Creepage and Clearance Distance Calculator (IEC 60664-1)
Two conductors at different potentials need enough air between them to survive a transient (clearance) and enough surface between them to survive long-term tracking (creepage). IEC 60664-1 sets both from the working voltage, the environment and the insulating material. This gives the general-purpose numbers; the isolation barrier in a transformer or a high-voltage PCB is exactly where they bite.
Formula
| F1, F2, F4 | IEC 60664-1 look-up tables for impulse, clearance and creepage |
| impulse | rated impulse withstand voltage (V peak) |
| k_altitude | clearance multiplier above 2000 m, Table A.2 |
| CTI | comparative tracking index, defining the material group |
What this model assumes, and where it stops
Assumptions
- The general-purpose values of IEC 60664-1 apply. Product standards (IEC 62368-1, IEC 61800-5-1, IEC 62109-1, IEC 60601-1) modify them and take precedence for their equipment.
- Inhomogeneous field - the ordinary case of pins, tracks and edges rather than deliberately rounded electrodes.
- The working voltage entered is the true worst case across this specific insulation, not the system voltage.
- The surface is uncoated. Conformal coating or potting is treated separately and can reduce the requirement.
- Frequencies up to a few hundred hertz. Creepage requirements rise for higher-frequency stress, which this table does not cover.
Limitations
- This is a design aid, not a compliance sign-off. The governing standard for your equipment is authoritative and its own tables, notes and test requirements decide the final numbers.
- The tables here stop at 2500 V working voltage, 12 kV impulse and 8000 m. Beyond them the page refuses rather than extrapolates, because extrapolating a safety distance would understate it. The standard itself continues further.
- Functional insulation is dimensioned here exactly like basic insulation, which is conservative. The standard permits dimensioning it for the actually expected overvoltage instead (Table F.8).
- Creepage is interpolated linearly between the rationalised working-voltage rows; the standard permits this but a certifier may round up to the next tabulated voltage instead.
- Material group IIIb is not recommended above 630 V and is treated as group III here; confirm against the standard at high voltage.
- The +1200 V rule for non-mains circuits is the simple case. Circuits with their own repetitive or switching transients need those peaks assessed directly.
- Coatings, cemented joints, grooves and ribs all change the effective distance and are governed by detailed rules in clause 6 that no single number captures.
- Pollution degree 4 (continuous conductivity) is outside this table and needs a different approach entirely.
When you need a 3D field solution instead
Closed-form models like the one above hold on idealised geometry. These are the cases where they stop being good enough and a full 3D electromagnetic and thermal solution is the only way to get a trustworthy answer:
- Field grading around sharp electrodes or triple points, where a rounded electrode earns back clearance the inhomogeneous-field table will not credit.
- Partial discharge inception in voids, potting and layered insulation, which is set by the local field rather than a spacing rule.
- High-frequency or high-dV/dt insulation stress in wide-bandgap converters, where the low-frequency creepage table stops applying.
- Encapsulated and multi-material barriers where the field crosses several permittivities and concentrates at the interfaces.
- Optimising a compact isolation barrier against a fixed test voltage, where a field solution shows how much margin the geometry really has.
Common questions
What is the difference between creepage and clearance?
Clearance is the shortest distance through the air between two conductors; creepage is the shortest distance along the surface of the insulation between them. Clearance stops a transient from arcing across the gap, while creepage stops a slow conductive track from forming on the surface. Creepage is never allowed to be smaller than the clearance, and a rib or slot can lengthen creepage without changing clearance at all.
What creepage and clearance do I need for 400 V?
For a 400 V working voltage in a normal indoor environment (pollution degree 2) on standard FR-4 (material group IIIa), basic insulation, IEC 60664-1 gives about 4.0 mm creepage. The clearance depends on the mains transient rather than the working voltage: a 230 V mains supply in overvoltage category II implies a 2.5 kV impulse and about 1.5 mm clearance. Reinforced insulation doubles the creepage to 8 mm and raises the clearance to 3 mm.
Does pollution degree or material group matter more?
Both move creepage a lot. Going from pollution degree 2 to 3 roughly doubles the required creepage, and dropping from material group I to IIIa raises it by about 60% at the same voltage. Clearance, by contrast, only cares about pollution degree through a minimum floor - the material group does not affect it, because arcing through air does not track along a surface.
When do I add margin for altitude?
Only above 2000 m, and only to the clearance. Thinner air breaks down at a lower voltage, so IEC 60664-1 applies a multiplier that reaches about 1.29 at 4000 m and 1.48 at 5000 m. Creepage is a surface-tracking phenomenon and is not corrected for altitude.
References
- IEC 60664-1 - Insulation coordination for equipment within low-voltage supply systems - principles, requirements and tests
- IEC 62368-1 - Audio/video, information and communication technology equipment - safety requirements
- IPC-2221B - Generic standard on printed board design - conductor spacing, Table 6-1
- TI SLUP419 - Demystifying clearance and creepage distance for high-voltage end equipment, Zhang & LaBella, 2024