The creepage distance describes the shortest insulating path along the surface of an insulating material between two electrically conductive parts. It prevents unwanted flashovers or leakage currents. IEC 60664-1 derives it from voltage, pollution degree and material group: for 400 V at pollution degree 2 it requires 2.0 mm of basic insulation with a group I insulating material and 4.0 mm with group III. A creepage distance may never be shorter than the associated clearance (Texas Instruments, SLUP419). This page gives the minimum values from 250 to 1,000 V, a seven-step dimensioning procedure, the measuring rules for grooves, ribs and joints, and shows how insulating and molded parts rescue a path that is too short.
In German, the colloquial Kriechweg and the standard term Kriechstrecke both mean creepage distance. Weidmüller, following the old DIN IEC 112, still calls the CTI the “comparative figure of creepage path formation”, where the word stands for the conductive track that a surface leakage current burns into the surface. How well a material resists that track is its tracking resistance. This page is about the length of the path your design offers the current.
Creepage distance and clearance protect against different failures
Two terminal tabs sit on an insulating plate with a 2 mm high rib between them. The clearance runs straight over the top edge of the rib. The creepage distance follows the plate surface to the rib, climbs it, crosses the edge and runs down the other side. The rib adds twice its height, 4 mm, to the creepage distance, while the clearance barely grows. The standard checks both dimensions separately because they guard against different failures.
| Characteristic | Creepage distance | Clearance |
|---|---|---|
| Path | along the surface of the solid insulation | straight through air |
| Failure mode | tracking from moisture and dirt under continuous voltage | flashover from a short overvoltage |
| Dimensioning quantity | RMS working voltage or rated voltage | rated impulse voltage from the overvoltage category |
| Material | CTI group I to IIIb is an input | irrelevant |
| Altitude above 2,000 m | no correction | correction factor, 1.48 at 5,000 m |
| Table in IEC 60664-1 | F.5 (F.4 in the 2007 edition) | F.2 |
| Example 400 V, pollution degree 3, group III, overvoltage category III | 6.3 mm | 3.0 mm at 4 kV |
The larger value sets the minimum. In the example it is the creepage distance, by more than a factor of two. In a clean environment it can be the other way round: at 230 V, overvoltage category II and pollution degree 2 the clearance needs 1.5 mm and the creepage distance with group I only 1.25 mm, so 1.5 mm applies to both. The altitude factor comes from Table A.2 of the standard (TI), and the 2020 edition also gives it in the new Table F.10. The full clearance table with case A and case B is part of our page on the dielectric strength of air.
Four inputs set the minimum creepage distance
Creepage distance is read from a table based on tests with polluted and condensing surfaces. There is no formula for it. The table needs four inputs:
- the RMS voltage across exactly this path, for mains-supplied circuits the rationalised voltage from Table F.3 (single-phase) or F.4 (three-phase), F.3a and F.3b in the 2007 edition, which in a 230/400 V supply is 250 V to earth and 400 V between phases
- the pollution degree at the insulation itself, not that of the whole building
- the material group of the insulating material according to its CTI
- the type of insulation, because reinforced insulation needs twice the creepage distance of basic insulation
| Pollution degree | What happens on the surface | Typical environment |
|---|---|---|
| 1 | no pollution or only dry, non-conductive pollution | air-conditioned clean rooms, sealed components |
| 2 | non-conductive pollution, occasionally conductive from condensation | homes, offices, shops, laboratories, test bays |
| 3 | conductive pollution, or condensation that makes dust conductive | industry, workshops, agriculture, unheated storage, boiler rooms |
| 4 | continuous conductivity from conductive dust, rain or wet conditions | outdoors, the standard specifies no creepage distance here |
The room examples come from the WAGO technical explanations. The CTI defines the material groups: group I from 600, group II from 400 to below 600, group IIIa from 175 to below 400 and IIIb from 100 to below 175. In the creepage table IIIa and IIIb share one column. The CTI comes from the data sheet, and if it is missing, assume IIIb. For orientation, from our comparison of electrically insulating plastics: PP, PE, PTFE and silicone reach CTI 600, PET sits at 350, polycarbonate at 250, FR4 at 175 to 249 and PEEK at only 150.
Creepage distance table to IEC 60664-1 from 250 to 1,000 V
The following values apply to basic and functional insulation outside printed boards, PD stands for pollution degree. They agree in the Weidmüller dimensioning guide, in the WAGO explanations of EN 60664-1:2008 and in Table F.5 of the current edition.
| Voltage (RMS) | PD 1, all groups | PD 2, group I | PD 2, group II | PD 2, group III | PD 3, group I | PD 3, group II | PD 3, group III |
|---|---|---|---|---|---|---|---|
| 250 V | 0.56 mm | 1.25 mm | 1.8 mm | 2.5 mm | 3.2 mm | 3.6 mm | 4.0 mm |
| 400 V | 1.0 mm | 2.0 mm | 2.8 mm | 4.0 mm | 5.0 mm | 5.6 mm | 6.3 mm |
| 500 V | 1.3 mm | 2.5 mm | 3.6 mm | 5.0 mm | 6.3 mm | 7.1 mm | 8.0 mm |
| 630 V | 1.8 mm | 3.2 mm | 4.5 mm | 6.3 mm | 8.0 mm | 9.0 mm | 10.0 mm |
| 1,000 V | 3.2 mm | 5.0 mm | 7.1 mm | 10.0 mm | 12.5 mm | 14.0 mm | 16.0 mm |
Four rules belong to this table:
- For reinforced insulation, double the value, so 4.0 to 8.0 mm at 400 V and PD 2.
