 30 MHz the answer stops being component values and becomes layout: the loop the capacitor is mounted in, where the heatsink is bonded, how the harness is routed. A component-value prediction carried to 100 MHz without that caveat is wrong. For the components either side of the Y capacitors, see the common-mode choke calculator and the capacitor self-resonance calculator.
Frequently asked questions
How big a Y capacitor can I use?
On the mains it is set by a current, not by a capacitance: the leakage is 2πfCV, and the limit is whatever your product safety standard allows for your equipment class. Enter that limit above and the page gives you the largest capacitance it permits at your voltage and frequency. The same capacitance is worth twice as much leakage at 60 Hz as at 30 Hz and eight times as much at 400 Hz, so an aircraft design gets far less than a mains one.
Why does my filter measure so much worse than its data sheet?
Two reasons, and this page shows both. The data sheet figure is measured to CISPR 17 between 50 Ω and 50 Ω, and your circuit is not 50 Ω at either end — a shunt capacitor working into a low-impedance bus has almost nothing to shunt. And above the capacitor’s self-resonance, which its series inductance and mounting loop set, the part is an inductor and the attenuation falls instead of rising.
Can I fit a Y capacitor from the 28 V return to chassis on a spacecraft?
That is a grounding architecture question, and no calculator can answer it. In a single-point-ground design the structure is deliberately not a current path; a capacitor to chassis is a radio-frequency connection to structure, and whether it belongs there is decided by the grounding diagram and the EMC control plan. This page computes what current it would pass and what impedance it would present, so that you can take real numbers into that conversation.
What is the difference between a Y1 and a Y2 capacitor?
The insulation each is qualified to bridge, not the capacitance. Y1 is qualified for double or reinforced insulation and Y2 for basic or supplementary insulation, with different impulse withstand tests behind them. You choose the subclass from the insulation the position requires. Fitting an ordinary ceramic capacitor rated for the voltage is not a substitute: the point of a Y-class part is that it is constructed and tested to fail open rather than short.
Does the leakage current change if the neutral is disconnected?
Yes, and that is exactly why the standards test it. IEC 60990 applies single-fault conditions one at a time, and clause 6.2.2.3 is the one that opens the neutral with earth intact, in normal and in reversed polarity. A design that relies on the two Y capacitors sharing the line voltage between them fails that test. Size for one capacitor carrying the whole voltage.
Is more Y capacitance always better for EMC?
No, in two separate ways. It is bounded by the leakage limit, and above the capacitor’s self-resonance more capacitance is actively worse because the part is behaving as an inductor and a larger part usually has more inductance and a lower resonance. When the Y capacitance is at its limit the next move is more common-mode inductance, a second stage, or reducing the common-mode source itself.
Related calculators
References
- IEC 62368-1, Audio/video, information and communication technology equipment — Part 1: Safety requirements. Clause 5.7, Prospective touch voltage, touch current and protective conductor current, with 5.7.2 measuring devices and networks, 5.7.4 earthed conductive accessible parts and 5.7.5 protective conductor current. Clause numbers verified against published CB test reports issued to the standard. The limit values are in the standard, which is copyrighted, and are not reproduced here.
- IEC 60950-1, Information technology equipment — Safety — Part 1. Clause 5.1, Touch current and protective conductor current, with subclauses 5.1.1 to 5.1.8 — 5.1.7 is titled Equipment with touch current exceeding 3.5 mA — and Annex D, Measuring instruments for touch-current tests. Verified against published CB test reports.
- IEC 60990:2016 Edition 3.0, Methods of measurement of touch current and protective conductor current. The measuring networks (the unweighted network, the network weighted for perception or startle reaction, and the network weighted for let-go / immobilisation) and clause 6.2.2, which applies single-fault conditions one at a time — 6.2.2.3 being the interrupted neutral, tested in normal and in reversed polarity.
- IEC 60384-14, Fixed capacitors for use in electronic equipment — Part 14: Sectional specification — Fixed capacitors for electromagnetic interference suppression and connection to the supply mains. The source of the X and Y subclasses and of the insulation each is qualified to bridge. Copyrighted; cited, not reproduced.
- MIL-STD-1275F, Characteristics of 28 volt DC input power to utilization equipment in military vehicles, 7 September 2022. A US Department of Defense standard, Distribution Statement A, approved for public release. It defines the steady-state and transient voltage environment at the equipment’s input terminals; it does NOT specify an isolation resistance between power return and hull, nor a structure current limit, which is why this page takes both as reader inputs.
- MIL-STD-1541A, Electromagnetic compatibility requirements for space systems, 30 December 1987. A US Government work, freely distributed. Cancelled by Notice 1 and superseded in practice by SMC-S-008, AIAA S-121 and MIL-STD-464; still the document most often named for single-point grounding of spacecraft, which is why it is named here with its status stated.
- ECSS-E-ST-20-07C Rev. 2, 3 January 2022, Space engineering — Electromagnetic compatibility. Clause 4.2.10 grounding and clause 4.2.11 electrical bonding requirements are the ones that decide whether a capacitor to structure is permitted. Freely downloadable from ECSS and copyrighted; cited by clause, not reproduced.
- CISPR 17, Methods of measurement of the suppression characteristics of passive EMC filtering devices. The source of the 50 Ω / 50 Ω insertion-loss convention this page deliberately reports alongside the figure for the reader’s own impedances.