MSHA - Code of Federal Regulations - 30 CFR 18.24
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MSHA - Title 30 CFR
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  |
            +------------------------+-----------+------------+