Minimum Creepage Distances
Phase to phase voltage | Points of measure | Minimum creepage distances (inches) for comparative tracking index (CTI) range1 | |||
CTI[GREATER THAN EQUAL TO]500 | 380[LESS THAN EQUAL TO]CTI[LESS THAN]500 | 175[LESS THAN EQUAL TO]CTI[LESS THAN]380 | CTI[LESS THAN]175 | ||
2,400 | 0-0 0-G | 1.50 1.00 | 1.95 1.25 | 2.40 1.55 | 2.90 1.85 |
4,160 | 0-0 0-G | 2.40 1.50 | 3.15 1.95 | 3.90 2.40 | 4.65 2.90 |
1 Assumes that all insulation is rated for the applied voltage or higher.
High-Voltage Minimum Free Distances (MFD)
Wall/cover thickness (in) | Steel MFD (in) | Aluminum MFD (in) | ||||
A1 | B2 | C3 | A | B | C | |
1/4 | 2.8 | 4.3 | 5.8 | 4 NA | 4 NA | 4 NA |
3/8 | 1.8 | 2.3 | 3.9 | 8.6 | 12.8 | 18.1 |
1/2 | * 1.2 | 2.0 | 2.7 | 6.5 | 9.8 | 13.0 |
5/8 | * 0.9 | 1.5 | 2.1 | 5.1 | 7.7 | 10.4 |
3/4 | * 0.6 | * 1.1 | 1.6 | 4.1 | 6.3 | 8.6 |
1 | (*) | * 0.6 | * 1.0 | 2.9 | 4.5 | 6.2 |
NOTE: * The minimum electrical clearances must still be maintained.
1 Column A specifies the MFD for enclosures that have available 3-phase bolted short-circuit currents of 10,000 amperes rms or less.
2 Column B specifies the MFD for enclosures that have a maximum available 3-phase bolted short-circuit currents greater than 10,000 and less than or equal to 15,000 amperes rms.
3 Column C specifies the MFD for enclosures that have a maximum available 3-phase bolted short-circuit currents greater than 15,000 and less than or equal to 20,000 amperes rms.
4 Not Applicable-MSHA doesn't allow aluminum wall or covers to be 1/4 inch or less in thickness (Section 18.31 ).
Where C is 1.4 for 2,400 volt systems or 3.0 for 4,160 volt systems, Isc is the 3-phase short circuit current in amperes of the system, t is the clearing time in seconds of the outby circuit-interrupting device and d is the thickness in inches of the metal wall/cover adjacent to an area of potential arcing.
30 C.F.R. §18.53