Safety factors cover several individual items. You seem to be primarily interested in the safety factor on the load itself. That is best described as the load factor inherent for each load type in an LRFD/LSD type load combination. For LRFD/LSD, the resistance factor (0.65 - 0.95 typically) covers things like design errors, fabrication errors, erection errors, material variations, etc.) For ASD design that all gets blended into the basic safety factor and is not distinguished between the root causes. Overall in LRFD, the net load factor generally averages out around 1.5. It is lower for loads such as dead load that you estimate with a high degree of accuracy and higher for loads (live load, snow load, wind load, etc.) that a guess at best for the proper value. Things like cranes, explosions, and other odd-balls have their own sets of factors again based on the expected knowledge of the load and its intensity.