For alternating current, most (63%) of the electric current flows between the surface and the skin depth, δ, which depends on the frequency of the current and the electrical (conductivity) and magnetic properties of the conductor. For aluminium, the skin depth is 10.6 mm at 60 Hz, and 11.6 mm at 50 Hz. As a result, the AC resistance of the conductor will be higher than the DC resistance (with the AC resistance at 60 Hz being slightly higher than at 50 Hz). Because of the relatively low frequency this effect is not large. For example, at 100 °C and 60 Hz, it results in only a 6.5% increase in resistance for a large conductor like 1092.5 mm2 (2152 kcmil) Bluebird and less than 1% for 402.8 mm2 (795 kcmil) Drake. The AC resistance will also be higher due to the proximity effect and hysteresis losses (detailed below). Other than the hysteresis losses noted below, the steel core has little effect on the resistance of the conductor because the steel core has much lower conductivity compared to aluminium. The American Society for Testing and Materials (ASTM) standard does include the conductivity of the steel core when calculating the DC and AC resistance of the conductor, whereas the (IEC) and The CSA Group standards do not.