Calculate high voltage air gap clearance or voltage from the other value, with unit conversions for in, ft, cm, m, V, and kV available.
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High Voltage Air Gap Clearance Formula
The calculator uses inches as the base unit for clearance and kilovolts as the base unit for voltage.
C = 1.25(V + 6)
V = (C / 1.25) - 6
- C = required air gap clearance, in inches
- V = voltage, in kilovolts
- 1.25 = clearance factor, in inches per kilovolt
- 6 = voltage offset used by the rule
If you enter voltage, the calculator uses C = 1.25(V + 6) to estimate the required clearance. If you enter clearance, it rearranges the same formula as V = (C / 1.25) – 6 to estimate the voltage associated with that air gap.
The calculator accepts clearance in inches, feet, centimeters, or meters, and voltage in volts or kilovolts. Inputs are converted to inches and kilovolts before the formula is applied, then converted back to the unit you selected.
Common Clearance Results by Voltage
The table below shows example clearances from the calculator formula. Values are rounded for readability.
| Voltage | Clearance | Clearance |
|---|---|---|
| 1 kV | 8.75 in | 22.23 cm |
| 5 kV | 13.75 in | 34.93 cm |
| 10 kV | 20.00 in | 50.80 cm |
| 25 kV | 38.75 in | 98.43 cm |
| 50 kV | 70.00 in | 177.80 cm |
| 100 kV | 132.50 in | 336.55 cm |
Examples
Example 1: Calculate clearance from voltage
Suppose the voltage is 15 kV.
C = 1.25(15 + 6)
C = 1.25(21) = 26.25 in
The required clearance is 26.25 inches.
Example 2: Calculate voltage from clearance
Suppose the air gap clearance is 40 inches.
V = (40 / 1.25) - 6
V = 32 - 6 = 26 kV
The estimated voltage is 26 kV.
FAQ
What does high voltage air gap clearance mean?
High voltage air gap clearance is the physical distance through air between two conductive parts at different electrical potentials. A larger voltage generally requires a larger air gap to reduce the chance of arcing or flashover.
Is this formula the same as an electrical code requirement?
No. This calculator applies the formula shown above. Actual required clearance can depend on AC or DC voltage, peak voltage, altitude, humidity, pollution level, insulation type, equipment category, and the applicable electrical standard. Use the result as a calculation from this rule, not as final design approval.
Why does the formula add 6 to the voltage?
The added 6 kV is an offset built into this clearance rule. It increases the calculated distance beyond a simple proportional value of 1.25 inches per kilovolt, especially at lower voltages.