Direct Current (DC) ratings for current and voltage are not easy to come by. A review of Industry Standards shows that AC current and voltage ratings are abundant while DC ratings receive little attention. So here are a few guidelines.
Ampacity ratings are essentially temperature ratings. The higher the current the higher the temperature of the conductor; as temperature increases the molecular structure in the insulation weakens and its ability to withstand voltage decreases. It does not matter if the current is continuous as in DC or alternating as AC, the results are the same. Therefore we can use the Ampacity Tables in NEC 310-15 for both DC and AC.
Voltage ratings however, are a different story. AC voltage ratings are different from DC voltage ratings and DC stresses are decidedly lower than AC stresses. A key is found in ASTM D3755-79 where it states that DC voltage ratings are 2-4 times greater than the AC- rms voltage ratings found in ASTM D-149-82, the test standard for AC voltage breakdown.
DC dielectric breakdown is measured in volts/mil. Two test methods are used. In one the temperature is held constant and the voltage is increased until failure (dielectric breakdown) occurs. In the other, the voltage is held constant and the temperature is increased until failure. Correlation of tests produces a composite voltage vs temperature curve for each insulating material. These two major stresses on a cables insulation, temperature and voltage, are inversely related. The higher the temperature the sooner the dielectric breakdown for a given applied voltage.
Insulation materials vary in their ability to withstand a voltage breakdown, but they withstand DC voltage much better than AC voltage. There are ICEA Stds for each insulating material type. ?Below 2000volts AC, the DC voltage rating is 3 times the AC rating. ??From 2000 volts to 5000 volts the ratio is?? 2 ? times the AC voltage.? And above 5000v the ratio is 2 X the ac voltage rating. . ?Thermoset insulations have higher voltage ratings than thermoplastics.? When in doubt, check AEIC CS6-79 and the ICEA Standards.
Today's cable specifier has a wide choice of insulating materials and selection frequently requires a trade-off in properties.? For example, to obtain good fire retardation, one must give up some dielectric strength; to obtain good abrasion resistance, bend radius is sacrificed and so on. The specifier then must consider his total operating environment when selecting a commercially available insulation material.