- If the working voltage lies between two rows, you may interpolate linearly. Round the result up.
- The standard does not recommend group IIIb at pollution degree 3 above 630 V.
- Since 2007 the standard gives no values for pollution degree 4. Tables based on VDE 0110-1 of 1997 with a PD 4 column, as still shown in the Weidmüller guide, no longer apply to it.
The table shows where a better material pays off. At pollution degree 2, group I halves the creepage distance compared with group III, at 400 V from 4.0 to 2.0 mm. At pollution degree 3 the same change saves only 1.3 mm, just over a fifth. At pollution degree 1 it saves nothing. Our recommendation is therefore: at pollution degree 2 choose the material first, at pollution degree 3 the geometry first. Some guides state a flat 8 mm for 400 V at pollution degree 3. That is the value in the 500 V row for group III and costs space the standard does not ask for.
The obvious objection to group I: PP and PE give up at 90 to 100 °C, and the heat-resistant thermoplastics PEEK and PPS sit in group IIIb. For hot locations there are thermosets that do both. Polyester glass mat such as GPO3 reaches CTI 600 and withstands 155 to 170 °C (see thermosets in electrical insulation). Mind the rule for mixed materials: if the path crosses two insulating materials, the lower CTI applies unless one section withstands the full voltage on its own.
Dimensioning creepage distance in seven steps
- Take the type of insulation from the product standard: functional, basic or reinforced.
- Determine the voltage across the path. Mains-supplied circuits use the rationalised mains voltage, internal circuits the highest RMS value under the worst operating condition.
- Set the pollution degree of the micro-environment. A sealed enclosure can create pollution degree 2 inside while 3 prevails outside, and the product standard decides whether that counts.
- Read the material group from the CTI in the data sheet, and assume group IIIb if none is given.
- Read the table value, interpolate for working voltages, and double it for reinforced insulation.
- Compare it with the clearance and use the larger value as the minimum creepage distance.
- Measure the geometry with moving parts in their least favourable position and with manufacturing and assembly tolerances, following the measuring rules in the next section.
Worked example: terminal carrier in a control cabinet on a factory floor
A polycarbonate terminal carrier separates two phases of a 230/400 V supply. The cabinet is ventilated and stands on a factory floor, so pollution degree 3 applies. Basic insulation at 400 V is required. Polycarbonate has CTI 250, that is group IIIa, column III. The table gives 6.3 mm. The clearance for overvoltage category III and 4 kV is 3.0 mm, so creepage sets the dimension.
The design offers 4.5 mm of plain surface between the terminals, 1.8 mm short. A group I material would lower the requirement to 5.0 mm and still not be enough. A rib 2 mm high and 1.5 mm wide in the middle extends the path to 4.5 + 2 × 2 = 8.5 mm. That leaves 1.5 mm to the terminal on each side, exactly the dimension X for pollution degree 3. A narrower gap between terminal body and rib would be bridged like a groove, and the rib would add only part of its height.
Grooves count only from width X, ribs shorten the table only at pollution degree 3
The standard measures creepage as if dirt filled every narrow recess. It sets a minimum dimension X for that:
| Pollution degree | Minimum width X |
|---|---|
| 1 | 0.25 mm |
| 2 | 1.0 mm |
| 3 | 1.5 mm |
If the associated clearance is below 3 mm, X may be reduced to one third of that clearance (Weidmüller). The measuring examples of the standard lead to these rules:
- A groove narrower than X is bridged straight across, and its depth does not count.
- A parallel-sided groove of X or more counts with its full contour, however deep it is.
- In a V-groove with an internal angle below 80° the contour counts, but the bottom is short-circuited by a link of width X.
- Over a rib, creepage follows the contour and clearance runs over the top edge.
- An uncemented joint between two parts behaves like a groove. If it is narrower than X, creepage runs straight through it.
- With a barrier pushed into a slot, the path through the joint at its base applies as soon as it is shorter than the path over the barrier.
- If an unconnected metal part such as a screw lies in the path, the sections before and after it add up. A section shorter than X counts as zero.
The standard credits the shape of the surface with an effect of its own only at pollution degree 3, where transverse ribs and grooves break the conductive layer of dirt. From a required creepage distance of 8 mm it allows the smaller bracketed values of the table at pollution degree 3 when a rib is used. At 1,000 V in group III the requirement drops from 16.0 to 12.8 mm, at 630 V in group III from 10.0 to 9.0 mm. The rib must be at least 20 % of the required creepage distance wide and 25 % high, so 2.6 mm wide and 3.2 mm high for 12.8 mm. WAGO lists the bracketed values for all voltages up to 10 kV.
