One advantage of direct current over AC is that DC current penetrates the entire conductor as opposed to AC current which only penetrates to the skin depth. For the same conductor size the effective resistance is greater with AC than DC, hence more power is lost as heat. In general the total costs for HVDC are less than an AC line if the line length is over 500–600 miles, and with advances in conversion technology this distance has been reduced considerably. A DC line is also ideal for connecting two AC systems that are not synchronized with each another. HVDC lines can help stabilize a power grid against cascading blackouts, since power flow through the line is controllable.