When the wiring in a house is overloaded with too many appliances drawing current, it overheats and can start a fire in the walls or attic. Old electric panels with round, glass screw-in type fuses use a simple, dependable technology to avoid that problem: a small metal strip, visible through the window in the center of the fuse, is calibrated to overheat and melt apart when amperage (current flow) exceeds the rating on the front of the fuse. If you can’t see the metal strip in the window, that means the fuse strip has “blown” open and shut off the circuit. 

    But there is a flaw in their design. Wiring for the household circuits in old houses is rated for a 15-amp fuse, but the base that the fuse screws into, called an “Edison base” or “Type T” and similar to one for a light-bulb, will also accept a 20, 25, or even a 30-amp fuse—twice the rated, safe load for the wiring.

    Homes that were built with fuse panels in first half of the 20th century had an electric system designed for the expected usage of the times. Just a few circuits were necessary for a refrigerator, range, a couple of fans, and some lighting. But the 1950s began the era of “Live Better Electrically,” and homes acquired TV, hi-fi stereo, a washing machine, dryer, multiple kitchen appliances, and eventually, air conditioning. Homeowners with the new, upgraded electric systems—with more amperage and circuit breakers instead of fuses—often got a gold medallion mounted at the front door by the builder to show that they had a home-of-the-future. But homes with an older system often had a problem with all those new appliances repeatedly blowing fuses, and residents discovered that replacing the 15-amp fuse that the circuit was rated to handle with a 25 or 30-amp one solved the problem.

    It also started a lot of house fires. So a new “Type S” fuse was created. It comes with its own base that locks permanently into the old Edison base and, once in place, only accepts a Type S fuse of the desired amperage rating. So after a 15-amp Type S fuse is installed, for example, no fuses with a higher rating can replace it.
    But even that was not foolproof. If you look closely at the photo above, of a panel in a 1930s Gainesville house we inspected recently, you can see two green-ringed fuses at the center that are rated for 25-amps. They have the old style “T” base that will accept any fuse rating, and will definitely allow too much current through the wires. The red-ringed fuse at right is a Type S, so that socket will only accept a 20-amp fuse like itself in the future, but the wiring is only rated for 15-amps—so the circuit is over-fused anyway, even with the new safety base, because it is the wrong rating. 

    Combine that issue with the fact that some older fuse panels are only rated at 60-amp total capacity and most have multiple wires clamped under the too-few fuse lugs, and you can see why insurance companies don’t want to write a policy for an older home with a fuse panel still in place. 

    As an old-time country realtor, “Miss Margaret” Hiers, of Chiefland, Florida, used to tell us, “If the house has got them fuses, you’ve got a problem!” Also, see our blog posts When did circuit breakers replace fuses in homes? and Why are old electrical components not always "grandfathered" as acceptable by home inspectors? 

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Here’s links to a collection of our other blog posts about ELECTRIC PANELS:

• What causes copper wires to turn green or black in an electric panel?  

• What is the maximum number of circuit breakers allowed in an electric panel?

• When should a corroded or damaged electric panel cabinet or disconnect box be replaced? 

• What is a tandem circuit breaker? 

• When did arc fault circuit interrupter (AFCI) breakers first become required?

• Can an electric panel be located in a closet? 

• Can an electric panel be located in a bathroom? 

• Can you add circuit breakers by different manufacturers to an electric panel if they fit?

• My circuit breaker won't reset. What's wrong?  

• What is a split bus electric panel?

• How do I identify a combination AFCI (CAFCI) circuit breaker? 

• What does a circuit breaker with a yellow or white test button indicate? 

• What is the maximum gap allowed between the front of a recessed electric panel box and the wall surface surrounding it? 

• What are the requirements for NM-cables entering an electric panel box?

• Why is a fuse box/panel an insurance problem for homebuyers? 

• Why is bundled wiring in an electric panel a defect?

• What is the difference between GFCI and AFCI circuit breakers? 

• Why are old electrical components not always "grandfathered" as acceptable by home inspectors?

   Visit our ELECTRIC PANELS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.