Depends on your skin type and if you burn easy.  If you burn easy (especially if your legs are still white - dont know what builders wear these days, not seen one in a while  

 ) if you burn easy start with a higher one, but if you tan well, I would start medium.  I never go lower than a 20 as I have fair skin and burn easy and with the higher reports of skin cancer now its a case of better safe than sorry.