Understanding how to read your fuse panel begins with understanding its origins and whether upgrades or other changes over time affect your fuse panel's accuracy. You might have a Westinghouse no fuse load center if your home was built around the 1930s or even later. The Westinghouse load center replaced the earlier Edison base fuse with circuit breakers that could be used over and over again by simply resetting them after they tripped open under an overload or short-circuit condition. Electricians no longer install the Edison base fuse panels during new construction or during service upgrades. There are Edison base fuse panels still in use in many older homes, so you need to know how to work with them if you have one.
The Problem With Over Fusing
If you live in an older home that still has the original Edison base fuse panel, you need to use care when reading it. The problem is that the screw shells were all the same size and homeowners took advantage of that to over-fuse a circuit. Your 15-ampere fuse might have been replaced with a 20- or 30-ampere fuse to keep the fuse from blowing from a constant circuit overload. These changes affect the readings you might get when checking your fuse panel.
Do Not Assume That a Fuse Marked 20-Ampere Is the Right Fuse
Before you replace a 20-ampere or 30-ampere fuse, check the size of the circuit conductor connected to the fuse holder with an American wire gauge (AWG) wheel to see if it is wired for a 20- or 30-ampere fuse. The thing that you need to remember as you check the size of the circuit wires is that the National Electrical Code limits the use of AWG 14, 12 and 10 copper wire to use on 15-, 20- and 30-ampere circuits, respectively. Check the size of a wire by sliding the slot in the gauge over a straight, bare section of the wire. The slot in the gauge should slide over the wire with little resistance when using the proper size slot.
Reading the Fuse Panel Directory
Every fuse panel, like modern circuit breaker panels, has a legend or branch circuit directory that tells what branch circuit each fuse protects. The problem with circuit directories is that few are up to date. Many times existing circuits are extended or new circuits are added, but the changes are not noted on the panel's directory. Never assume that the information you read on a circuit directory is totally accurate. Directories also become illegible with time, and you may need to prepare a new one for your panel. An accurate circuit directory could mean the difference between life and death during an electrical emergency.
If you discover that you have over-fused circuits, you need to upgrade your electrical service to handle new branch circuits. Until the service is upgraded, reduce the loads on the existing circuits and install the proper size fuses.