Creepage distance on printed boards has its own, smaller values
For printed wiring the table has two separate columns. At 230 V mains the 250 V row applies: 0.56 mm at pollution degree 1 and 1.0 mm at pollution degree 2. The column for pollution degree 2 applies to all material groups except IIIb, which makes the CTI of the base material the first question in the layout.
| Voltage | PCB PD 1 | PCB PD 2, group I to IIIa | Other parts PD 2, group III |
|---|---|---|---|
| 250 V | 0.56 mm | 1.0 mm | 2.5 mm |
| 400 V | 1.0 mm | 2.0 mm | 4.0 mm |
| 630 V | 1.8 mm | 3.2 mm | 6.3 mm |
| 1,000 V | 3.2 mm | 5.0 mm | 10.0 mm |
FR4 usually sits at CTI 175 to 249 and therefore in group IIIa. If the data sheet gives no value, assume IIIb, and the right-hand column applies, with spacings two to two and a half times as large. Solder resist does not count as insulation (Mikrocontroller.net, trace spacing). A conformal coating reduces spacings only where the product standard allows it, for example via IEC 60664-3 or Table G.13 of IEC 62368-1. A milled slot must be at least X wide or it is bridged. If it is wide enough, the shorter path along the slot contour or around the slot end counts, and across the slot only clearance counts.
IPC-2221B is a generic design standard for printed boards and does not replace a safety standard. At 400 V peak it requires 0.25 mm on inner layers, 2.5 mm on uncoated outer layers and 0.8 mm on coated outer layers. Texas Instruments advises taking the larger value where it differs from IEC 60664-1, here 2.5 instead of 2.0 mm.
Which standard sets creepage for your product
IEC 60664-1 is a basic safety publication. It covers equipment up to 1,000 V AC and 1,500 V DC, up to 30 kHz and up to 2,000 m altitude, and it applies directly where no product standard sets its own values. The German edition DIN EN IEC 60664-1 has been current since July 2022 (DKE), and a draft amendment A1 appeared in August 2026. Product standards adopt the method and set their own tables:
- IEC 62368-1 for audio, video, IT and communication equipment, creepage in Table 17
- IEC 61800-5-1 for adjustable speed drives, Table 10
- IEC 62109-1 for photovoltaic inverters, Table 14
- IEC 61010-1 for measurement, control and laboratory equipment
- IEC 60335-1 for household appliances
- EN 50124-1 for railway applications
- UL 840 as the North American counterpart of IEC 60664-1, UL 1059 for terminal blocks
Above 1 kV the system changes. Outdoor insulators in high-voltage insulation are dimensioned by specific creepage distance in millimetres per kilovolt, set out in IEC TS 60815.
Where space runs out, an insulating part extends the creepage distance
A creepage distance that is too short can almost always be fixed without a new housing. The tool is a flat or formed insulating part that forces the path over its edge. Four designs cover most cases:
- An insulating film with overhang under a busbar. The path runs from the edge of the bar to the edge of the film and around it, so it measures overhang plus film thickness. Subtract the positional tolerance of assembly from the overhang.
- A barrier or partition between two potentials. It counts only if the joint at its base is not the shorter path, so it has to be bonded, self-adhesive or seated in a slot wider than X.
- A cap or cover over terminals and connections, folded or thermoformed as an insulating molded part.
- A group I material exactly where the path runs, such as a flame-retardant PP film with CTI 600 on a group IIIb carrier. The section on the film then has to hold the voltage on its own.
How a film is shaped matters too. If film edges are slit for assembly, every slit shortens the creepage distance. Patent EP1385180A1 describes exactly this problem with transformer films and folds the film instead of slitting it. At GOBA the die pre-embosses fold edges in the same stroke, and the part bends into its final shape along these lines. Where a slit only serves assembly, a fold edge is therefore the better choice. The same applies to busbar insulation: a closed sleeve beats any slit one.
GOBA makes these parts in stamping and forming from films, papers and laminates from 0.023 to 3.0 mm thick and up to 1,000 × 2,000 mm, with steel rule dies or solid steel tools, folded, embossed, bent or thermoformed, self-adhesive on request. We deliver from a single piece, within two weeks at most when the material is in stock. For a quote we need the drawing, quantity and thickness. Add the required creepage distance, pollution degree and voltage, and we select the material from our insulation materials to match.
GOBA takeaway: geometry or material first, coating last
Creepage distance hinges on three numbers in the specification: voltage, pollution degree and CTI. The order of levers follows from them: at pollution degree 2 the material first, at pollution degree 3 the rib, the barrier with a sealed joint or the film with overhang first. Coating and potting look cheaper but count only where the product standard allows them and they have been tested accordingly. Fix the creepage distance while the geometry is still open, and measure it with tolerances before the tool is ordered.
