Summary:
The implied warranty of habitability calls for an electrical system that was installed according to applicable codes at the time of installation. If it's working properly, landlords are under no obligation to update it, but they must repair any defects that create a hazard.
About Landlordology:
Landlordology features a number of in-depth articles, guides and practical tips for landlords and property owners, especially those with 50 rental units or less.