30 CFR § 18.24
Electrical clearances.
Minimum clearances between uninsulated electrical conductor surfaces, or between uninsulated conductor surfaces and grounded metal surfaces, within the enclosure shall be as follows:
MINIMUM CLEARANCES BETWEEN UNINSULATED SURFACES
+------------------------+------------------------+
| | Clearances (inches) |
| |-----------|------------|
| Phase-to-Phase Voltage | | Phases-to- |
| (ms) | Phase-to- | Ground or |
| | Phase | Control |
| | | Circuit |
+------------------------+-----------+------------+
| 0 to 250...............| 0.25 | 0.25 |
| 251 to 600.............| 0.28 | 0.25 |
| 601 to 1000............| 0.61 | 0.25 |
| 1001 to 2400...........| 1.4 | 0.6 |
| 2401 to 4160...........| 3.0 | 1.4 |
+------------------------+-----------+------------+
